Teardown evaluation of a B737 stabilizer done at the National Institute for Aviation Research (NIAR) at Wichita State University.
Life Extension Strategies As Mike Menard states, well-maintained aircraft can be kept in service almost indefinitely. This said, most aircraft are certified for a set lifespan known as the Design Service Goal (DSG). It is generally based on a predetermined number of flight hours and ‘flight cycles’ (pressurization/ depressurization cycles) that the aircraft has been designed to endure. “The whole aircraft structure is designed to withstand every load which occurs during its operation at least for this time,” says Halske. “If the aircraft shall be operated beyond the above mentioned service goal, the whole aircraft needs to be certified for this life extension by the OEM.” OEMs are amenable to providing such life extension programs. For instance, Airbus’s ESG1 (Extended Service Goal) program makes it possible for A320 operators to boost the aircraft’s life span from 48,000 flight cycles and 60,000 flight hours up to 60,000 flight cycles and 90,000 flight hours.
Making this life extension requires “a
huge effort,” says Jean-Marc Lenz, SR Technics’ SVP of Aircraft Services. “(It) includes a new full scale fatigue test of the fuselage and the wing. The operator who
wants to operate in ESG1 has to embody certain structural modification latest at the original DSG.”
Engines do not face the same limits as airframes. “There is no restriction on how many times an engine can be overhauled,” says Julie Busque, spokesperson for Pratt & Whitney. “Nor is there a time restriction on when the engine should be removed from service.” To be specific, “since every part of the engine can be replaced by new or repaired ones during its overhaul, there is no point in time when you can’t ‘keep them running anymore’,” says Halske.
Restrictions on Rejuvenation All of these positive points notwithstanding, aircraft are like cars. The older they get, the more problems they can present to their operators as parts wear out. For operators and MROs, it is the number of flight cycles that can pose the most serious challenges. The constant pressurizing and depressurizing of the cabin puts the airframe under repeated stress. The more cycles, the more likely that metal fatigue will occur; just as a piece of strip of metal bent back and forth in a person’s hands will eventually crack and snap.
36 Aviation Maintenance |
avm-mag.com | April / May 2012
In new aircraft, this isn’t an issue. But as the flight cycles and flight hours accumulate, “Many aircraft will develop fatigue cracking in specific areas over time,” says Menard. “The design engineers, through analysis and testing, are able to predict the onset of such cracking,” adds Dr. Laubach-Hock. “Yes, unanticipated cracking scenarios do arise, but these are often a product of change in the flight profile; namely different stresses than predicted, analyzed and tested for.” The rule of thumb is, “The older the
aircraft, the higher the potential risk to encounter severe structural findings while performing fatigue tests,” Lenz says. The aircraft’s operating environment also has an impact. “Salty, coastal areas will create corrosion, as will flying through heavily polluted areas like densely populated areas in Asia,” Menard notes. “Some corrosion issues arise with certain areas due to the design, where condensation water gathers in a low point in the airframe.” This brings us to a key reality of older
aircraft. “They tend to use a lot more sheet metal than modern aircraft fitted with composites,” says Jonathan Berger, VP of ICF International; an aviation consultancy that serves commercial and government clients.
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