Another TWA 841 Theory Emerges

The TWA 841 mystery lives on. I was recently contacted by the author of a newly released book that offers a different theory about TWA 841. While I don’t agree with the author’s theory, I am glad to share it here. You can decide for yourself. Below is the email exchange with the author. Some of these exchanges have been edited and moved from the comments section of another post.

Steve Shaojie Tang

Captain Hoot Gibson was a hero to save all lives on Flight 841. The root cause for this accident was due to the nonlinear instability phenomenon which has been discovered recently by me. There has been a fundamental mistake made in aviation industry for aircraft dynamics. That was why NTSB said one thing, Boeing said another thing, and pilot said a different thing. The autopilot should be blamed for this accident. I included my scientific findings in my book, Nonlinear Instability and Inertial Coupling Effects – The Root Causes Leading to Aircraft Crashes, Land Vehicle Rollovers, and Ship Capsizes, ISBN 9781732632301. The book information is available at http://www.faiteve.com. I made a scientific analysis for TWA Flight 841 accident in my book.

Steve,

I’d love to read your analysis of TWA 841. Can you send me that? Thanks.

Emilio

Steve Tang

On 4 April 1979, aTWA Boeing 727-31 Flight 841 entered an uncommanded roll at FL390(39,000 feet) altitude near Saginaw, Michigan. There was no turbulence when the flight was cruising at FL390 in visual flight condition. The aircraft subsequently dived to about FL50 in about 63 seconds before the flight crew regained the control by extending the landing gear. During dive, the aircraft completed 360 degrees of roll [24]. The flight crew made an emergency landing, and the 89 lives on board were saved.

The captain mentioned that he was flying the aircraft on autopilot mode with the altitude-hold selected. He experienced a light buffet and noticed that the autopilot was commanding a left turn with the control wheel displaced accordingly while the attitude director indicator (ADI) showed the aircraft was in 20 degrees to 30 degrees bank to the right. However, the ADI showed that the aircraft did not followed the command and continued to bank to the right at a slightly higher than normal rate of roll. The pilot then disconnected the autopilot and applied more left control to fight the roll[24]. This fact tells us that the autopilot already failed to fight the uncommanded roll before the pilot took over and continued trying. As discussed in Section 5.8 above, this kind of fighting was useless and just put more energy into the system and made the dynamic system to achieve its steady state earlier, meaning larger roll and yaw motions would follow.

The author of this book believes that the probable cause of this accident was due to the pitch instability discovered in this book. Two factors may play roles in the pitch instability in this accident. They were the substantial decrease of roll damping at high altitude (FL390) and the lower yaw damper failure.

As we know from the discussion in Section 5.9.1, roll damping of aircrafts reduces as altitude increases. At FL390 with the true airspeed (TAS) of 458 kn(0.8 Mach), the roll damping of Flight 841 would reduce about three times comparing with that at the ground level. In addition, the flight crew noticed a warning light showing the failure of “A” hydraulic system and a warning flags showing that the lower yaw damper was inoperative after regaining control of the aircraft at lower altitude. The above factual information suggested that the roll damping and yaw damping of the aircraft at the beginning of the upset may be substantially decreased simultaneously. According to the formula , the pitch threshold may be in turn substantially decreased and was most likely exceeded. In this case uncommanded yaw and roll would happen. This uncommanded yaw oscillation, i.e. heading oscillation, was recorded by the FDR at the time between 2147:40 to 2147:55 as shown by the FDR plot in[24]. This yaw upset was the starting point for all the following consequences. The roll motion at the same time may also show a similar oscillation, but unfortunately, the FDR did not record the roll motion. However, the pilot statement proved that the roll motion and autopilot’s counteracting command of roll were involved at the beginning of the upset.

Duringthe pilot counteracting maneuver after the upset, the upper yaw damper may completely be suppressed due to the rudder to the (blown down) limit because the pilot stated that despite the almost full deflection of the left aileron and full displacement of the left rudder, the aircraft continued to roll to the right. In this case, the pitch threshold amplitude may probably be very close to zero. This means that all the pitch energy may have to be transferred to roll and yaw. That may be the reason why the aircraft experienced violent 360 degrees roll and 180 degrees yaw motions.

With an inoperative lower yaw damper and much lower roll damping at FL390, the chance for the pitch instability to occur was much higher. In addition, the autopilot may also help to pump energy into the system. The “symptom”of the uncommanded roll and yaw indicated the pitch instability and the inertia induced roll and yaw resonance. This symptom was observed not only for TWA Flight 841 but also for many other aircrafts.

It is worth to mention that the flight crew of TWA Flight 841 used the right approach to fight the pitch instability at later time although they might not know the mechanism. By extending the landing gear they in fact changed the roll moment of inertia of the aircraft. As a result, the roll natural frequency was changed accordingly so that the roll resonance at least may be reduced. At the same time by decreasing the altitude although not intentionally, the TAS was decreased, and the roll damping increased. Therefore, the pitch instability threshold was increased. In such a situation, the aircraft might become controllable. It was surprised to know that the flight crew finally regained the control after the roller-coast dive and safely landed the substantially damaged aircraft. It was the flight crew that saved the 89 lives on board at the end. 

From Author Steve Shaojie Tang 

I believe that you are also a pilot and should be able to understand the situation we are facing now in the industry. For more information, go to www.faiteve.com.

From the book Nonlinear Instability and Inertial Coupling Effects

Steve Tang created the following video to demonstrate his theory. https://www.youtube.com/watch?v=gG2-mu6I11A&feature=youtu.be