United Airlines Flight 811

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, killing 9 passengers. The aircraft involved was a Boeing 747-122 (c/n 19875/89, reg N4713U), delivered to United Airlines on October 20, 1970.

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. Its flight crew consisted of Captain David Cronin, First Officer Al Slater 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 seat-belt sign lit. Around this time (02:08) the plane had been flying for approximately 16 minutes and was passing between 22,000 and 23,000 feet (6,700–7,000 m). In the business-class section, a grinding noise was heard, followed by a loud thud which rattled the whole aircraft — 1½ seconds later the forward cargo-door blew out abruptly. The pressure differential caved in the floor above the door, causing two rows of seats (8G-12G and 8H-12H) and an individual in 9F to be ejected from the cabin, resulting in nine fatalities and leaving a gaping hole in the aircraft. The fatalities were: Anthony and Barbara Fallon, Harry and Susan Craig, Lee Campbell, Dr. J Michael Crawford, John Swan, Rose Harley and Mary Handley-Desso. Mae Sapolu, a flight-attendant in the Business-Class cabin, was almost pulled out of the plane, but was seen by passengers and fellow crew clinging to a seat leg; they were able to pull her to safety inside the cabin, although she was severely injured. The pilots began an emergency descent to get the aircraft rapidly down to breathable air, while performing a 180-degree left turn to take them back to Honolulu. The decompression had damaged components of the on-board emergency oxygen supply system, which is primarily located in the forward cargo sidewall area, just aft of the cargo door. The debris ejected from the plane 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 low EGT and EPR, leading the crew to deactivate it. At 02:10, an emergency was declared, and the crew began dumping fuel to get the plane's weight down to an acceptable landing weight. Initially, they pushed the number 4 engine slightly to help force the plane down faster, but once they noticed it was giving almost no N1, high EGT, and was emitting flames, they shut it down also. Some of the explosively ejected debris damaged the right wing's LEDs (Leading Edge Devices), dented the horizontal stabilizer on that side, and even struck the tailfin. NTSB reports found human remains in the fan blades of Number 3 engine, bringing a cold comfort that some of the victims died almost instantly as they were pulled out of the plane. During the descent Captain Cronin had ordered Flight Engineer Randal Thomas to tell the flight attendants to prepare for an emergency landing, but he was unable to contact the flight attendants. Thomas asked the captain if he could go down and 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 as he now saw the gigantic hole in the side of the plane. Thomas came back to the cockpit, visibly pale, and reported that large section of fuselage aft of the Number 1 exit door was open. He concluded that it was probably a bomb, and considering the condition of the plane, it would be unwise to exceed 250 knots. The plane's stall speed was around 240 knots, producing a narrow operating envelope. As the plane 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 kn. Regardless, Captain Cronin was able to get the plane to a halt without going off the end of the runway. Fourteen minutes had elapsed since the emergency was declared. Evacuation was carried out and all passengers and flight attendants were off in less than 45 seconds, though every flight attendant suffered some injury during the evacuation, ranging from scratches to a dislocated shoulder.

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. 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 until much later. The 747's cargo door utilized a series of electrically-operated latch cams with which the latch pin locks in, the cam then rotating into the closed position. A series of L-shaped arms called locking sectors, actuated by the moving of a lever to close the door, are designed to reinforce the 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. 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 unlock; the pressure differential and aerodynamic forces then blew the door off the fuselage, causing the massive decompression.

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. 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 20000 ft, 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 in 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. Prior to Flight 811 incident Boeing 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.

Outcomes
The NTSB issued a recommendation for all 747-100s in service at the time to replace their cargo door latching mechanisms with new, non-faulty 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. United Airlines ran a simulation through a flight simulator and were, despite many attempts and variable tweaks, unable to successfully land a plane after losing the forward cargo door. 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, but abandoned in 2001 during overhaul maintenance at Plattsburgh International Airport. The plane was broken up for parts in 2004.