* For Internal Circulation Only SAFE WINGS Flight Safety Magazine of Air India, Air India Express and Alliance Air Issue 59, APRIL 2017 This issue WAKE-UP TO WAKE TURBULENCE
SAFE WINGS April Edition 59 1 P a g e
April Edition 59 SAFE WINGS WAKE-UP TO WAKE TURBULENCE :-By Capt Vivek Kulkarni W hen turbulence is forecast, expected or encountered either in clouds or clear air the crew puts on the seat belt signs and sets turbulence speed and remains alert. However a wake turbulence encounter can be a surprising experience for both crew and passengers. All turbulence injuries involve people who are not properly seated and do not have their seat belt fastened, particularly when the seat belt sign is switched off. Severe Wake turbulence has caused aircraft upset incidents. A Jet Upset is defined as: Pitch attitude greater than 25 Nose Up, or Pitch attitude greater than 10 Nose Down, or Bank angle greater than 45 or Within above parameters but flying at airspeeds inappropriate for the conditions. If an Upset results in a stall/stick shaker activation, recovery from the stall should be accomplished first by applying and maintaining Nose Down elevator until stall recovery is complete and stick shaker activation stops. A stall condition can be recognized by stick shaker activation accompanied by one or more of the following Buffeting Lack of Pitch or Roll control Inability to arrest descent rate 2 P a g e
SAFE WINGS April Edition 59 After completing the stall recovery, using the recovery procedure given in the QRH, the Upset recovery is accomplished as per the QRH procedure depending on the Nose high, Nose low aircraft state. Wake turbulence Wake turbulence is turbulence that forms behind an aeroplane as it passes through the air causing wing-tip vortices. These vortices are formed any time an aerofoil (Figure 1) is producing lift as lift is generated by the creation of a pressure differential over the wing surface. The air accelerates over the upper surface of the wing having to travel a longer distance as compared to the air flowing under the wing. This causes low pressure over the upper surface relative to the lower surface.the lowest pressure occurs over the upper surface of the wing and the highest pressure under the wing (Figure 2 and 3) Aerofoil Section Figure: 1 3 P a g e
April Edition 59 SAFE WINGS Figure:2 Figure 3 High pressure air below an aircraft s wing flows around the wingtips to the low pressure air on top of the wing (Figure 4). This results in vortices being generated from the wings (Figure 5). When viewed from behind, the left wing vortex rotates clockwise and the right wing vortex rotates counterclockwise (Figure 6). The region of rotating air behind the aircraft is where the wake turbulence occurs. Every aeroplane produces wake vortices however the strength of the vortices is predominantly determined by the weight, speed and wingspan of the aircraft. Figure 4: 4 P a g e
SAFE WINGS April Edition 59 Figure 5: Wake vortices Figure 6: Wake vortices 5 P a g e
April Edition 59 SAFE WINGS Wake vortices spread laterally away from the aircraft and generally descend at an initial rate of about 300 to 500 feet per minute for about 30 seconds. The descent rate decreases and eventually approaches zero when about 500-900 ft below the flight path (Figure 7). Figure 7: Movement of wake vortices The life span of the wake vortices is about 30 seconds for a wind speed of 5-10 kts; up to 85 seconds for wind speeds less than 5 kts; and up to 100 seconds in still air. The vortices spread laterally and downward from the rear of the aircraft (Figure 8). Figure 8: 6 P a g e
SAFE WINGS April Edition 59 A380 wake vortex In June 2008, the A380 Wake Vortex Steering Group published the Safety Case for Wake Vortex Encounter Risk (WVE) due to the Airbus A380. The results indicated that, based on data collected over 2 years, the WVE risk was considered acceptable and the vertical or horizontal separation criteria used by ATC were adequate. It also indicated that the WVE risk from an A380 during the cruise was not noticeably different from that of aircraft such as the Boeing B747-400, Airbus A340-600 and B777-300. However during this study the number of A380s flying were negligible compared to B777-300s and B747-400. Since then the number of B747-400 and A340-600s have reduced in number and the A380s have grown in number especially in the Gulf region with the largest customers of A380 being the Big three of the Gulf namely Emirates,Etihad and Qatar. Moreover we have not had any adverse reports of wake turbulence from B747-400, B777-300 or A340-600. On the other hand we have had numerous experiences of encountering wake of the A380. A380 wake turbulence classification The wake turbulence separation standards applied by ATC are determined by grouping aircraft into four categories (light, medium, heavy and Super) based on the aircraft s maximum certified take-off weight. The A380 is the only aircraft currently assigned the category of SUPER. The Airbus A380, with a maximum take-off mass in the order of 560 000 kg, is the largest passenger aircraft ever to enter into revenue service. The aircraft is in the HEAVY wake turbulence category. However, as vortices generated by the A380-800 are more substantial than for other aircraft in the HEAVY wake turbulence category, the guidance recommends an increase in relation to the wake turbulence separation minima published in the PANS-ATM. This is intended to ensure that aircraft operating near an A380-800 do not encounter wake vortices of a greater 7 P a g e
April Edition 59 SAFE WINGS magnitude than are generated by other aircraft in the HEAVY wake turbulence category. Wake Turbulence Separation Recommendations: Arriving Aircraft The following non-radar separation minima should be applied to aircraft landing behind an A380 MEDIUM aircraft behind an A380 3 minutes; Departing Aircraft A minimum separation of 3 minutes should be applied for a LIGHT or MEDIUM aircraft and 2 minutes for a non-a380 HEAVY aircraft taking off behind an A380 when the aircraft are using: 1. the same runway; 2. parallel runways separated by less than 2 500 ft, 3. crossing runways if the projected flight path of the second aircraft will cross the projected flight path of the first aircraft at the same altitude or less than 1000 ft below; 4. parallel runways separated by 2 500 ft or more, if the projected flight path of the second aircraft will cross the projected flight path of the first aircraft at the same altitude or less than 1 000 ft below. A separation minimum of 4 minutes should be applied for a LIGHT or MEDIUM aircraft when taking off behind an A380 from: 1. an intermediate part of the same runway; or 2. an intermediate part of a parallel runway separated by less than 2500 ft. 8 P a g e
SAFE WINGS April Edition 59 Radar Wake Turbulence Separation Minima The following wake turbulence radar separation minima should be applied to aircraft in the approach and departure phases of flight behind an A380: Non 380 HEAVY...6 NM Medium...7 NM Light... 8 NM WAKE TURBULENCE ENCOUNTERS There have been several reports of severe wake turbulence encounters suspected to be caused by A380.Recently, one B737 aircraft encountered severe wake turbulence of an A380 at FL 320, with bank angle reaching about 70.Most severe wake turbulence encounters were in the Gulf areadue to the presence of a large number of A380s in this region, but this could happen elsewhere too. Although Air Traffic Control is ensuring adequate separation between Super (A 380) Category Aircraft and other aircraft during departure / approach and landing, most of the encounters in recent past have been in cruise. The worst cases have been when passing 1000 ft below (reciprocal, same direction or crossing) an A380 where the effect of wake is maximum. Safeguards: Crew could maintain situational awareness by listening out to flight call signs suffixed by Super. Monitor TCAS displays and be alert when an aircraft is crossing you especially on reciprocal track and 1000 ft above. If it is a Heavy aircraft expect some turbulence and if it is an A380 then anticipate moderate to severe wake turbulence. A regular routine reminder to passengers to keep their seat belts loosely fastened when seated is a good practice. 9 P a g e
April Edition 59 SAFE WINGS We have taken up this matter with DGCA and with UAE authorities and have suggested lateral separation of A380 from the routes followed by other aircraft. 10 P a g e
airsafety@airindia.in or Safewingsmagazine@gmail.com PROMISING A SAFER SKY, AIR INDIA, AIR INDIA EXPRESS & ALLIANCE AIR Editorial: Capt V Kulkarni, Bhavish B S,LP Shenoy Designed by Bhavish BS