United Airlines Flight 811
N4713U after the cargo door tore off in flight and caused an explosive decompression resulting in 9 deaths.
|Date||February 24, 1989|
(Cargo door design flaw and failure), Engines damaged by debris
near Honolulu, Hawaii
|Aircraft type||Boeing 747-122|
|Flight origin||San Francisco Int'l Airport
San Francisco, California, United States
|1st stopover||Los Angeles Int'l Airport
Los Angeles, California, United States
|2nd stopover||Honolulu International Airport
Honolulu, Hawaii, United States
|Last stopover||Auckland Airport
Auckland, New Zealand
United Airlines Flight 811 experienced a cargo door failure in flight on February 24, 1989, after its stopover at Honolulu International Airport, Hawaii. The resulting decompression blew out several rows of seats, resulting in the deaths of 9 passengers.
Flight 811 took off from Honolulu International Airport bound for Auckland, New Zealand with 3 flight crew, 15 flight attendants, and 337 passengers at approximately 01:52 HST. Its flight crew was extremely experienced and consisted of Captain David Cronin (59), First Officer Al Slader (48), and Flight Engineer Randal Thomas (46). Captain Cronin had logged more than 28,000 flight hours, of which approximately 1,650 were in Boeing 747 aircraft. The first officer and flight engineer had logged 14,500 flight hours and 20,000 flight hours, respectively.
During the climb, the crew made preparations to detour around thunderstorms along the aircraft's track; anticipating turbulence, the captain kept the seatbelt sign lit. After the plane had been flying for approximately 16 minutes, and was passing between 22,000 and 23,000 feet (6,700–7,000 m), a grinding noise was suddenly heard in the business-class section, followed by a loud thud which rattled the whole aircraft. One and a half seconds later, the forward cargo door blew out abruptly. The door swung out with such force that it was forced past its normal stop and slammed the side of the fuselage, bursting it open. Pressure differentials and aerodynamic forces caved in the cabin floor, causing ten seats (G and H of rows 8 through 12), as well as an individual seated in 9F whose armrest failed, to be ejected from the cabin. All 9 passengers seated in these locations were killed (seats 8G and 12G were unoccupied). A gaping hole was left in the aircraft and a flight attendant in the Business Class cabin was almost pulled out of the airplane and was seen by passengers and fellow crew members clinging to a seat leg; they were able to pull her to safety inside the cabin, although she was severely injured. Another flight attendant in the Business Class Cabin hung on to the steps leading to the upper deck, and was dangling from them when the decompression occurred.
The pilots initially believed a bomb went off inside the airplane, as this accident happened just ten weeks after Pan Am Flight 103 was blown up over Lockerbie. To reach an altitude where the air was breathable, they began an emergency descent, while also performing a 180-degree left turn to fly back to Honolulu. The explosion damaged components of the on-board emergency oxygen supply system, as it was primarily located in the forward cargo sidewall area, just aft of the cargo door.
The debris ejected from the airplane during the explosive decompression caused severe damage to the Number 3 and 4 engines, causing visible fires in both. The crew did not get fire warnings from either of them. Engine 3 was experiencing heavy vibration, no N1 reading, and a low EGT and EPR, so the crew shut down Engine 3. At 02:10, an emergency was declared and the crew began dumping fuel to reduce the aircraft's landing weight. Initially, they pushed the Number 4 engine slightly but once they noticed that its N1 reading was almost zero, its EGT reading was high, and it was emitting flames, they shut it down as well. Some of the explosively ejected debris damaged the right wing's Leading Edge Devices, dented the horizontal stabilizer on that side, and even struck the tailfin.
During the descent, Captain Cronin ordered Flight Engineer Thomas to tell the flight attendants to prepare for an emergency landing, but Thomas was unable to contact them through the intercom. He asked the captain for permission to go down to find out what was happening; Cronin agreed. Thomas saw severe damage immediately upon leaving the cockpit; the aircraft's skin was peeled off in some areas on the upper deck, revealing the frames and stringers. As he went down to the lower deck, the magnitude of the damage became apparent as he saw the large hole in the side of the cabin. Thomas returned to the cockpit, visibly pale, and reported that a large section of fuselage aft of the Number 1 exit door was open. He concluded that it was probably a bomb and that, considering the damage, it would be unwise to exceed the airplane's stall speed by more than a small margin.
As the airplane neared the airport, the landing gear was extended. The flaps could only be partially deployed as a result of damage sustained following the decompression. This resulted in a high landing speed of around 190–200 knots (350–370 km/h). Cronin was able to bring the airplane to a halt without overrunning the runway. Fourteen minutes had elapsed since the emergency was declared. Evacuation was carried out and all remaining passengers and flight attendants exited the plane in less than 45 seconds. Every flight attendant suffered some injury during the evacuation, ranging from scratches to a dislocated shoulder.
The accident was most likely caused by improper wiring and deficiencies in the door's design. Unlike a plug door which opens inwards and jams against its frame as the pressure outside drops (making it impossible to accidentally open at high altitude), the Boeing 747 was designed with an outward-hinging door. While this increases the cargo capacity, it requires a strong locking mechanism to keep the door closed. Deficiencies in the design of wide-body aircraft cargo doors were already known since the early 1970s from flaws in the DC-10 cargo door. Despite the warnings and deaths from the DC-10 incidents, and early Boeing attempts to solve the problems in the 1970s, the problems were not seriously addressed by the aircraft industry and the National Transportation Safety Board until much later.
The 747's cargo door utilized a series of electrically-operated latch cams which the door-edge latch-pins closed into. The cams then rotated into a closed position, holding the door closed. A series of L-shaped arms, called locking sectors, actuated by the final manual moving of a lever to close the door, were designed to reinforce the now unpowered latch cams and prevent them from rotating into an unlocked position.
The locking sectors were made out of aluminum and were of too thin a gauge to be able to keep the latch cams from moving into the unlocked position against the power of the door motors. If an electrical switch designed to cut electrical power to the cargo door when the outer handle was closed was faulty, the motors could still draw power and rotate the latch cam to the open position. The same event could happen if frayed wires were able to power the cam-motor, even if the circuit power was cut by the safety switch.
It appeared in this case that a short circuit in the aging plane caused an uncommanded rotation of the latch cams, which forced the weak locking sectors to distort and allow the rotation, thus enabling the pressure differential and aerodynamic forces to blow the door off the fuselage, ripping away the hinge fixing structure, the cabin floor and side fuselage skin, causing the massive explosive decompression.
Lee Campbell, a native New Zealander returning home, was one of the casualties on Flight 811. After his death, his parents Kevin and Susan Campbell investigated the cause of the decompression independently of the National Transportation Safety Board. The Campbells' investigation led them to conclude that the design of the aircraft's cargo door latching mechanism was flawed.
As early as 1975, Boeing realized the aluminum locking sectors were of too thin a gauge to be effective and recommended the airlines add doublers to the locking sectors. In 1987 Pan Am Flight 125 outbound from London Heathrow Airport encountered pressurization problems at 20,000 feet (6,100 m), causing the crew to abort the flight and return to the airport. After the safe landing, the aircraft's cargo door was found to be ajar by about 1.5 inches (3.8 cm) along its ventral edge. When the aircraft was examined in a maintenance hangar, all of the locking arms were found to be either damaged or sheared off entirely. Boeing initially attributed this to mishandling by ground crew. To test this concern, Boeing instructed 747 operators to shut and lock the cargo door with the external handle, and then activate the door-open switch with the handle still in the locked position. Since the S-2 switch was designed to deactivate the door motors if the handle was locked, nothing should have happened. Some of the airlines reported the door motors did indeed begin running, attempting to force the door open against the locking sectors and causing damage to the mechanism.
Soon after the Pan Am incident in 1987, Boeing had issued a Service Bulletin notifying operators to replace the aluminum locking sectors with steel locking sectors, and carry out various inspections. In the United States, the FAA mandated this service by means of an Airworthiness Directive (AD) and gave US-flag airlines 18 months to comply with the AD. After the Flight 811 incident, the FAA shortened the time to 30 days.
In 1991, an incident occurred at New York's John F. Kennedy International Airport involving the malfunction of a United Airlines Boeing 747 cargo door. At the time, United Airlines' maintenance staff were investigating the cause of a circuit breaker trip. In the process of diagnosing the cause, an inadvertent operation of the electric door latch mechanism caused the cargo door to open spontaneously. This incident led to latch damage similar to that observed on the cargo door of Flight 811.
Two pieces of the Flight 811 cargo door were recovered from the Pacific Ocean on September 26, 1990, and October 1, 1990 from 14,100 feet below the ocean surface.
The NTSB issued a recommendation for all 747-100s in service at the time to replace their cargo door latching mechanisms with new, redesigned locks. A sub-recommendation suggested replacing all outward-opening doors with inward-opening doors, which cannot open in flight due to the pressure differential. No similar fatality-causing accidents have officially occurred on this aircraft type, although other investigations indicate the possibility that other old Boeing 747s were afflicted.
In 1989, the flight crew received the Secretary's Award for Heroism for their actions. The aircraft was successfully repaired, re-registered as N4724U in 1989, and returned to service with United Airlines in 1990. In 1997, the aircraft was registered with Air Dabia as C5-FBS and abandoned in 2001 during overhaul maintenance at Plattsburgh International Airport. David Cronin died on October 7, 2010, aged 81.
The story of the disaster was featured on the first season of Canadian National Geographic Channel show Mayday (known as Air Emergency in the US, Mayday in Ireland and Air Crash Investigation in the UK and the rest of world). The episode is titled "Ripped From The Sky" (Air Crash Investigation: "Unlocking Disaster").
- American Airlines Flight 96 - explosive decompression caused by a cargo door malfunction on a DC-10
- Turkish Airlines Flight 981 - explosive decompression caused by a cargo door malfunction on a DC-10
- Aloha Airlines Flight 243 - explosive decompression caused by metal fatigue in the fuselage
- AAR92-02 NTSB report
- "Unlocking Disaster." Mayday. Cineflix Productions. Aired Discovery Channel Canada, 2003.
- Moira Johnston. "The Last Nine Minutes: The Story of Flight 981". pp. Page 29 Image. Retrieved December 3, 2008.
- Moira Johnston. The Last Nine Minutes: The Story of Flight 981. pp. Google Books Search on '747'. Retrieved December 3, 2008.
- Judith Valente (February 27, 1990). "Roots Of Tragedy -- Parents Seek Reasons For Death Of Son". Wall Street Journal. Retrieved June 24, 2009.
- Kolstad, James L (August 28, 1991). "Safety Recommendation Document A-91-83 and -84". et al. Washington, DC, USA: National Transportation Safety Board. Archived from the original on January 26, 2013. Retrieved January 26, 2013.
- "Half of Door From Stricken Jetliner Recovered Off Hawaii". Los Angeles Times. October 3, 1990. Retrieved February 25, 2014.
- Aircraft Accident Report Number AAR-92/02 Executive Summary, National Transportation Safety Board.
- Honoring the Crew of United Airlines Flight 811, House of Representatives, Page H1798, May 10, 1989, Retrieved from the Library of Congress.
- "Air Dabia C5-FBS (Boeing 747) (Ex N4713U N4724U )". Airfleets. Retrieved May 18, 2009.
- "Photo Air Dab".
- "EAA mourns loss of Dave Cronin, heroic pilot and EAAer".
- First NTSB Aircraft Accident Report AAR-90/01 (superseded by Report AAR-92/02)
- Second NTSB Aircraft Accident Report AAR-92/02
- Accident description at the Aviation Safety Network
- Transcript of cockpit voice recorder
- Eyewitness Report: United 811
- Pre-accident photos of N4713U