Impact of Aging on Airplane Maintenance

As an aircraft mechanic, airplane maintenance is sometimes not covered in adequate detail in aircraft mechanic school. In all aircraft, there is an aircraft life cycle, which can be described as  the “bathtub curve“. The overall reliability of a system or component through out its life cycle involves three phases:

  • infancy/break-in
  • useful life
  • wear out

This is shown in the accompanying figure, adapted from The Australian Transport Safety Bureau.

Airplane Maintenance Bathtub Curve

Source: ATSB (2008)

During the very early flight years of an aircraft, the failure rate decreases over time, as many failures are due to material flaws or problems in manufacture. This phase is less relevant when considering aging aircraft. In the useful-life phase, failures due to initial flaws gradually decrease while failures due to wear-out gradually increase. Therefore, the average number of failures remains relatively constant throughout the useful-life phase. During the wear-out phase, failures will increase as the product reaches the end of its useful life.

The FAA has in place a system of continuing airworthiness.  When it fails, the consequences might be dramatic.  Here is an example of what happens when manufacturer bulletins for maintenance are not fully known.  In Australia, in December 2000 and April 2001, Ansett Australia’s fleet of Boeing 767 aircraft were taken out of service,  because of problems in the Australian airworthiness system.

In 2000,  Ansett’s  fleet of Boeing 767 aircraft was among the oldest in the world. These aircraft were used on relatively short domestic flights, and had accumulated a large number of flight cycles. The Ansett fleet of Boeing 767 aircraft was grounded, not once but twice, because some fatigue damage inspections had not been carried out.  In June 1997, Boeing introduced an Airworthiness Limitations Structural Inspection Program for the Boeing 767 aircraft. This program was part of the damage tolerance requirements and was designed to detect fatigue cracking in susceptible areas that had been identified through tests and in-service experience. There was a Boeing requirement to carry out some of the inspections before 25,000 cycles.  However, Ansett staff  did not originally recognize this requirement and at the time of the “Airworthiness Limitations Structural Inspection” program introduction, some of the Ansett Boeing 767 aircraft had already flown more than 25,000 flight cycles . In June 2000, further inspections at 25,000 cycles were introduced by Boeing. These inspections included the Body Station 1809.5 bulkhead.  Ansett initially did not act on these inspections.  In fact, some aircraft had developed dangerous cracks in their wing front spars.

A Case Study in Airplane Maintenance

The following diagram, published by the Australian Transport Safety Bureau (http://www.atsb.gov.au/publications/2007/b20050205.aspx ) from “How Old is Too Old? The impact of ageing aircraft on aviation safety”, 2008,  shows Australian Ansett Airlines Boeing 767 wing front spar cracks, which, if left untreated, may have lead to catastrophic failure.  Fortunately, the cracks were discovered in time, and the manufacturer maintenance recommendations were applied before any problems developed.

Boeing 767 bulkhead and wing front spar pitchload fittings

What can we learn from this nearly catastrophic aging process?

Managing Airplane Maintenance

There are two basic approaches for managing the airplane maintenance aging process. The first option is to replace the aircraft, while the second option is adequate maintenance of the aircraft. In Australia, high-capacity  operators have generally controlled aircraft age by investing in the acquisition of new aircraft to replace their older aircraft before maintenance problems become overwhelming. The resultant savings in maintenance costs balance the expense of acquiring new aircraft.  While the new aircraft still require maintenance, this is less demanding and thus less expensive than maintaining an aging aircraft.

The second method of controlling aircraft aging, namely ongoing additional and specific maintenance, is generally most common for general aviation and low-capacity aircraft. These sectors of the aviation industry operate under tight economic constraints, with limited capacity to acquire new aircraft, and  the absence of suitable new production aircraft restricts their options for acquiring new aircraft. When maintenance is chosen as the mechanism to control aircraft aging, the program needs to take into account in-service defects as well as analysis of flight-critical components. Manufacturer support is important to ensure the thoroughness of the maintenance program.

Source: AAP Image Library

Consider the disastrous consequences of poor maintenance with Aloha Airlines Flight 243.  As a result of corrosion over time, a number of small fatigue cracks formed in the same longitudinal skin splice and rapidly joined. These small fatigue cracks were too small to be detected by inspection. Yet, when the cracks joined up, they travelled across multiple frames, thus violating the design assumption that a small fail-safe triangular tear and safe blow-out would happen. This type of failure mechanism, where small cracks form and rapidly join, producing failure, is known as multiple site fatigue damage. The Aloha Airlines accident showed that the lead-crack design method is not compatible with multiple site fatigue damage because the cracks joined across multiple frames. As a result of this dramatic accident in 1988,  the hazardous combination of aircraft fatigue and corrosion was brought to the attention of the aviation industry. The safety of aging aircraft became a priority, with hearings on aging held in the Aviation Subcommittee of the United States Congress. This airframe corrosion issue is understood now and completely resolved. The take-away from Aloha Flight 243 and the Ansett Airline fatigue cracks is to always follow manufacturer manuals and technical bulletins, and be vigilant when performing safety inspections. Airplane maintenance is yet another technical issue to understand in aircraft mechanic school.

by Steve Adams

 

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One Response to “Impact of Aging on Airplane Maintenance”

  1. A and P License Says:

    The Aloha Airlines Flight 243 disaster could have been prevented with proper NDT inspection techniques