What is Boeing 737 MAX MCAS?
When Boeing set out to develop the 737 Max, engineers had to find a way to fit a much larger and more-fuel efficient engine under the wing of the single-aisle jet’s notoriously low-riding landing gear. By moving the engine slightly forward and higher up and extending the nose landing gear by eight inches, Boeing eked another 14% improvement in fuel consumption out of the continually tweaked airliner.
That changed, ever so slightly, how the jet handled in certain situations. The relocated engines and their higher thrust caused an upward pitching moment — in essence, the Max’s nose was getting nudged skyward.
Boeing quietly added a new system “to compensate for some unique aircraft handling characteristics during it’s (sic) Part 25 certification” and help pilots bring the nose down in the event the jet’s angle of attack drifted too high when flying manually, putting the aircraft at risk of stalling, according to a series of questions and answers provided to pilots at Southwest Airlines, the largest 737 Max operator reviewed by The Air Current.
The Maneuvering Characteristics Augmentation System (MCAS) was designed to address this, according to Boeing engineers and pilots briefed on the system, now at the center of the inquiry into the crash of Lion Air 610, a brand new Boeing 737 Max 8. MCAS is “activated without pilot input” and “commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall.”
“Its sole function is to trim the stabilizer nose down,” according to the system’s description to pilots, who were learning about it for the first time this week.
The system activates when the sensed Angle of Attack (AOA) “exceeds a threshold based on airspeed and altitude.” That tilts the 737 Max’s horizontal stabilizer upward at a rate of .27 degrees per second for a total travel of 2.5 degrees in just under 10 seconds. How much the stabilizer moves depends on Mach number. At higher Mach the stabilizer moves less, at slower speeds it moves more. The trim system under MCAS is not stopped by simply moving the control yoke.
The LionAir Crash
Flight data recorder has revealed some facts about the crash which has shown that the alpa vane or angle of attack vane located on the nose of the max on the captain or first officer’s side was probably faulty which lead to erroneous data.
Failure of this component could lead to IAS disagree, altitude disagree, angle of attack disagree and possibly a stick shaker on the affected side.
The MCAS protection system possibly kicked in after sensing a stall and in an attempt to prevent the stall, trimmed the aircraft nose down.
If this is left unchecked can lead to a potential nose heavy situation where it becomes almost impossible to manually raise the nose.
Possible Preventive Actions
The system can be deactivated if pilots trim the aircraft manually to override the MCAS’s attempt to automatically pitch the jet’s nose down.
The memory item for stabilizer runaway is basically to turn off the stabilizer cut out switches located to the right of the engine start levers and this also would have stopped the stabilizer trim.
The MCAS hasn’t been properly communicated to pilots of the max fleet which is the reason the FAA came out with an emergency Air Worthiness Directive.
Sadly these pilots on that faithful day never had this information and the system wasn’t mentioned in the flight crew operations manual (FCOM) that governed the master description of the aircraft for pilots.
These systems have to be made known to the pilots flying these aircrafts because we as pilots always operate on situational awareness and technical situational awareness is also crucial, with this we can know why a system is behaving in a particular way and the possible ways to fix it.
This is an explanation of the rumors that’s floating around presently and is subject to further corrections. Please feel free to drop your opinions.