The Boeing 737 Max crisis
edge and the trailing edge of the wing) and the direction of the relative wind. ’ 43
Stall is a phenomenon where ‘ the trailing edge of the wings no longer have 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. 44 To tackle this potentially fatal flaw, Boeing engineers came up
with a system called the Manoeuvring Characteristics Augmentation System (MCAS). This system relies on the readings from the AoA probes on each side of the aircraft. If the AoA sensors show a steep angle, the system will trigger an Automatic Nose Down (AND) by manipulating the angle of the horizontal stabilizer mounted at the back of the aircraft. When the horizontal stabilizers are trimmed downwards, it pitches the aircraft up; and when the horizontal stabilizers are trimmed upwards, it pitches the aircraft down. 45
Figure 10: Stall
Collins Aerospace, the company which has been developing avionics for B737 aircraft since 2003, are also responsible for providing the MCAS software. Since AoA probes are susceptible to damage from external objects easily, for example, bird strikes and hailstones, it is possible that these sensors show erroneous data. Previous B737 aircraft had some type of AoA indicator in the cockpit, directly linked to the external sensors.
Figure 11: The AoA probe of an aircraft
In fact, the Federal Aviation Administration (FAA) issued an Airworthiness Directive (AD) for Cirrus SF50 jets on 16 th April 2019 regarding AoA issues: there were three reports ‘ where stall warnings and stick shakers were activated by normal systems in a normal flight ’. 46 Unlike the B737 MAX, the Cirrus system of Stall Warning and Protection System (SWPS), or Electronic Stability & Protection System (ESP), can be used to intervene by the pilots quickly, thus resulting in no casualties. The aircraft were ungrounded after software fixes. 47 However, when selling the B737 MAX aircraft, Boeing decided to sell these cockpit indicators separately for $80,000 on each aircraft, ‘as part of an ‘ Error Notification Kit ’ […] Southwest were offered this same option at $50,000’ . 48 This means that pilots might not know whether or not they are experiencing stall, especially when visual references are unavailable. At the same time, ‘ Boeing made it a standard requirement on all MAX airplanes with MCAS, that an AoA Disagree warning should be displayed in the cockpit whenever the AoA sensors disagree by 10° for ten seconds. However, […] Collins Aerospace […] tied the AoA Warning to the $80,000 kit. ’ As a result, more than 80% of the MAX operators 49 did not
43 Anderson et al. 2010, Understanding Flight, 2 nd ed : 13. 44 Coughlin 2020, Crashing the 737 MAX, Revision 1 : 10. 45 JxJ AVIATION 2020, What is a Trimmable Horizontal Stabilizer (THS)? : 6:16. 46 Wikipedia 2022, Cirrus Vision SF50 .
47 Calderwood 2019, Cirrus SF50 Vision jet grounded . 48 Coughlin 2020, Crashing the 737 MAX, Revision 1 : 12. 49 Coughlin 2020, Crashing the 737 MAX, Revision 1 : 13.
115
Made with FlippingBook - Online catalogs