On 28 April 1988, a Boeing 737-200, being operated by Aloha Airlines experienced sudden structural failure of the fuselage and a consequent explosive depressurisation whilst en route at FL240. Approximately 5.5 metres (or 18 feet) of cabin skin and structure was lost from the aircraft an one of the cabin crew was fatally injured. The flight crew carried out an emergency descent and made a landing at Kahului Airport on the Island of Maui, Hawaii.
This is an extract from the official accident Report AAR/89/03 by National Transportation Safety Board (NTSB):
As the airplane leveled at 24,000 feet, both pilots heard a loud “clap” or “whooshing” sound followed by a wind noise behind them. The first officer […] stated that debris, including pieces of grey insulation, was floating in the cockpit. The captain observed that the cockpit entry door was missing and that “there was blue sky where the first class ceiling had been.” [Figure 1] The captain immediately took over the controls of the airplane. He described the airplane attitude as rolling slightly left and right and that the flight controls felt “loose”.
Figure 1. Illustration of B737-200 part of fuselage that was separated during flight. Source:
FAA Lessons Learned from Transport Airplane Accidents (http://lessonslearned.faa.gov/
Because of the decompression, both pilots […] donned their oxygen masks. The captain began an emergency descent. […] Because of ambient noise, the pilots initially used hand signals to communicate. The first officer stated that she observed rate of descent of 4,100 feet per minute at some point during the emergency descent. The captain stated that he actuated the passenger oxygen switch. The passenger oxygen manual tee handle was not actuated.
When the decompression occurred, all the passengers were seated and seat belt sign was illuminated. The No. 1 flight attendant reportedly was standing at seat row 5. According to passenger observations, the flight attendant was immediately swept out of the cabin through a hole in the left side of the fuselage. The No. 2 flight attendant, standing by row 15/16, was thrown to the floor and sustained minor bruises. She was subsequently able to crawl up and down the isle to render assistance and calm the passengers. The No.3 flight attendant, standing at row 2, was struck in the head by debris and thrown to the floor. She suffered serious injuries […].
The first officer said she turned the transponder to emergency code 7700 and attempted to notify Honolulu Air Route Traffic Control Center (ARTCC) that the flight was diverting to Maui. Because of the cockpit noise level, she could not hear any radio transmissions, and she was not sure if the Honolulu ARTCC heard the communication.
The controller observed the emergency transponder indication and tried to make contact with the aircraft several times without success. After the aircraft descended through 14,000 feet, the first officer contacted Maui Tower, declared an emergency situation, and stated the need for emergency equipment.
During the descent, the No. 1 engine failed and the captain attempted to restart it, without success. The captain was able to make a normal touchdown and landing rollout. He used the No. 2 engine thrust reverser and main landing gear brakes to stop the aircraft. The aircraft was evacuated on the runway.
To access the video showing the accident B737 on the runway, click here. (The video is courtesy of FAA Lessons Learned from Transport Airplane Accidents)
After the accident, a passenger stated that as she was boarding the airplane […] she observed a longitudinal fuselage crack. The crack was in the upper row of rivets along the S10L [See Figure 2] lap joint, about halfway between the cabin door and the edge of the jet bridge hood. She made no mention of the observation to the airplane ground personnel or flightcrew.
Figure 2. Front view of the fuselage, showing the sections of the fuselage and the missing area. Source: FAA Lessons Learned from Transport Airplane Accidents
According to the official accident Report, two inspectors working on separate shifts conducted inspection as required by Boeing service bulletin (SB) and related airworthiness directives (AD) after work that has been done on the aircraft fuselage skin prior the accident. An inadequate maintenance program was found to be the reason for the fuselage section separation during flight. The maintenance program failed to detect the presence of significant disbanding, corrosion, and fatigue damage. The process that was used to bond the overlapping fuselage skins together was poorly performed, and led to early disbonding.
The disbonding in turn resulted in what is known as a "knife-edge effect". This effect created a poor fatigue detail in the skin and many adjacent fastener holes started to crack.
This form of cracking is known as multiple site damage, which leads to widespread fatigue damage (WFD) in its advanced stage. By definition, WFD is a condition in which the airplane is no longer able to carry the required residual strength loads.
The Report further states:
The Safety Board […] believes that Aloha Airlines had sufficient information regarding lap joint problems to have implemented a maintenance program to detect and repair joint damage. The information available to Aloha Airlines on lap joint problems included the following:
- The B737s in the Aloha Airlines’ fleet were high-cycle airplanes accumulating cycles at a faster rate than any other operator;
- Aloha Airlines operated in a harsh corrosion environment;
- Aloha Airlines previously had discovered a 7.5 inch crack along lap joint S-10L on another B-737 airplane;
- Boeing had issued, and records indicate that Aloha Airlines was aware of, a SB covering lap joint inspection and repair in 1972, revised in 1974 and upgraded to an ASB in 1987; and
- The FAA had issued an AD in 1987 requiring inspections of the lap joints along S-4 and referencing the Boeing ASB, which called for inspection of other lap joint location, including along S-10.
In the Operational Consideration section of the Report it is stated that the flightcrew actions were consistent with simulator training situations which minimise the exposure to physiological effects. The Report focuses on following three operation areas:
- Assessment of in-flight structure damage:
The IAS [Indicated airspeed] used in the decent [280 to 290 kts], although it minimized the time at altitude, increased the manoeuvring loads and subjected the passengers to flailing and windburn from the effect of exposure. The open fuselage break was also subjected to high dynamic pressure from the wind force. […] The Operators Manual, Emergency Descend procedure (and emergency checklist) states that if structural integrity is in doubt, “limit airspeed as much as possible to avoid high maneuvering loads.” […]
- Air Ground Emergency Communications:
In the course of the accident, ATC changed frequency for primary radio contact with the airplane during the emergency. […] after some difficulty, the crew established contact with Maui Tower; Maui Tower was initially not apprised of the full nature of emergency or the structural damage. After notification of the emergency, ATC [Maui Tower] directed a frequency change to Maui Approach control. […] The Safety Board wishes to reiterate that ATC must make every effort to minimise the workload of a crew during an emergency. Further, an error during the handoff could result in lost communications and a possible loss of positive traffic control of the emergency aircraft.
- Emergency Ambulance Response:
[…] Had the ambulance service been notified earlier by the control tower and been waiting at the airport when the airplane landed, the seriously injured passengers could have been treated and transported to the hospital 13 minutes sooner. It is incumbent on these persons making a judgment for notification of emergency services to be aware of the circumstances and possibilities of each scenario.
The National Transportation Safety Board (NTSB) determined the following Probable Cause:
[…] the probable cause of this accident was the failure of the Aloha Airlines maintenance program to detect the presence of significant disbonding and fatigue damage which ultimately led to the failure of the lap joint at S-10L and the separation of the fuselage upper lobe.
The following contributing factors to the accident are identified in the Report:
- failure of Aloha Airlines management to properly supervise the Aloha Airlines maintenance program and to assess the airline's inspection and quality control deficiencies.
- failure of the FAA to broaden the scope of Airworthiness Directive 87-21-08, and mandate the inspection of all the lap joints, as was proposed by Boeing Alert Service Bulletin (SB) 737-53A1039 (while the SB mandated the inspection of all the lap splices, the AD only mandated the inspection of the Stringer 4 location).
- lack of a complete terminating action (neither generated by Boeing nor required by the FAA) after the discovery of early production difficulties in the B-737 cold bond lap joint. These difficulties consisted of low bond durability, corrosion, and premature fatigue cracking.
The NTSB Report provides a number of recommendations about the accident. The key recommendations are listed below.
- Provide specific guidance and proper engineering support to Principal Maintenance Inspectors (FSDO) to evaluate modifications of airline maintenance programs and operations specifications that propose segmenting major maintenance inspections.
- Revise the regulations governing the certification of aviation maintenance technician schools and the licensing of airframe and power plant mechanics to require that the curriculum and testing requirements include modern aviation industry technology.
- Require operators to provide specific training programs for maintenance and inspection personnel about the conditions under which visual inspections must be conducted. Require operators to periodically test personnel on their ability to detect the defined defects.
- Revise the National Aviation Safety Inspection Program objectives to require that inspectors evaluate not only the paperwork trail, but also the actual condition of the fleet airplanes undergoing maintenance and on the operation ramp.