The concern was that an increased risk of the nose being pushed up at low airspeeds could cause the plane to get closer to the angle at which it stalls, or loses lift, Mr. Ludtke said. After weighing many possibilities, Mr. Ludtke said, Boeing decided to add a new program — what engineers described as essentially some lines of code — to the aircraft’s existing flight control system to counter the destabilizing pitching forces from the new engines. That program was M.C.A.S. M.C.A.S., according to an engineer familiar with the matter, was written into the so-called control law, the umbrella operating system that, among other things, keeps the plane in “trim,” or ensures that the nose is at the proper angle for the plane’s speed and trajectory. In effect, the system would automatically push the nose down if it sensed that the plane’s angle was creating the risk of a stall. Both M.C.A.S. and the so-called speed trim system — the automatic stabilizer controls used on the 737 NG and earlier versions — operate primarily via the horizontal section of the 737’s tail fin, which consists of a relatively narrow “elevator” in the back and a larger surface called a stabilizer in the front. In manual flight, pilots move the nose up and down by pulling or pushing on a control column, also called a yoke, to pivot the elevator one way or the other. Ordinarily, the stabilizers accomplish a more subtle task, making sure that the up or down forces on the tail keep the plane balanced around its center of gravity. Either pilot can control the stabilizers electrically using switches at the top of the yoke. M.C.A.S. was written to use the stabilizers in a different way. The modified system’s first task was to automatically offset the stall risk created by the change in the size and location of the engines. “M.C.A.S. was necessary then for the airplane to be certified by the F.A.A. to have met all of the regulatory design requirements for stability and control,” Mr. Ludtke said.
Pilots were not retrained to fly FAA’s upgraded 737 Max after the company told the Federal Aviation Authority the craft handled like its predecessor, and the decision has become centre of attention in the investigation of October’s crash of a Lion Air 737 Max, which killed 189 people. FAA and the Boeing had also decided not to tell pilots about a control system change which offset a stalling risk created by the size and placing of the aircraft’s engines. The cockpit voice recorder recovered from the wreckage is being examined, along with a history of maintenance problems at Lion Air.