It was never going to be an easy fix. As a result of the accidents in 2018 and 2019 the FAA discovered that the Boeing 737 MAX did not comply with the stability requirements for a large aeroplane. This wasn’t a minor detail or a software problem. Aerodynamic stability is a function of the basic design of the aeroplane, i.e. its shape.
In order for an aircraft to be safe and controllable it must have longitudinal stability. If the aircraft is flying level and something makes the aircraft pitch up and start climbing, then it should naturally pitch down and return to level flight. If the pilot pulls back on the control column then the nose of the aircraft pitches up but when the pilot lets go the nose drops back to where it was before. On the B737 MAX there is a point where if the pilot pitches up and then lets go the nose will continue pitching up. If this pitch-up is unchecked then the aeroplane will slow down, stall and stop flying. If an issue like this was discovered during the design of a new aircraft, then you’d expect the designers to go back to the drawing board and fix the design. The issue with the MAX is that it wasn’t designed as a new aircraft.
The B737 MAX is the latest derivative of a design that started flying in 1967. The original B737 had engines with a diameter of 125cm that were mounted underneath the wings. The aircraft was 30m long and had great longitudinal stability. The MAX is 39m long and has engines with a diameter of 176cm. To fit these big engines on they had to be mounted further forward and higher up. The position of the engines on the MAX means that at a certain ‘angle of attack’ the engine nacelles generate a lift component that is forward of the centre of gravity. This produces the undesirable pitch up. To fix this problem Boeing came up with the ‘Manoeuvring Characteristics Augmentation System’ (MCAS). If MCAS detects that the aircraft is approaching a dangerous angle of attack, then it activates the stabilizer trim.
Stabilizer trim is used constantly by the pilots, or autopilot, during flight. It changes the angle of the tailplane. At different speeds and altitudes, the tailplane needs to be set to different angles to keep the aeroplane flying level without the pilots pushing or pulling on the controls.
There are problems with using stabilizer trim to counteract longitudinal instability. If MCAS winds in a bunch of stabilizer trim to get the nose of the aeroplane to pitch down, then it will put the aircraft ‘out of trim’. When the nose drops, and the aircraft returns to level flight, the pilot will have to pull really hard on the controls to stop it from going into a dive.
The pilots can disable stabilizer trim and, therefore, MCAS by isolating the electric motor that drives it. Trim is then controlled manually by winding a big handle on the flight deck. MCAS drives the trim really fast so if the pilots take a bit of time to react then the aircraft will be badly out of trim by the time MCAS is disabled. If the aircraft is too far out of trim, then it may not be possible for the pilots to exert enough force on the trim handle to move it back to where it should be. If this happens at high altitude then the pilots should be able to recover, if it happens close to the ground then maybe not.
To compound the problem Boeing allowed MCAS to be controlled by a single sensor so that one failure could lead to an ‘undesired’ MCAS activation. They also decided that pilots didn’t need to know about the system so there was no special training provided. A lot of the press coverage has concentrated on these additional issues, but the fundamental problem is aerodynamics and the shape of the aeroplane, not software and sensors.
With hindsight it’s clear that the B737 MAX should have been grounded after the first accident (or ideally before). At the time when it was grounded, I was surprised. I thought it was an over-reaction. I had too much confidence in Boeing, who built the aircraft and in the FAA, who certified it (I also have a VW Golf diesel). What is hard to understand is why, once a fundamental problem had been discovered, Boeing decided to build 400 more aircraft to the same design.
Andrew McKechnie is a B737 pilot, aviation consultant and aviation authority inspector. This article was written based on information published in the press and represents Andrew’s view in December 2019. Andrew is not an expert in aircraft design.