Overlooked clues

Of the many problems associated with the investigation of TWA 841, one of the biggest oversights was the failure of the investigators to explain several important clues. For example, no attempt was made to explain the importance of the yaw damper fail flag noticed by the crew after the recovery. Neither was there any attempt to explain the aircraft’s tendency to bank to the left during the recovery. These were very important clues that should have been followed and addressed but were not.

Another important clue that got scant attention was the freeplay found in the right outboard aileron post flight. At first the investigators discarded this anomaly by claiming that the fractured bolt that led to the freeplay had happened as a result of the high G dive and did not precipitate the dive. Later, when the logic of the NTSB’s delay scenario, the scenario involving the crew not taking any corrective action for 16.5 seconds, was called into question, the NTSB changed their minds and said that the freeplay in the right outboard aileron would have reduced lateral control. The reduced lateral control would mean that the plane would become uncontrollable if the crew did not take corrective action in 12.5 seconds instead of the original 16.5 seconds. The extra four seconds did little to make the delay theory any more plausible. What they were implying was that the crew had performed an unauthorized procedure, and then when the plane started to roll to the right, they didn’t do anything for 12.5 seconds. In that amount of time the plane would have been in a 60 degree bank to the right.

The change of heart over the right aileron freeplay is just one example of how the investigators were quick to adopt evidence that supported their claims while discarding or explaining away evidence that was contrary to their theories.

The right aileron freeplay might very well have been the initiator of the upset. In a number of rudder hardover incidents, rudder input by the pilot or the yaw damping system preceded the rudder hardover or reversal. In the case of United Flight 585 in Colorado Springs, the aircraft experienced windshear prior to the upset. In the case of USAir Flight 427, the upset was preceded by an encounter with wake turbulence. It is highly likely that the freeplay in the right aileron caused that aileron to upfloat just prior to the incident. This very well could have been the slight buzz reported by Hoot. The upfloating right aileron would have caused the aircraft to want to turn and bank to the right. The autopilot, however, was trying to maintain the current heading. It did this by commanding a bank to the left. Once the control wheel reached a certain limit the spoiler on the left wing would have been raised to aid in roll control. That could have caused the slight buffet described by Hoot.

I am not an engineer and don’t know the inner workings of the 727 yaw damping system, but it’s easy to see how the yaw damping system might have received mixed signals over this cross control situation.

In the short video below, TWA investigator David Haase talks about the missed opportunity in following the clue involving the right outboard aileron.