Why were pilots worried about the Boeing 737 MAX?

Passengers who feel a degree of anxiety about flying will be forgiven for feeling more than usually concerned about safety, after the uncoordinated shambles of banning the Boeing 737 MAX from the air.

Following the tragic loss of Ethiopian Airlines flight ET302 on Sunday, a patchwork of ground-stop orders began to develop. From China to the Cayman Islands, national safety officials banned the latest version of the 737 from their runways and airspace.

Two Turkish Airlines planes were most of the way to the UK from Istanbul when the Civil Aviation Authority issued its ban. They turned around over central Europe to head back to the airline’s hub.

Passengers were waiting to board the return legs of those flights, from Birmingham and Gatwick airports to Istanbul. Absurdly, as they were told that the plane was deemed too dangerous to carry them, across the Atlantic travellers were boarding the very same jet – with the planemaker saying: “Safety is Boeing’s number one priority and we have full confidence in the safety of the 737 MAX.”

Within 24 hours the US Federal Aviation Administration (FAA) had changed its tune and banned the aircraft. Boeing said: “We are supporting this proactive step out of an abundance of caution.”

Grounding a plane is an extreme step, taken very reluctantly. I have spent the week talking to pilots and aviation safety experts about their concerns.

They start with the difference between the Boeing 737 MAX and previous versions of this very popular and successful jet.

In order to reduce fuel burn and harm to the environment, Boeing needed bigger and more efficient engines. They have to be carried higher on the wing, and moved a little forward to comply with minimum ground clearance requirements.

That changes the balance of the aircraft, and Boeing calculated that in some circumstances the new design could increase the possibility of a stall, in which an aircraft loses lift.

The best indicator of an imminent stall is an excessive “angle of attack”. This is the angle between the wing and the airflow and is one of the fundamentals of flying. If it is too high, the plane slowly loses speed then swiftly loses lift and altitude.

Boeing installed a “pitch trim system” to protect against the risk. This stall-protection measure kicks in when the angle-of-attack sensor indicates danger. By operating an elevator in the tail, it automatically nudges the nose downwards. The pilots can then take action to address the problem.

Could something designed to enhance safety actually put the plane at risk?

From reading the preliminary accident report into the loss of Lion Air flight 610 in October 2018, shortly after take-off from Jakarta, it appears the answer is that it could.

The report warns that a single faulty angle-of-attack sensor can cause “increasing nose-down control forces”. In other words, the plane’s brain – its flight control system – can wrongly be told that a stall is imminent, tilt the nose down in response and increase resistance to the pilots’ efforts to correct it.

The accident report makes clear that there is a simple solution: operate two switches marked “Stab Trim Cutout”.

But serving pilots and safety experts are uneasy.

Some say that Boeing failed to be sufficiently upfront about what they believe is a very significant design difference between the MAX and previous versions. Pilots are accustomed to encountering all manner of potential disaster scenarios in flight simulators, and learning how to deal with them. But not one where the plane takes on a mind of its own and keeps tilting the nose down despite you using all your strength to fight against it.

“This condition, if not addressed, could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain,” said the FAA in an Emergency Airworthiness Directive issued shortly after the Lion Air crash.

When things start going wrong on a flight deck, the most valuable commodity is time – which generally correlates with altitude. Plenty of height should provide time to assess the situation calmly and take the appropriate action. But the Lion Air jet was barely a minute into its flight and less than 1,000 feet off the ground when the pilots started reporting problems.

Over the next 10 minutes, the aircraft’s height varied wildly as the increasingly fraught pilots were confronted with conflicting information.

The profile of the Ethiopian Airlines flight turns out to have been similarly erratic. Which is why air safety regulators grounded the Boeing 737 MAX until more is known about the second tragedy. But they did so messily, which will spark insecurity among nervous fliers and perhaps deepen the grief of those who are mourning their loved ones.

Air safety regulators have a common aim, and they should speak with a united voice.