The flight data recorder (FDR) on TWA 841 recorded just four parameters: altitude, g loads (vertical acceleration), heading, and airspeed. Today’s modern digital flight data recorders (DFDR) can record 28 parameters and even more on fly by wire aircraft. When the NTSB investigators decided that the crew was not being truthful due to the supposed CVR erasure, they excluded their sworn testimony from the investigation. So rather than rely on the three expert witnesses they had on hand, they decided to rely solely on the only physical evidence they had, which was the damaged aircraft and the FDR readout. Below is the actual FDR readout from level off at 39,000 feet to the upset and subsequent recovery. Be sure to click on the image for an expanded panoramic view.
The jumbled mess at the end was what the investigators were trying to interpret. If you look closely, you’ll notice that the heading trace appears to go back to the left. Neither the investigators nor the manufacturer of the flight data recorder had an explanation for this other than a worn mechanism. The reason for the backward time shift was actually caused by the gimble controlling the heading trace hitting its maximum limit.
The TWA 841 FDR wasn’t designed to accurately record a maneuver involving two complete rolls and a spiral dive of over 30,000 feet per minute. Yet the NTSB was sure that they could determine what had caused the TWA 841 upset by trying to duplicate the traces above. So they connected an FDR like the one used on TWA 841 to a simulator and set out to recreate the upset. The FDR traces on their initial attempts didn’t come close to duplicating the traces above. So they kept trying. The NTSB investigators flew 118 simulator trials and could not duplicate the FDR traces above on any attempt.
They had two that were close, but in order to come somewhat close to the traces above they had to fly the simulator in a way that no pilot would do. A case in point involves the abrupt heading changes shown just prior to the upset. In order to duplicate that trace the simulator pilot had to let the plane roll to the right with about 60 degrees of bank than rapidly roll the aircraft to the left until it was inverted. Hoot had always maintained that the aircraft had never banked to the left. The final NTSB report still has the plane banking right, then left, then back to the right.
Everyone was so fixated on the slat as the cause of the upset that no one saw that the heading excursions were evidence of a yawing aircraft. It would take more than a decade before someone actually took the time to compare the FDR traces to what Hoot described. In the image to the right, prepared by Leigh Johnson, the author of the 1990 ALPA petition for reconsideration, you can see how an aircraft that yaws right, pauses, yaws right could account for such abrupt heading changes.
This is just one example of how the NTSB used faulty analysis to help bolster their biased conclusions. They misread the heading trace before the upset. They misunderstood the heading trace after the upset. And then in an effort to show a correlation between a test flight and the accident aircraft with regard to the acceleration trace just prior to the upset, they improvised a new technique involving magnifying the acceleration trace by 200 times. They then used this unproven technique to claim that they had matched the oscillations in frequency and amplitude from the flight test aircraft with the oscillations on the accident aircraft. In the book I will show how this new technique was flawed. Like most of the investigation by the NTSB, it was a case of garbage in garbage out.