Aloha 243 — Corrosion and Fatigue Tore the Roof Off a 737 in Flight

Summary

At 13:46 Hawaii time on 28 April 1988, Aloha Airlines Flight 243 — a 19-year-old Boeing 737-297 climbing through 24,000 feet between Hilo and Honolulu — suffered an explosive decompression in which roughly 18 feet of the upper fuselage skin and structure peeled away in flight; chief flight attendant Clarabelle "C.B." Lansing was swept overboard and never recovered, eight people were seriously injured, and the cause was not a bomb, a bird, or pilot error but fatigue cracking and crevice corrosion that had linked along a cold-bonded lap joint until the cabin tore open like a tin. The aircraft, registration N73711, had flown about 89,680 pressurization cycles — among the highest of any 737 in service — on Aloha's short inter-island hops, and each cycle had loaded the joint that failed.

The fracture began at the longitudinal lap joint along stringer S-10L, on the upper row of rivets where the upper fuselage skin overlaps the lower. The joint had been assembled with a cold-bonded epoxy-scrim adhesive intended to share the pressurization load across the bonded area rather than through the rivets alone. When that bond disbonded — a known defect in early 737 production — salt-laden humid air entered the gap, crevice corrosion attacked the faying surfaces, and the entire hoop load funneled into the rivet holes. There, the countersunk "knife-edge" rivet design left a thin, sharp lip of metal at each hole, an ideal site for fatigue cracks to initiate. Cracks formed at many adjacent holes at once.

This was multiple-site damage: not one large crack growing slowly toward a detectable size, but dozens of small subcritical cracks, each individually below the inspection threshold, growing in parallel along the rivet row. When the ligaments between them failed, the cracks linked instantaneously into a single running fracture and a "flap" of fuselage unzipped. The National Transportation Safety Board, in report AAR-89/03, fixed the probable cause as the failure of Aloha's maintenance program to detect the disbonding and fatigue damage at S-10L — a detection failure, not merely a structural one. The metal had behaved exactly as fracture mechanics predicted; the system meant to catch it had not. Inspections ran at night under poor lighting, crews were untrained to find disbonds, a Boeing service bulletin and an FAA Airworthiness Directive on the books had a scope too narrow to mandate the joint that failed, and a passenger had seen a crack while boarding and said nothing. Aloha 243 became the founding case of the aging-aircraft era in commercial aviation.

Timeline

The Build — A Bonded Joint That Was Supposed to Never Reach the Rivets

The 737's fuselage is a pressurized cylinder, and the longitudinal seams where the curved skin panels overlap are its most heavily loaded detail. Every climb pumps the cabin up; every descent lets it down. The hoop stress trying to split the cylinder open along its length is reacted at these lap joints, and on the early 737 Boeing carried that stress two ways at once: through rows of rivets, and through a continuous cold-bonded adhesive layer — an epoxy-impregnated scrim cloth cured between the overlapping skins. The intent was elegant. If the bond carried most of the load over its whole area, the rivets would see little cyclic stress, fatigue would be negligible, and the joint would effectively last forever.

The intent depended on the bond holding, and it did not always hold. The cold-bond process of that era was prone to disbonding, and once a disbond opened, two failures cascaded from it. First, the load the adhesive had carried transferred abruptly onto the rivets, so the rivet holes now saw the full cyclic hoop stress the design assumed they would be spared. Second, the disbond created a sealed crevice between two aluminum faying surfaces — and Aloha flew its aircraft through warm, salt-saturated Pacific air, parking them in it overnight. Moisture wicked into the crevice and crevice corrosion ate the mating surfaces, thinning the metal and prying the skins apart through the expansive volume of its own corrosion product, the so-called pillowing effect. The joint designed never to stress its rivets was now stressing them fully, in a corroding environment, on an airframe accumulating cycles faster than almost any other 737 in the world. The rivet detail compounded it: the holes were countersunk in thin skin, leaving a sharp "knife-edge" of residual material at the inner surface — a built-in stress concentration where a fatigue crack needs only cyclic load and time. The joint had all three.

The Failure Sequence — Many Small Cracks That Linked at Once

Fatigue cracks initiated at the knife-edge rivet holes along the upper row of the S-10L lap joint, and not at one hole but at many — the defining feature of the accident. Conventional damage-tolerant design assumes a single dominant crack grows slowly and predictably, passing through a window in which it is large enough to find on inspection but still far from critical. Multiple-site damage defeats that assumption. Dozens of small cracks, each shorter than the detectable threshold and each individually harmless, grow in concert along the rivet line while the ligaments between them quietly thin. As long as the ligaments hold, the joint looks intact; when they let go, every crack joins its neighbors in the same instant, and an invisible flaw field becomes a foot-long running fracture with no intermediate, findable stage.

On 28 April 1988, climbing through 24,000 feet, that linkage happened. The cabin pressure drove the hoop stress past what the cracked ligaments could carry; the cracks coalesced and an entire panel of upper fuselage — about 18 feet of skin, stringers, and frame from behind the cockpit aft over the forward cabin — was torn away in the slipstream. The decompression was explosive and total, the cabin open to a 300-knot wind at altitude. C.B. Lansing, standing in the aisle, was swept out and lost; passengers held by seatbelts were pinned in the blast and eight were seriously hurt. The airplane held together below the failure line — the floor beams and lower fuselage kept it flyable — and the crew brought it down at Kahului on Maui in about twelve minutes, losing one engine on the way but landing 94 survivors. The structure had failed catastrophically and the outcome was, against the mechanism, very nearly not fatal at all.

Contributing Factors

Aftermath

The toll was one death and eight serious injuries, and the airplane was a write-off, but the lasting product of Aloha 243 was regulatory and structural, not statistical. The accident proved that a transport aircraft could reach an age and cycle count at which multiple-site fatigue, accelerated by corrosion, outran the inspection regimes then in force — and the industry responded by inventing the discipline of managing aging aircraft. The FAA convened an Airworthiness Assurance Task Force, and structured aging-aircraft inspection programs were developed with Boeing and the operators. In October 1991 Congress passed the Aging Aircraft Safety Act, driving mandatory supplemental structural inspection programs, corrosion-prevention and control programs, and the retroactive application of damage-tolerance analysis to airframes that predated those rules. The concept of widespread fatigue damage entered the certification vocabulary, ultimately producing limit-of-validity requirements that cap how long an airframe may fly before fatigue is presumed to threaten the structure fleet-wide; cold-bonded lap joints were inspected, repaired, and on many types eliminated. In aviation memory "Aloha 243" is shorthand for what happens when a high-cycle airframe meets multiple-site damage that no single inspection was built to find — the case that turned "aging aircraft" from a phrase into a federally mandated engineering program.

Lessons

References

• Aircraft Accident Report AAR-89/03: Aloha Airlines Flight 243, Boeing 737-200, N73711, near Maui, Hawaii, April 28, 1988 National Transportation Safety Board
• Aloha Airlines Flight 243 Wikipedia
• Aloha Airlines Flight 243, Boeing 737-200, N73711, 28 April 1988 Federal Aviation Administration — Lessons Learned from Transport Airplane Accidents