Not everyone who encountered the MAX 8’s fatal flaw was killed by it
|National Post 21 Mar 2019 at 08:52|
Amid evidence that a software flaw brought down two Boeing 737 MAX 8s, other pilots are reporting that they too encountered the deadly defect, but managed to battle the plane’s computer in order to stay aloft.
The reports illustrate a disturbing facet of the MAX 8 saga: Hundreds of airline passengers around the world may have narrowly missed the same fate that eventually killed 346 people.
In the crash of both Lion Air Flight 610 and Ethiopian Airlines Flight 302, early evidence suggests that both aircraft were doomed by an error that caused the plane to spontaneously pitch itself into a deadly nosedive.
Subsequent reports have revealed that a potentially fatal “nose down” scenario also struck a pair of other Lion Air flights as well as at least two passenger flights in the United States.
In all those cases, pilots were able to override the error before it spun out of control. Quite likely, passengers may not have even known that they were briefly aboard another potential MAX 8 disaster.
The suspected problem with the 737 MAX 8 is a feature known as the Maneuvering Characteristics Augmentation System (MCAS).
The MAX 8 has slightly heavier engines than prior 737 variants, causing the plane to pitch upwards upon takeoff. Rather than retrain pilots on this quirk, Boeing simply installed a piece of software that would automatically pitch the airliner forward to compensate.
If an external sensor is malfunctioning, however, the aircraft’s computer will continually plunge itself into a nose dive in the mistaken belief that it is staving off a dangerously steep climb.
In both the Lion Air and Ethiopian Airlines crash, the aircraft’s final moments were spent pitching wildly up and down in what appears to have been a desperate fight for control between pilots and the software.
MCAS doesn’t have an on/off switch, and pulling back on the aircraft’s control wheel (as appears to have been the strategy in both crashes) only makes the problem worse.
However, MCAS can be overcome by shutting off electric control to the MAX 8’s horizontal stabilizer; the small wing-like appendages on the aircraft’s tail that can be tilted up or down to level-out a plane in flight.
A trim wheel on a 737. If electric control to the horizontal stabilizer is disabled, this wheel can be used to control the stabilizers manually. Quora.com
When MCAS plunges an aircraft into a dive, it’s doing this by tilting the stabilizer. So, all a pilot has to do is cut power to the stabilizers and then control them manually using a big wheel in the cockpit that’s connected to the aircraft tail by steel cable.
On March 9, a day before the Ethiopian Airlines crash, a Lion Air MAX 8 began launching into nosedives due to a suspected MCAS error. In that case, the plane appears to have been saved by an off-duty pilot and knew to cut power to the stabilizers.
The same procedure also appears to have been used to save the 737 that was ultimately involved in the October 29 crash of Lion Air Flight 610. Only one day before its plunge into the sea off Indonesia, a flight crew encountering the “nose down” problem quickly ran through an emergency checklist that pointed them to the solution.
“The copilot flew the rest of the flight using manual controls and without autopilot,” reads a report in the engineering journal IEEE Spectrum .
The Aviation Safety Reporting System, an anonymous incident database maintained by NASA, similarly recorded two instances of American pilots having to outwit a bucking 737 MAX 8. In both those instances, pilots recovered control of the diving aircraft by disabling the autopilot.
“The Captain immediately disconnected the autopilot and pitched into a climb. The remainder of the flight was uneventful,” read a November incident report.
If a pilot knows what to expect, overriding an MCAS error only takes a few seconds. However, MCAS isn’t even mentioned in the 737’s flight manual. If it malfunctions, a flight crew is left to diagnose the problem from scratch while also desperately keeping the plane aloft and dealing with a cockpit full of warning alerts.
It’s not uncommon that planes will crash because of an easily solved problem that is overlooked in the heat of an emergency. In 2007, a Boeing 737 flying for the Indonesian airline Adam Air crashed for the simple reason that pilots had been so preoccupied with a malfunctioning navigation system that they hadn’t noticed the plane rolling onto its side .
The flight recorder from Lion Air Flight 610 reveals a chaotic scene of the crew desperately thumbing through flight manuals to diagnose their plunging airplane. At no point in their final minutes did either figure out that they were fighting a computer.
“They didn’t seem to know the trim (pitch adjustment of the aircraft) was moving down,” a source who had heard the recording told Reuters . “They thought only about airspeed and altitude. That was the only thing they talked about.”
View of the horizontal stabilizer on a 737. The whole stabilizer is rotated to control pitch, and a malfunctioning software glitch is believed to have used the horizontal stabilizers to pitch two 737 MAX 8’s into deadly dives. Flickr.com/Cory W. Watts
Boeing has acknowledged a software flaw in the MAX 8, but has maintained that it could always be safely shut off by a flight crew if it went on the fritz.
MAX 8 pilots, in turn, have complained that they were not told by Boeing that they were flying aircraft whose controls could be commandeered by a computer. “The Flight Manual is inadequate and almost criminally insufficient,” one pilot wrote in a November report to the Aviation Safety Reporting System.
Another pilot, flying after the crash of Lion Air Flight 610, wrote of having to alter his flying routines in order to remove the “MCAS threat.”
Given that the MCAS threat was well-known at the time of the Ethiopian Airlines crash, scrutiny has descended on why pilots may not have known how to disable the software.
“After this horrific Lion Air accident, you’d think that everyone flying this airplane would know that’s how you turn this off,” Steve Wallace, the former director of the U.S. Federal Aviation Administration’s accident investigation branch, told Bloomberg News this week.