DAMAGE TOLERANCE BASED SID DOCUMENTS AND AGING AIRCRAFT: CESSNA S APPROACH TO MAINTAINING CONTINUED AIRWORTHINESS Robert Held and Beth Gamble Cessna Aircraft Company, Wichita, Kansas Abstract Many of Cessna s twin-engine airplanes designed and built from the 1950 s through the early 1980 s are still flying today. Several of these airplanes carry passengers in scheduled commuter service. Because of the lack of new affordable replacement aircraft, there are clear financial incentives for both Cessna and the aircraft operator to invest in supplemental inspections, repairs and modifications to extend aircraft operational life. Cessna s approach to maintaining continued airworthiness, by establishing damage tolerance based inspections and structural modifications, is somewhat unique in the general aviation industry. This paper presents the details of Cessna s activities for developing and implementing damage tolerance based Supplemental Inspection Documents (SIDs) and structural modifications for aging twin-engine airplanes. At the time many of these aircraft were certificated and put into service, there were no FAA requirements to establish airframe component replacement lives or damage tolerance based structural inspections. As a result, there was minimal design data available to use with today s state-of-the-art analytical methods. Development of the SID documents utilized data from new full-scale ground tests, airframe finite element models, da/dn and fracture toughness tests, flight strain surveys and fatigue and damage tolerance analyses as well as service experience. The first SID document was developed for the Model 402 under FAA contract as a part of the FAA s National Aging Aircraft Research Program (NAARP). Last year, the Model 402 SID document was incorporated into the aircraft maintenance manual and several operators have successfully implemented the SID inspections into their maintenance programs. In addition, Cessna has used the knowledge gained in the development of the Model 402 SID to create similar structural maintenance documents for many of Cessna s other 300/400 series aircraft. Background This paper focuses on Cessna s approach for continued airworthiness of small twinengine airplanes. The basic concepts for the majority of the airframes were developed in the 1950 s and 1960 s. These airplanes were in production from the 1950 s to the early 1980 s and have been successfully used in a wide range of operations. Some models, such as the Model 402C shown in Figure 1, have experienced a wide range of usage: from owner-operated airplanes that fly a few hours a year to airplanes used in commuter operations that fly over a thousand hours a year. Page 1 of 12
Figure 1. Typical Cessna Model 402C Airplane Several steps have been taken over the years to support continued airworthiness. In the 1970 s the maintenance manuals were revised to identify specific locations on the airframe for periodic inspections. In the early 1990 s another action was taken to focus attention on continued airworthiness. Continued Airworthiness Program (CAP) documents were created for each series of twin-engine airplanes. In the late 1990 s the continued airworthiness of these airframes were investigated taking into account fleet experience and employing current state-of-the-art analytical methods. These airframes were designed and certified to the standards of Civil Aeronautics Manual 3 (CAM 3) for normal category. The only requirement covering fatigue prior to 1956 was a paragraph in the regulations that required the design to avoid points of stress concentration, in so far as practical, where the variable stresses above the fatigue limit were likely to occur in normal service. In 1957, a fatigue requirement was added for pressurized fuselage structure. Later, the requirements were subsequently revised to include fatigue requirements for wing and empennage structures. Since 1996, FAR 23 airworthiness standards require a damage tolerance evaluation of the airframe structure for commuter category airplanes. Overview of Cessna Fleet Although Cessna s 300/400 series aircraft were produced from the 1950 s to the early 1980 s, the lack of new affordable replacement aircraft has resulted in operators continuing to use these airplanes. Several of these aircraft are used in scheduled airline service to transport tourists to resort areas in Massachusetts, Hawaii, Florida, and the Caribbean. Figure 2 shows the age and number of Cessna 300/400 series aircraft still registered in the United States as of 7/1/01. Page 2 of 12
1000 800 Total Aircraft Manufactured U.S. Registered Aircraft, 7/1/01 Number of Aircraft 600 400 200 0 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 Aircraft Age in 2001 (Years) Figure 2. Age of Cessna s 300/400 Series Fleet The highest usage airplane in Cessna s 300/400 series fleet is the Model 402. Several Model 402 (M402) aircraft in the United States have exceeded 25,000 hours. The estimated average number of hours on U.S. registered airplanes for both typical usage airplanes and those in commuter service is shown in Figure 3. However, British (CAA) and Australian (CASA) aviation authorities established lives for the wing structure of 8200 and 7700 hours for the M402/402A/402B (these airplanes will be referred to as the Model 402 through B ) and Model 402C (referred to as the M402C) respectively based on fatigue analyses. To date, six failures of the Model 402 through B wing main spar cap have occurred, the earliest at 8,000 hours. None of these failures resulted in an accident. However, one M402C wing main spar cap failed after 20,400 hours, resulting in an in-flight wing separation. Currently, FAA Airworthiness Directives Average Hours 20000 16000 12000 8000 4000 402 Through "B" Aircraft 402C Aircraft Typical Aircraft Commuter Aircraft CAA & CASA LIFE LIMIT 0 1995 2001 1995 2001 Year Figure 3. Estimated M402 Average Hours Page 3 of 12
AD 79-10-15 and AD 2000-23-01 require repetitive inspections of the M402 through B and M402C wing structures respectively to inspect for cracks so that catastrophic failure will not occur. In order to assure the continued airworthiness of the wing as well as the rest of the airframe, Cessna has developed separate Supplemental Inspection Documents (SID) for the M402 through B and M402C. Before reviewing the development of the SID documents, the economic and regulatory reasons to develop SID documents are presented in the following paragraphs. General Aviation Revitalization Act of 1994 Cessna s incentive to develop SID documents is to assure the continued airworthiness of its fleet. In 1994, the U.S. Congress passed the General Aviation Revitalization Act (GARA), which, in general, limits the liability of an aircraft manufacturer to 18 years after aircraft delivery. Figure 2 shows that in 2001, fewer than 250 airplanes in Cessna s 300/400 fleet are less than eighteen years of age. GARA does have some important exceptions, however. First, GARA only protects a company against lawsuits filed in the U.S. Second, it does not apply to the medical passenger aboard an air ambulance flight nor does it apply to an airplane operated in scheduled passenger-carrying commuter service. In 1995, there were 150 M402 aircraft flying in commuter service. In 2001, as many as 250 Cessna M402 aircraft are operated in commuter service in the United States, while at least sixty Cessna 300/400 series aircraft (Models 340, 414, 421 and 441) are used as air ambulances (Figure 4). Number of Aircraft 80 60 40 20 Number of M402 Aircraft Registered in US, 7/01/01 Number of M402 Aircraft Used in Commuter Service Number of 300/400 Aircraft Used as Air Ambulance 0 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Aircraft Age in 2001 (Years) Figure 4. Cessna Aircraft in Commuter/Ambulance Service Page 4 of 12
Congressional Mandate In addition to safety and economic incentives, there are regulatory reasons to develop SID documents. As a result of several accidents and incidents, most notably the fatigue failure of an Aloha Airlines Boeing 737 on April 28, 1988, Congress passed the Aviation Safety Research Act of 1988. In response to the Aviation Safety Research Act and concerns related to the increasing age of aircraft fleets, the Federal Aviation Administration (FAA) developed the National Aging Aircraft Research Program (NAARP) to ensure the structural integrity of high-time high-cycle aircraft. One of the three major components of the NAARP is the development of supplemental structural inspections for commuter aircraft. SID documents were mandated for aging large transport aircraft in the 1980 s and have proven effective in ensuring the structural integrity of these aircraft. In the 1980 s and early 1990 s there was much discussion as to whether it was practical to conduct a damage tolerance assessment on the airframes of small commuter airplanes. In 1995, the FAA subsidized the development of Supplemental Inspection Documents (SID) for the two aircraft most commonly used in commuter service, one of which was the Cessna Model 402. The development of the SID document for the Cessna 402 is discussed in the following section. In April 1999, the FAA issued NPRM 99-2 which proposed that SID documents be required for all airplanes carrying ten or more passengers operated under 14 Code of Federal Regulations (CFR) Part 121, all U.S. registered multiengine airplanes operated under 14 CFR Part 129, and all multiengine airplanes used in scheduled operations conducted under 14 CFR Part 135. The proposed rule would require implementation of a damage tolerance based structural inspection program for these airplanes older than 14 years by December 20, 2010. Introduction The Model 402 SID Development Program The Cessna Model 402, a twin-engine piston non-pressurized airplane capable of seating up to nine passengers, was first produced in 1967. Three models which are structurally identical, the 402, 402A and 402B, (referred to as the Model 402 through B ) were produced. These models are equipped with tip tanks and have a dry wing, with optional fuel lockers located in the wing. The design gross weight is 6300 lb. Figure 5 presents a schematic of the Model 402 through B. In 1979, the Model 402C was introduced with a higher gross weight of 6850 lb., a redesigned wet wing without tip tanks and a redesigned vertical stabilizer. Figure 5 presents a schematic of the Model 402C. Both the Model 402 through B and the Model 402C airframes were addressed in the damage tolerance assessment. Over 2000 Model 402, 402A, 402B and 402C airplanes had been built when production was terminated in 1985. Page 5 of 12
Figure 5. Cessna Model 402 Through B (left) and M402C The Model 402 SID was the result of a two-year development program subsidized by the FAA. The program focused on developing a supplemental inspection document (SID) for all variants of the Cessna Model 402 based on state-of-the-art damage tolerance analysis techniques. New development tests, service experience and applications of current technology in the areas of loads, stress, fatigue and fracture mechanics were utilized to identify and establish structural inspections and modifications necessary to maintain safety and provide for continuing structural integrity and airworthiness. These items were accomplished and the SID was developed in three phases. A summary of each phase is presented in the paragraphs below. Phase 1 In Phase 1, most of the data required to perform a damage tolerance analysis was collected. Phase 1 tasks are shown in Figure 6. The first task was to identify the principle structural elements, that is, the components that carried significant flight and ground loads and whose failure could result in catastrophic failure of the airframe. In order to determine the principal structural elements, detailed geometry and material information [0] was collected for each airframe component. Service experience data [1] was collected by surveying Model 402 owners and by reviewing Cessna service bulletins and FAA Service Difficulty Records. Finite element models [2] were developed for both the Model 402 through B and Model 402C airframes. As an example, the M402C wing and fuselage finite element model is shown in Figure 7. The model is presented in two pictures for clarity. Also, new limit load ground tests [3] were conducted on a Model 402 obtained from a salvage yard to provide finite element model verification data [4]. Page 6 of 12
[0] Structural [3] Ground [1] Service [9] Flight Definition Test Strain Experience Strain Survey Survey [2] Developed FEM [4] Validated FEM [5] Selected Analysis Locations [10] Developed Stress Equations [6] Usage [7] Flight [8] Load Survey Profiles Spectra Developed Stress Spectra Figure 6. Phase 1 Tasks Phase 2 Figure 7. M402C Wing and Fuselage Finite Element Model Next, the critical areas of each of the principal structural elements were identified [5]. High stress levels determined from the finite element models and ground tests, fatigue test results, service experience, inspectability, and susceptibility to corrosion and to accidental damage were all considerations in choosing analysis locations. A total of 26 analysis locations were chosen for the Model 402 through B while 25 analysis locations were chosen for the M402C. Page 7 of 12
After the analysis locations were selected, the stress spectra for input into the fatigue and damage tolerance analyses were developed. The first step in developing the stress spectra was to determine how Model 402 commuter operators were utilizing their aircraft. To accomplish this, a survey [6] was developed and mailed to the M402 operators identified by the FAA as carrying either passengers or cargo in commuter service. Flight length, number of passengers, gross weight, payload, fuel loading, flight altitudes and speeds were obtained from the survey in order to develop the flight profiles. As an example of the data gathered from the survey, the flight lengths are shown in Figure 8 that represent a total of 1777 flights per week from surveys received on 85 airplanes. Since there was a wide variation in usage, three flight profiles [7] were developed to represent the customers missions as closely as possible: a short profile of 25 minutes in length, a severe profile representing mountainous conditions and a typical profile representing an average flight length of one hour. Flights per Week 105 90 75 60 45 30 15 0 1269 98 91 26 82 41 41 25 25 15 15 5 5 3 5 5 3 402/402A/402B 402C Short Flight Operator 20 3 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Flight Time (hours) Figure 8. M402 Flight Length Summary, Commuter Usage Load spectra [8] were developed for the wing, fuselage, empennage, engine support and the landing gear using publicly available measured data. Maneuver, vertical and lateral gust, engine thrust, balancing tail loads, taxi and ground-air-ground were considered. A flight strain survey [9] was undertaken to measure stresses at each analysis location as a function of flight parameters. Using the stresses obtained from the flight strain survey, stress equations [10] were developed for the taxi, maneuver and gust segments defined in the load spectra. From these stress equations a flight-byflight stress spectrum was developed. The stresses within each flight of the spectrum were cycle counted using a method similar to the rainflow method. Page 8 of 12
Phase 2 During Phase 2 the damage tolerance analysis was conducted and inspection methods and intervals established. Phase 2 tasks are shown in Figure 9. The first step in conducting the damage tolerance analysis was to locate the required material properties for each component of the airframe [1]. Most data was found in public sources. For 301 (1/4 hard) steel, da/dn and fracture toughness (K c ) data were developed through testing [2]. Also spectrum-loaded coupons were tested to develop the parameters for the Willenborg-Chang retardation model in the damage tolerance analysis. CRACKS95 (Reference 1) [3] was used to carry out the crack growth calculations based on a.05 inch typical quarter-circular crack size [4] using either the Walker or NASGRO 2.0 equation (Reference 2). A classical fatigue analysis using Miner s rule was also conducted [5]. [1] Collected [4] Establish [5] Fatigue Material Initial Flaw Analysis Properties Sizes [9] Determined Onset of Widespread Fatigue Damage Material Properties [3] Conducted [7] Established [8] Determine Crack Growth Analyses Inspection Thresholds Inspection Methods [2] Conducted [6] Material Phase 1 Testing Fail Safe Tests Phase 3 Figure 9. Phase 2 Tasks Before the inspection intervals were established, ground tests were conducted on the airframe obtained from the salvage yard to determine which airframe components were failsafe [6]. A single spar cap element was severed and limit load was applied. The M402C empennage and wing structures and the M402 through B empennage were shown to be failsafe. For those components, which were proven to be fail-safe, the initial inspection [7] was based on classical fatigue analysis results divided by a scatter factor. For components, which were not fail-safe, initial inspection intervals were based on crack growth analyses. Inspection methods (eddy current, fluorescent penetrant, magnetic particle and visual) were then chosen [8]. Recurring inspection intervals were based on crack growth analysis results. Page 9 of 12
The last item in Phase 2 was to determine the onset of widespread fatigue damage [9]. Since the M402 is not pressurized the wing structure was evaluated using evaluation guidelines presented in the final report of the AAWG committee on Widespread Fatigue Damage (Reference 3). Sources of data used for the investigation included teardown evidence from full scale and component cyclic test articles, FAA Service Difficulty reports, and teardown evidence from high time field aircraft. No evidence was found for the potential of widespread fatigue damage. Phase 3 Phase 3 consisted of three tasks as shown in Figure 10. First, recommended modifications for the Model 402 through "B" and Model 402C wings were developed [1]. Results of the fatigue and damage tolerance analyses performed in Phase 2 indicated that modifications needed to be made to the wing for both the Model 402 through "B" and the Model 402C to ensure continued airworthiness. The modifications that were developed for the Model 402 through "B" were analyzed in Phase 3. The design modifications analyzed for the Model 402C, cold working the holes in the main spar cap, were revised after the completion of the contract. The final recommended modifications for both wings incorporate an external strap on the lower wing surface to reinforce the lower main spar cap. Installation of the spar strap for the Model 402 through "B" is mandatorily recommended by Cessna for all airplanes with more than 6,500 hours, while the spar strap is recommended for all Model 402C aircraft with more than 15,000 hours. Phase 2 [1] Develop & Analyze Modifications [2] Develop SID [3] Document Write Final Report Implement SID Phase 1 Figure 10. Phase 3 Tasks Page 10 of 12
Second, the SID for the Model 402 was developed [2]. The SID is presented in four sections: 1) Introductory and General Information, 2) Inspection Requirements in Chart Form, 3) Planning and Technical Data for Each Inspection Area and 4) NDI procedures. Many of the inspections related to the Model 402 primary structure originated from the Cessna twin engine aircraft Continuing Airworthiness Program Document (CAP). When necessary, the inspections were modified to reflect new inspection intervals determined in Phase 2 or to incorporate the latest state-of-the-art NDI inspection procedures. Also, since some new locations were analyzed, new inspections were developed for these locations for inclusion in the SID. A total of fifty inspections covering the wing, fuselage, engine beam and empennage structures are in the SID. The SID also incorporates the strap modifications to the wing structure. Finally, the Final Report for the Model 402 SID program was developed in Phase 3 [3]. The final report summarizes all activity performed during the development of the Cessna Model 402 SID (Reference 4). SID Implementation After the SID document was completed in fulfillment of the FAA contract, the SID was officially published in the Model 402 Maintenance Manual in November 2000. The SID has been successfully implemented in several countries outside of the United States. To date, the SID has had the most dramatic effect in Australia. CASA has removed the airworthiness directive AD/Cessna 400/40 on Cessna 400 series aircraft if the airplane is inspected per the SID requirements. This AD required that the airplane be retired after two lifetimes (16,400 hours for the Model 402 through "B" and 15,400 hours for the Model 402C). Using a pre-release draft of the SID document, the first complete airframe inspection was accomplished in October 2000 in Ballarat, Victoria, Australia. Approximately 525 hours of labor were required to complete the SID inspections without the strap modification. A photograph of an airplane getting ready to undergo the SID process is shown in Figure 11. To accomplish the SID the first time requires removal of Figure 11. M402C aircraft pre-sid inspection (left) and SID inspection disassembly. Page 11 of 12
the engines and wings. The amount of disassembly varies between operators. One operator has chosen to completely disassembly and rebuild the airplane as shown in Figure 11. The customers who have completed the SID inspections believe that the inspections are reasonable and necessary. Every operator who has performed the SID inspections has found either cracks and/or corrosion. Conclusion The Model 402 assessment shows that a damage tolerance based evaluation is feasible for small twin-engine airplanes. A combination of field experience, fatigue tests and fatigue and damage tolerance assessments was used to define inspection intervals and modifications. Cessna s method of relaying the inspections and wing modifications to operators is through a SID document published in the aircraft maintenance manual. The SID will be revised as required to reflect the latest field information. The Model 402 SID is the first of several SID documents to be published. In the next two years, SID documents will be published for the other Cessna 300/400 series aircraft. Cessna plans to eventually extend the SID concept to the early Citation fleet as well. References 1. Gallagher, J.P., Medlar, P.C., Cross, C.W., and Papp, M.L., CRACKS95 System, University of Dayton Research Institute, Dayton, OH, November 1995. 2. Forman, R.G., Fatigue Crack Growth Computer Program NASA/FLAGRO, JSC- 22267A, NASA/Lyndon B. Johnson Space Center, Houston, TX, August 1986, Revised March 1989. 3. Final Report of the Airworthiness Assurance Working Group Industry Committee on Widespread Fatigue Damage, Mr. Ronald Wickens Chairman, Structural Fatigue Evaluation for Aging Airplanes, October 1993. 4. DOT/FAA/AR-98/66 Supplemental Inspection Document Development Program for the Cessna Model 402, dated March 1999. (available for download at http://aar400.tc.faa.gov/acc/accompdocs/98-66.pdf). Page 12 of 12