Brazilian Air Force aircraft structural integrity program: An overview

This paper presents an overview of the activities developed by the Structural Integrity Group at the Institute of Aeronautics and Space - IAE, Brazil, as well as the status of ongoing work related to the life extension program for aircraft operated by the Brazilian Air Force BAF. The first BAF-operated airplane to undergo a DTA-based life extension was the F-5 fighter, in the mid 1990s. From 1998 to 2001, BAF worked on a life extension project for the BAF AT- 26 Xavante trainer. All analysis and tests were performed at IAE. The fatigue critical locations (FCLs) were presumed based upon structural design and maintenance data and also from exchange of technical information with other users of the airplane around the world. Following that work, BAF started in 2002 the extension of the operational life of the BAF T-25 “Universal”. The T-25 is the basic training airplane used by AFA - The Brazilian Air Force Academy. This airplane was also designed under the “safe-life” concept. As the T-25 fleet approached its service life limit, the Brazilian Air Force was questioning whether it could be kept in flight safely. The answer came through an extensive Damage Tolerance Analysis (DTA) program, briefly described in this paper. The current work on aircraft structural integrity is being performed for the BAF F-5 E/F that underwent an avionics and weapons system upgrade. Along with the increase in weight, new configurations and mission profiles were established. Again, a DTA program was proposed to be carried out in order to establish the reliability of the upgraded F-5 fleet. As a result of all the work described, the BAF has not reported any accident due to structural failure on aircraft submitted to Damage Tolerance Analysis.

as well as the status of ongoing wark related la the lije extensian pragram lar aircrafl operated by the Brazilian Air Force BAF The first BAF-operated airplane to undergo a DTA-based life extension was the F-5 fighter, in the mid 1990s. From 1998F-5 fighter, in the mid 1990s. From to 2001, BAFworked on alife extensionprojectfor theBAFAT-26 Xavante trainer. All analysis and tests were performed at lAE. The fatigue critical locations (FCLs) were presumed based upon structural design and maintenance data and also from exchange of technical information with other users ofthe airplane around the world. Following that work, BAF started in 2002 the extension ofthe operationallife ofthe BAF T-25 "Universal ", The T-25 is the basic training airplane used by AFA -The Brazilian Air Force Academy. This airplane was also designed under the "safe-lífe" concepto As the T-25 fleet approached its service life limit, the BrazilianAir Force was questioningwhether it could be kept in flight safely. The answer came through an extensive Damage Tolerance Analysis (DTA) program, briefly described in this paper. The current work on aircrafl structural integrity is being perfonnedfor the BAF F-5 E/F that underwent an avionics and weapons system upgrade. Along with the increase in weight, new configurations and miss ion profiles were established. Again, a DTA program was proposed to be carried out in arder to establish the reliability ofthe upgradedF-5fleet. As a result ofall the work described, the BAF has not reported any accident due to structuralfailure on aircrafl submitted to Damage Tolerance Analysis. Key words: Fatigue, Damage tolerance, Structure, Service life.
The first Brazilian flight was reported on 23 October 1906, and was performed by Alberto Santos Dumont in an airplane named 14 bis. At that time, not much was known about other achievements, so that initial1y Santos was considered to be the first man to fly a fixed-wing craft, capable of taking off, flying and landing under its own power (Hoffinan, 2003). Another important milestone in Brazilian aeronautics was the flight ofthe Demoiselle (Fig.  1). It was the first ultra-light airplane in history (Winter, 1998). Santos Dumont flew the Demoiselle for the first time in 1908.
Airplanes manufactured in the first half ofthe 20th century were primarily based on the static strength point of view. The aircraft industry's approach to achieving structural integrity was significantly modified as a result of failures that occurred in the 1940s and 1950s. The most significant were the fatigue failure of the wing of the Martin 202, in 1948, and the Comet fuselage failures that occurred in 1954. At that time, the concept known as safe life was introduced. According to this philosophy, an articleneeded to betested to prove its fatigue strength.
The safe life approach is based on the life-to-failure test divided by a scatter factor, usually three or four, tú account for uncertainties in material properties, manufacturing and assembly processes, and applied loads. When strictly enforced, the safe life approach imposed a severe penalty: if the service life was reached, then the fatigue life was deemed expended and the aircraft removed from service. However, if a flaw existed in the structure, the safe life did not ensure flight safety. The main steps of a DTA Program can be surnmarized as follow: • Flighldalasurvey This task is necessary in order to obtain the information on how the airplane is benig used. Sorne data musl be available for further analysis: load faclors, altilude, speed, angular accelerations, etc. This body ofinformation is called LIESS (Load/Environmenl Speclra Survey)

This is how the stress in each Fatigue Critical Location
(FCL) relales lo the LlESS. The equalions come from slress analysis and strain gauged aircraft.

• Stress Spectra
For eachFCL the stress spectra are determined, based on the LlESS and the stress-lo-load ratio.

• Residual Strength
Based on the material properties, structure geometry and fracture mechanics parameters, how the structural strength decreases with crack size is determined. The minimum strengthrequired defines the critical crack siz Research Instilule (SwR1) was conlracled lhrough the USAP (United Slales Air Force) lo perform the analysis (Wieland el al., 1996). Their work included hands-on job trainnig for BAP personnel. The load spectrum was oblained from 5 F5-Es operating at the Sanla CruzAir Force Base, Brazil.
In the lale 1990s, a group was formed al the Instilule of Aeronautics and Space (IAE) wilh the objective of developing human resources and technical expertise for inhouse damage tolerance analysis. This group is now part of lhe Slructural Integrily Subdivision (ASA-l) al the Aeronautical Syslem Division (ASA) atthe 1AE.
The firsl in-house DTA work was wilh lhe BAP AT-26 "Xavante", This aircraft was originally designed under a safe-life slruclural concep!. lt is a light ground-altack, reconnaissance and training subsonic jet aircraft. It was produced by EMBRAER under license from the ltalian manufaclurer Aermacchi SpA. The firsl plane flew ni 1971.
More recentIy, in the mid-90s, 4 accidents were reported two wilh BAF airplanes and 2 olhers worldwide involving lhis type of aircraft. All of lhem were related lo slruclural failure in lhe wing spar (Fig. 2). Subsequenl analysis showed that the failure was due lo manufacturing quality problems caused by improperly drilled holes in a critical area (MelloJr, 1999). Following Ibis work, the BAP Slruclural Integrity Group was also responsible for performing the DTA for the T-25 Structural Integrity Subdivision began DTA assessment on the AT-26. The main requi rements were to have a more reliable structure as well as to keep the flying status beyond the original, and proven unsafe, service life. The main tasks for theAT-26 DTA were: data review, flight data recording, processing and evaluation, strain gauged airplane for flight test campaign, coupon tests, structural FEM (Finite Element Modeling) and fracture mechanics analysis, and non-destructive inspection development. Figure 3 shows several piclures represenling lasks accomplished during lhe AT-26DTA. Finite element models of selected FCLs were constructed in order to obtain the stress levels at the locations most likely to develop fatigue cracks (Mello Jr., 1999). These local stress levels were experimentally confirmed and adjusted through an extensive flight test programo Strain gauges were installed at the main aircraft FCLs for stressto-load ratio validation. Retardation parameters from coupon tests were then used as input for the crack growth analysis lo obtain the crack growth curves (Mello Jr., 1998) and to define the adequate intervals for the non-destructive inspectionmethod (NDI) applicable for eachFCL.
A total of 15 fatigue criticallocations were analyzed and for each one a proper ND1 was scheduled. For the wing spar, considered the most critical spot, anew ND1 procedure had o be implemented, The critical region and surroundings are inspected by eddy current technique wilh lhe help ofvideo pack equipment (Fig. 4). This inspection is backed up by X-Ray ofthe entire region, The main achievement of this project was to give the Brazilian Air Force the necessary time to transition from tlris aircraft to its replacement and increase the structural reliability oftheAT-26.

• T-25 "Universal" life extension
The T-25 is the basic training airplane made by the Brazilian Aviation Company NENA, and used by AFA -theBrazilian Air Force Academy. This airplane was also designed under lhe safe-life concep!. As the T-25 fleet approached its service life limit, the BrazilianAir Force began to question whether it could be kept flying safely. The answer was provided by an extensive Damage ToleranceAnalysis. This airplane entered service at the beginning ofthe 70s. Unlike the previous project, no catastrophic structural accident was reported during its service life. Additionally, the safe-life full scale test was slopped after 7,000 effective flight hours without any reported major damage.
The main goal was to extend the T-25 service life by 5,000 flight hours, retaining the integrity ofthe structure. For the complete analysis, the same steps presented for lhe AT-26 were followed. A total of 25 fatigue criticallocations were considered: 11 in the wing, 10 in the fuselage, 3 in the horizontal stabilizer and 1 in the vertical stabilizer (Mello Ir. et al., 2004). Over 700 flight hours were collected from the BAF Academy planes, covering all the missions and typical idiosyncrasies of a variety of trainee pilots. For proper calibration ofthe stress-to-load ratio equations, 16 FCLs were strain gauged and a test flight campaign was conducted.
The DTA included an interactive phase with the Maintenance Depot and End User. This was considered a key step for the final result, since all the changes and recommendations must be maintainable and operational. Among the suggested changes in maintenance procedure were the new inspections in the center box wing spar and in the attachment lug region of the extemal/intemal wing. Strapreinforcement at the root wing lower sparcap was also proposed to be carried out during a major Depot overhaul (Mello Ir. et al., 2005).

F-5MSTRUCTURALINTEGRITYPROGRAM
The BAF F-5, versions E and F, are being modernized, The upgrade inc1udes new avionics and weapons system. It is clear that the operational effort will be changed considering Journal ofAerospace Technology and Management the new system in a more capable aircraft. Also, the weight and its distribution have been modified. All of lhis made a new structural analysis mandatory to ensure a reliable service life. A complete proposal for a 2-year DTA program was submitted to the Brazilian Air Command. The program started onAugust 2007.
The fleet is now referred lo as F-5EMIFM. Among the various changes lhat affect the structurallife is the cutting of the forward fuselage at station 103 (60.5 for F-5F) to include a bigger radar antenna and the removal of one gun, This was necessary to make room for the new avionics.
After all the modifications during the upgrade process, the "new" aircraftincreased in weight, leading to changes in the structural load distribution. Also, new configurations and mission profiles were established.
One example ofthis configuration/profile change is thenew airplane's capability lo perform ripple, having the CCRP (Continuous Computer Release Point). An extensive gjump in flight campaign was carried out to certify this type of mission as safe for the F-S. Strain gauges and accelerometers were instal1edto measure the instantload at critical spots.
With the new avionics, all airplanes are eligible for LlESS (Load/Enviromnent Spectra Survey). At the end, the DTA will determine what will be the impact ofthe upgrade on the maintenance schedule and workforce to support the F-5EMIFM fleet.
Also, it will show how new avionics and weapons systems affect the usage ofthe same platform in aspecific Squadron. Due to changes in weight and balance, comparisons are being made between stress spectrum profiles for sorne of the aircraft fatigue critical locations. Final1y, a maintenance schedule will be proposed based on what the analysis predicts. So far, more than 700 flight hours have been collected from the BAF F-5 Squadrons. Several computer codes and pre-analyses have been written, in order to substantiate the upcoming F-5EMlFM DTA Final Report.

PROSPECTlVE WORK
The BrazilianAir Force is already anticipating the need for alife extension ofthe A-1 "AMX". The AMX is an attack jet that provides the required performance to acquaint pilots with the demands of modern combat scenarios. The complete structural reassessment will be performed after a mid-life avionics upgrade to be carried out over the next fewyears.
The A-29 "Super-Tucano" (Fig. 5) is a combination of a turboprop with fourth-generation avionics and armament systems. The Super-Tucano has an outstanding humanmachine interface, fully compatible NVG Gen 111 cockpit lighting innovation. It can operate in the most hostile environments and from unprepared runways, by day or Mello Jr.,A.W.S., Garcia, A. N ., Ribeiro, F. N ., Mattos, D.F.V.
night.1ts first flight for the BAF was onAugust 2004.
The BAF requirements included a comprehensive fatigue life evaluation, following all the structural integrity and reliability certifications. EMBRAER has contracted the IAE's Structures Laboratory to perform the durability and damage tolerance full scale fatigue test for this aircraft. A three service life time has been simulated, and now cracks will be induced inthemost critical areas.
Another two service lives will be tested with monitoring of the induced cracks. The damage tolerance analysis is being performed by EMBRAER. There are several other projects that may be implemented over the next few years by the IAE Structural Integrity Group.Among them are C-130 DTA update, T-27 "Tucano" DTA andA-4 (Brazilian NAVY) DTA assessment.

CONCLUSION
Brazilian aviation has a history that is deeply involved in the history of manned flight. Since 1906, Brazil has been very active in the efforts to make flying better and safer. The new concepts of DTA to assure structure integrity were adopted by the BAF in the 1990s and since then there has been a significant improvement in safety for the projects lhat adopted that philosophy, with no reported accident due lo structural failures on aircraft subjected to DTA. The IAE has proven that it has the necessary tools and technical personnel needed to analyze, propose changes and perform the follow-up for any structural problem faced by the BrazilianAir Force.
New operational requirements from the BAF will provide a long-term demand for human resources involved in structural analysis and design from the IAE Aeronautical Systems Division, Structural Integrity Group.

ACKNOWLEDGMENTS
The Authors would like lo thank the Brazilian Air Command for having sponsored the Structural Integrity Group, always providing the necessary means to carry out the service.