Feb. 08, 2019, ©. Leeham News: In our run through of the pitch stability problems of an airliner we covered high and low-speed problems in the last Corners and before it deep stall.
Now we go back to the region just before and during a stall, to look at more measures to help the pilot besides stall warning and stick pushers.
The stall is a dangerous area of the pitch envelope. The wing’s aerodynamic flow breaks down to an unstable flow which is easy to influence by outside factors.
Despite a lot of investigations during the creation of an airliner using CFD (Computer Fluid Dynamics tools) and wind tunnels followed by flight tests, it can still go terribly wrong when an aircraft enters stall.
The flight test investigations are made with a number of cases, but for a commercial airliner, no flight tests are driven into a deeply stalled state.
An unfortunate stall entry can have a lot of pitch and yaw inertia pushing the aircraft deep into stall with an asymmetric aerodynamic flow situation. This is a recipe for a spin, a dangerous rotating stall with asymmetric flow over the wings, and it takes a long time to get out of a spin.
Fighters go there during their flight test but they are made for flying on the limit and the pilots have an ejection seat just in case. Airliner’s whole business is to bring their passenger to their destination with a maximum of safety, this is what their design and testing teams focus on.
Thus airliner designers focus on helping the pilots not to enter stall and the certification process has strict requirements how the airliner shall behave in the region before stall entry and during stall.
Figure 1 shows an airliner flying at a trimmed Angle of Attack (AoA) of 9° like it does just before landing (green line crossing the zero moment line at 9° AoA). If the pilot for reasons of an emergency has to do a tight turn when on the approach, the aircraft comes into the light grey area of AoA.
Here the aircraft shall have the same yoke force resistance to increasing the AoA as before. It shall not suddenly be easier to pull the nose up (red curve).
The 737 MAX was a bit too easy to pull into a stall when flying with high AoA and making abrupt maneuvers. The larger engines for the MAX hung further forward from the wing, added a destabilizing aerodynamic area ahead of the center of gravity, destabilizing the pitch moment curve at high AoA.
Boeing and the certification authority, FAA, decided added margins was called for. Boeing added a pitch augmentation at high AoA called Maneuvering Characteristics Augmentation System, MCAS.
The aircraft should trim nose down to increase the stick force needed once it passed into the light grey area where the base aircraft had a region of less stability. Before the augmentation, the pilot felt if the aircraft wanted to fly into the stall, it got easier to increase the AoA after 12°AoA. With the augmentation the felt extra force was the same for the first and last part of the curve before the maximum lift was achieved at stall (and stall warning kicked in).
So far so good. It’s common an aircraft’s flight control system has fixes to stability margin changes in different parts of the flight envelope. The implementation for the 737 MAX had two problems, however:
As described in the previous Corner, 737 pilots are used to the aircraft trimming in the background both in manual and autopilot flight. They feel the trimming in the yoke when flying manually and see the trimming of the autopilot on the rotating trim wheel in the cockpit.
MCAS behaves the same once it’s activated. The only pilot which commented on background trimming during the three previous flights (which all seemed to have had the same problem) was the Captain on the Denpasar to Jakarta flight, the one before JT610. And he saw it as the normal trimming being confused by the speed and altitude measurement problems. He wrote in his log “ Airspeed unreliable and ALT disagree shown after takeoff. Speed Trim System also running to the wrong direction, suspected because of speed difference”.
He shut off the trim because he thought it better this way, but he did not identify the problem as a trim runaway. He thought it was Speed Trim working off bad information. Therefore he didn’t report trim runaway to the mechanics nor to his colleagues after the flight. Nor did any other of the five pilots who flew with the problem before JT610.
We can see how the fact the pilots were used to the aircraft trimming and the MCAS not behaving differently masked to the pilots the aircraft had a new system which could go wrong. The action to take for the unknown problem was not obvious. If it would have been, the six pilots before the fatal flight would have reacted, they didn’t.
To introduce MCAS was logical. Its reliance on a single signal trigger was probably accepted because the system covered a remote corner of the flight envelope. The action to counter any faults was deemed obvious. In practice, it wasn’t.