Eastern Air Lines Flight 401 - Cause of The Crash

Cause of The Crash

The NTSB investigation discovered that the autopilot had been inadvertently switched from altitude hold to CWS (Control Wheel Steering) mode in pitch. In this mode once the pilot releases pressure on the yoke (control column) the autopilot will maintain the pitch attitude selected by the pilot until he moves the yoke again. Investigators believe the autopilot switched modes when the captain accidentally leaned against the yoke while turning to speak to the flight engineer, who was sitting behind and to the right of him. The slight forward pressure on the stick would have caused the aircraft to enter a slow descent, maintained by the CWS system.

Investigation into the aircraft's autopilot showed that the force required to switch to CWS mode was different between the A and B channels (15 vs 20 pounds respectively). Thus it was possible that the switching to CWS in channel A did not occur in channel B thus depriving the first officer of any indication the mode had changed (Channel A provides the captain's instruments with data, while channel B provides the first officer's).

After descending 250 feet from the selected altitude of 2000 feet a C-chord sounded from the rear speaker. This altitude alert, designed to warn the pilots of an inadvertent deviation from the selected altitude, went unnoticed by the fatigued and frustrated crew. Investigators believe this was due to the crew being distracted by the nose gear light, and because the flight engineer was not in his seat when it sounded and so would not have been able to hear it. Visually, since it was nighttime and the aircraft was flying over the darkened terrain of the Everglades, there were no ground lights or other visual indications that the TriStar was slowly descending into the swamp. It was also discovered that Captain Loft had an undetected tumor in his brain, and this was later found to be in an area controlling vision, but this was not thought to have contributed to the accident.

The final NTSB report cited the cause of the crash as pilot error, specifically: "the failure of the flight crew to monitor the flight instruments during the final four minutes of flight, and to detect an unexpected descent soon enough to prevent impact with the ground. Preoccupation with a malfunction of the nose landing gear position indicating system distracted the crew's attention from the instruments and allowed the descent to go unnoticed."