United Airlines Flight 811

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United Airlines Flight 811

Illustration of Flight 811 just after decompression
Accident summary
DateFebruary 24, 1989 (1989-02-24)
TypeExplosive decompression
(Cargo door design flaw and failure), Engines damaged by debris
SitePacific Ocean
near Honolulu, Hawaii
Passengers337
Crew18
Injuries38
Fatalities9
Survivors346
Aircraft typeBoeing 747-122
OperatorUnited Airlines
RegistrationN4713U
Flight originSan Francisco International Airport, San Francisco, United States
1st stopoverLos Angeles International Airport, Los Angeles, United States
2nd stopoverHonolulu International Airport, Honolulu, United States
Last stopoverAuckland Airport, Auckland, New Zealand
DestinationSydney Airport, Sydney, Australia
 
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United Airlines Flight 811

Illustration of Flight 811 just after decompression
Accident summary
DateFebruary 24, 1989 (1989-02-24)
TypeExplosive decompression
(Cargo door design flaw and failure), Engines damaged by debris
SitePacific Ocean
near Honolulu, Hawaii
Passengers337
Crew18
Injuries38
Fatalities9
Survivors346
Aircraft typeBoeing 747-122
OperatorUnited Airlines
RegistrationN4713U
Flight originSan Francisco International Airport, San Francisco, United States
1st stopoverLos Angeles International Airport, Los Angeles, United States
2nd stopoverHonolulu International Airport, Honolulu, United States
Last stopoverAuckland Airport, Auckland, New Zealand
DestinationSydney Airport, Sydney, Australia

United Airlines Flight 811 experienced a cargo door failure in flight on Friday, 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.

The aircraft involved was a Boeing 747-122 (c/n 19875/89, reg N4713U), delivered to United Airlines on October 20, 1970.

Contents

Incident

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.[1] Its flight crew consisted of Captain David Cronin, First Officer Al Slader, and Flight Engineer Randal Thomas.

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 pressure differential caved in the main cabin floor above the door, causing ten seats (G and H of Rows 8 to 12), as well as an individual seated in 9F, to be ejected from the cabin, and leaving a gaping hole in the aircraft. Nine fatalities resulted (Seats 8G and 12G were unoccupied).[1] 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 began an emergency descent to get the aircraft down into breathable air rapidly, while performing a 180-degree left turn to fly back to Honolulu. The decompression damaged components of the on-board emergency oxygen supply system, which was primarily located in the forward cargo sidewall area, just aft of the cargo door.[2]

Location of N1 and N2 in a turbofan engine, and a diagram of EPR, EGT, N1 and N2 indicators. Many turbine engines have rotating assemblies called spools which can turn at different speeds, requiring multiple tachometers. These tachometers are referred to as N1 and N2.

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, although Engine 3 was experiencing heavy vibration, no N1 reading, and a low EGT and EPR. This led the crew to deactivate Engine 3. At 02:10, an emergency was declared, and the crew began dumping fuel to get the airplane to an acceptable landing weight. Initially, they pushed the Number 4 engine slightly to help force the plane down faster; once they noticed that its N1 reading was almost zero and its EGT reading was high, and that it was emitting flames, they shut it down also. 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. 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 obvious to him as he saw the gigantic hole in the side of the cabin. Thomas came back 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 250 knots (460 km/h). The airplane's stall speed was around 240 knots (440 km/h), producing a narrow operating envelope.[2]

As the airplane neared the airport, the landing gear was extended. The flaps were only partially deployed, as a result of damage sustained following the decompression. This resulted in a landing speed between 190–200 knots (350–370 km/h). Regardless, Captain Cronin was able to bring the airplane to a halt without going off the end of the runway. Fourteen minutes had elapsed since the emergency was declared.[2] Evacuation was carried out, and all passengers and flight attendants were off in less than 45 seconds. Every flight attendant suffered some injury during the evacuation, however, ranging from scratches to a dislocated shoulder.[1]

Cause

The accident was most likely caused by improper wiring and deficiencies in the door's design. Unlike a plug door which opens inwards and essentially jams against its frame as the pressure outside drops, the Boeing 747 was designed with an outward-hinging door which, while increasing capacity, required a 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.[3][4] 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.[5]

The 747's cargo door utilized a series of electrically-operated latch cams that the door-edge latch-pins close into, the cam then rotates into the 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, are designed to reinforce the now unpowered latch cams and prevent them from rotating into the unlocked position.

The locking sectors were made out of aluminum and 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 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 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 decompression.[2]

Personal investigation

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.[6]

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.[2]

Cargo door recovered by US Navy divers

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.[2]

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.[7] 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.

Outcomes

The damage to the aircraft, as seen from the cabin.

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.[8] 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.[9]

In 1989, the flight crew received the Secretary's Award for Heroism for their actions.[10] United Airlines ran a simulation through a flight simulator and were, despite many attempts and various tweaks, unable to successfully land a plane after losing the forward cargo door.[11]

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,[12] and abandoned in 2001 during overhaul maintenance at Plattsburgh International Airport.[13] The plane is visible on the ground at Plattsburgh on Google Maps/Earth at coordinates 44°40'00.76" N 73°28'13.33" W.

Dramatization

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 Apart" (Air Crash Investigation: "Unlocking Disaster").

See also

References

External links

Coordinates: 20°41′24″N 158°40′34″W / 20.69°N 158.676°W / 20.69; -158.676