November 15, 2019, ©. Leeham News: We continue the series on analyzing the Lion Air JT610 crash. We now analyze the initial part of the flight. In the last Corner, we analyzed what went wrong in the aircraft. The left Angle of Attack sensor had a 21° bias failure.
How such a rather limited failure could bring a new Boeing 737 MAX down is what we try to understand in this series. To assist us, we have a detailed final accident report from the Indonesian Safety Board.
We will look at the initial part of Flight JT610, the accident flight, in this Corner. Then we continue with the final part in the next Corner. We will compare the accident flight, JT610, with the previous flight, JT043, to gain more insights, as this flight flew with the same problem but managed to complete the flight.
We will specifically discuss the pilots’ reactions to the effects the single Angle of Attack (AoA) wrong bias could have for these flights and how such a limited problem could finally bring down a modern airliner. The facts around the subjects are taken from the final report, and I add my thoughts and conclusions.
We know today the aircraft had exactly the same configuration and problem on the flight before JT610, flight JT043. The fault messages seen during this flight were unreliable airspeed (IAS DISAGREE) and altitude (ALT DISAGREE) on the Captain’s Primary Flight Display (PFD) and a roof placed warning light for a problem with the flight control feel pressure (FEEL DIFF PRESSURE).
All these failures stemmed from incorrect AoA values as the ADIRU (Air Data Inertial Reference Unit) uses the AoA to correct the values for Airspeed and Altitude for measurement faults due to the AoA. The wrong correction for the left side instruments gave a difference in the values to the right side instruments and the difference was detected by the safety functions of the avionics system.
The 737 MAX, unlike the 737 NG, didn’t have a functional AoA disagree warning for the pilots’ displays. It should have been there but due to a programming failure by the Flight Control Computer (FCC) supplier, which also did the software, it was not displayed on MAXes that didn’t have the option for the AOA display on the pilots’ displays. It wasn’t recognized until the aircraft had entered service. Then any change was too late and the introduction was scheduled for the first Avionics software update 2021.
This meant the JT043 pilots had not been able to analyze what was wrong with the aircraft. The maintenance log listed the three warnings which had been shown, but did not reveal the aircraft was difficult to control nor that the crew used the trim cutout switches to stop a repeated trimming nose down of the aircraft.
The Captain thought it was Speed Trim which was confused by the erroneous airspeed which was trimming. He wrote in Lion Air’s flight crew incident system after the flight “ Speed Trim trimmed in the wrong direction”.
With the aircrews having no information about MCAS, this was the most plausible explanation for what happened. The crew did not classify the trimming as “Trim Runaway” as this is when the trim runs uninterrupted in one direction until one stops it. Here MCAS trimmed for seven to nine seconds (dependent on Mach) and then stopped, started again, then stopped in an endless repeat cycle.
The aircraft was handed over as repaired by the flight line mechanic to JT610’s Captain. The mechanic had done the cleaning actions of the pitot system as mandated by the maintenance manual based on the communicated fault indications from JT043.
There was no discussion of the flight control problems on the previous flight as there were no comments from the crew in the aircraft maintenance log other than notes about IAS DISAGREE, ALT DISAGREE and the FEEL DIFF PRESS warnings.
At rotation, as the values from the AoA sensors become valid (they are unreliable at lower airspeed), the left stick shaker activated for the Captain who was flying. He notified the First Officer (FO, the Pilot Monitoring, PM) about it.
After a short while, IAS DISAGREE appeared on the Captain’s display. He asked the FO to read out the memory checklist items for “Unreliable Airspeed”. The FO didn’t have the checklist in memory and it took a while until he found the checklist in the Quick Reference Handbook (QRH) and started reading it.
Item number eight on the list says “Crosscheck left, middle (backup) and right airspeed indications when the aircraft is trimmed and stabilized”. By now, the flaps were up and the Captain was fighting MCAS. Hence no stabilized condition was reached and no airspeeds were compared (they only differ with about 10 to 15 knots on displays showing 200 to 250 knots so you need a calm aircraft and a careful analysis to reliably identify any difference).
The non-completion of the IAS DISAGREE checklist meant it was never established which ADIRU system caused the warnings. The crew, therefore, could not realize the problem was one-sided and the FO could have flown the aircraft on his display with the middle backup display as a backup.
In the JT043 flight, the IAS DISAGREE checklist could be completed and the airspeed difference could be traced to the Captain’s side. This was possible as the FO held against MCAS without trimming which gave the calm to do the readings. A third pilot on the clap seat helped with the analysis of the problems as well. The FO said he could almost not hold the aircraft (he had a stick force of 105lb which is very high) and the Captain subsequently told him to trim.
Once the fault was isolated the Trim Cutout Switches were tried, it stopped the trimming. Then they were re-engaged and the nose down trimming re-appeared. The switches were then put in the cutout position for the rest of the flight. The use of the switches to quiet a confused Speed Trim was not documented in the Maintenance log after the flight, as “fixing the IAS DISAGREE problem would fix the Speed Trim confusion” according to a follow-up discussion with the JT043 Captain.
The JT610 Captain held against MCAS nose-down trim each time it triggered and then trimmed to neutral stick force. As the Captain trimmed, MCAS was reinitialized and it trimmed nose down again after a five-second wait. His reaction time before trimming was first 10 seconds then faster, finally settling at three seconds (he countered MCAS 22 times before handing over the flying to the FO).
We will halt at this point and draw the first conclusions:
In subsequent Corners, we will analyze the final part of the JT610 flight, which is from where the First Officer was given the control of the aircraft.