Winch Launch Illustrated Training Guide

Transcription

1 Winch Launch Illustrated Training Guide By Bill Daniels November, 2013 Revision Copyright 2012 "There is nothing more frightful than ignorance in action." - Goethe Special thanks are due the following people for offering helpful suggestions. Don Ingraham CFI-G & Owner Cross Country Soaring Faribault, MN George Moore BSEE, CFI-G Spokane, WA Alfonso Ossorio CFI-G, DPE, Soaring Society of Boulder. Boulder, CO Frank Whiteley, Winchmeister Extraordinaire Greeley, CO Major Carl Keil, CAP Colorado Wing Glider Program Officer CFI-G Denver, CO Col. Jack Buschmann CAP National Glider Program Officer Denver, CO Bill Daniels is a CFI-G with a FAI Diamond Badge. Bill has more than 6000 hours in more than 50 types of gliders and holds a Commercial SEL with Instrument rating. He serves as Chairman of the SSA Growth and Development Committee and is a CAP glider program instructor with the rank of Lieutenant. Bill took his first winch launch in 1960 and made thousands of auto tows at El Mirage Dry Lake in the 1960 s. In the May, 1969 issue of Soaring Magazine, Bill wrote up his flight test of Jim Marske s Pioneer flying wing which was entirely done on auto tow. When the dramatic increase in fuel costs drove up the cost of aero tows, he became again interested in ground launch as a means of reducing the cost of glider flying. His presentation High Performance Winch Launch 40 Seconds to Freedom at the 2005 SSA Convention led to an article in the December, 2005 issue of Soaring Magazine by the same name. Another Soaring magazine article Ground Assault! appeared in the January 2013 issue. Further presentations on Advanced Winch Design and Pilot Training for Winch Launch at the 2011 SSA Conference in Philadelphia served to popularize winch launch in the US. Bill travels with Frank Whiteley to train clubs and commercial operations in winch launch operations. 1

2 Introduction Universal training guides for winch launch are difficult to write since there are many differences between launch sites, winches and gliders, however, this document will try to present as close to a universal viewpoint as possible. Each site must develop a winch training manual and syllabus specifically addressing the requirements of their operation. This is neither that manual nor that syllabus although it can be a guide for creating them. It is the view of many winch qualified instructors, including myself, that a sufficiently knowledgeable, alert and skilled pilot can safely cope with any situation arising from winch launch. This guide will focus on winch launch hazards and techniques for their avoidance in hopes the reader, with the assistance of a qualified flight instructor, will achieve such a level of skill and knowledge. Such a pilot need have no fear of winch launch. Any group setting out to provide winch launch training should understand a winch launch is of very short duration compared to aero tow. Things happen very quickly too quickly for traditional instruction methods. The student should be thoroughly briefed on the items to be demonstrated or practiced before the launch and the review or critique should be reserved until after release, or after landing. The short duration of the launch makes it reasonable for the instructor to fly at least the first three launches in order for the student to become familiar with the physical sensations and views from the cockpit. This allows the student to observe the instructors good technique and, hopefully, adopt it. The instructor should demonstrate each technique and maneuver until the student clearly understands it before asking the student to practice it. Allowing a student to attempt a maneuver without a demonstration is neither advisable nor safe. The training syllabus should plan no more than six consecutive flights with each student in a session. Most students will be saturated after six and little can be gained from more. In order to maintain focus on the launch, this document will address the subject from the student and instructor point of view. For a guide on winch field operations, see Winch Operations Guidelines. This document assumes the reader has read that document. It should become clear winch launch demands a level of skill and alertness which might not have been taught in aero tow training. This is both a challenge and an opportunity. Pilots who master winch launch will become better at almost everything else. Certainly, they will become expert at accuracy landings and soaring away from low altitudes. They may also become safer aero tow pilots by adopting the same level of alertness required by winch launch. Winch launch is highly repetitive and the list of skills demanded is not lengthy. However, those skills which are required must be learned extremely well. 2

3 Human Factors It almost goes without saying that winch launch is not for everyone, but no healthy person willing to undertake a formal instruction program should have problems. In addition to the usual soaring hazards of dehydration and fatigue, winch launch can put stress on a pilot in the form of mental anxiety. It is incumbent on instructors to keep a close watch on their students and call a break in flight instruction if a student seems stressed. Instructors themselves are particularly vulnerable to dehydration and fatigue since they spend hours in the cockpit talking loudly which causes them to exhale moisture as well as helping students push gliders. Most glider trainers have little space in the cockpit for water containers so it is vital the ground crew take the initiative of offering water to instructors at regular intervals. Ground crews would not be out of line to suggest an instructor take a break if he appears fatigued. Instructor Note 1: Task Switching Costs Experiments show subjects are slower and less accurate at performing a complex task when they suddenly switch from another complex task especially if the rules for task 2 are different than for task 1. The same experiments show task switching is a teachable skill. (For more information, search the Internet for "Task switching costs".) Instructors must be alert for students having difficulty switching from "Launch Mode" to "Landing Mode" and take steps to help them learn task switching skills. Condor flight simulator can be a useful aid. Instructor Note 2: Low G sensitivity Instructors have observed that some individuals react unpredictably when first exposed to zero G. Since this is a normal situation encountered in rope break recoveries, it's necessary for them to become accustomed to reduced or zero G so they can stay focused on flying rope break recoveries accurately. Teaching proper pushover technique resolves the issue. Have the student fly a series of dive, zoom and zero G pushovers at several thousand feet AGL until they can do so accurately. The average healthy human has a reaction time of seconds to actuate a control like a rope release handle or initiate a pushover. This is more than quick enough to perform safely on winch launch. See: If the instructor has any reason to suspect a student reacts too slowly, he should conduct a test with the above web site. While a student may react slowly simply because they lack a clear understanding of the action expected and is not yet reacting instinctively, slow reaction time can also be a symptom of medical problems which needs attention from a physician. The instructor should discuss the issue with the student and if understanding is not the problem, gently suggest the student take steps to deal with the issue before instruction continues. There are individuals who complain loudly about the 'extreme' acceleration encountered in a winch launch. However, simple arithmetic will show the maximum possible acceleration is less than 1.5G with the limit being the weak link. While this is much greater than with aero tow, it is not "extreme" by any means. Naval aviators encounter 6G on catapults. Race drivers routinely encounter more than 3G's. Lying flat on one's back generates 1G of "eyeballs in" acceleration which is typical for winch launch. Complaints about acceleration may indicate a pilot is anxious about winch launch. Such anxiety is likely to reach a peak during the seconds before a launch begins manifested by physiological issues such as breath holding, hyperventilation and hypoxia (Google: "anxiety physiology"). If not reassured by a calm, confident instructor, the student may perform erratically. Many otherwise inexplicable accidents might be explained by pilots suffering anxiety induced hypoxia. The 3

4 instructor should be constantly alert for anxiety and make every effort to reassure and calm students. Suggesting a student "relax, take a deep breath" just before each launch is an effective strategy. Difficulty switching tasks quickly can also be a serious issue. (See Instructor Note 1) Examples are difficulty switching from "sitting" to "flying" mode during the quick ground roll which can lead to a dropped wing and the probable ensuing groundloop or when dealing with launch failures where the pilot must instantly switch from "launch mode" to "landing mode". It is possible task switching are involved in many winch accidents. Fortunately, task switching skills improve with practice and the instructor can hasten this skill by using software such as Condor Flight Simulator (See Page 17). The concept of a pilot being "behind the aircraft" is highly applicable to winch launch. A winch training program should emphasize getting "ahead" of events and controlling them. Equipment - Winches Winches may be divided into two categories: 1. Speed Controlled (SC) with a strong throttle vs. rope speed relationship which means the winch operator controls airspeed. 2. Automatic Tension Controlled (ATC) winches which allow the pilot to control airspeed. Classic SC winches tend to use automotive engines and transmissions which, in their original role, were designed to accelerate a heavy vehicle and, when cruising, resist slowing under increasing load if the throttle is held constant - i.e. torque increases with load. In its new home, the power train tries to maintain rope drum RPM under increasing load even though rope speed must slow down with increasing climb angle to maintain a constant airspeed. The automatic transmission was highly tuned for the donor vehicle but is wildly mis-matched to launching gliders so the critical initial acceleration is very difficult for the winch operator to control accurately. ATC winches use electric or hydrostatic Continuously Variable Transmissions (CVT), a tensiometer and a microcontroller or Process Logic Controller (PLC). The winch automatically controls rope tension as measured by a tensiometer and the pilot controls airspeed with elevator inputs just as in free flight; i.e. nose up reduces airspeed and nose down increases airspeed. No speed signals are needed between pilot and an ATC winch since the pilot controls airspeed. Unlike automotive drive trains, ATC winches reduce torque with increasing load. An ATC winch controls rope tension so it can never exceed weak-link strength effectively eliminating weak-link failures and greatly reducing rope breaks. It will automatically increase power if the glider encounters a downdraft and decrease power in updrafts easing the pilot s workload while producing a higher and safer launch. With SC winches, the winch operator controls the glider airspeed with the winch throttle. If the pilot wishes to change airspeed, the pilot must signal the winch operator to increase or decrease speed/power. Although the winch operator is charged with controlling the glider's airspeed, he has no airspeed instrument and must proceed on feel or signals from the pilot. The pilot can only control the steepness of the climb by changing angle of attack. If no angle of attack (AoA) indicator is installed in the glider, the pilot is also relying on feel. As a result, no two launches will be alike. It's like a couple trying to dance with each listening to different music. With some SC winches, changes in climb steepness can have an unpredictable effect on airspeed. If the SC winch is less powerful or not operating at peak power, increased climb steepness will load 4

5 the engine and slow the winch resulting in decreasing airspeed. If the winch is powerful, the additional pull of a steeply climbing glider will have little effect on rope speed in which case the glider airspeed must increase in response to a steeper climb. SC winches may even present alternating speed behaviors in the same launch. Pilots should expect unusual behaviors. Not surprisingly, pilot training is highly specific to a particular SC winch and its characteristics. In the author s experience, no two SC winches are alike nor is the behavior of a particular winch the same with different gliders or under different wind conditions. Winch design is undergoing a slow shift from SC winches towards ATC winches which will allow standardization of winch behavior and therefore training syllabi. ATC winches make the whole winch training experience easier, safer and far more pleasant. Equipment Gliders Modern glider trainers such as the ASK-21 or DG505 exhibit exemplary behavior during winch launch. These trainers have a CG release location and elevator authority balanced so that little elevator input is needed to achieve an ideal launch profile. These gliders make winch launch a joy. However, certain older gliders can present problems. Those with only nose hooks will exhibit "porpoising" behavior which makes them unsuitable for winch launch. Gliders with deep fuselages, high wings and/or limited elevator authority can present special problems. If the CG is far above the hook, a strong nose up "Inertia coupling" is induced during acceleration. When combined with a small tail, the pilot may find it difficult to limit premature rotation. Gliders with a 'compromise hook', neither at the nose or CG, will need full up elevator to climb and will experience a degree nose "pop-up" if the rope breaks as discussed in the section on Launch Failure. These "Problem gliders", if launched at all, should be handled with care. When combined with classic SC winches, they make for a poor winch transition experience. Equipment Winch Rope In the past, winches used stranded steel cable or solid wire but UHMWPE fiber rope (Spectra TM, Dyneema TM ) offers so many operational and safety advantages; there is no reason to consider steel. In fact, most insurance policies will not cover using steel cable. Pre-Launch Preparation Before accepting the rope, the pilot must make certain both self and glider are ready for launch. Extra care must be taken to insure there are no loose objects in the cockpit. Seat cushions must be firm enough the pilot will not sink into them under the force of acceleration. Rudder pedals should be adjusted closer than normal. The pilot should push hard on the pedals to determine if there is danger of sliding aft under acceleration such that critical controls become unreachable. The glider must be staged pointing exactly at the winch. If this is not done, the lateral component of rope tension can cause a dangerously unstable transition into the rotation phase. As with aero tow, care should be taken to insure air brakes and canopy are locked. While a glider will climb satisfactorily with open spoilers, they shift span-wise lift distribution toward the tips which greatly increases the bending moment on the wing spar. The pilot must be certain the weak link specified in the glider's POH is used and the correct ring set is attached to the CG release - never the nose hook. 5

6 Pilots should have a target airspeed specific for the glider type in mind and know its maximum winch launch airspeed. They should have estimated a critical altitude below which there is sufficient runway to land straight ahead and above which there is enough altitude to circle back to the departure runway. The pilot must also have a plan for dealing with very low level launch failures. There's no time to plan these things during the launch. Finally, the pilot should place the left hand on the release knob, but not gripping it, so it can be pulled without delay should the pilot feel the ground roll is not precisely nominal. Controls should be in the neutral position. It's considered good form to tell passengers to stiffen their neck muscles just before the ground roll begins. The Normal Launch Figure 1 Normal Launch The launch will begin with an acceleration of 1 G gaining 19 knots every second. After 1 second, the glider will have traveled 16 feet and have control authority. After 2 seconds, the glider will be at 38 knots and just lifting off the runway surface. At 2.6 seconds, the glider will begin smoothly rotating to capture the "Target Airspeed". During the rotation phase, the pilot will be controlling the pitch rate with reference to the ASI such that the glider will stabilize at the pre-selected "target airspeed" - the same technique as in high performance airplanes rotating into a climb to capture the V Y (Best rate-of-climb airspeed). The initial acceleration tells the pilot what rotation rate will be required to "capture" the target airspeed - low acceleration, slow rotation, higher acceleration, faster rotation. The pilot may allow the airspeed to exceed the target during rotation to achieve a greater stall margin and bring it down once in the climb. At no time during the rotation phase will the combination of Airspeed, Attitude and Altitude (AAA envelope) be such the pilot could not pitch over and land straight ahead with generous safety 6

7 margins should a launch failure occur. During the remainder of the launch, the pilot will make small adjustments to pitch attitude to maintain the target airspeed. With rope tension set to equal the glider's weight, (Tension Factor = 1) the climb angle will peak at 45 degrees. Lower tension results in shallow climbs - higher tension requires steeper climbs. The pilot will take care to track down the runway centerline as the glider climbs or, if there is a crosswind, slightly upwind of the runway. Figure 2 Normal Launch As the glider reaches the round over phase, the angle between the rope and the longitudinal axis of the glider will approach 70 degrees. Tost CG releases are designed to automatically release ( Back-release ) the rope at this angle. Good winch operator technique is to cut power just before the automatic release occurs so there is little tension on the rope as the release occurs. Cutting power will increase the sag of the rope so an automatic release is triggered. The pilot must still pull the release anyway to assure rope separation and instill good habits. As the ring set leaves the release mechanism, the launch is over and free flight begins. The nose will be slightly high so the pilot must lower it to achieve a normal glide. The glider will be at a height above the runway equal to about half the length of the rope used. If launched into a headwind, the height will be greater. No "clearance" turn is required - the pilot may continue straight ahead or turn either way. A launch to 2000' AGL will take about 40 seconds. Operation at large airfields with long rope lengths have achieved release heights of 5700 feet AGL with the glider climbing for over two minutes. The height achieved depends mainly on the length of rope used and the headwind component. The winch operator will recover the rope by winding it onto the rope drum before it hits the ground. The rope should never pose a risk to anyone or anything. 7

8 Ground Roll Technique & Hazards The first 2.5 seconds of a winch launch require a high level of alertness. If at any time the pilot feels things are not precisely normal, an immediate release is mandatory. Never try to "save" a launch or pick up a dragging wing tip - RELEASE! More so than with aero tow, the glider must track straight with wings level. As noted in the section on normal launch, roll and yaw control will be achieved in less than a second so it is difficult to imagine a wing dropping to the ground unless there is aileron input. As part of the cockpit preflight check, the pilot should consciously center the ailerons and rudder. The only exceptions are rudder input when the CG release is located off center-line on the fuselage side as with the K-13 or when a crosswind component is present and even this should be removed before the rotation begins. Failure to release when things start to go wrong may result in a particularly ugly ground loop. If a wing tip digs in as in the photo below, the opposite wing will instantly accelerate, generating enough lift to drive the tip further into the ground making a save impossible. In the worst case, the glider may roll inverted with fatal results. The pilot should reject the launch if the takeoff area is not level; the grass not cut short or a crosswind exceeds the POH limit. Figure 3 Dragging Wingtip Leads to Groundloop The picture on the right resulted from a pilot holding aileron into a crosswind which exceeded the POH limit, on sloping ground, in tall grass and failing to release when the wing hit the ground. It is not possible to determine conclusively, but it appears the rope is still attached to the CG hook. The winch driver prevented a much worse accident by chopping power as shown by the open parachute striking the glider s nose. The glider suffered major damage but, thankfully, neither pilot was injured. 8

9 Liftoff and Rotation - Technique & Hazards Figure 4 Low Airspeed Plus Fast Rotation Leads to a Stall The stall margin is reduced during the rotation phase as the wing is loaded by the need to accelerate the glider to a vertical speed of over 45 Knots even as it is beginning to take on the load of the rope pull. As a result the loaded stall speed increases significantly during the rotation phase. If a highly loaded wing stalls while there is left-right asymmetric control inputs, a violent snap roll may ensue with probable fatal results. There is a short window of vulnerability from tail wheel liftoff to V W-MIN where up elevator combined with the inertial nose-up couple can cause the nose to rise faster and higher than a pilot intended. See Appendix A. Figure 5 Stall Plus Asymmetric Controls Leads to Snap Roll 9

10 Most gliders will exhibit some tendency to pitch up due to the low CG release location. Any such tendency must be opposed with down elevator during the ground roll and early rotation to prevent too early and too rapid rotation. As the glider rotates into the climb, the rope pull vector will move closer to the CG so less down elevator is needed. In a steep climb, the pull vector may pass ahead of the CG so up elevator is needed. If the pilot is controlling airspeed with pitch attitude, the pilot will cope with trim changes as a matter of course. Stall hazards may be avoided by carefully monitoring airspeed during the takeoff roll and rotation into the full climb while resisting any tendency to rotate into the climb prematurely. Airspeed may be permitted to exceed the target during the rotation phase for extra stall margin. Figure 5 Inertia Couple forces tail down Figure 7 is from a British Air Accident Investigation Board (AAIB) report on a fatal winch launch stall/snap roll accident. It relates rotation rate to stall speed. The British Gliding Association (BGA) insists the Figure 6 Rotation Rate vs. Stall Speed problem is rotation rates in excess of 10 degrees/sec even though it is clearly possible to stall at 10 deg./sec. More likely, the pilot forced the glider to begin rotation at less than 40 Kts and was caught in the AIUR window of opportunity. See Appendix A The US view emphasizes airspeed saying the pilot should not initiate rotation until ~50 knots. Then, smoothly increase the rotation rate as the airspeed continues to increase then reduce the rotation rate to zero as the target airspeed is captured. During the entire rotation, the airspeedrotation rate locus must remain safely above stall as shown by the red line added by the author. With sufficient speed in hand, rotation rates more than 20 degrees/sec can be used safely. Higher airspeed in the rotation also makes recovery from a low launch failure much safer. 10

11 This section provides the best available guidance on flying the rotation phase safely using indicated airspeed and the pilot s estimate of rotation rate. However, it should be clear to any reader a pilot trained to use an AoA indicator could simply be told, "Don't stall the glider" and "Fly best L/D AoA". Airspeed and rotation rates would be subsumed into the AoA data. Crosswind Technique Gliders tend to 'weather vane' into a crosswind more easily with a CG hook than with a nose hook. For this reason, it is critical no winch launch should be attempted with a crosswind component greater than the demonstrated crosswind component listed in the glider's AFM/POH. Aero tow pilots have been trained to hold the upwind wing low during the takeoff roll to help maintain position behind the tug. For winch launch, this technique introduces the possibility of dragging the upwind wing tip leading to a groundloop as addressed in the section on ground roll. Fortunately, winch ground roll is very short and there's no tug to follow so holding wings level and allowing the glider to drift a few feet downwind is of little consequence. The best technique is to use only as much downwind rudder as needed to maintain a straight ground roll and allow the wing runner to balance the glider by tilting it slightly into the crosswind while the pilot holds exactly neutral ailerons. All asymmetric control inputs must be removed during the rotation phase. Winch pilots should always be aware of airfield surface conditions with particular attention to sloping ground in the takeoff area. High ground on the upwind side reduces wingtip clearance even without a bank into the wind. Banking into the wind almost guarantees wing tip contact. Once into the main climb, lowering the upwind wing will cause the glider to track into a crosswind. "Crabbing" into a crosswind is ineffective. Keeping the ground track somewhat to the upwind side of the runway during the launch is advisable. The winch operator and rope retrieve crew will appreciate this as it makes the post-release rope recovery easier and safer. Takeoff Roll and Rotation Summary 1. Set up straight and square on level ground with a straight rope and the glider aimed at the winch. 2. Never attempt a launch with a crosswind greater than the POH/AFM limit. 3. The glider must roll straight with wings balanced - otherwise RELEASE IMMEADIATELY. 4. Pilot must start roll with neutral ailerons. 5. The pilot's left hand should be on the release but not gripping it. 6. Pilots should be aware of the AIUR window of vulnerability and avoid up elevator until V W-MIN 7. Rotate smoothly so airspeed continues to increase until it stabilizes at the V Y "target airspeed". 8. A stall during rotation is EXTREMELY DANGEROUS. 11

12 V Speeds and Airspeed Excursions In an otherwise normal launch there will be airspeed deviations due to a variety of causes including pilot error, winch behavior, winch operator error, turbulence or wind layers. Whatever the cause, airspeed errors, in themselves, represent less of a threat than ill considered actions by the pilot which may increase the risk. Managing airspeed excursions while still on the rope is preferable. A winch pilot is well advised to become very familiar with the various airspeed limitations listed in a glider's manual such as V S and V W-MAX and a new V speed appearing in flight manuals V W-MIN (Minimum safe winch airspeed) and V Y (best climb airspeed) It will surprise some that pilots can have a large degree of control over airspeed and should use it before signaling for power changes. The concept of "pilot controlled airspeed" will seem strange to aero tow only pilots who are accustomed to merely following a tow plane while the tow pilot controls airspeed. It may be useful to consider winch operator and pilot as a flight crew with the winch operator acting as a "flight engineer" controlling power with a throttle and the pilot controlling airspeed with pitch attitude. The winch driver should select throttle settings for the glider type and wind akin to setting "takeoff power" on heavy airplanes. The pilot accepts power so provided and controls airspeed with pitch. With a high-power, high-acceleration winch, it is not unusual for the airspeed to be higher than (V Y) at the end of the rotation phase. The pilot should then be able to smoothly reduce airspeed to (V Y) by momentarily increasing pitch attitude. Instructor Note 3: 180 Turns Aero tow trained pilots will have an instinct to make a 180 turn to land downwind after a low launch failure. Since the glider will still be near the approach end, a 180 turn will position the glider with the runway behind and nowhere to land. In fact, most of the runway would have been available for a straight ahead landing. Instructor Note 4: Head-up illusion This results from the utricle and the saccule in the vestibular system responding to a sudden forward linear acceleration where the pilot perceives the illusion the nose of the glider is pitching up. This may cause some winch pilots to complain of an acceleration induced pitch-up which did not, in fact, occur. In the worst case, a pilot suffering from this illusion will respond by nosing the glider into the runway. The instructor must insist students rely on visual cues for the determination of pitch attitude. Instructor Note 5: Low Turns Aero tow pilots are taught '180' turns to a downwind landing on the departure runway from a height of 200'. In fact, this is closer to one 225 degree turn followed by a 45 degree turn in the opposite direction to align with the runway. The second 180 turn described here has a greater safety margin. If the airspeed is below V Y the pilot need only lower the nose slightly to reduce the load on the winch allowing RPM, and therefore airspeed, to increase. To decrease airspeed, the pilot should raise the nose to increase the load on the winch engine. This is exactly like controlling an airplane's airspeed during a full power climb. Pitch changes are the only action required to control airspeed with an ATC winch. With an SC winch the pilot may need to signal the winch operator requesting a change in power in addition to making pitch changes. 12

13 To request an increase in speed/power, the pilot lowers the nose and the winch operator, seeing the glider is not climbing normally, will increase throttle to compensate. As with an ATC winch, lowering the nose itself will cause the airspeed to increase. To request a reduction in speed/power, the pilot yaws the glider left and right vigorously with the rudder. The winch operator will reduce throttle roughly 5% in response to each signal. Do not lower the nose when signaling for a reduction in speed/power as this will cause a further increase in airspeed and counter the effect of reduced throttle. If these signals fail to produce the desired result, delay the release until at a safe altitude and fly a normal landing pattern. Some German clubs are utilizing airspeed radio telemetry so the winch operator can see and control the glider's airspeed accurately. The pilot then controls the climb profile but he has no indicator to show how to do this properly. If the pilot had a rope tension and AoA display, he could just increase pitch attitude until either the tension was near the weak link breaking point or the AoA was at best L/D to achieve an optimum launch. Increasing pitch would cause the winch driver to increase power to maintain airspeed. While this scheme is workable, controlling tension as with an ATC winch and letting the pilot control airspeed is a better solution. Managing a Launch Failure Figure 8: Flight trajectory for 100 foot AGL rope break The term Launch Failure includes a variety of failure modes. Most commonly it is a rope break but may also include failure of the weak link or winch. It is the winch equivalent of "Premature Termination of Tow". Whatever the reason for the failure, the pilot response is always the same - Unload the wing and Fly the Glider! Virtually all accidents from launch failure are pilots failing to fly the glider. If the nose is up 45 degrees at the failure, the glider will be losing airspeed at about 12 knots per 13

14 second so the pilot must react swiftly. The correct action is to unload the wing by vigorously pushing over in a zero G ballistic trajectory. At zero G, "stall speed" has no meaning - airspeed can be, and often is, well below V S without stalling. At zero G the wing is producing no lift thus has no induced drag. If the Altitude, Attitude and Airspeed at failure fall within an envelope from which a safe recovery can be made, all is well. Outside of this envelope, at low altitude, low airspeed or high pitch angle, there will be insufficient height for a recovery. Like a CG envelope, the AAA Envelope will vary from glider to glider and, to a degree, on the experience of the pilot. A winch pilot should study Figure 9 to determine the AAA Envelope best suited for his experience and glider. Note the selection of 1.5 x V S as a recovery airspeed in Fig. 9 introduces an illogical result. One would not expect to lose 100 feet with a failure at 50 knots and 10 degrees nose up. Actually, one would pitch to 10 deg. nose down, recover to yellow triangle airspeed with little height loss, then simply flare and land. If a glider requires lots of up elevator in the climb, expect the nose to 'pop up' degrees with a rope failure which may take the glider outside the AAA envelope - use higher airspeed and lower climb angle in these gliders. The key determinant of height loss is the minimum airspeed at the top of the ballistic trajectory. A zero G pushover will minimize airspeed loss and therefore the height lost. See Appendix 2. Figure 9: Typical AAA Envelope in Red Box The zero G push over should continue until the nose is as far below the horizon as it was above it when the rope break occurred. The words as far below the horizon as it was above have been carefully chosen to provide the maximum safety margin. If a rope break occurs at 20 feet with the nose up 10 degrees, the glider will strike the runway if the pilot attempts a 45 degree dive but at 500 feet with the nose 45 degrees above the horizon, lowering the nose only 10 degrees below the horizon will not result in safe airspeed recovery. With the nose at the correct angle below the horizontal the pilot must wait for a safe airspeed ( xv S) before taking further action. It is critical no turn be initiated or spoilers opened until a safe airspeed is achieved. If an AoA indicator is installed, the pilot can safely recover at a significantly greater height by smoothly reloading the wing starting at the top of the trajectory at an AoA corresponding to best L/D in level unaccelerated flight. 14

15 Once a safe airspeed is achieved, the pilot must decide whether to land ahead on the remaining runway or circle to land. Figure 10 Profile view - Straight Ahead Landing It's useful to divide launch failures into high and low where low failures end with a straight ahead landing on the runway and high failures require a circle to land. Each wind speed-runway-winchglider combination will have a Critical Altitude below which a straight ahead landing can be made safely using spoiler and slip (Figure 10). Above the Critical Altitude the pilot can safely circle back to land into-the-wind on the departure runway (Figure 11). Under most conditions, the land ahead and circle back options overlap by several hundred feet making the choice obvious and relatively benign. Figure 11 Map View - Circle to land A circle to land from a "high" launch failure is done in phases. The first, as with a low failure, is a zero G pushover and safe airspeed recovery - only then enter a 45 degree bank, 180 degree turn to the downwind side of the runway if a crosswind component is present. The downwind turn direction is chosen so the bank angle will be decreasing as the glider performs the final 180 degree turn to align with the runway. (Figure 11) 15

16 As the glider approaches the 180 degree point, the pilot must assess the situation and decide if a short downwind leg is needed. If the height AGL is near 200 feet, the turn should be continued through 360 degrees so the glider is again lined up with the runway. 200 feet AGL is the same height students are taught to turn back when dealing with a aero tow rope break. If the height is greater than 200 feet AGL at the end of the first 180 turn, a short downwind leg is flown until the height reaches 200 feet AGL where another 180 degree turn is made to align with the runway. Take care not to continue the downwind leg past the approach end of the runway unless at or above the normal pattern altitude. At all times, the airspeed must be closely controlled and the gilder perfectly coordinated to maximize the margin of safety. Precision flying is very important in the circle-to-land maneuver. Obviously, the aggressiveness required in the circle to land maneuver depends on the height above the "Critical Altitude". At >800 feet AGL a normal landing pattern can be flown. Teaching a return to the departure runway has many advantages. It will work at all airfields, no off field landing need be considered and the landing will be into the wind. However, if other runways are available, their use should not be excluded. The purpose of the circle to land maneuver is not to fly the glider back to the start line but to allow a safe landing on any part of the runway. Stretching the maneuver to stop at the start line (Winch launch equivalent of "Get-Home-Itis") should be strongly discouraged. A safe landing is always preferred over a convenient one. Extremely low launch failures Launch failures just after liftoff requires extremely fast "task switching" from a takeoff mindset to a landing mindset. The good news is the glider will be perfectly positioned for a landing straight ahead. This situation is exactly like a "balloon" on landing - the glider will be slightly nose up gaining height and losing airspeed. The pilot must lower the nose enough to prevent further height gain and airspeed loss, stabilize the glide, and then flare for touchdown. Glider Release Failure True release failures are thankfully extremely rare. Slightly more common is the rope fouling the landing gear which results in an attachment point other than the CG hook. For this reason, if the glider is jerked forward over the rope, the launch should be stopped, and the ring set attachment to the CG hook re-checked. To prevent fouling, the lead rope (Strop) between the weak link and the ring set is made very stiff so it can't bend around the wheel. If, despite this, the launch has begun with the rope fouled on the wheel the pilot will have no way to release. The pilot should overfly the winch on the runway heading so the winch operator will see the glider has not released. This will tell the winch operator to trigger the guillotine cutting the rope at the winch. With the rope cut, UHMWPE rope, unlike steel cable, will trail nearly horizontally behind the glider like a thread of spider silk. The pilot should fly a high pattern and steep approach to the runway with touchdown planned near the departure end. This should prevent a trailing rope from fouling anything on the ground. 16

17 Winch Failures Winches can have many failure modes. Unlike automobiles, a winch can't be driven around the block to diagnose problems. For this reason, any odd behavior, no matter how slight, should be cause to discontinue launches until the issue can be identified and eliminated. For the pilot, the main concern is the rapidity of failure. Instead of the sharp thump of a rope break, the rope tension may slowly fade away as the winch loses power. If the pilot is controlling airspeed, this will cause him to lower the nose as the power fades which is exactly the right response even if he doesn't recognize the problem as a winch failure. If the pilot is focused on attitude while the winch controls speed, he may allow the airspeed to decay until the glider stalls. It is good form for the winch operator to transmit "Abort, Abort, Abort" as soon as a failure is detected. The pilot should treat a suspected winch failure just like a rope break but should avoid landing on the rope if at all possible. If the glider lands on or near the rope, there is the possibility of the winch suddenly regaining power and pulling the parachute into the glider from behind. Summary of Launch Failure Techniques 1. Expect every launch to fail - Fly the Glider! 2. Estimate the "Critical Altitude" for conditions then plan emergency landings before launch. Include wind direction and strength in your emergency planning. 3. Always stay within a safe AAA Envelope. 4. React instantly to launch failure by pushing over at zero G - or even slightly negative G. (Note that airspeed below unaccelerated Vs is safe at zero G.) 5. Height lost or gained is primarily determined by airspeed at the top of the ballistic trajectory. 6. Establish level flight at a safe airspeed ( Vs) before opening spoilers or turning. 7. Pull release only after safe airspeed is achieved. This is not a critical item. 8. Execute landing straight ahead if below critical altitude or circle to land if above. 9. Pay extremely close attention to coordination and airspeed control when circling to land. 10. Concentrate on landing safely anywhere on the airfield - never try for a 'convenient' landing. 17

18 Instructor Note 6: Use of Condor Fight Simulator This amazing PC program can simulate almost all the techniques and hazards to a quite realistic degree. Like any simulator, Condor allows exploration of dangerous situations without risk. On the "NOTAM" screen select "winch launch" then set wind and weather conditions. The instructor should spend considerable time learning to fly the simulator so maneuvers look realistic. It helps the student visualize the aerodynamics if the smoke is on ( T key toggle). The simulator can be paused with the P key and the F2, F3, and F4 keys show external views with the arrow keys on the number pad shifting the view and the + and - keys zooming in and out. The "S" key captures a screen shot as a JPEG image which can be incorporated into training manuals. Unless a complete cockpit with accurate control placement and 180 degree view is available, the best instructor technique is to use Condor as an animated white board where the instructor flies the simulator and the student watches. The goal is to help the student visualize a maneuver. The simulated winch is an ATC winch with a tension factor of about 70% of the glider's weight. This allows the 'pilot' to control the glider airspeed with pitch as with an ATC winch. Airspeed control: Demonstrate raising the nose will decrease airspeed and lowering it will increase it. Tip drag leading to Ground loop: Bring a wing tip into contact with the ground just before the glider reaches flying speed. The glider will groundloop and, if the timing is just right, may flip and crash inverted. Stall and snap roll: Use any 15m glider or the PW-5. Use full up elevator to rotate into the climb then apply rudder as the 'glider' stalls. This is very convincing. Crosswinds: Set the weather for wind direction and speed. Rope breaks: On the NOTAM screen, set the rope failure probability to >50% and the program will provide random breaks or set the release function to the keyboard and have someone else activate the release without warning the 'pilot'. Practicing rope breaks on Condor is great training for those wishing to improve their "Task Switching" abilities. An infinite variety of rope break situations can be simulated. Condor Limitations: 1. The program does not support a "back release". 2. There are no "high density altitude airports" although users can create them. 3. Slow power loss cannot be simulated. 4. Rope tension is limited to 70% of the glider weight. 5. With only a single PC monitor, lateral vision is lacking so holding the wings level at steep climb angles is difficult. It helps to set the "weather" so clouds are in the field of view or set a turn point off the departure end and make it visible in options. 18

19 Suggested Winch Launch Transition Syllabus: (20 36 launches) Session one: Winch launch familiarization (Six launches) Instructor flies three normal launches and the transition candidate observes. Then the candidate flies three normal launches under close observation by the instructor. If a launch failure occurs, the instructor will take the controls and fly the recovery. The instructor then debriefs the candidate on the ground offering critique with suggestions for the next session. Session two: Further launch practice and zero G training. (One to six launches) The candidate flies up to six launches under instructor observation and tries to contact lift. If lift is found, the student will gain sufficient altitude for zero G pushover practice. Pushover practice is diving the glider to approximately 90 knots and then zooming upward at 45 degrees. When the airspeed drops to 65 knots, the instructor will yell Rope Break! and the candidate will push over at zero G until the nose is 45 degrees below the horizon. This allows the candidate to experience zero G and learn the control inputs needed to achieve it. If no lift is available, it may be necessary to take an aero tow to gain enough height. The instructor will debrief the candidate offering critique. Session three: High launch failure training. (Four to Six launches) The instructor will fly a launch to pattern altitude and pull the release simulating a rope break. He will demonstrate a recovery with zero G pushovers and airspeed recovery. The candidate will then fly the next 3-5 launches with the instructor pulling the release without warning simulating a rope break at successively lower altitudes but always above the Critical Altitude ending with a tight, low altitude 360 pattern to an into-the-wind landing. The instructor will debrief the candidate and offer critique. Session four: Low launch failure training. (Four to Six launches) The instructor will fly a launch to a height below the Critical Altitude and pull the release simulating a rope break. The instructor will fly a ballistic airspeed recovery and land straight ahead. The candidate will fly the next 3-5 launches with the instructor pulling the release without warning at progressively lower altitudes ending with a simulated break just after liftoff. The instructor will debrief the candidate and offer critique. Session Five: Winch failure training. (Three to six launches) The student will fly six launches with the winch operator simulating various winch failure modes. These should include over speed, under speed, slow power loss, delayed power cut and unusual gear changes if the winch is equipped with a transmission. The instructor and winch operator will debrief the candidate together and offer critique. Session Six: Crosswind training. (Two to six launches) This session may be skipped if the preceding sessions encountered significant crosswinds. Otherwise, it should be delayed until crosswinds are available. The instructor will demonstrate wings level takeoff roll with minimum rudder input and centering controls before rotation. The instructor will also demonstrate tracking to upwind so the rope falls on the runway. The student will practice under supervision. 19

20 Appendix A Acceleration Induced Uncommanded Rotation (AIUR) Does it really happen? It has been an article of faith among some winch launch groups that excessive acceleration will cause uncontrollable pitch up if the glider's CG is well above the CG hook. In support of this idea, a diagram is often used such as Figure 1 borrowed from the BGA safe winching initiative slide show. No doubt this drawing was made with the best intentions, but as we shall see, it can be misleading. One would expect that such a dangerous scenario would leave a significant record of accidents. However, detailed examinations of 20 years of accident records from a several countries, including the UK, failed to turn up even one accident where acceleration was listed as a primary or contributing cause. While the lack of relevant accident data is highly suggestive, absence of evidence is not evidence of absence. More analysis is needed. Why is this important given the lack of accidents? Figure 1 may lead to dangerously slow acceleration such that wing-drop cartwheel accidents become a significant risk. Cartwheel accidents do, in fact, appear frequently in the accident record. Counter-intuitively, it may even be that low acceleration is a hidden contributor of rotation stalls and flick rolls. That would indeed be a sad state of affairs. Figure 1 shows a glider that has somehow been levitated above the ground before a strong Cable pull is applied. It ignores the effect of tail wheel contact with the runway and does not identify the airspeed the diagram is depicting but it appears to be just above stall. A black arc is shown with a label stating this is the Maximum Elevator Pitch-Down Moment available without saying at what airspeed although again, it would have to be very slow. Elevator authority increases with the square of airspeed so if the glider were a bit faster, it might well be adequate. Figure 6 What exactly is the point of Figure 1 and to whom is it addressed? If it's telling pilots not to climb without safe airspeed, that's both valid and laudable. If it's telling pilots acceleration is bad, it's both incorrect and aimed at the wrong audience since pilots can't control their acceleration. Taken at face value, it seems to be telling pilots if you find yourself in this situation, you're screwed but they'll probably know that anyway. Possibly, the artist just want's to tell pilots how dangerous it can be and gets it wrong in the process. The most plausible scenario leading to the situation depicted in Figure 1 is one where cable pull began gently and ramped up slowly to a strong pull over about 10 seconds. As the glider reached lift-off speed, the pilot allowed it to float up to about 10 feet AGL where increasing cable pull overcame inadequate elevator authority causing uncommanded pitch-up. This situation could never develop if the initial acceleration had been strong and uniform from the beginning of the takeoff roll. 20

21 The following is a more detailed analysis of the acceleration phase at one second or smaller intervals in a timeline starting at the beginning of the takeoff roll and continuing to V Y. I'll use a Discus 2b single-place glider similar to the one shown in Figure 1 and stipulate 1G (19kts/sec) acceleration starting at T=0. Keep in mind that for a glider to pitch-up before the wheels leave the runway, the center of rotation must be at the tail wheel since the runway surface will prevent the tail from swinging down. Rotating about the tail wheel also means the height of the glider's CG above the hook has no influence since both would be swinging up along the same arc. Figure 2 shows the glider at T+1 second where the glider has reached 19 knots, traveled 16 feet and achieved solid aileron control authority. If the glider began the takeoff roll with the wings balanced, and the ailerons have remained neutral, it's difficult to imagine a wing drop could occur in such a short interval. If one balances, then releases the wing on a stationary glider, it will take longer than this to fall. The nose-up moment depicted in Figure 1 is in full force at T+1 but pitch-up is impossible since the tail wheel is in contact with the runway, and the wing can't yet lift the glider. The height of the glider's CG above the hook has no effect at T+1 except to increase the download on the tail wheel. This has the desirable effect of increasing resistance to weathervaning. Figure 3 shows the glider at T+2 seconds where it has traveled 64 feet and reached 38 knots. Elevator authority is quite good and rapidly improving with the square of airspeed. The rotation center remains at the tail wheel so the height of the glider's CG above the hook still has no effect. Figure 2 T+1 Second 1G, 19 Kts, 16 feet and Aileron Control Speed Wing lift is just now generating enough force to overcome the weight of the glider and starts it rotating upward about the tail wheel. The pitch-up is impeded by inertia and the Spur Roll effect shown in Figure 4. T+2 is the beginning a short 0.2 second Spur Roll phase. Figure 3 T+ 2 seconds 1G, 38 Kts, 64 feet The term Spur Roll colorfully describes a glider rolling along on its tail wheel with the main wheel in the air resembling a spur on a boot. 21

22 Figure 4 shows an exaggerated Spur Roll to explain the forces at work. Note the bent yellow line formed by the tail wheel, CG hook and winch. Even as wing lift greater than that required to lift the glider's weight (green arrow) is pulling a bend into the line, more than 1,000 lb-f of rope tension is trying to force it straight again (Red arrow). The system will reach equilibrium at an angle determined by rope tension. Greater tension equals a lower angle of attack and less tension equals a higher angle of attack. The angle shown in Figure 4 could only be reached under relatively low rope tension. Figure 4: Exaggerated Spur Roll The Spur Roll effect explains why gliders launched with extremely high acceleration leave the ground in a shallow, low angle of attack trajectory whereas slowly accelerating gliders tend to reach higher angles suggesting slow acceleration might play a part in rotation stall and flick roll accidents. Figure 5 shows our glider at T+2.2 seconds where it has reached 42 knots and lifted both wheels off the runway. Due to the Spur Roll effect the deck angle is only slightly above horizontal. The glider will reach VW-MIN in another 0.4 seconds. It's only in this 0.4 sec. lift-off to VW-MIN interval where the glider is vulnerable to the nose-up moment depicted in Figure 1. It's difficult to imagine it could overcome the lateral axis inertia and pitch damping to cause much trouble in this interval unless the pilot is using full-up elevator. Figure 5 T+2.2 seconds, 42 Knots, 78 feet and Liftoff Figure 6 at T+2.6 our glider has reached VW-MIN, where nose-up rotation becomes desirable. Elevator authority is far more than needed to control pitchup. As the rotation phase progresses from VW-MIN through the rotation phase to the initial climb attitude and airspeed (VY), the rope pull vector at the CG hook moves into alignment with the glider's CG eliminating the nose-up moment described in Figure 1 altogether. Figure 6 T+2.6 seconds 50 Kts, 111 feet and VW-MIN 22

23 The above scenario assumes a constant 1G acceleration from T=0 to T+2.6. In fact, some ramp-up starting at T=0 would be needed although it would likely be as short as 0.1 second. It also ignores the effects of density altitude and headwinds. The former will increase times and distances and the latter will reduce them. So, what would be the effect of reducing acceleration to 0.5G? The opportunity for a wing drop will double. Lower rope tension would allow a larger deck angle in the Spur Roll. It would then enter the AIUR window of vulnerability at a higher angle of attack and that window would last twice as long. There seems to be no safety benefit to reducing acceleration. To the contrary, it results in a very real increase in risk. Are there scenarios where AIUR is more likely? Only two are plausible. One is the Loft gently, then accelerate hard scenario. This might happen if a winch produces unsteady or oscillatory rope tension but the more probable cause is very timid initial acceleration followed by full throttle after the glider is airborne. This seems to be the scenario described in Figure 1. The second scenario is full-up elevator in the lift-off to V W-MIN AIUR window of vulnerability. A fair question is why some people feel so strongly that high acceleration causes AIUR. Two possible answers suggest themselves. One is X-axis acceleration is rarely experienced outside motor sports and the military so its sudden onset can be startling. It's likely to be even more startling when a nosewheel glider tips sharply back onto its tail wheel. (It's interesting that all AIUR susceptible gliders mentioned on newsgroups have nose-skids.) When a person who strongly believes in the Figure 1 explanation is rattled by X-axis, G-onset, they will be predisposed to experience a well known vestibular system effect called the Head-Up Illusion. The Head-Up Illusion (Figure 7) involves a sudden forward linear acceleration where a pilot perceives a powerful sensation that the nose of the aircraft is pitching up. The illusion is typically experienced by pilots under instrument meteorological conditions where no horizon is visible. A lost horizon can also happen as a glider's nose rises to block the pilots view. It's easy to believe that a real pitch-up (tipping back onto the tail wheel) combined with a powerful Illusion of it could convince an otherwise observant person they are experiencing AIUR when it isn't actually happening. Figure 7 False Climb or "Head-up" illusion None of the above is to say premature rotation and rotation stalls can't occur and, if they do, result in serious, even fatal accidents. A plausible explanation is impatient pilots induce premature rotation by holding aft stick as a slowly accelerating glider enters the AIUR window of vulnerability where the inertial couple plus up elevator causes the nose to rise much faster and further than the pilot intended. It also seems plausible this impatience would more likely in slow-acceleration situations. So, back to the title question. Does AIUR really happen? Yes, but only in two extreme situations that should never happen in the first place. Both are significant causes of accidents in their own right. Lofting glider into the air at just above stalling airspeed is asking for an accident. Under uniform 1G 23