The Aging Aircraft Safety Act of 1991 told the FAA to make rules ensuring that older airliners stay airworthy, and to inspect them and review their records as part of every heavy maintenance check after their 14th year in service. It followed the Aloha Airlines Flight 243 accident of 1988, which showed that an aircraft can pass its inspections and still carry hidden fatigue damage. Later rules set a hard limit on how long an airliner’s structure can be relied on.
What happened on Aloha 243
On 28 April 1988 Aloha Airlines Flight 243, a Boeing 737-200 built in 1969, was levelling at 24,000 feet between Hilo and Honolulu when about 18 feet of the cabin roof and walls tore away. Flight attendant Clarabelle “C.B.” Lansing was swept out of the aircraft and never found. Captain Robert Schornstheimer and First Officer Madeline “Mimi” Tompkins landed at Kahului on Maui 13 minutes later. The other 94 people on board survived, eight of them seriously injured.
What passengers and crew experienced in those seconds is covered in our guide to explosive and rapid decompression.
Corrosion, disbonding and fatigue cracks, in plain words
A fuselage is a tube of thin aluminium skin. Panels overlap along lengthwise seams called lap joints and are riveted together. On early 737s, including this one, the overlap was also glued with a cold-bond adhesive. The FAA says this process had manufacturing difficulties, and Boeing stopped using it in production in 1972.
When the glue let go (disbonding), the rivets had to carry the load the bond was meant to carry. The rivet countersinks went through the full thickness of the upper skin, leaving a sharp edge and a high stress at each hole. Each time the cabin was pressurised and released, small cracks grew at many neighbouring rivets. This is multiple-site damage. The FAA calls it insidious: many small, hard-to-find cracks can link up suddenly into one long, critical crack. When that happens across a structure, it is called widespread fatigue damage.
That is what happened at the lap joint at stringer S-10L on Flight 243.
Metal fatigue, in short, is damage that builds up from repeated loading, even when each load is well within the metal’s strength. For a fuselage, the repeated load is pressurisation, once per flight.
Why the inspections missed it
The aircraft had flown 89,680 flight cycles. Three things made that number dangerous, according to the FAA’s summary of the NTSB findings.
Cycles, not hours. Aloha flew very short hops between the Hawaiian islands. Its heavy check came every 15,000 flight hours, which looked reasonable next to Boeing’s recommended 20,000. But the airline was building up cycles at about twice the rate Boeing assumed, and fatigue cracks in a pressurised fuselage grow with cycles, not hours.
A heavy check split into pieces. Aloha divided its heavy (D) check into 52 separate work packages done overnight. The NTSB said this was an inappropriate way to carry out such a check.
A narrow directive. In 1987 the FAA had issued an airworthiness directive (AD 87-21-08) requiring inspection of only the lap joints at stringer 4, although Boeing’s service bulletin covered other stringers too. After the accident, cracks were found at the stringer 4 joint that the required eddy current inspection should have detected. Aloha’s training records showed little formal training in these inspection techniques.
Across the fleet, inspectors found bulging skin, popped rivets and flaking paint along the lap joints of almost every Aloha 737. The NTSB noted that pilots and line maintenance staff had come to accept these classic signs of corrosion as normal.
The NTSB’s probable cause was the failure of Aloha’s maintenance programme to detect significant disbonding and fatigue damage, which led to the failure of the lap joint at S-10L and separation of the upper fuselage. It named as contributing factors Aloha management’s supervision of maintenance, the FAA’s decision not to require inspection of all the lap joints, and the lack of a complete fix for the early cold-bond joint problems (NTSB/AAR-89/03). Our guide to maintenance errors sets Aloha beside other cases.
The FAA’s response and the 1991 Act
The FAA’s response came in layers. Its lessons-learned summary lists:
- mandatory corrosion control programmes, so that every operator has prevention and inspection systems good enough to ensure hazardous corrosion never occurs;
- airworthiness directives requiring inspections and modifications of fuselage lap joints, and in 2002 the removal and replacement of certain lap splice skin;
- a rule requiring operators to assess repairs to pressurised fuselages;
- a change to the certification rule for new aircraft (14 CFR 25.571, Amendment 96) requiring full-scale fatigue test evidence that widespread fatigue damage will not occur within the aircraft’s design service goal.
Congress acted too. The Aging Aircraft Safety Act of 1991, now 49 U.S.C. § 44717, says the FAA “shall prescribe regulations that ensure the continuing airworthiness of aging aircraft”. Those rules must require the FAA to inspect each aircraft an airline uses, and review its maintenance and other records, to decide whether it is in safe condition and properly maintained. Airlines must make the aircraft and records available. The inspections and reviews are carried out as part of each heavy maintenance check after the aircraft’s 14th year in service.
Widespread fatigue damage and the limit of validity
The last big piece came in 2010. The FAA’s widespread fatigue damage rule (75 FR 69746, effective 14 January 2011) requires the makers of affected airliners to set a limit of validity, or LOV. This is the point, in flight cycles or flight hours, up to which the engineering data behind the aircraft’s structural maintenance programme can be trusted. The maker must show that the aircraft will be free from widespread fatigue damage up to that limit.
Operators must build the LOV into their maintenance programmes and may not fly an aircraft beyond it unless an extended LOV is approved. In other words, an airliner’s structure now has a stated working life, backed by testing.
Other structural cases: a different problem and a bad repair
Not every structural failure is about age.
American Airlines Flight 587, 2001. The vertical stabiliser of an Airbus A300 separated about 90 seconds after take-off from New York’s JFK airport, and all 260 people on board and 5 on the ground were killed. The NTSB found the fin failed under loads beyond its ultimate design load, created by the first officer’s unnecessary and excessive rudder pedal inputs, with the rudder system’s design and the airline’s training contributing (NTSB/AAR-04/04). This was an overload failure, not a problem of fatigue or age. See American Airlines Flight 587.
Japan Air Lines Flight 123, 1985. A Boeing 747’s rear pressure bulkhead ruptured where it had been repaired after a tail strike. The FAA says the repair was installed improperly, which caused early fatigue failure. Here, fatigue came from a repair, not from the original design.
The structural failure topic page lists every event on this site in which the airframe itself failed.
What passengers can take from it
Flight 243 is one reason older airliners are watched more closely than they used to be. US law now requires inspections and records reviews of airliners after their 14th year in service. Repairs to pressurised fuselages must be assessed. And affected airliner types have a structural limit of validity, set by the maker and enforced by the regulator.
Investigators often use Aloha to show how many defences had to fail at once, from the glue on the production line to the scope of one directive. Our guide to the Swiss cheese model uses it as an example, and Is flying safe? puts changes like these in the context of the long fall in accident rates.
Sources
- NTSB. NTSB/AAR-89/03, Aloha Airlines Flight 243, Boeing 737-200, N73711, near Maui, Hawaii, 28 April 1988 Source
- FAA. Lessons Learned: Aloha Airlines Flight 243 (N73711) Source
- US Code (Cornell LII). 49 U.S.C. § 44717, Aging aircraft (from the Aging Aircraft Safety Act of 1991, Pub. L. 102-143, title IV, 28 October 1991) Source
- FAA (Federal Register via GovInfo). Aging Airplane Program: Widespread Fatigue Damage, final rule, 75 FR 69746 (15 November 2010) Source
- NTSB. NTSB/AAR-04/04, In-Flight Separation of Vertical Stabilizer, American Airlines Flight 587 Source
- FAA. Lessons Learned: Japan Air Lines Flight 123 (JA8119) Source