What happened
Following the completion of cargo loading at Baltimore/Washington International Airport, Flight 931 departed the gate at 0-40 and took off from runway 28 at 01:10. The aircraft, a Lockheed Electra, initially received clearance to climb to 13,000 feet before being cleared further to FL220 by Washington ARTCC.
At approximately 01:32, the crew identified a malfunction involving the number 2 gyro. To address this, they switched to using the number 1 gyro to drive both approach horizons, which initially appeared to resolve the issue. Shortly after being cleared direct to the Dryer VOR by Cleveland ARTCC, the aircraft entered a turn.
During this maneuver, the first officer's approach horizon may have provided inaccurate pitch and roll data. This likely resulted in the flightcrew receiving conflicting information from the two horizons while attempting to correct an unusual attitude. In the process of trying to recover the aircraft, the crew overstressed the airframe, leading to a spiral descent. The Lockheed Electra broke apart in mid-air; while some debris struck residential structures, the majority of the wreckage landed in unpopulated, wooded terrain.
Findings
Investigations into the accident focused on the inability of the crew to properly assess the flight condition before a total loss of control occurred. A failure within the number 2 vertical gyro system—potentially involving the amplifier or related circuitry—is believed to have provided incorrect data to the co-pilot's approach horizon. The structural disintegration of the aircraft was caused by aerodynamic overstress during the recovery attempt.