All 260 people onboard Flight 587, plus 5 on the ground, were killed when the plane crashed into a residential area on November 12, 2001. Flight 587 took off shortly after another large aircraft. The plane experienced turbulence. According to the NTSB, the pilot’s overuse of the rudder mechanism, which had been redesigned and as a result was unusually sensitive, resulted in such high stress that that vertical stabilizer separated from the body of the plane.
This event is an example of an Aircraft Pilot Coupling (APC) event. According to the National Research Council, “APC events are collaborations between the pilot and the aircraft in that they occur only when the pilot attempts to control what the aircraft does. For this reason, pilot error is often listed as the cause of accidents and incidents that include an APC event. However, the [NRC] committee believes that the most severe APC events attributed to pilot error are the result of the adverse APC that misleads the pilot into taking actions that contribute to the severity of the event. In these situations, it is often possible, after the fact, to analyze the event carefully and identify a sequence of actions the pilot could have taken to overcome the aircraft design deficiencies and avoid the event. However, it is typically not feasible for the pilot to identify and execute the required actions in real time.”
This crash is a case where it is tempting to chalk up the accident to pilot error and move on. However, a more thorough investigation of causes identifies multiple issues that contributed to the accident and, most importantly, multiple opportunities to increase safety for future pilots and passengers. The impacts to the goals, causes of these impacts, and possible solutions can be organized visually in cause-and-effect relationships by using a Cause Map. To view the Outline and Cause Map, please click “Download PDF” above.
The wake turbulence that initially affected the flight was due to the small separation distance between the flight and a large plane that took off 2 minutes prior (the required separation distance by the FAA). This led to a recommendation to re-evaluate the separation standards, especially for extremely large planes. In the investigation, the NTSB found that the training provided to pilots on this particular type of aircraft was inadequate, especially because changes to the aircraft’s flight control system rendered the rudder control system extremely sensitive. This combination is believed to be what led to the overuse of the rudder system, leading to stress on the vertical stabilizer that resulted in its detachment from the plane. Specific formal training for pilots based on the flight control system for this particular plane was incorporated, as was evaluation of changes to the flight control system and requirements of handling evaluations when design changes are made to flight control systems for previously certified aircraft. A caution box related to rudder sensitivity was incorporated on these planes, as was a detailed inspection to verify stabilizer to fuselage and rudder to stabilizer attachments. An additional inspection was required for planes that experience extreme in-flight lateral loading events. Lastly, the airplane upset recovery training aid was revised to assist pilots in recovering from upsets such as from this event.
Had this investigation been limited to a discussion of pilot error, revised training may have been developed, but it’s likely that a discussion of the causes that led to the other solutions that were recommended and/or implemented as a result of this accident would not have been incorporated. It’s important to ensure that incident investigations address all the causes, so that as many solutions as possible can be considered.