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Tuesday, March 12, 2019

Atlas Air 5Y3591 Dove because of Nose-Down Elevator Command

The NTSB has revealed that Atlas Air 767 Freighter flight 5Y3591 dove based on nose-down elevator command. The nose was pushed over 49 degrees nose down. The engines were brought up to full thrust. There was no stall warning that preceding these actions. The dive reached 430 knots before impacting. Pitch attitude rose from -49 deg. to -20 deg. in the dive.
UPDATE 1:40pm
NTSB changes pitch down from elevator command - striking the phrase column input.
UPDATE 12:32pm:
Revised to reflect the actions may also be consistent with upset recovery.
UPDATE 8:22am
NTSB cautions no probable cause has been issued, 
statements to that effect are premature and speculative.

From the NTSB update:
Also, about this time, the FDR data indicated that some small vertical accelerations consistent with the airplane entering turbulence. Shortly after, when the airplane’s indicated airspeed was steady about 230 knots, the engines increased to maximum thrust, and the airplane pitch increased to about 4° nose up and then rapidly pitched nose down to about 49° in response to column input nose-down elevator deflectionThe stall warning (stick shaker) did not activate.

FDR, radar, and ADS-B data indicated that the airplane entered a rapid descent on a heading of 270°, reaching an airspeed of about 430 knots. A security camera video (figure 4) captured the airplane in a steep, generally wings-level attitude until impact with the swamp. FDR data indicated that the airplane gradually pitched up to about 20 degrees nose down during the descent.
The revision in the NTSB update from column input to nose-down elevator command is an entirely different spin. This new wording is much more in line with some fault of the elevator control system (unintentional command). It does not rule out intentional command.

The NTSB does not make any claim this was an intentional act. The NTSB responded directly to this post.
Please note, investigative updates do not state probable cause, are not a final report and our investigation is ongoing. As such, conclusions/statements about probable cause are premature and speculative.
The application of full thrust is contrary to actions consistent with recovery in a dive, where airspeed is already building up.

If the pitch down were caused by a failure of the elevator control system, and if stabilizer trim was not available, then the crew might use thrust to pitch the airplane back up as a loss-of-control (LOC) technique that would be a part of Upset Prevention and Recovery Training (UPRT). What is troubling with this scenario is that thrust was added BEFORE the pitch down.

The NTSB will not make any official finding until they have completed their investigation. 


The NTSB made no mention of any failure indication or anomaly so far. NTSB will conclude what failures were encountered as part of their investigation. Of significance will be any failure in the elevator control system.

NTSB have stated that the CVR area mic recording was difficult to understand.
Crew communications consistent with a loss control of the aircraft began approximately 18 seconds prior to the end of the recording.
NTSB does not make any statement that the CVR indicated a struggle between the pilots for control. The NTSB has not released a transcript. The CVR would be the first and most compelling factor in capturing evidence of a crew struggle.  The singular statement supports a flight crew working together and does not suggest anything else.

"Column Input" is an abstract term. Was the pilot applying column input, or was there some failure of the column that provided input by virtue of the failure? The NTSB has not provided any clarity on that point, which has led to misinterpretation. Strictly speaking, the NTSB has left open the scenario that the column itself was failed in someway. There are two columns, each with significant mechanical features.

The stabilizer jackscrew appeared to be trimmed nose down when observed sitting on the warehouse floor. A stabilizer trimmed nose-down does not appear to align with recovery from a failure that results in nose-down elevator command. The simplest recovery from a failure that leads to nose down elevator command would be by trimming the stabilizer nose up.

In summary:
  1. While descending for 3000, the airplane briefly leveled at 6200
  2. The airplane climbed up to 6300 feet and leveled
  3. Turbulence was encountered
  4. At 230 knots, full thrust was applied.
  5. The airplane pitched up to 4 deg nose up.
  6. Nose was pitched down (49 deg nose down) due to "column input" nose-down elevator command
  7. Full thrust was maintained in the dive
  8. Airspeed reached 430 knots
  9. No stall warning
  10. No failures or system anomalies have been identified
  11. Post crash, stabilizer jackscrew is observed to be trimmed nose down
While an intentional act is an explanation, there are others relating to upset recovery, and in those the flight crew members are heroically doing everything they can. The NTSB investigation will bring compelling evidence one way or the other. I can only imagine the emotional roller-coaster for anyone related to or involved with this investigation. The NTSB initial use of column input to describe the forcing functions is regrettable and has led to misunderstanding.

EgyptAir 990

EgyptAir 990 involved nose-down elevator command into a dive. NTSB found no evidence of any failure condition within the elevator system of the accident airplane that would have caused or contributed to the initial pitchover or prevented a successful recovery.
The National Transportation Safety Board determines that the probable cause of the EgyptAir flight 990 accident is the airplaneís departure from normal cruise flight and subsequent impact with the Atlantic Ocean as a result of the relief first officer's flight control inputs. The reason for the relief first officer's actions was not determined.



Potential Causes for Elevator Movements 
During the Accident Sequence, the investigation ruled out all but four possible anomalies and failure scenarios as potential factors in the accident because they diverged too far from what was reflected on the accident flight's FDR to warrant further consideration. Analysis showed that the effects of four failure scenarios (each of which involves dual failures) bore some resemblance to some portions of the accident flight's FDR data. Specifically, initially it appeared that each of these failure scenarios could potentially cause nose-down elevator movements or a split elevator condition that might resemble those recorded on the accident flight's FDR. Those four failure scenarios were (1) disconnection of the input linkages to two of the three PCAs on the right elevator surface, (2) a jam of the input linkages or servo valves in two of the three PCAs on the right elevator surface, (3) a jam of the input linkage or servo valve in one PCA and the disconnection of the input linkage to another PCA on the right elevator surface,91 and (4) a jam in the elevator flight control cable connecting the right-side control column to the right aft quadrant assembly combined with a break in the same cable. Therefore, the wreckage from the accident airplane was examined for possible evidence of PCA anomalies, and the predicted elevator movements resulting from these failure scenarios were evaluated and compared with the data from the accident flight.



Stay tuned!


Peter Lemme

peter @ satcom.guru
Follow me on twitter: @Satcom_Guru
Copyright 2019 satcom.guru All Rights Reserved

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.

8 comments:

  1. You have the sense of the jackscrew backwards. The picture I saw had the jackscrew about 3/4 of the way to full nose up trim on the airplane.
    The jackscrew shows the nose of the stabilizer was down, pushing down on the tail and raseing the nose. Remember nose down on the stabilizer is nose up on the airplane.


    ReplyDelete
    Replies
    1. I included a picture of the jackscrew installed from another 767 repair, which shows the actuator on top. The position is near the top, stabilizer leading edge up, airplane trim nose down. Other readers feel the position is not near the stop. I am awaiting the preliminary report, as it is not certain to me know where it is in its travel.

      Delete
  2. Is it possible, that Boeing has implemented MCAS not only in B3xM but also silently during maintenance updates in other types like this freighter B767, because they thought, this is a security feature.

    ReplyDelete
    Replies
    1. NO! In fact, I was part of the Pitch Augmentation Control System group which was a dual computer to trim the stabilizer airplane nose down when approaching a stall. It was abandoned in 1982 during flight test in preference to vortex generators. PACS was removed.

      Delete
  3. There is no MCAS on the commercial version of the 767. In the 1980s, Boeing’s engineers considered using a pitch augmentation system for the commercial version of the 767, but dropped the idea after finding that vortex generators provided adequate control.
    Both the KC-767 and KC-46 fleets delivered to air forces in Italy, Japan and the U.S. rely on the MCAS to adjust for pitch trim changes during refueling operations.

    ReplyDelete
    Replies
    1. I was a Boeing flight control engineer that worked on Pitch Augmentation Control System for 757 and 767 in 1981 and 1982.

      Delete
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