Lion Air flight 610: The Final Minutes
Last week, we started the sequence of events that happened inside the cockpit of Lion Air flight 610. The primary resource for this sequence of events is the final report released by the Komite Nasional Keselamatan Transportasi (KNKT). We left off with the cockpit still blaring with alarms and alerts.
The Maneuvering Characteristics Augmentation System (MCAS) had been triggered and was pushing the nose down while the captain, as the pilot flying, interrupting the inputs and then correcting for them. He was also getting both overspeed and underspeed alerts, both of which needed immediate but opposite actions.
For the MCAS to activate, three things must be true:
- the aircraft’s AOA value (as measured by the AOA sensor) must exceed a specific threshold,
- the aircraft must be in manual flight (autopilot not engaged),
- the flaps must be fully retracted.
The captain did not know about MCAS or that a misaligned AOA sensor had been installed onto the aircraft. He had asked the first officer to perform the Airspeed Unreliable checklist and to ask for clearance to a holding point in order to buy time to troubleshoot and he had spotted one important point: when the flaps were extended, the constant nose-down adjustments stopped.
As a result, he had extended the flaps fully as he tried to work out which, if any, of the instruments could be trusted. He did not explain any of this to the first officer who appeared to be struggling all but the most simple tasks.
The aircraft was climbing at a rate of 1,500 feet per minute with a pitch attitude of 3° nose up as the flaps position reached 5. On the captain’s display, the low-speed barber pole and the overspeed barber pole merged into one. The control column stick shaker activated again, warning of an impending stall, and continued to vibrate.
At the same time, the overspeed barber pole appeared on the first officer’s flight display, with the bottom of the pole at about 340 knots. The low-speed barber pole did not appear. Neither pilot spoke to the other about what they were seeing on their respective flight displays.
The captain shouted “memory item, memory item” but didn’t say which list, or why.
The automatic trim activated nose down again for one second and then again. The flaps were fully extended, so this was not the MCAS but likely a manual input, perhaps to reduce the climb in case the aircraft really was entering a stall.
The first officer responded with the memory items he had completed. “Feel differential already done, auto break, engine start switches off, what’s the memory item here?”
Rather unhelpfully, the captain snapped a single word response. “Check!”
The automatic trim activated nose down for two bouts of two seconds, making for four slight nose-down movements in seven seconds. Then it activated nose down again twice more as the aircraft stopped the left turn, rolling level on a heading of 100°.
Three minutes had elapsed since the aircraft’s departure. Throughout the flight, the aircraft had been reacting in ways mostly incomprehensible to both flight crew. The core issue was the faulty sensor but the AOA DISAGREE warning had not illuminated: unknown to the crew, this alert was not functional on the aircraft. They were left to diagnose a dizzying array of symptoms on their own to a backdrop of alarms and alerts. At the time, no one had heard of MCAS or that the 737-MAX would automatically trim nose-down at higher angles of attack in ways that had not been seen in earlier models. The captain of the previous flight had worked out that the STAB TRIM CUT-OUT disconnected the MCAS from trimming nose-down in response to the faulty AOA sensor. This captain, 31 years old with 6,028 hours (mostly on the Boeing 737) had noticed that retracting the flaps had added to the problem but overall, he struggled to make sense of the situation. He called out for support but inconsistently and often incoherently: he didn’t seem to be able to work with his less experienced first officer for effective trouble-shooting. The repeated sounding of alarms and constant clacking of the stick shaker only added to the stress in the cockpit.
The first officer asked “Flight control?”
The captain’s voice, “yeah,” was followed by the sound of paper pages being turned as the captain trimmed the aircraft nose up for one second.
The first officer called out “Flight control low pressure” just before another warning sounded in the cockpit: an altitude alert tone. They had set the desired altitude to 5,000 feet but the actual altitude had deviated from this. The captain’s altitude showed 4,110 feet and the first officer’s showed at 4,360 feet.
The automatic trim activated nose down for one second. The aircraft began to turn to the left. The captain trimmed nose-up for one second. The first officer called out “feel differential pressure”. The captain responded that he should perform the checklist for Airspeed Unreliable, which the first officer acknowledged. The aircraft continued to climb at a rate of 1,600 feet/minute, with the captain’s altimeter showing 4,900 feet and the first officer’s altimeter showing 5,200 feet.
The first officer informed the captain that he was unable to find the Airspeed Unreliable checklist in the manual.
Another altitude alert tone sounded: they were continuing to climb at a rate of 460 feet per minute and the captain’s display showed 5,310 feet and the first officer’s 5,570 feet. The sound of pages turning filled the cockpit again.
Meanwhile, the air traffic controller added a tag to the aircraft’s radar display to say FLIGHT CONT TROB so that anyone looking at the radar target would be aware that the aircraft had reported flight control trouble. This shows how quickly everything was happening; writing this from the comfort of my own desk, it seems like a very long time since the first officer reported their issue to the controller, but in reality only one minute had passed.
The flight crew seemed close to being able to gain control of the situation. Then the flaps started retracting again from position 5 to position 1.
There was no discussion related to the flap position. There is no way of knowing who retracted the flaps. However, as the captain had extended the flaps without a word of explanation, it seems likely that the first officer took it upon himself to retract them.
The first officer repeated the controller’s instruction to fly heading 350° for the captain’s benefit and complained that there was no Airspeed Unreliable checklist.
The flaps began to travel from position 1 to fully retracted, again with no comment or discussion.
The conditions for the MCAS to automatically adjust the pitch nose down in response to the faulty AOA data were about to be fulfilled again.
While the flaps were still retracting, the first officer said “ten point one.” In the Quick Reference Handbook, 10.1 is the page that the Airspeed Unreliable checklist is on. He’d found it.
The first item on the checklist is to disengage the autopilot, if engaged. It was not engaged but this would have emphasised that the crew should not use the autopilot in an attempt to deal with the issues they were facing.
As he began reading out the Airspeed Unreliable checklist, the MCAS activated the automatic trim nose-down for two seconds. The captain interrupted it by commanding nose up trim for 6 seconds. The pitch trim recorded 6.19 units.
Because the configuration of the aircraft will define what “in trim” means, those units can’t be used to understand if the aircraft is pitching up or down but it does allow us to have a relative understanding of how the trim changed during the flight. The units of pitch trim are constantly recorded by the FDR: I won’t list every pitch trim change, but it is useful to look at the units at key points, so we can get a feeling for how pitch trim, and thus the pitch, were changing during the flight.
A few seconds later, the aircraft trimmed nose-down again for seven seconds. The captain interrupted it again, commanding nose-up trim for six seconds. MCAS activated the trim to pitch nose down again for four seconds and the captain commanded nose up for four seconds. The aircraft trimmed nose down for three seconds and the pilot commanded nose-up trim for three seconds. The pitch trim was now 5.0 units.
The aircraft was still turning left and they blew straight past the 350° heading given by the controller. As they reached a heading of 015°, the controller called to tell them they needed to turn right for a heading of 050° and maintain 5,000 feet. The first officer, who was still reading aloud from the Airspeed Unreliable checklist, stopped to repeat the controller’s instructions.
MCAS activated the automatic trim nose-down for three seconds and the captain commanded nose-up for six seconds. The automatic trim activated nose-down for five seconds, interrupted again by the captain trimming nose-up for six seconds. At the same time, the controller instructed the flight crew, who were on a heading of 023°, to turn right, heading 070°, to avoid traffic.
The flight crew had not declared an emergency, so the controller had no way of knowing that in the chaos of the cockpit, the constant heading changes were adding to an already impossible workload.
The first officer, who up until now had been handling radio communication in his role as Pilot Monitoring, did not respond to the controller’s heading change nor did he acknowledge the traffic. He was focused on the Airspeed Unreliable checklist, reading aloud for the benefit of the captain that the flight path vector and the pitch limit indicator might be unreliable. The controller called twice more, finally receiving an acknowledgement from the first officer.
Someone commanded nose-up trim for seven seconds, almost certainly the captain.
MCAS activated again, trimming nose-down for about five seconds until it was interrupted by a nose-up trim for five seconds. The first officer continued to read aloud from the Airspeed Unreliable information until he was interrupted by the controller telling them to turn right to heading 090°, which the first officer acknowledged. At that point, the aircraft was on a heading of 038°.
The controller almost immediately called back to ask them to stop the turn and fly heading 070°, which the first officer acknowledged.
The captain commanded nose-up trim for five seconds. The MCAS activated again with five seconds of nose-down trim, which was interrupted by the captain again trimming nose up for six seconds. The MCAS trimmed nose down for four seconds and the captain trimmed nose up for four seconds. Nose down for four seconds, nose up for three. At this stage, the trimming nose-up and nose-down has become constant; the captain was entirely focused on fighting the MCAS attempts to bring the nose down.
Finally, the first officer finished reading the Airspeed Unreliable checklist and told the captain that he was going to move on to Performance Inflight. The MCAS trimmed nose-down for seven seconds, which the captain interrupted with seven seconds of nose-up trim. Through out this, the captain was just about managing to keep the aircraft from descending; the aircraft’s pitch trim varied between 5.30 and 5.83 units.
The first officer called for the senior cabin crew member to come to the cockpit for an update. The controller called with traffic information. The captain trimmed the nose up for one second.
The cabin crew member came into the cockpit where the captain told her to please get the engineer from the cabin. The first officer repeated the instruction and the cabin crew member left.
MCAS trimmed nose-down for one second, the captain trimmed nose-up for three seconds. As soon as he stopped, the MCAS trimmed down for another four seconds and the captain trimmed nose-up for six. Trimming nose-up is the correct way to interrupt the MCAS automatic trimming, but as the AOA sensor was still feeding bad data in to the system, the MCAS was reacting anew to the situation every time the captain stopped adjusting the trim.
The cockpit door opened, presumably the engineer coming into the cockpit. The captain said simply, “Look what happened.”
At the same time, the controller instructed the flight crew to turn left heading 050º and maintain 5,000 feet. The first officer acknowledged the call, although the aircraft had never made the turn to 070° and they were already at a heading 045° when the controller made the request for 050°. The aircraft needed to turn right by a further 5° rather than left. The first officer was no longer monitoring and the constant battle for control of the trim distracted the captain from following the controller’s headings.
MCAS activated nose-down trim for four seconds, interrupted by the Captain commanding nose-up for 6 seconds. The cabin crew member used the interphone to share with the others that there was a technical issue in the cockpit.
MCAS activated nose down for six seconds, interrupted by nose-up trim for five seconds. MCAS activated trim adjusted nose down for seven seconds, the captain trimmed nose up for three. The first officer, briefly recollecting that he was the Pilot Monitoring on this flight, called out that the landing gear was up and that they were at an altitude of 5,000 feet.
He had barely stopped speaking when the altitude alert sounded again. The captain’s altimeter showed 4,770 feet while the first officer’s showed 5,220 feet.
The captain trimmed up for six seconds, bringing the pitch trim up to 5.4 units.
As the aircraft’s altitude dropped on the radar screen, the controller asked if they were descending. The first officer repeated that they had a flight control problem and that they were flying the aircraft manually.
The MCAS pitched nose down for four seconds, interrupted by the captain trimming nose-up for seven.
The controller asked the flight crew to maintain heading 050° and to change frequency to arrivals, which the first officer acknowledged. The aircraft was at that stage on heading 059°.
At this point, the flight had been airborne for 9 minutes. It’s probably taken you longer than that to read the sequence of events.
The MCAS pitched nose-down for six seconds countered by the captain commanding nose up for three. Nose-down for six, nose-up for three. The first officer contacted arrivals and advised them that they were experiencing a flight control problem. The pitch trim was 4.5 units.
The arrivals controller told the flight crew to prepare for landing on runway 25 and to fly heading 070°. The first officer read back the instruction. Their current heading was 054°.
The MCAS trimmed nose down for two seconds and the captain trimmed nose up for nine. Nose-down for five, nose-up for three.
This was when the captain asked the first officer, again, to take over as Pilot Flying.
It is somewhat of a controversial decision for which there is no right answer. On the one hand, the captain is the most experienced pilot and thus should maintain the responsibility for the flight for the landing, rather than the potential confusion of changing pilots in the middle of an emergency. On the other hand, relinquishing control would allow the captain a better chance of trouble-shooting the situation, which the first officer was clearly not managing to do. By now, the captain must have felt that it was clear that they could maintain the flight by constantly interrupting and correcting the nose-down trimming.
However, he said none of this. The first officer commanded aircraft nose-up trim for three seconds and called out, “I have control.”
The captain took over communications, contacting arrivals to request that they continue via a specific waypoint, instead of direct, in order to avoid weather. The controller immediately approved this.
MCAS activated the nose-down trim for eight seconds. The pitch trim, which had varied between 5.8 and 4.5 under the captain’s control, was now at 3.4 units.
The first officer had no idea what was going on or even how to describe it. “Wah! It’s very—” He commanded nose-up trim for one second, bringing the pitch trim to 3.5 units.
The captain was still speaking to the controller, explaining that he could not determine their altitude as all of the aircraft instrustments were indicating different altitudes. The captain was also becoming overloaded: he identified their flight as Lion Air six five zero, instead of 610.
“Lion Air 610, no restriction,” said the controller, by which he probably meant that he was not going to ask them to maintain a specific height.
MCAS activated the nose-down trim for about three seconds until it was interrupted by the first officer, who commanded nose-up trim for one second. The first officer’s column sensor recorded 65 pounds of back pressure, so he was pulling the nose up hard. Unlike the captain, he was not correcting the aircraft pitch using the trim; after interrupting the the MCAS, he was trying to keep the now-out-of-trim aircraft level through sheer brute force. The pitch trim dropped to 2.9 units.
The first officer commanded nose-up trim for another four seconds, bringing the pitch trim up to 3.4 units. He had trimmed the aircraft but not enough, which meant he still needed forcibly pull back on the column to keep the nose up.
The captain asked the arrivals controller to block 3,000 feet above and below them for traffic avoidance, effectively ensuring that they had a buffer as they were struggling to control the aircraft he did not know their true altitude.
Unfortunately, this wasn’t clear to the controller, who responded by asking the captain for their intended altitude.
MCAS activated the nose-down trim for eight seconds. The pitch trim dropped to 1.3 units and the 737 pitched down to -2°. They were descending at 1,920 feet per minute as the first officer’s column registered 82 pounds of back pressure. He pulled back even harder, fighting against the trim, and called out to the captain that the aircraft was flying down.
Fixation is a psychological condition where the pilot is unable to process new information. The pilot’s attention is “fixed” on a single source of data, usually in response to overload, and no longer perceives important data from other sources. Studies have shown that aural information is most commonly lost first. We can see this effect in the captain, who did not react at all to the first officer or make any move to arrest the aircraft’s descent. Instead, he responded to the arrivals controller question instead repeating his previous request. “Five thousand feet.”
As the controller approved the request, the first officer shouted loudly that the aircraft was flying down. The captain finally responded. He said, “It’s OK.”
It was most assuredly not OK.
The first officer commanded nose-up trim for two seconds, setting the pitch trim to 1.3 units. The MCAS activated for four, dropping the pitch trim to 0.3 units. The first officer’s control column recorded 93 pounds of back pressure.
Although the first officer seemed to almost grasp the issue that the 737 MAX was trimmed for descent, he did command any further nose-up trim. By now, the captain’s side altitude showed 3,200 feet and the first officer’s side showed 3,600 feet. They were no longer “flying down”, they were diving at a rate of over 10,000 feet per minute.
The Extended Ground Proximity Warning System sounded urgently. TERRAIN – TERRAIN! SINK RATE! The overspeed clacker sounded; a manual system which could not be fooled by the faulty AOA sensor.
The MCAS commanded one final nose-down trim but this one was interrupted by the Boeing 737 crashing into the water.
The arrivals controller called out to the aircraft and received no response. The controller called out again. The flight had disappeared from the radar screen. The arrivals controller and the Terminal East controller both tried four more times between them to contact the aircraft.
Then, unable to deny that the Lion Air 610 must have crashed, the controller asked any aircraft available to hold over the last known position for a visual search.
About half an hour later, a tug-boat crew found floating debris about 33 nautical miles northwest of Jakarta.