Semantron 23 Summer 2023

The Boeing 737 Max crisis

3. Electric Trim: This is a mode of trimming. Pilots need to make an input, for example, typically a thumb-control switch to either trim the horizontal stabilizers up or down. This is widely used on Boeing aircrafts. It sends an electric signal to the jackscrew (the actuator) at the back of the aircraft and performs the desired action. On Airbus aircrafts, the aircraft is automatically trimmed by the fly-by-wire system.

Figure 22: Captain’s yoke on a B737 aircraft. T he thumb-control trim is shown on the top left corner of the yoke.

4. Leading and Trailing Edge: Leading edge is the smooth edge on a wing that comes in contact with air first, while Trailing edge is the rear edge of a wing, where airflow separated from the leading edge meet.

Figure 23: Depiction of the leading edge, trailing edge, camber and chord line.

Figure 24: A trim wheel on a B737 aircraft. Pilots can unfold the handle if manual wheel trim is needed. 5. Manual Wheel Trim: This is a mode of trimming. Typically on a Boeing 737 aircraft, the primary mode of trimming is using electric trim. Wheel trim acts as a secondary method of trimming. Pilots can rotate the trim wheel, and it uses the mechanical effect to trim the horizontal stabilizers.

6. Memory Items: In an emergency, there are certain actions that need to be done immediately to prevent loss of aircraft control. In these cases, there will not be time for pilots to reference to the QRH. Therefore, as part of their training, all pilots are required to memorize some of the immediate actions listed on the QRH. Some examples are runaway stabilizer, stabilizers inoperative, rejected takeoff, engine fire, rapid depressurization and more.

Figure 25: The Stab Trim Cut-out switches on a B737 aircraft. It is located in the centre column in the cockpit. 7. Stab Trim Cut-out: This is a set of switches on B737 aircrafts, which disables electric trim, meaning that pilots will have to manually use the trim wheel. On B737 MAX aircrafts, enabling Stab Trim Cut-out is the only way to stop MCAS from intervening normal operations, as MCAS works by sending AND or ANU commands to the electric jackscrew.

8. Runaway Stabilizer: It describes the continuous uncommanded stabilizer trim movement, according to the QRH.

9. Stall: A phenomenon when ‘ the trailing edge of the wings no longer has enough smooth air flowing over them ’. This occurs when the speed and pitch of the aircraft combine to disrupt airflow, causing not enough lift to overcome the weight.


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