The Reactive Versus Proactive Approach to Flight Safety
When considering the Federal Aviation Administration’s (FAA) latest attempt to assure structural integrity of airliners, it may be useful to recall ignored developments of the recent past. The contrast will underscore the FAA’s inadequate efforts.
The recent action concerns an FAA-issued airworthiness directive (AD) requiring inspections of a bulkhead frame on the B737-600 and -700 twinjets. AD #2015-08-09 was published in the Federal Register on 30 April 2015, with an effective date of 4 June 2015. The details of the AD are tedious for a general audience, but a couple quotes suffice to capture the essence of the problem:
“This AD was prompted by reports of cracking in the body station (STA) 727 bulkhead lower frame. This AD requires a detailed and open hole high frequency eddy current (HFEC) inspection of the left- and right-side lower frame webs and inner chords for cracking, and corrective actions and preventive modifications …
“We are issuing this AD to detect and correct cracking in a bulkhead lower frame web and inner chord, which could result in a severed frame and induced skin cracks, which could lead to rapid decompression of the fuselage…
Note first that the cracking was not predicted but was found on an in-service airliner. Body station 727 is approximately where the wing trailing edge meets the fuselage.
Note second that the frame could be cracked clear through, thus negating any strength or support it is supposed to provide.
Note third that the result can be lost skin integrity and blowout of pressurization; if this occurs at cruise altitude, an immediate emergency descent to lower altitude is necessary to get the airplane to dense air before the passengers’ emergency oxygen is exhausted.
Note fourth that the cracking may not revealed by visual inspection but requires HFEC to detect.
Note fifth that even if microscopic cracking is detected and repaired on one side, the opposite side requires strengthening modifications. This is due to the fact that repairing the cracked side induces new strains on the structure, with necessary compensating strengthening to the opposite side. Failure to do so could lead to excessive stress and resultant fatigue cracking on the unaffected side.
Note sixth that the AD effects 489 airplanes, approximately one tenth of the nation’s total commercial airliner fleet.
Note seven, and the intent of this discussion, is that this AD is one of many issued over the years addressing structural cracking on jetliners. Every one of these ADs was issued after cracking was discovered during maintenance or in flight. Every single AD was issued after the structural problem had progressed from a latent threat to an overt and un-ignorable hazard to flight safety.
This is a sad state of affairs, given what has been possible for years.
This latest inspection requirement illustrates the technological lag between military and commercial practice. Many aircraft in military service — including transports — are equipped with “fatigue meters” (recording accelerometers); through these devices the aging of each aircraft is “managed” according to a fatigue index (FI) derived from an ongoing, measured analysis if the airplane’s structure.
When a military aircraft reaches a certain FI, it undergoes planned rework in particular areas for continued airworthiness. This planned rework is performed before failure occurs, not after, as is the case here with the latest AD on B737 structure.
Moreover, it is possible to built a fuselage to safely fly, and resist fatigue loads, for 200 years — far longer than an airliner’s current 30 or 40-year design life. According to a 1997 paper by engineers L. John Hart-Smith and Richard Miller, the difference is in the details. Focusing on riveted fuselage lap splices, they believe they can be built to safely serve through four million flights — the rough equivalent of 200 years of airline service. Hart-Smith and Miller pointed out that most of the attention in airliner structural durability focuses on prolonging the last 10 percent of the life of the aircraft. It is in the last 10 percent of structural life that most fatigue cracks have grown to the point where they are detectable. The paper authored by Hart-Smith and Miller focused on extending the first 90 percent of the aircraft’s structural life.
“New test data [show the] benefits to fuselage lap splices from proper rivet installation under controlled high squeeze force. Higher squeeze force alone is shown to raise fatigue life of a conventional 3-row countersunk-rivet lap splice from more than 100,000 [flight] cycles to failure with today’s installation standards to almost 500,000 cycles to failure, at the same stress level … with exactly the same rivets and joint geometry.
“Improving the design, by using … rivets installed from the inside out at both the top and bottom rows in the joint … further [increases] the fatigue lives to 1,000,000 cycles with the same number of rivets of the same diameter and the same stress in the same skin. This would correspond with a service life of 200 years before any crack grew to a detectable size.
“Combining both improvements with an increase in overlap, to reduce bending stresses, while keeping the same three rows of rivets … [results] in a life of almost 4,000,000 cycles at the same stress level.”
To be sure, increasing slightly the amount of overlapped skin in the spliced joint would add weight, the designer’s bugaboo. But, as the authors explained:
“Let us suppose … that the splices add 5 percent to the weight of the aircraft’s structures. This means that the weight of a splice could justifiably be doubled if doing so permitted the skin stresses to be raised by as little as 5 percent. The longer splice tested here would indeed double the splice weight, but the total weight saving on the skin would be far greater than 5 percent — and still without reducing the fatigue life below 200 years!”
Here lies the problem: the FAA is chasing fatigue cracks, after they are found, one AD at a time, when the military uses a pro-active fatigue index to prevent cracking, and aircraft structural engineers have already developed a method of vastly increasing the durability of riveted components.
The FAA approach is totally reactive — ordering fixes after failures are discovered. The agency has not ordered the installation of accelerometers to record an airplane’s fatigue index, with repairs and modifications made before cracks are discovered. The FAA has not ordered manufacturers to implement structural practices that assure crack-free integrity beyond the usual life in service of an airliner.
The FAA has mastered the art of looking over its shoulder rather than taking a forward-looking approach that would effectively end fatigue cracking in airliner structures. It’s the aviation equivalent of accepting food-borne pathogens when procedures have been developed to circumvent the threat to public health but are not implemented.