United Airlines Flight 811

United Airlines Flight 811

N4713U after the cargo door tore off in flight and caused an explosive decompression resulting in nine deaths.
Accident summary
Date	February 24, 1989 (1989-02-24)
Summary	Explosive decompression (Cargo door design flaw and failure), engines damaged by debris
Site	Pacific Ocean near Honolulu, Hawaii
Passengers	337
Crew	18
Injuries (non-fatal)	38
Fatalities	9
Survivors	346
Aircraft type	Boeing 747-122
Operator	United Airlines
Registration	N4713U (became N4724U after repairs)
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
Destination	Sydney Airport Sydney, Australia

United Airlines Flight 811 was an airline flight that 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 nine passengers. The airliner returned to Honolulu, where it landed safely.

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

Incident

N4713U, photographed in 1982 at LAX, prior to losing the cargo door and part of the fuselage.

Flight 811 took off from Honolulu International Airport bound for Auckland, New Zealand, with three 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. Flight 811 was his second-to-last scheduled flight before his mandatory retirement. 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 heard in the business-class section, followed by a loud thud that rattled the whole aircraft. One and a half seconds later, the forward cargo door blew out. The door swung out with such force that it passed its normal stop and slammed into the side of the fuselage, bursting the fuselage open. Pressure differentials and aerodynamic forces caused the cabin floor to cave in, and ten seats (G and H of rows 8 through 12) were ejected from the cabin. All eight passengers seated in these locations were killed (seats 8G and 12G were unoccupied), as was Anthony Fallon in seat 9F, whose seatbelt failed. A gaping hole was left in the aircraft, through which a flight attendant in the business-class cabin was almost blown out of the aircraft. Passengers and crew members saw her clinging to a seat leg and were able to pull her back inside the cabin, although she was severely injured. Laura Brentlinger, the senior flight attendant, 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 eight weeks after Pan Am Flight 103 was blown up over Lockerbie, Scotland. 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.

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. 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 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.

Despite extensive air and sea searches, no remains were found at sea of the nine victims lost in flight. Multiple small body fragments and pieces of clothing were found in the Number 3 engine, indicating at least one victim was ejected from the fuselage into the front of the engine, but it was not known whether the fragments were from one or more victims. Because Anthony Fallon was seated farthest from the hole in the plane, he was likely the last victim ejected and with the least velocity; he was also the only victim not ejected with his seat (a seat entering the engine would have caused immediate destruction of the engine); therefore, it is likely that the remains found inside the Number 3 engine were his. Given the aircraft's altitude and speed, the force of the ejection and the fact that debris impacted much of the aircraft's external structure, it was deemed highly unlikely that any victims were alive during their four-minute descent to the ocean's surface.

Victims

• Lee Campbell, Wellington, New Zealand
• Harry Craig, Morristown, New Jersey
• Susan Craig, Morristown, New Jersey
• Dr. J. Michael Crawford, Sydney, Australia
• Anthony Fallon, Long Beach, California
• Barbara Fallon, Long Beach, California
• Mary Handley, Bay City, Michigan
• Rose Harley, Hackensack, New Jersey
• John Swann, Sydney, Australia

Cause

The accident was most likely caused by improper wiring and deficiencies in the door's design. Unlike a plug door which opens inward 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 used 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.

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 cause of the incident was not human error but instead the combination of an electrical problem and an inadequate design of the aircraft's cargo door latching mechanism. They later presented their findings to the safety board.

The cargo door recovered by manned deep sea submersible Sea Cliff.

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 U.S.-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 halves of the Flight 811 cargo door were recovered from the Pacific Ocean on September 26, 1990, and October 1, 1990, from 14,100 feet (4,300 m) below the ocean surface. The cargo door had fractured length-wise across the center. Recovery crews reported that no other debris or evidence of human remains had been discovered.

Outcomes

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.

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. In 2004 the aircraft was scrapped for spare parts.

Captain David Cronin died on October 4, 2010, aged 81.

Passenger John F. Stephenson would later die in a separate plane crash, when his light plane plummeted into a residential area of South-Eastern Melbourne in 2014.

Dramatization

The story of the disaster was featured on the first season of Canadian Discovery 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").

See also

• 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 of a 737
• British Airways Flight 5390 - explosive decompression caused by an improperly installed windscreen on a BAC 1-11

External links

• 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