Afraid to Fly? Technical Information on the Lion Air Crash

This crash has troubled anxious fliers because it involved an almost new plane.

Posted Nov 07, 2018

The crash of the Lion Air 737 MAX has caused an increased fear of flying because it was a new plane. They ask how could something go wrong with a new plane? Does this mean there is a problem with the 737 MAX?

Reportedly, pilots had had trouble with the plane on several previous flights in a row. A maintenance log entry was leaked to the internet citing unreliable speed information and uncommanded operation of the motor that positions the stabilizer. Operation of the stabilizer motor has led to speculation about an angle of attack sensor problem. What is an angle of attack sensor? I'm sure you are familiar with a weather vane. When the wind blows, it streamlines with the wind and points to the direction the wind is coming from. Airliners have a similar device called the angle of attack sensor. Its purpose is to measure the angular difference between the wing and the air flowing over the wing.

Think of putting your hand out the window of a car when going down the highway. The wind is flowing straight past your car. Think of your hand as a wing. If you hold the palm of your hand level, air flows straight by your hand. There is no angular difference. In aviation terms, we would say your hand's angle of attack is zero. And you would notice that the wind is not pushing your hand up, or down.

Now, give your hand a slight angle of attack. Tilt the edge of your hand that is hitting the air slightly up. That causes the air to push your hand up. Tilted up, your has a positive angle of attack against the wind. To make a plane go up, the pilots raise the nose a bit. That, of course, increases the angle at which the wing cuts into — or attacks — the airflow.

An angle of attack of three degrees is needed to maintain level flight, while an angle of five or six is needed for a gentle climb. The angle of attack is much larger — about 12 degrees — for a rapid climb after takeoff.

But, on most airliners, pilots do not refer to angle of attack. Some planes have an angle of attack indicator in the cockpit; some don't. Instead, pilots use an instrument called the attitude indicator. Attitude is defined as "the arrangement of the parts of a body or figure." So, in flying, an attitude indicator shows whether the plane's nose is aimed at the horizon (or just slightly above the horizon to maintain altitude), is aimed above the horizon (for climb), or aimed below it (for descent).
As to the speed of the plane, the throttles are adjusted to produce the desired amount of power. Engine speed is referred to as a percentage of max power. For cruise, the engine speed is around 88%. For descent, the throttles are back at idle, and idle speed is around 40%. For takeoff, the engine speed is around 93%. That's far below max power. The lower power setting is used to extend the life of the engines.

Any competent pilot knows what engine speed is needed to take off, climb, or descend, as well as the angle the nose needs to be at for takeoff, climb, and descent. Again, the angle is judged by using the attitude indicator or by eye. The angle of attack indicator is not used.

So what is the angle of attack indicator used for? It sends information to the computers used to adjust the elevator at the tail end of the plane. And here is where it seems things went wrong on the Lion Air 737 MAX. Erroneous signals from the angle of attack sensor may have caused a motor to operate that changes the position of the front half of the elevator, which is called the stabilizer. This fault, or similar faults, are said to have been reported by the pilots on four flights in a row before the crash.

Reportedly, maintenance did work intended to correct in-flight problems. Yet, faults reappeared on the following flights.

A motor changes the position of the front half of the elevator, the stabilizer. If the motor runs the front half of the elevator down, it lowers the nose of the plane. However, since the pilots control the rear half of the elevator, they can apply pressure on the controls and hold the nose up, so the plane does not descend.

On the 737 MAX, the autopilot can cause the motor to run for up to ten seconds. Runtime is limited to make sure the motor, if operating when it shouldn't, can't move the front of the elevator more than the pilots can override by applying pressure on the controls in the cockpit.

The motor can be operated by using a switch on the control wheel. This video shows the switch, the wheel that spins when the motor is operating, and movement of the stabilizer.

Now, about the operation of the motor when it should not be operating: There could be a problem with the switch or the wiring that turns the motor on. That's very unlikely because the switch is actually two switches, both of which must be turned on simultaneously to cause the motor to operate. Developing an electrical fault in both at the same time is pretty near impossible. There could be a problem with the autopilot that turns the motor on improperly. Here is where the angle of attack sensor comes in. A problem with the angle of attack sensor could cause the autopilot to operate the motor.

Neither of these conditions should be a problem. We have a term for the problem: "runaway stabilizer trim." We have a procedure for the problem. Every airline pilot is supposed to be trained to react to "runaway stabilizer trim." First, how do you recognize it? When the motor is operating, a wheel in the cockpit rotates. If the motor is operating when it should not be, how do you stop it? This video shows how simple it is. You stop the motor by grabbing the rotating wheel. Then, turn off the switch that sends power to the motor. End of problem.

Let's talk about — not the spinning wheel — but about media spin. The airline doesn't want to be criticized so they are blaming Boeing. Boeing responded by pointing out there is a procedure for the problem by citing "existing flight crew procedures to address circumstances where there is erroneous input from an AOA sensor."

Was this an admission by Boeing that there is a problem with the 737 MAX? No. We have had the same or similar systems all the way back to the original Boeing airliner, the 707, and at every airline I know of, every pilot trained hands-on to deal with runaway stabilizer trim.

Any airline should be able to diagnose and correct system faults. Any properly trained pilot can recognize and deal with this problem.

So drop the concern about the 737 MAX. A fault in the stabilizer system can develop on any plane, and the procedure for dealing with it is simple. The question is why the fault wasn't cleared by maintenance and why the pilots on the crashed plane didn't deal with it as the pilots on the previous flights had.