By: Prem Mahendranathan
The Boeing 737 MAX comes with a software called MCAS (Maneuvering Characteristics Augmentation System).
It is a longitudinal stability enhancement software that is designed to counteract non-liner lift produced by the aircraft’s engines nacelle. MCAS is not a stall prevention software.
Boeing 737MAX has two Large LEAP-1B (68″ diameter) engines attached slightly up and forward, than the CFM56-7 (61″ diameter) engines on its predecessor 737 NG.
Larger diameter nacelle and its location on the aircraft generate vortex flow and produces non-linear lift at high Angle of Attack (AoA)
As the AoA increases during takeoff and climb, this non-linear lift causes a slight pitch-up effect because the engines nacelle are in front of aircraft’s centre of gravity.
The Pitch up effect due to non-linear lift reduces stick force, and that makes the pilot to exert more back pressure on the yoke, leading the aircraft towards stall. ( Note: This is not allowed under FAR §25.173 “Static longitudinal stability”.)
MCAS software senses this condition, through one of the two AoA sensors called “Alpha Vane” located either side of the fuselage.
Then it directs the horizontal stabiliser trim to move the aircraft back to nose down position. Once the MCAS senses AoA falls below the stable threshold it will command the stabilizer trim to return the aircraft back to state before MCAS was activated.
MCAS activates without pilot’s command and only operates on manual and flaps up flight.
If the sensor or software is at fault, then the aircraft has a chance of crash.
FAR §25.173 Static longitudinal stability.
Under the conditions specified in §25.175, the characteristics of the elevator control forces (including friction) must be as follows:
(a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or VFC/MFC,whichever is appropriate, or lower than the minimum speed for steady unstalled flight.
(b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in §25.175 (a), (c), and (d), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in §25.175(b), when the control force is slowly released from any speed within the range specified in paragraph (a) of this section.
(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots.
(d) Within the free return speed range specified in paragraph (b) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.
[Amdt. 25-7, 30 FR 13117, Oct. 15, 1965]