To the uneducated reader it seems strange the Boeing 737 MAX 8 MCAS system only operates when autopilot is off. I understand the reasoning behind why it was implemented to operate when autopilot is off. My question is, is the operation of MCAS automated system when autopilot is off an exception for the 737 line, or are there other automated systems providing direct control surface actuator inputs when autopilot is off on the Boeing 737s? If so, what are these?
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Are there other automated systems providing direct control surface actuator inputs when autopilot is off on the Boeing 737s?
One such system on the 737 Classic and NG is the Speed Trim System:
As you can see above, when certain conditions are met, of which 5 seconds has passed since a pilot trim input is made, the STS trims the plane if required.
The system's classification would be stability augmentation system. When light, rear loaded, and with plenty of power (during the climb phase of the takeoff or during a go-around), the 737 has a tendency to pitch (nose) up (since the engines are below the wing and below the center of mass); the STS would limit that.
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$\begingroup$ So the Speed Trim System responds to the airspeed and not the Angle of Attack? $\endgroup$– DLHMar 19, 2019 at 16:19
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1$\begingroup$ @DLH: As it reads: Stab pos, speed, V/S, and thrust lever position. $\endgroup$– user14897Mar 19, 2019 at 16:21
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2$\begingroup$ It might be worth adding that MCAS does differ from the other systems that make automatic changes to trim and existed on earlier versions in that all the other systems can be overridden by simply making opposite input, while MCAS does not (for obvious reasons as opposing excessive pitch up command is the purpose, but it is a significant difference). … $\endgroup$ Mar 20, 2019 at 21:40
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1$\begingroup$ … See avherald.com/h?article=4c534c4a: “The source added the B737 NG simulator was not able to reproduce different trim handling by both aircraft: on the NG aircraft automatic trim (e.g. by the SRS) could be counteracted by an (intuitive) elevator opposite control input (e.g. on a nose down trim a nose up elevator input would stop and disable the autotrim system) unless a double failure was inserted by the sim instructor whereas on the MAX the intuitive counter acting elevator input no longer stops the automatic trim in order to permit MCAS to work.” $\endgroup$ Mar 20, 2019 at 21:40
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$\begingroup$ Thanks. Would be great to have a fairly complete list of such systems in the same answer. Question: the Boeing STS the equivalent of an autothrottle or does it also have a separate autothrottle (apparently autothrottles will sometimes make pitch changes)? Also @user71659's answer raised yaw damping, which does appear to be an always on separate system on the 737 - b737.org.uk/theruddersystem.htm. $\endgroup$– spinkusMar 21, 2019 at 0:02
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All large swept-wing aircraft have a yaw damper, which is always active. It takes an input from a yaw gyro and applies rudder inputs in order to counter Dutch roll.
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Many aircraft have stick pushers. These push the stick or yoke forward to prevent a stall and that actuates the elevator nose down. For example, take a look at the Bombardier Q400 or this FAA AC:
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1$\begingroup$ This answer seems almost completely unrelated to the question in its current form. $\endgroup$– 0xddMar 19, 2019 at 16:57
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$\begingroup$ The 737 Max is not exceptional in operation, in that many aircraft use a stick pusher when the autopilot is off. $\endgroup$– AdamMar 19, 2019 at 20:04
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2$\begingroup$ A stick pusher is explicitly (and very) different from the MCAS. $\endgroup$– 0xddMar 19, 2019 at 20:05