March 15, 2019, ©. Leeham News: With the crash in the weekend of Ethiopian Airlines Flight 302 we take a break from the Yaw and Roll stability discussions to look at what happened Sunday.

The 737 MAX 8 with 157 persons onboard crashed six minutes after takeoff. Here is what we know.

Flight path

The aircraft crashed after having taken off from Runway 7R on Addis Abebba Bole airport. FlightRadar 24 (FR24) has captured the ADS-B signal from the flight from runway roll to 2 minutes and 45 seconds into the flight, Figure 1, the Yellow-Green curve. When FR24 lost tracking of the flight it continued for another three minutes until it crashed at the end of the red curve from about 1000ft above ground.

The flight’s FR24 ADS-B altitude and speed traces are shown in Figure 2 and 3.

The FR24 ADS-B receivers had bad reception so there are dropouts in the one-second rate emissions from the ADS-B transmitter on the aircraft. The Authorities have in the meanwhile been able to complement the ADS-B trace all the way out to the 6 minutes point of the crash by Satellite receivers picking up the ADS-B signal from the flight. We don’t have access to this data.

There are several things to note from the traces and information which has been given from Ethiopian Airlines:

The crew declared an emergency after takeoff demanding to return to the airport. The crew reported they had unreliable airspeed indications and difficulties to control the aircraft.

The aircraft climbed very slowly and only to about 1,000ft above ground before crashing.

The airspeed was high, however. Normal would be to stay at 250kt below 10,000ft. The graph in Figure 3 shows the aircraft’s Ground speed, which is different to its speed through the air, but any wind affecting the real speed of the aircraft should have been below 20kts at the altitude flown.

which is different to its speed through the air, but any wind affecting the real speed of the aircraft should have been below 20kts at the altitude flown. It’s likely the crew retracted the takeoff flap somewhere around local time 5:39:40 as the speed thereafter rises above the maximum Flap speed for the 737 MAX of 250kts.

As the flaps are retracted any too high Angle of Attack signal, like in the Lion Air case, it will then trigger repeated MCAS nose down actions.

There are irregularities in the vertical speed data from FR24, but I would caution to interpret it as the MCAS nose down trim followed by the Pilot counter trimming, then MCAS trimming nose down again for the Lion Air flight. The FR24 data has dropouts of 10 to 15 ADS-B emissions between its values, so it’s difficult to draw any firm conclusions from the data.

The vertical speed trace in Figure 3 is calculated from adjacent altitude ADS-B values. Once again with large dropouts in the data, one shall be cautious with conclusions from this trace.

What conclusions can be made?

We can conclude from the radio communication from the crew and from the FR24 data the Crew had unreliable airspeed problems like the Lion Air flight.

If this is caused by a faulty AoA signal the air data computer on the side with the faulty AoA will do an erroneous correction of the airspeed and altitude it feeds the Pilots display for this side of the aircraft. This will trigger a master warning in the aircraft of unreliable airspeed as there will be a difference in the airspeed of the affected side and the co-pilots airspeed. There will also be a difference to the third independent backup indicator’s airspeed.

If it’s a too high AoA signal causing the unreliable airspeed, we can assume the stall warning will be on for the affected pilot from liftoff. This will cause the artificial Yoke feel system to increase the Yoke force. The flying pilot will feel he needs an increased back force on the Yoke to hold the aircraft level.

If the flaps are retracted, the MCAS system will start trimming nose down. It will be interrupted by the pilot trimming nose up to compensate but will start again after a 10-second delay, once the pilot has stopped trimming.

In this situation, the pilots shall shut off the aircraft’s trim system and trim manually. If this is done and what exactly happened until the crash we will know when the French DEA has read out the data from the data and voice recorders the Ethiopian authorities sent them and the Ethiopians have decided to brief the public on the results.

The Seattle Times today wrote the horizontal stabilizer trim system, a jackscrew which pushes the horizontal stabilizer up or down, has been found at the crash site. Apparently, it was in the full nose down position. This should point to this being another MCAS accident, with the aircraft’s powerful pitch trim going to full nose down position.

The aircraft can then not be held level with the pilot’s elevator control; he needs to trim against the MCAS trimming to keep pitch authority. Why the pilots didn’t use the trim cutout switches to shut down MCAS trimming, we will learn from the voice recorder read-outs.

Where is the fix to the problem?

The fix to the problem of the MCAS kicking in when it shouldn’t will be ready in April, according to Boeing. The updated software for the MCAS function has several changes. Here how Boeing communicated the changes at 12th of March:

For the past several months and in the aftermath of Lion Air Flight 610, Boeing has been developing a flight control software enhancement for the 737 MAX, designed to make an already safe aircraft even safer. This includes updates to the Maneuvering Characteristics Augmentation System (MCAS) flight control law, pilot displays, operation manuals and crew training. The enhanced flight control law incorporates angle of attack (AOA) inputs, limits stabilizer trim commands in response to an erroneous angle of attack reading, and provides a limit to the stabilizer command in order to retain elevator authority.

The following is noteworthy from Boeing’s communication:

The triggering Angle of Attack signal gets verified against other data to check the signal is valid.

There will only be one instance of nose down trimming, not repeated ones like today.

There will be a limit to how far the MCAS can trim the horizontal stabilizer nose down, so the aircraft’s elevator can compensate and control the aircraft even against a malfunctioning MCAS.

One now asks why such check and limits were not implemented in the first place? There are several areas of question marks over Boeing’s problems with the 737 MAX’s sensors and stability augmentation system:

It’s strange the AoA signal to the Air data computer gets corrupted on brand new aircraft with only months between presumably two cases of it happening.

It’s also strange there were no balances or checks in the system to check the signals were correct and not erroneous. The signals were used to trigger powerful and potentially dangerous functions in the flight control system.

Finally, it’s strange how MCAS was allowed to trim the horizontal tailplane full nose down. It’s not needed to fulfill its intended stabilizing function in a remote part of the flight envelope. By allowing this to happen, MCAS can overpower the pilot’s elevator control and render the aircraft uncontrollable.

There will be a lot of debate around the implementation of the MCAS function for the 737 MAX. With the fix, Boeing and FAA now show what was wrong with the implementation in the first place.

Edit:

There are many questions put in the comments section which are answered by previous articles on this subject. For those who want to read more, here are links to these articles. Just click on the Headline or the light grey “Continue reading” of the synopsis: