China's Flight 5735 crash: mechanical failure or human action?

On Monday, March 21, China Eastern Airlines flight 5735 took off from Kunming in western China on a flight to Guangzhou, on the southern coast. After take-off, the plane rose to a cruising altitude of 29,000 feet and levelled off, traveling east. A little more than an hour into the flight, the plane abruptly took a nosedive and crashed into a muddy mountainous region after falling about five miles in a minute and a half.

Recovery crews at the scene confirmed there were no survivors among the 123 passengers and nine crew members. The two flight recorders ("black boxes") were recovered and are undergoing analysis, but Chinese officials emphasize that it is too early to reach any conclusions about the cause of this tragedy, which is the worst air transport disaster in China since 2010.

As a precaution, China has grounded over 200 of its Boeing 737-800 series airliners for heightened safety inspections. So far, no other country has taken similar steps, although the Boeing 737-800 is reportedly the most popular airliner of its size class in the world. One reason may be that the circumstances of the crash do not seem to indicate any particular mechanical failure.

Many types of mechanical problems in aircraft give some sort of warning before causing life-threatening conditions. For example, the software problems that led to the crashes of two 737 MAX airplanes in 2019 were known to cause unexpected behaviour of the aircraft in certain conditions, but the manufacturer assumed (wrongly) that training would enable pilots to take the appropriate actions to counteract the plane's tendency to nose up and stall.

The information we have so far about Flight 5735 shows no sign that anything was wrong prior to the moment that the plane took a nosedive. Even more oddly, despite repeated attempts by air traffic controllers to contact the pilots during the 90-second dive, no replies were received. The lack of response would be understandable if an explosion or other violent breakup of the plane occurred in mid-air, but eyewitnesses and the crash scene testify that the plane was largely intact until it hit the ground, except for a small piece found some 10 km away that might have broken off during the dive. Investigators estimate that the plane's velocity could have approached the speed of sound, which is beyond its design speed and could have broken small parts off before the crash.

Although Chinese officials have not addressed the question directly, one possibility that must be addressed is that one or more of the pilots deliberately crashed the plane. While very unlikely, such a thing has happened before.

In 2014, Malaysian Airlines Flight 370 disappeared over the Indian Ocean with 239 people on board, and despite two extensive searches, the plane has never been found, although small pieces have washed ashore at various places. In an extensively researched article in the Atlantic, William Langewiesche makes a strong case for the idea that the pilot, Zaharie Ahmad Shah, somehow overcame the other crew members, depressurized the cabin at an altitude that would have killed everyone on board outside the oxygen-supplied cockpit, and then let the plane crash in the ocean, possibly after committing suicide himself.

It is too early to draw any definite conclusions from the preliminary data we have about Flight 5735 and we may have to wait quite a while before the official investigation produces any results. Under United Nations rules, the country in which an airplane crash occurs is in charge of the crash investigation, and while there are ties to US investigation agencies by virtue of the fact that Boeing made the plane, they must play secondary roles.

Information from the cockpit voice recorders, if any, will be extremely helpful in determining what was going on before and during the last minute and a half of the flight. Reports state that no conceivable malfunction of the autopilot systems would cause the plane to take and persist in such an extreme nosedive, which apparently can only be induced by the pilot's manual intervention.

This raises a more general question: should new planes have software in place that would make it impossible to take such actions that would lead inevitably to the loss of the aircraft?

In discussions about how the MCAS system, which is mostly software, led to the crashes of two 737 MAX planes in 2019, it looked like we had already gone too far in the direction of a robot taking over a plane. In any case, all new airliners are "fly-by-wire" in the sense that the pilot's manipulations of the controls pass through software systems that then execute the actual movement of control surfaces.

In that view, it just becomes a matter of judgment on the part of the computer whether to let what the pilot is telling the computer to do take precedence over what the computer knows is best for the plane, or at least what might lead the plane to crash.

Perhaps the rarity of a pilot going berserk is enough to allow us to lodge our ultimate trust in the person actually flying the plane and not in some software that probably hasn't been tested under the conditions that would apply if a rogue pilot was determined to crash the plane.

Besides, a pilot that's smart enough to fly a plane safely is probably smart enough to outwit software designed to keep him from doing it.

In any case, we will simply have to wait until Chinese accident-investigation officials reach their considered conclusions about what caused the crash of Flight 5735. In his article on the Malaysian Air crash, Langewiesche says that important personal information about the pilot Shah was suppressed by accident investigators, who were reluctant to admit the possibility that one of their own pilots deliberately caused the crash, which officially still has no determined cause. Political considerations can be a factor in accident investigations, and if the Chinese investigation comes up empty-handed, it may be because stating the most likely cause would be politically embarrassing.

Politics aside, if a mechanical failure was at fault, it's important to get to the bottom of it so that all the other users of 737-800s can avoid such disasters in the future.

This article has been republished with permission from the author's Engineering Ethics blog


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