The scene provided important clues. The engine and pylon assembly had separated near the runway, while most of the aircraft wreckage was located near the crash site. Investigators were able to examine the separated assembly and the damaged attachment structure rather than relying only on fragments from the impact area.
The investigation also had to establish the order in which failures occurred. An aircraft crash can leave evidence of multiple kinds of damage: some caused before impact, some created when the aircraft strikes the ground, and others resulting from the post-crash fire. Separating these categories is essential to identifying the original failure.
Evidence From the Aircraft and Its Systems
The physical evidence showed that the engine and pylon had separated during takeoff rotation. Investigators examined the pylon structure and identified damage and cracking that had reduced its strength.
They also studied the aircraft's hydraulic and electrical systems. The separation had severed hydraulic lines and interrupted electrical power to equipment associated with the left engine. These failures explained why the left-wing slats retracted and why certain warning systems did not provide the indications the crew would normally expect.
The flight data recorder provided another important source of evidence. By examining the recorded parameters, investigators could reconstruct the aircraft's speed, altitude, attitude, and movement during its short flight.
The cockpit voice recorder and other evidence helped establish what the crew experienced and when key events occurred. Investigators compared the available information with witness observations and the physical evidence to develop a consistent sequence.
The work was not simply about finding a single broken part. It was about understanding the chain of events that connected the maintenance operation to the aircraft's loss of control.
Why Investigators Look Beyond the Final Failure
When a major component separates from an aircraft, it may seem that the cause is obvious. But identifying the immediate failure is only the beginning.
Investigators must ask why the component failed, whether the failure could have been detected, whether maintenance procedures were adequate, and whether other design or operational factors allowed the failure to become catastrophic.
In Flight 191's case, the engine separation triggered other failures that made the aircraft difficult to control. The loss of warning systems reduced the crew's ability to recognize the developing danger. Existing takeoff procedures also had to be examined in light of the aircraft's altered configuration.
The NTSB's final report identified several contributing factors beyond the immediate maintenance-induced damage. These included vulnerabilities in the pylon attachment design, vulnerabilities in the leading-edge slat system, deficiencies in regulatory surveillance and reporting, failures of communication among the operator, manufacturer, and Federal Aviation Administration, and the limited tolerance of the prescribed operating procedures for the unusual emergency.
