Repairs can cover and conceal evidence of structural weakening
Fatigue damage under patches known as doublers can be deadly, and an expanded hunt must be mounted to find spreading cracks before they cause a catastrophe.
That is the central message in a battery of recommendations issued April 8 by the National Transportation Safety Board (NTSB). “Hidden damage [can be] crucial because fatigue cracking in the pressurized compartments of an airplane … could lead to catastrophic structural failure,” the NTSB warned.
The safety board’s call to action is an outgrowth of the fatal May 25, 2002, crash of a China Airlines B747-200 over the Taiwan Strait. All 225 passengers and crew aboard were killed in what appears to be a case of sudden structural failure from multiple-site damage (MSD). In this type of widespread fatigue damage, the ability of the aircraft structure to carry its design load is drastically reduced.
The NTSB is assisting the Aviation Safety Council (ASC) of Taiwan in investigating the case, which involves an airliner that literally fell apart because of an old repair that proved to be inadequate. As such, the crash of China Air Flight CI-611 has ramifications that go well beyond the accident airplane. At the very least, preliminary findings suggest shortcomings in the supplemental structural integrity program (SSIP) to assure the airworthiness of high-time aircraft.
Briefly, the airplane blew apart – into four major pieces – from explosive decompression at an altitude of 34,600 ft., about 20 minutes into the planned flight from Taipei to Hong Kong.
Investigators have come to focus on the maintenance history of the airplane. About six months after the B747-200 was delivered new to China Airlines, it suffered a tailstrike while landing at Hong Kong. The aft belly skin suffered “serious abrasion damage” from this February 1980 incident. According to the maintenance records, the damage was repaired May 25, 1980, with the installation of two doublers that were riveted over the damaged area. As an indicator of the extent of damage, each doubler measured 10 ft. long by about 2 ft. wide, and the aluminum was 0.1 inch thick.
Too much force was applied to the rivets, creating stress points which, over time, can weaken structure. In this case, the overstressed rivets were not the worst of it.
The scored skin underneath the patches was not removed and replaced. The scratches and gouges in the underlying skin were not even polished out. The Boeing repair manual called for the damaged skin to be removed and replaced before installation of a doubler.
It now appears that the maintenance documents do not match the airplane. In the bureaucratic equivalent of the student’s excuse “the dog ate my homework,” the NTSB said, “China Airlines has not provided investigators with any detailed repair documentation (such as work cards or inspector signoffs) for the permanent repair.”
Shoddy as it was, the repair held, for 22 years to the day, when it failed, unable to contain the force of internal pressurization pushing outward. The shivered aluminum remains, pulled out of the waters of the Taiwan Strait, showed the telltale evidence of multi-site fatigue damage. In this type of damage, cracks creeping steadily outward from along the rivets, and toward each other along that line, can connect with sudden, lightning-like speed. In this “fast fracture” phenomenon, the structure unzips with explosive force. The fast-fracture result of MSD was evidenced in the explosive decompression of a high-time Aloha Airlines B737 in 1988. That case, though, did not involve a faulty repair, but it did send a message like a shot of chilled water to the heart of the industry about the structural safety of old jets. With the top third of the fuselage gone, the pilots were able to land the jet, preserving evidence of the weakening effect of MSD.
The China Airlines case raises the specter of MSD again, with dark overtones of lax inspection, maintenance and record-keeping procedures. In this case, too, the evidence was recovered. Had the B747 gone down far over the Pacific, the mute evidence of MSD might have been lost forever. That evidence, to employ a variation of a presently popular metaphor, may be the mother of all fatigue cracks. Metallurgical examination revealed a seven and a half foot-long section of MSD, according to the NTSB. The fatigue cracking originated at the unremoved scratches just outside of the rivet line and propagated completely through the skin.
Declining durability
All of it was under the doubler and therefore was not visible from the exterior of the airplane. That is, except for a crack creeping some five inches out from under the doubler’s edge. One wonders how this clear hint of worse cracking underneath apparently went unnoticed during innumerable pre-flight walkaround inspections.
On Nov. 26, 2002, six months after the crash of Flight CI-611, Boeing [BA] issued an alert service bulletin, calling upon B747 operators to remove doublers installed on B747s to repair tailstrike damage, and to inspect underlying skin for scratching damage (SB 747-53A2489).
The Federal Aviation Administration (FAA) followed two months later with a Jan. 24, 2003, airworthiness directive making the Boeing bulletin mandatory (AD 2003-03- 19).
According to the NTSB, the scope of these remedial actions is “too narrow.” The inspections and repairs need to be expanded to cover all aircraft, not just the B747 fleet. Doublers need to be removed “immediately” to determine whether hidden MSD is present. And, if found, proper repairs must be made.
Indeed, the NTSB’s recommendations follow a March 21 call from the ASC to the International Civil Aviation Organization strongly recommending “immediate inspection” of all aircraft not covered by the Boeing service bulletin for any hidden damage (see http://www.iasa.com.au/asc.htm).
Why didn’t the FAA mandate inspections of all aircraft types with doubler repairs? An FAA official responded, “The AD was limited to 747s because the only evidence of a potential safety problem was on the accident aircraft. There must be an indication of at least a ‘potential’ unsafe condition to put out an AD on an aircraft model.”
The NTSB recommendations address two other important issues, dealing with metal and manpower. The first is the limitation of nondestructive inspection (NDI) methods. They are not able to consistently detect cracks in metal under a doubler, or to detect cracks on the reverse side of the metal being inspected. Second, maintenance personnel are not warned sufficiently about the potential for catastrophic damage from improper repairs and hidden damage. For example, no mention was made in the Boeing alert service bulletin of the dire implications of shoddy repair work. The NTSB urged that the bulletins, manuals, training programs, maintenance guidance and such must include warnings to all concerned that hidden damage can be deadly.
Apart from the NTSB’s pleadings, a number of related issues boil out of this case:
- Tailstrikes are not only expensive, they can also have long term ramifications.
- A workable solution to prevent tailstrikes, or the damaging effects therefrom, would be a boon to safety.
- Other faulty and undocumented repairs may be lurking. Recall that the deadliest crash on record remains that of a Japan Air Lines B747 in 1985, which was attributed to a faulty repair of the rear pressure bulkhead. When it blew apart, tearing hydraulic lines, the complete loss of hydraulic systems left the pilots helpless to prevent the airplane’s impact with a mountain. That repair, too, followed a tailstrike.
- Strain gauges in strategic locations, as are used in some military aircraft, may be a useful means of tracking structural strength and the weakening ravages of MSD, corrosion and so forth on commercial airliners.
At this time, the ASC chooses to characterize the faulty repair to the China Airlines jet as a “risk factor” rather than a “causal factor.” The ASC is still in the fact-finding phase of its accident postmortem.
Meanwhile, China Airlines earlier this month announced that Singapore Airlines Engineering Co. had been retained to restructure the Taiwan-based carrier’s engineering and maintenance operations. When in doubt, get help. (ASW note: NTSB recommendations at http://www.ntsb.gov/Recs/letters/2003/A03_07_10.pdf)
Thoroughly Bad Idea
In its recent draft report on ETOPS (extended operations) an Aviation Rulemaking Advisory Committee (ARAC) recommended contingency fuel for 15 minutes of holding at an alternate airport (see ASW, March 24). Some believe that 30 minutes fuel for holding is more prudent (see ASW, March 31). Support for the latter, as a minimum, comes from a 1997 emergency landing of a Delta Air Lines MD-11 trijet on a planned flight from Atlanta to London’s Gatwick Airport. Due to bad weather at Gatwick the airplane diverted to Manchester. To get priority handling with other traffic at Manchester, the crew had to declare an emergency because the Federal Aviation Administration (FAA) approved terminology for priority, “minimum fuel,” is not recognized in the UK.
Fuel planning for the flight was based on the FAA requirement (FAR 121 – 645) for 30 minutes flying at holding speed above the alternate airport, as well as the 30 minutes holding fuel called for in Delta’s fuel policy. The incident report by the UK’s Air Accidents Investigation Branch (AAIB) exemplifies why the removal or paring of contingency fuel may be a thoroughly bad idea. One is reminded of the aphorism that it may be better to pay for the fence at the top of the hill than the stretcher at the bottom. >> See http://www.aaib.dft.gov.uk/bulletin/jan98/n805de.htm <<