Mohawk Airlines Flight 405

Investigation

The National Transportation Safety Board (NTSB) launched a full investigation into the accident, which included a three-day public hearing in Albany on April 25 through April 27, 1972, and a deposition in Washington, D.C., on May 19, 1972. Both the flight data recorder and the cockpit voice recorder were recovered from the wreckage, and their recorded data was found to be intact and usable. The investigation revealed that as the flight crew attempted to reduce thrust on the left engine during the final approach, they were unable to remove the 'cruise pitch lock' mechanism that is used to maintain a cruise thrust setting. When they subsequently attempted to perform an emergency feathering and shutdown procedure on that engine, they were able to shut down the engine but unable to achieve a feathering of the propeller. This eventually resulted in the left propeller creating a high amount of asymmetric drag while windmilling; so much so, that the other engine operating at full power was not able to arrest the resulting uncontrollable descent.

The NTSB, despite investing substantial investigative resources trying to uncover the reasons behind the two unusual and seemingly separate propeller-related malfunctions, was unable to shed light on either one. It was not able to replicate the 'pitch lock stuck' malfunction, nor adequately explain why the crew subsequently failed to effect the standard feathering procedure to properly shut down and reduce the thrust and drag on the left side.

In effect, by not being able to properly secure the left engine, an unwanted asymmetric high thrust situation turned into an irreversible unwanted high asymmetric drag, which eventually resulted in an inevitable and premature descent and crash.

In its final report, issued on April 11, 1973, the Board determined the following Probable Cause for the accident:

The inability of the crew to feather the left propeller, in combination with the descent of the aircraft below the prescribed minimum altitudes for the approach. The Board is unable to determine why the left propeller could not be feathered.

The Board also found the following Contributing Factors:

Contributing causal factors for the nonstandard approach were the captain's preoccupation with a cruise pitch lock malfunction, the first officer's failure to adhere to company altitude awareness procedures, and the captain's failure to delegate any meaningful responsibilities to the copilot which resulted in a lack of effective task sharing during the emergency. Also, the Board was unable to determine why the propeller pitch lock malfunctioned during the descent.

In subsequent correspondence between the NTSB and the Federal Aviation Administration (FAA), included in the final report, the NTSB questioned the then-available operating procedures and manuals for the aircraft. The NTSB found that there was insufficient guidance to pilots in the handling of "Cruise Pitch Lock Stuck" condition. For example, it was not clear based on existing instructions and guidelines whether a missed approach would be indicated and/or possible under these circumstances, and if so, what would be the recommended procedure to successfully execute the maneuver. Also, the condition of a shut down but unfeathered engine, i.e. windmilling propeller with high asymmetric drag and minimum control implications, which was encountered in this accident, was insufficiently covered, according to the NTSB.