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== Further reading ==
== Further reading ==
* {{Cite web |url= https://www.boeing.com/commercial/737max/737-max-software-updates.page |title= The 737 MAX MCAS Software Enhancement |publisher= Boeing}}
* {{cite news |url= https://www.washingtonpost.com/business/2019/05/06/safety-was-just-given-inside-boeings-boardroom-amid-max-crisis/ |title= ‘Safety was just a given’: Inside Boeing’s boardroom amid the 737 Max crisis |author= Douglas MacMillan |date= May 5, 2019 |work= Washington Post}}
* {{cite news |url= https://www.washingtonpost.com/business/2019/05/06/safety-was-just-given-inside-boeings-boardroom-amid-max-crisis/ |title= ‘Safety was just a given’: Inside Boeing’s boardroom amid the 737 Max crisis |author= Douglas MacMillan |date= May 5, 2019 |work= Washington Post}}
* {{cite news |url=https://www.seattletimes.com/business/boeing-aerospace/engineers-say-boeing-pushed-to-limit-safety-testing-in-race-to-certify-planes-including-737-max/ |title= Engineers say Boeing pushed to limit safety testing in race to certify planes, including 737 MAX |date= May 5, 2019 |author1= Dominic Gates |author2= Mike Baker |work= The Seattle Times}}
* {{cite news |url=https://www.seattletimes.com/business/boeing-aerospace/engineers-say-boeing-pushed-to-limit-safety-testing-in-race-to-certify-planes-including-737-max/ |title= Engineers say Boeing pushed to limit safety testing in race to certify planes, including 737 MAX |date= May 5, 2019 |author1= Dominic Gates |author2= Mike Baker |work= The Seattle Times}}

Revision as of 04:56, 25 May 2019

The MCAS is under scrutiny after the Ethiopian Airlines Flight 302 crash

The Maneuvering Characteristics Augmentation System (MCAS) is a flight envelope protection system developed for the Boeing 737 MAX to provide similar handling qualities to previous versions. It lowers the nose without pilot action when it determines the aircraft pitch attitude is too nose-high, based on input from airspeed, altitude and angle of attack (AoA) sensors. However, it is susceptible to erroneous activation, as evidenced in the deadly crashes of Lion Air Flight 610 and Ethiopian Airlines Flight 302. The 737 MAX is indefinitely grounded until regulators believe the aircraft is airworthy pending a software update.

Description

Purpose

The engines of the Boeing 737 MAX are larger than in previous models and are extended further forward and higher up. Their nacelles have an increased aerodynamic effect, which increases the tendency to pitch up at high angles of attack (AOA). The MCAS is intended to mimic the pitching behaviour of previous models, in order to enhance handling characteristics when flying at a high angle of attack, and thus avoid the need for significant pilot retraining.[1][2][3]

The flight crew operations manual (FCOM), the basis for airlines' documentation and training, does not mention the MCAS. According to Boeing, "a pilot should never see the operation of MCAS" in normal flying conditions,[1] and it is "fundamentally embedded in the handling qualities" and "not a separate system to be trained on",[3] hence there is no need to inundate average pilots with more information than they need or can digest.[4] In the rare cases when the system activates, its operation will "likely go unnoticed by the pilot."[1]

It is thus distinct from an anti-stall device, such as stick pusher which physically moves the pilot's control column forward when the airplane is approaching a stall: Boeing CEO Dennis Muilenburg said "it has been reported or described as an anti-stall system, which it is not. It's a system that's designed to provide handling qualities for the pilot that meet pilot preferences."[3]

Operation

The MCAS detects when the aircraft is approaching a high AOA condition and retrims the horizontal stabilizer to pitch the nose back down again and restore the need for pilot force on the control column to achieve high AOA.[1] The system is explicitly designed to override pilot action and prevent them from regaining control authority, in order to avoid an inadvertent stall.[5]

Sensors on the aircraft provide airspeed, altitude and AOA information. MCAS uses these to calculate the aircraft flight condition. In the case of the AOA sensors mounted on the aircraft's exterior, MCAS polls them alternately to update its data and acts on input from only the current one.[6][1] In the event that an AOA sensor developed a fault, an AOA "disagree" alert was intended to display. However it was mistakenly made dependent on the presence of an optional AOA monitor display, and as such could not function If the option was not installed.[7][8] The option was only ordered by 20% of the airlines, so most aircraft do not have a functioning disagree alert.[9]

If the calculated flight condition indicates an approaching stall, MCAS then trims the tailplane nose down.[5][1]

The older 737 cockpit has separate on/off control switches for independent electrically-assisted and automatic trim systems. On the 737 MAX, a combined switch is provided and the pilot cannot turn off the MCAS without also disabling electronic trim. A manual trim wheel is provided, but is not powerful enough to adjust the stabilizer in all flight conditions. Activating the powered trim system can be necessary and this also activates the MCAS.[10]

Software upgrade

A software upgrade has been developed to fix some perceived design issues. It "ensures the system takes input from two sensors, instead of one", and notifies pilots in case of a sensor failure. The AOA-disagree alert will be displayed even if the AOA indicator option is not installed. MCAS will "activate only once, not multiple times, if the sensor reading remains stuck at a high value". It limits the trim authority of the MCAS so that "the pilot can always pull back on the control column with enough force to counteract any automatic nose-down movement".[11][12][13] MCAS activation requires both sensors to be in agreement within 5.5 degrees of each other.[14]

Scrutiny

Altitude and speed of Lion Air Flight 610

The MCAS has been under scrutiny following the fatal crashes of Lion Air Flight 610 and Ethiopian Airlines Flight 302 soon after takeoff and a number of functional issues have been raised.[15][16]

The use of a single AOA sensor at any one time creates a single point of failure and leads to the possibility that a single malfunctioning sensor could produce erroneous data, cause the MCAS system to pitch the nose downward and force the aircraft into a dive.[17][1]

Due to the amount of trim the system applies to the horizontal stabilizer, aerodynamic forces increase the pilot control effort needed to raise the nose. As long as the faulty AOA readings persist, a human pilot "can quickly become exhausted trying to pull the column back".[5]

Boeing and the FAA decided that cockpit displays of the AOA and an AOA disagree alert, which signals if the sensors give different readings, were not critical features for safe operation and could be considered optional.[18] Boeing charged extra for these alerts.[19][11]

It has been suggested that features of the MCAS system and failure to operate as intended may have contributed to the crashes which led to the grounding.[20][21]

Boeing CEO Dennis Muilenburg has stated that there was "no surprise, or gap, or unknown here or something that somehow slipped through a certification process."[22] On April 29, 2019 he stated the design of the aircraft was not flawed and reiterated that it was designed per Boeing's standards.[23] Nevertheless the aircraft have been grounded pending two crash investigations and the software upgrade remains under scrutiny by the FAA. A veteran software engineer and experienced pilot suggested that software changes may not be enough to counter the 737 MAX's engine placement.[24]

Software certification

MCAS software modifications identified before the accidents were completed on March 28.[25] However a delay was announced on April 1.[26] The changes to the software and pilot type rating requirements will be independently reviewed the FAA and regulators in other countries. Canada and the European Union stated that they will conduct their own evaluations.[27] The Civil Aviation Administration of China stated that it would independently review software updates.[28]

On May 16, 2019, Boeing announced that the new flight software has completed 360 hours of flight testing and is awaiting approval from the FAA.[29]

Operational conditions

The Air Line Pilots Association (ALPA), the largest pilot union, is recommending MCAS training to be incorporated into routine flight simulator sessions that are part of pilot recurrent training, although it is not asking for initial simulator training on the MCAS before returning the aircraft to service. The FAA's Flight Standardization Board (FSB) draft report recommends computer-based training, rather than simulator training to transition from the 737 Next Generation.[30]

On May 17, after discovering 737 MAX flight simulators could not adequately replicate the crashes conditions,[31] Boeing corrected their software to improve the force feedback of the manual trim wheel and to ensure their realism.[32] Only Air Canada and Boeing have a MAX simulator, Southwest Airlines and American Airlines both expect to receive their first MAX simulator in late 2019.[33] While the 737 NG and MAX simulators are different, Boeing would prefer to limit difference training to be computer-based, also deemed sufficient for the US Airline Pilots Association or Southwest Airlines pilots, but simulator time seems necessary for American Airlines pilots and Transport Canada.[34]

References

  1. ^ a b c d e f g Ostrower, Jon (November 13, 2018). "What is the Boeing 737 Max Maneuvering Characteristics Augmentation System". The Air Current. Retrieved March 14, 2019.
  2. ^ Bazley, Tarek (March 11, 2019). "Control system under scrutiny after Ethiopian Airlines crash". Al Jazeera.
  3. ^ a b c Zhang, Benjamin (29 April 2019). "Boeing's CEO explains why the company didn't tell 737 Max pilots about the software system that contributed to 2 fatal crashes". Business Insider.
  4. ^ "Boeing Withheld Information on 737 Model, According to Safety Experts and Others". Wall Street Journal. November 13, 2018.
  5. ^ a b c Travis, Gregory (April 18, 2019). "How the Boeing 737 Max Disaster Looks to a Software Developer". IEEE Spectrum.
  6. ^ Baker, Mike; Gates, Dominic (March 26, 2019). "Lack of redundancies on Boeing 737 MAX system baffles some involved in developing the jet". The Seattle Times.
  7. ^ Gelles, David; Kitroeff, Natalie (2019-05-05). "Boeing Believed a 737 Max Warning Light Was Standard. It Wasn't". The New York Times. ISSN 0362-4331. Retrieved 2019-05-11.
  8. ^ "FAA considered grounding some Boeing 737 Max planes last year: source". news.yahoo.com. Retrieved 2019-05-11.
  9. ^ Gelles, David; Kitroeff, Natalie (2019-05-05). "Boeing Believed a 737 Max Warning Light Was Standard. It Wasn't". The New York Times. ISSN 0362-4331. Retrieved 2019-05-11.
  10. ^ Mike Baker and Dominic Gates (May 10, 2019). "Boeing altered key switches in 737 MAX cockpit, limiting ability to shut off MCAS". The Seattle Times.
  11. ^ a b Tabucho, Hiroko; Gelles, David (March 21, 2019). "Doomed Boeing Jets Lacked 2 Safety Features That Company Sold Only as Extras". The New York Times. Retrieved March 21, 2019.
  12. ^ "Boeing to upgrade 737 Max flight control software". Flight Dashboard. FlightGlobal. March 12, 2019. Retrieved March 12, 2019.
  13. ^ Johnson, Eric M (November 29, 2018). "Boeing eyes Lion Air crash software upgrade in 6-8 weeks". Reuters.
  14. ^ "Boeing details changes to MCAS for 737 Max". The Air Current. 2019-03-27. Retrieved 2019-05-24.
  15. ^ Gates, Dominic (March 17, 2019). "Flawed analysis, failed oversight: How Boeing and FAA certified the suspect 737 MAX flight control system". The Seattle Times.
  16. ^ Fehrm, Bjorn (2019-04-05). "Bjorn's Corner: ET302 crash report, the first analysis". Leeham News and Analysis.
  17. ^ Baker, Mike; Gates, Dominic (March 26, 2019). "Lack of redundancies on Boeing 737 MAX system baffles some involved in developing the jet". The Seattle Times.
  18. ^ Freed, Jamie; Johnson, Eric (November 30, 2018). "Optional warning light could have aided Lion Air engineers before crash: experts". Reuters.
  19. ^ Newburger, Emma (March 21, 2019). "Crashed jets reportedly lacked key safety features because Boeing charged extra for them". CNBC. Retrieved March 26, 2019.
  20. ^ Mike Baker and Dominic Gates (May 10, 2019). "Boeing altered key switches in 737 MAX cockpit, limiting ability to shut off MCAS". The Seattle Times.
  21. ^ Sean Broderick (May 10, 2019). "Ethiopian MAX Crash Simulator Scenario Stuns Pilots". Aviation Week Network.
  22. ^ Daniel McCoy (Apr 24, 2019). "Boeing CEO: Nothing slipped through in original 737 MAX certification". Wichita Business Journal.
  23. ^ Dominic Gates (Apr 29, 2019). "Facing sharp questions, Boeing CEO refuses to admit flaws in 737 MAX design". Seattle Times.
  24. ^ George Leopold (March 27, 2019). "Software Won't Fix Boeing's 'Faulty' Airframe". EE Times.
  25. ^ "Boeing Releases Software Update, Other Changes for Embattled 737 MAX 8". interestingengineering.com. March 28, 2019.
  26. ^ Gelles, David (April 1, 2019). "Boeing Fix for 737 Max Software Is Delayed". NYTimes.
  27. ^ Reints, Renae (March 21, 2019). "Canadian and European Regulators to Certify Boeing 737 Max Changes in Addition to FAA". Fortune.
  28. ^ "Chinese regulator suspends airworthiness certificate of the Boeing 737 MAX". Airlinerwatch. March 27, 2019.
  29. ^ Wallace, Anna-Maja Rappard and Gregory. "Boeing says it has completed 737 Max software fix". CNN. Retrieved 2019-05-17.
  30. ^ Sean Broderick (May 8, 2019). "ALPA: Simulator Time Not Needed To Un-Ground 737 MAX". Aviation Week Network.
  31. ^ Natalie Kitroeff (May 17, 2019). "Boeing 737 Max Simulators Are in High Demand. They Are Flawed". The New York Times.
  32. ^ "Boeing says it has corrected simulator software of 737 MAX jets". Reuters. May 17, 2019.
  33. ^ "Boeing acknowledges flaw in 737 MAX simulator software". Agence France-Presse. May 19, 2019.
  34. ^ Scott Hamilton (May 20, 2019). "Pontifications: Mandate sim time for MAX return to the skies". Leeham News.

Further reading

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