Wednesday, February 27, 2019

More Questions Raised for Atlas 5Y3591 Loss of Control

Atlas 767 flight 5Y3951 was descending for 3,000 feet when it appears to have briefly leveled off at 6,200 feet, before nosing over and plunging to the ground.

The events transpiring in the last 35 seconds are uncertain except for the ending. All that is known are questions. The following observations are just to highlight areas of interest. Nothing conclusive can be gleaned from this very limited snapshot. The NTSB will have the flight data recorder to provide a much more comprehensive readout and supporting information. There is nothing to substantiate a wind shear, it is mentioned for completeness and remains a factor for consideration. There isn't a wind shear that would lead to this flight profile and the weather observations do not support such a violent act. 

The reasons for the level off and subsequent dive are unknown. Structural failure in the pitch axis control system (elevator, stabilizer) will be investigated by the NTSB. The Flight Data Recorder will have information to correlate as well. There is nothing to confirm or rule anything out yet.

Times are approximately from the time of impact. 

33 seconds: the airplane starts to level off for 6,200 (pressure altitude) from its steady descent.

20 seconds: the airplane has leveled off, but a bit firmly, high pitch rate at the end.

20 seconds: the ground speed begins to increase. This could be that the airplane is accelerating due to thrust, if so the airspeed is increasing. This could also be due to wind shear, in which case the airspeed would be decreasing. The magnitude of the change and the duration would indicate an aggressive change in either case.

17 seconds:  There is a speed blip. Uncertain if it is data artifact.

16 seconds:  The airplane begins to descend again.

15 seconds:  A very firm nose over, high pitch rate, negative vertical speed begins to pick up.

12 seconds: Ground speed starts to decrease. With the airplane descending, this would be by pulling back thrust and maybe adding spoilers.  Wind shear is always another answer, in which case it would be increasing headwind, increasing airspeed.

9 seconds: Extremely aggressive nose over into a steep dive. There is a speed blip, but otherwise speed builds up to the point of impact. 

5 seconds: the push over moderates slightly and then increases again. As if pitch rate stopped briefly and then continued over.

With the three victims recovered, the search for the flight data recorder and cockpit voice recorder, as well the pitch control components, can proceed as a top priority. With the bottom layer on the crash scene reported very soft, it may take days or even weeks to gather. All of the debris will be recovered and inspected for signs of damage not related to the impact, and for supporting any failure scenarios. At this point, the next step for understanding what happened will come after the recorders are recovered and parts are inspected. Assuming recovery is rapid, a preliminary report should be available before the end of March.

Stay tuned!

Peter Lemme

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Peter Lemme has been a leader in avionics engineering for 38 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chaired the SAE-ITC AEEC Ku/Ka-band satcom subcommittee for more than ten years, developing ARINC 791 and 792 characteristics, and continues as a member. He contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Media Independent Secure Offboard Network.

Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).

An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.


  1. What is the source of the data presented here? Maybe a simulation match based on ?what data?

  2. This is flightradar24 data (link in the article) from the accident flight. I have processed the data to create the derivatives. I also removed some spurious data points.