Note: Loss of the system A engine-driven hydraulic pump, and a heavy demand on system A, may cause A/P A to disengage.

Just finding it difficult to understand why the A/P would disengage in this case.

  • $\begingroup$ If they are hydraulic actuators then the A/P may well disengage when it commands a movement and doesn’t get feedback that it happened. $\endgroup$
    – Jim
    Jun 4, 2022 at 19:52
  • $\begingroup$ Doesn't the 737 have a hydraulically powered pitch feel system? $\endgroup$
    – John K
    Jun 4, 2022 at 19:57
  • $\begingroup$ @Jim that would require multiple simultaneous failures, which aren't catered to normally. $\endgroup$
    – John K
    Jun 4, 2022 at 19:59
  • $\begingroup$ @Jim that makes sense, but that means Either A/P would disengage not just A? $\endgroup$ Jun 4, 2022 at 22:50
  • $\begingroup$ Not necessarily. The system I worked on (not commercial jet) had two actuators in series controlled by different systems (perhaps system B here) which means hydraulic pump from system B might still be working allowing A/P B to still send commands to its actuators. $\endgroup$
    – Jim
    Jun 4, 2022 at 23:17

1 Answer 1


The Boeing 737 has two independent autopilots that use two independent hydraulic servos for controlling the aircraft:

Autopilot Flight Director System (AFDS)

The AFDS is a dual system consisting of two individual flight control computers (FCCs) and a single mode control panel. The two FCCs are identified as A and B. For A/P operation, they send control commands to their respective pitch and roll hydraulic servos, which operate the flight controls through two separate hydraulic systems.

(Boeing 737 NG FCOMv2 4.20.1 - Automatic Flight - System Description, emphasis mine)

You can see the two autopilots A and B connected to hydraulic systems A and B in the hydraulic system overview:

737 NG Hydraulic System

(Boeing 737 NG FCOMv2 13.20.1 - Hydraulics - System Description)

Without the engine-driven hydraulic pump, the respective hydraulic system relies on the AC electrical pump to pressurize the system:

A and B Hydraulic System Pumps

Both A and B hydraulic systems have an engine–driven pump and an AC electric motor–driven pump. [...] An engine–driven hydraulic pump supplies approximately 4 times the fluid volume of the related electric motor–driven hydraulic pump.

(Boeing 737 NG FCOMv2 13.20.2 - Hydraulics - System Description)

Since the engine-driven pump can supply 4 times the fluid volume of the electric pump, the system pressure can drop during times of heavy demand (e.g. gear operation or flap extension/retraction) when only the electric pump is available. The respective autopilot servos can therefore (temporarily) not function as intended and the autopilot will disengage itself.

This could in principle affect both system A and B. However, the PTU (power transfer unit) can use system A hydraulic pressure to assist system B:

Power Transfer Unit

The purpose of the PTU is to supply the additional volume of hydraulic fluid needed to operate the autoslats and leading edge flaps and slats at the normal rate when system B engine–driven hydraulic pump volume is lost. The PTU uses system A pressure to power a hydraulic motor–driven pump, which pressurizes system B hydraulic fluid. The PTU operates automatically when all of the following conditions exist:

  • system B engine–driven pump hydraulic pressure drops below limits
  • airborne
  • flaps are less than 15 but not up.

(Boeing 737 NG FCOMv2 13.20.4 - Hydraulics - System Description)

Since these conditions are met during flap and slat operation (time of heavy demand on system B), the PTU would likely provide enough pressure to keep autopilot B operational after engine-driven pump 2 has failed. Unlike the PTU on the A320, the system does not work in the other direction. Low pressure in system A is not increased by the PTU and therefore autopilot A is at a higher risk of failing during electric hydraulic pump operation.

  • $\begingroup$ What a Rube Goldberg design that system is. Cobbled together by designers who weren't quite comfortable with going all hydraulic. That had to wait for designs like the 767. $\endgroup$
    – John K
    Jun 5, 2022 at 17:47
  • $\begingroup$ Well, 737 is an archaic relic, and recently Boeing has not been very impressive with engineering and manufacturing anyway. $\endgroup$
    – Jpe61
    Jun 5, 2022 at 19:12
  • $\begingroup$ The diagram and the description you quoted seems to suggest that the 737 PTU only feeds the slats/LE flaps, not the whole B system. I would thus naively expect that the PTU would have no effect in terms of compensating for heavy demand like extending the (TE) flaps and keeping the B autopilot operational. Did I misunderstand something? $\endgroup$
    – TooTea
    Jun 6, 2022 at 8:16
  • $\begingroup$ @TooTea That's correct, but the time of highest demand is when the LE and TE devices are moving at the same time (flap lever positions 1 to 10). Afterwards, only TE devices extend and the PTU is no longer operating (starting at position 15). It would seem that the electric hydraulic pump is capable of providing enough fluid volume for TE flap extension and autopilot B at the same time. $\endgroup$
    – Bianfable
    Jun 6, 2022 at 10:41

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