Thursday, March 14, 2019

Comparing Ethiopian ET302 to Lion Air JT043, JT610

Did Ethiopian ET302 succumb to the same situation as Lion Air JT610? The FDR and CVR data is being processed as I write this, with a public release in some form expected by Monday March 18. Aviation authorities have grounded the Boeing 737 Max before receiving a report from this recorder data. The only information to the public is from commercial ADS-B data brokers, notably Flightradar24.com, whose recording is incomplete. Aireon has shared space-based ADS-B data with several parties, but not to the public. Canada and the US make reference to the Aireon data as a factor in their assessment, with the claim that there were sufficient similarities to be concerned.

The FAA issued an airworthiness directive within a few days of the Lion Air JT610 accident.


This AD was directly addressed to the MCAS malfunction due to erroneous angle of attack sensor reading. The FAA made an assessment that corrective action to emphasize the response to uncommanded stabilizer motion by using the cutout switches.


The AD provided very useful information. It showed that an AOA vane can cause stick shaker, altitude disagree, airspeed disagree, incorrect minimum speed, elevator feel diff pressure alerts. The stabilizer motion is highlighted, by both increasing control forces and the need to cutout the electric stabilizer trim.

The AD did not mandate an AoA disagree alert if there was no AoA display. Without the alert, there may be no fault identified to the AoA directly. There was no mention that one stick shaker activated and the other not is also an AoA disagree situation.



While the pilots received this information, there was no guidance in the AD provided for maintenance fault isolation. The actions taken to repair the airplane with these faults the night before JT610 were wholly inadequate, yet are described perfectly with this AD. The stick shaker issue was not reported.


The FAA reserved the option to revise the AD in the future.

 
A Boeing revision to the MCAS FCC software is not available, even at this time, to require dual AoA sources, to limit the authority, and to remove the ability to retrigger.

The FAA was fully cognizant of the hazards presented by an erroneous AoA vane. The AD was their response, believed to be sufficient and adequate. This AD was carried forth globally. The AD addressed the biggest concern raised by the pilot community which was to bring awareness of the function. The whole point of an AD is to ensure that no future accident would result from these circumstances.

The autopilot is disabled if the stabilizer trim is cutout. Otherwise the only other aspect is the need to manually trim using the flight deck trim wheel. The cutout switches are easily reached by either pilot.  Thus the action is easy, but the response increases pilot workload due to manual flight and trim. JT043 appears to have been flown without any difficulty once selecting the cutout switches.

Thus, the FAA stares incredulously (as do I) to associate Ethiopian ET302 to the same factors that the AD specifically addresses. The recognition is easy, the action is easy. How could this still be a factor? We will know once the flight data recorder is readout.

At this point, the flightradar24.com ET302 data can be compared for any obvious correlation to other flights.

Lion Air JT610 appears to have crashed from loss of control, ultimately from failing to cutout malfunctioning MCAS stabilizer nose down trim commands. JT043 flew the same airplane the day before and encountered the exact same circumstances except in this case, they cutout the stabilizer trim and proceeded to their destination.  Ethiopian ET302 crashed shortly after takeoff.

The data used in this analysis for the Lion Air flights comes from NTSC accident preliminary report and an earlier partial release.  Data for ET302 is based on Flightradar24 ADS-B receivers. The ET302 data stops after about three minutes and does cover the rest of the flight.

Aireon has provided space-based ADS-B data to the investigation. This data will be complete for the whole flight and may offers some additional insights not available to the public.

Reports that information emerging from the recovery of evidence from the accident site that could support a common factor between Ethiopian and Lion Air may be referring to the stabilizer jackscrew. It may be able to visually determine the last stabilizer trim position.  A significant nose-down position would correlate the most closely to JT610.

What is common with ET302?

At what appears to be the transition to flaps up, the airplane lost altitude (500 feet) and accelerated 50 knots. The point of the descent, the depth of the descent, and the speed increase in the descent; matches well to both JT043 and JT610.

What is new with ET302? 

  1. The takeoff slope is flatter than normal and has at least two occurrences of zero climb rate within 500 feet AGL. The initial climb out should maintain positive rate of climb. ET302 took off from about a 9300 foot density altitude which does not compare to the Lion Air field conditions. Comparing ET302 to other airplanes taking off from the same runway shows that ET302 overall takeoff climb rate was well below normal.
  2. ET302 accelerated to speeds well above normal in the level off and last climb out data points.

JT043

refer to the event number in the figure below

1) Stick shaker goes off on liftoff. Stays on for the duration of the flight.
2) Flaps Up
3) Autopilot is engaged
4) Autopilot is disengaged (which enables MCAS)
5) MCAS starts to trim nose-down
6) MCAS trim is stopped by manual nose up trim
     Trim is left nose down
     Heavy pull column forces
7) Airplane descends about 1000 feet as a result of nose-down trim.
    Airplane speed increases by about 70 knots
8) Two more MCAS events, light dip in climb out slope.
     Stabilizer trim is cutout.
9) MCAS event
10) Stab trim cutout


JT610

refer to the event number in the figure below

1) Stick shaker goes off on liftoff. Stays on for the duration of the flight.
2) Flaps Up
     MCAS trim nose-down
     Heavy pull column forces
     Airplane descends about 400 feet as a result of nose-down trim.
     Airplane speed increases by about 70 knots
3) Flaps Down
    Trim event from FCC, but with flaps down it should not be MCAS
     and it goes the wrong way for speed trim (still working what this is)
4) Flaps Up
5) MCAS event
     light dip in climb out slope.
6) MCAS event
     light dip in climb out slope.
7) MCAS event
     light dip in climb out slope.




ET302

refer to the event number in the figure below 

1) Airplane descends 500 feet after climbing out to 1000’ AGL
    Airplane gains about 50 knots
2) Airplane levels out
    Airplane gains about 50 knots
3) Airplane climbs



refer to the event number in the figure below 

1) After takeoff, about 200’ AGL, Zero Rate of Climb
2) After takeoff, about 500’ AGL, Zero Rate of Climb
3) After takeoff, after 1000’ AGL, descend at -2000 fpm peak vertical speed
4) Level
5) Climb
6) +2500 fpm vertical speed




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.

5 comments:

  1. What does the electric trim thumb switch do once MCAS is on? Can it move the nose up, or only stop MCAS from moving the nose down.

    If so, once MCAS moves the tailplane nose down, the only way to recover the tailplane trim to a neutral flyable position is manually with the handle and wheel? That seems like a big item to know, in addition to in addition to shutoff electric trim motor.

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  2. the electric trim command from the yoke switches are in priority to MCAS. It is apparent MCAS stops its trim command if a manual trim command is encountered. The electric trim is workable unless the cutout switches are thrown. From that point, the stab must be manually trimmed with the wheel.

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