by Erik Dolson
Boeing has released information to airlines on how to convince customers and crew that the 737 Max planes are “safe” after two crashed, killing 346 people. One point made was that passengers are more emotional than rational:
Perhaps Boeing should focus on fixing the planes and telling the truth, rather than manipulating emotions.
Most aircraft fly in a “balance” of forces acting on wings and tail that rotate the plane around the “center of lift.” At any given speed through the air, wings push up with a certain force, and the tail pushes with a different force. Change the speed of air over wings and the balance will change, the plane will rotate up or down, finding a new equilibrium.
If the nose rises too far and the angle of the wing to the air flow (angle of attack) is too great, the wing will stop flying. This is called a “stall.” Usually, the main wing will stall before the tail, which will cause the nose to drop, the angle of attack to improve, airspeed to increase, and the wing (and plane) plane can start flying again.
Boeing put new engines on the 737 Max, but had to move them forward on the wing. Engine pods have their own lift. Being farther forward, there was more of a “lever arm” of lift from the engines, and this changed the balance between wing and tail, especially at high angles of attack.
It’s possible that the new configuration allowed the main wing to have more lift and not stall before the tail. If the main wing does not stall before the tail, neither wing nor tail can provide control. The airplane could fall out of the sky.
Or, it’s possible that at a certain angle of attack, lift from the engine pods might overwhelm the control of the tail surfaces, causing the nose to suddenly flip up.
In designing the plane, one solution would have been to change the wing. But a new wing would have required new certification, higher costs and delay. So Boeing installed the Maneuvering Characteristics Augmentation System (MCAS) software that prevented the plane from approaching extremely high angles of attack.
In most planes, including the older 737s, if a plane’s nose drops one of the first things a pilot will do is pull back on controls to bring the nose up and increase power to gain more lift from the main wing. This appears to have happned in the two tragedies.
But changes Boeing made to the behavior of the new planes were not highlighted, nor were recovery procedures if the software was misbehaving. In those situations, pilots had to turn off the new software before they could regain control of their airplane because the software pushing the nose down was stronger than pilots.
It appears pilots of the two doomed 737 Max planes did not know this because Boeing did not want airlines to have to retrain pilots to fly the new Max. This would have increased airline costs and made the new plane less competitive, so Boeing downplayed the impact of the software and did what they could to avoid calling attention to this new characteristic, both with their airline customers and the FAA.
Boeing also included a single angle-of-attack sensor as standard equipment on the 737 Max, despite being required by military buyers to install three sensors. Airbus planes also have three sensors. The reason for three is that if two sensors disagree, which one is right? Boeing said the pilots themselves would be the “redundant” system, unless airlines wanted to spend the money on an additional sensor. Add-ons were a profit center for the plane maker.
One has to think that if Boeing, one of the largest manufacturing companies in the world, could fix this issue with a software tweak that would have been done long ago, and 737 Max planes that were piling up before Boeing ended production a couple of weeks ago would be on their way to customers. Why hasn’t this happened?
One guess is that a software tweak won’t fix a fundamental problem with the plane and the placement of the new engines. Will it still fly if computer systems failed, the MACA system was not there to babysit, and the plane encountered a condition of extremely high angle of attack where lift of engine pods destabilized the plane to the point of loss of control?
Would a plane built to those specifications be allowed to fly passengers?
If not, remember that Boeing believes “a human connection will be more effective than rational appeals.” The company may need a hug.
(I welcome comments on this topic by professional pilots, especially test pilots and/or aeronautical engineers)
