Aircraft Accidents and Lessons Unlearned XXVII: United 811

On February 24, 1989, while climbing out of Honolulu, Hawaii, passing 22,000 for 23,000 feet of altitude at 300 knots of indicated airspeed, United flight 811, a Boeing B747-122, tail number N4713U, suffered an explosive decompression. The forward cargo door separated from the lower fuselage. At its departure, the door damaged the right wing, numbers three and four engines; the right wing’s leading-edge devices, the fuselage, the right horizontal stabilizer and the vertical stabilizer as it fell away. Due to the violent nature of the door’s separation, a fuselage section above the cargo door, measuring thirteen feet lengthwise and fifteen feet vertically – to the first-class passenger windows’ level – tore free and away from the aircraft. The captain executed an emergency descent with a left turn to return to Honolulu where the plane landed safely, per Accident report AAR-92/02; accident number DCA89MA027.

Navy radar in the vicinity of Honolulu tracked the debris as it fell into the Pacific Ocean. The forward cargo door had been split in two pieces; the two sections were recovered from the ocean floor nineteen months later, on October 1, 1990. Within a year and a half, the National Transportation Safety Board (NTSB) had the necessary pieces to connect the dots, figure out how the door failed. More importantly, they could learn, Why.

Cargo doors are intricate devices, whether they are hydraulically operated main cargo doors, manually operated narrow body belly doors or electrically operated wide body belly doors; precise mechanical motions are synchronized to take place at specific times. However, the fundamental workings of a cargo door are simple: sequential order. The first sequence is the door open/close actuator which raises and lowers the door. As the closing door reaches the doorframe, the door actuator cuts out; a second sequence occurs where the door is pulled in tight against the frame. This signals the third sequence: the locking device; the door is captured and secured in place.

Three forces would have worked on Flight 811’s forward cargo door, causing the catastrophic results. The first is the internal pressure of the aircraft acting to open the door; over one hundred pounds per square inch were pressing against the 99” by 110” door’s inside surface. Simultaneously Bernoulli’s principle explains the second force acting on the door’s exterior, like lift on a wing: “An increase in the velocity of a stream of fluid [air] resulted in a decrease in pressure;” the decrease in pressure on the outside pulled the door away from the fuselage. Once the door was ‘pushed’ open, the third force, the airstream around the plane, tore the door away at 300 knots.

Much of AAR-92/02 report’s damage descriptions were confusing; the pictures used were poor quality and suggested that the NTSB investigators should have employed more technical drawings that would have translated better. Unfortunately, the NTSB failed to use maintenance experienced investigators to explore the maintenance issues.

The report’s maintenance records were documented between December 5, 1988 and December 30, 1987. Eleven times between these dates, the forward cargo door was written up as non-functioning electrically, resulting in the alternate method being employed: manually closing the door, a proven safe practice. However, United ignored the repetitious nature of these write-ups and failed to fix the problem, instead they postponed the fix indefinitely. The NTSB investigators did not pick up on this history finding, then failed to show any forward cargo door maintenance history from December 30, 1988 to the accident.

Instead, the NTSB maintenance investigator focused on a trivial matter as stated in Finding 16, “The smooth wear patterns on the latch pins of the forward cargo door installed on N4713U were signs that the door was not properly aligned (out of rig) for an extended period of time, causing significant interference during the normal open/close cycle.” As stated above, the door was under several forces during each flight. The cargo door’s latches were going to make ‘smooth wear patterns’ as the door leans into the latches. The latches also contacted the pins as the latches moved around them. This was normal.

AAR-92/02 related to a similar incident involving Pan Am flight 125, a B747-122, where, during flight, the cargo door was open one and a half inches along the bottom edge with the latch cams unlatched. Pan Am 125 returned to its origin due to a problem with pressurization. The NTSB used this incident as a ‘told-you-so’ moment to both Boeing and the Federal Aviation Administration (FAA), to say that the NTSB’s warning was unheeded, that the same thing happened with United 811.

However, Pan Am 125 and United 811 were fundamentally different. Was the event of Pan Am 125 dangerous? Yes. But to compare the two flights was a distraction; it focused away from United 811’s unique issues, that would have pointed to United 811’s root cause. On Pan Am 125, the door was never completely closed, pressurized air bled out of the gap, decreasing the pressure that would have pushed the door open. The pressurization forces were entirely different because the opening never allowed the forces to build up on the door. The NTSB’s attempt to draw comparisons did nothing to increase safety or knowledge of the root cause; instead it wasted time.

Throughout the report, the NTSB investigators stumbled through the various conclusions, some findings having had nothing to do with the accident, e.g. Fire and Rescue truck paint schemes. With the cargo door’s recovery, the root cause should have been definitive. Instead the NTSB resorted to a guessing game, probable causes were made in error that focused on one or two inconclusive reasons for the explosive decompression but, in the end, AAR-92/02 established nothing.

What, then, caused the forward cargo door to depart United 811? Without the evidence available at the time to study, all one can do is speculate and speculation is useless. It was clear that the use of megaphones (Finding 24) or the Fire and Rescue trucks’ camouflage paint job (Finding 23) never contributed to the accident. However, as a former aircraft mechanic, one who had worked many cargo doors, there were several issues I expected the NTSB should have focused their attention on.

The last maintenance performed on the cargo door was the replacement of a mid-span latch pin due to gouging. Wearing was a common consequence of normal cargo door use but gouging meant the door may have had a single latch out of rig, perhaps more than one. Gouging could have also signaled a timing problem, that the latch was out of sync with the other latches. Did the NTSB ask to see the replaced pin, if that was possible? What about the new pin; was it showing early signs of gouging. According to the maintenance records, since 1984, “… a full cargo door rigging check had not been accomplished.” If an NTSB investigator experienced in maintenance were available, he would have found the gouges to be an item of interest, certainly of a more focused examination.

The most unusual item in the maintenance history, however, was the adjustment of the S-8 switch on December 14, 1988; the NTSB investigator did not give this a second look; the S-8 switch and the gouged pin never made it into the Findings. The S-8 switch was the Hook Close Switch; this switch sequenced the two ‘Pull in’ hooks to rotate over the doorframe pin, which pulled the cargo door flush with the fuselage. If they resembled the B727 main cargo door’s ‘Pull in’ hooks they had duck bill shaped grooves that captured the pin and, as they rotated, pulled the door tight against the doorframe. The locking cams then easily rotated to the closed position, which secured the cargo door in place.

What if the S-8 switch was adjusted incorrectly on December 14th? What if the ‘Pull in’ hooks rotated early or late; would it have captured the pin or, instead, would the hooks have slid behind the pin? Why did the NTSB not assure the S-8 switch had been inspected by a quality control inspector? If it was not, was that a policy violation; a preventable cause? We will never know.

AAR-92/02 was typical of many accident reports reviewed on this site: heavy on the blame but light on the root cause. Later accidents continued to display signs of previous accidents, not because FAA inspectors did not do their jobs, or the industry did not do what’s right but because time and energies were misdirected; opportunities were wasted on inexperienced guess work. Aviation will not become safer when the resources to make it safer are ignored or ill-used.