I am travelling with Thomson on a Boeing 737-800 and wondered if it had a RAT?


1 Answer 1


No it does not, it does not need one, there is a mechanical connection to the flight controls that can be used if all else fails.

  • The B737 flight controls are hydraulically powered.
  • There are three hydraulic systems: System A, System B, and Standby. Only one main system (A or B) is required for hydraulically flying the aircraft, during normal operation they are both operational.
  • The two main hydraulic systems have an Engine Driven Pump (EDP), which can continue delivering hydraulic pressure when the associated engine is windmilling. All three hydraulic systems are also powered by their own Electric Motor Driven Pump (EMDP).
  • In case of dual engine failure the APU can power the electrical systems for the EMDPs, still delivering full hydraulic power.
  • If the fuel has run out and the APU cannot operate, two batteries provide at least 60 minutes of backup power for the electrical systems. The EDMPs can be powered in this stage, however they provide a high load.
  • If all fuel is gone and the batteries are depleted, the aircraft can be flown by hand, directly overcoming the aeroforces by pulling hard! This is called manual reversion.
  • In manual reversion, the aileron trim tabs now function as geared tabs, assisting in overcoming the aeroforces. Elevators will have high aeroforces, high friction forces, and freeplay around centre point. Stabiliser trim wheels provide additional pitch control. The rudder has no manual reversion.

The A320 flight control system are powered in a pretty similar way except for the manual reversion - as a last resort the RAT is deployed for powering the backup hydraulic system. The A320 has no direct mechanical link to the elevators and ailerons, only to the rudder.

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  • 36
    $\begingroup$ I'd be pretty nervous to fly on a plane with schematics labeled in Comic Sans. $\endgroup$
    – user14
    Aug 3, 2017 at 2:10
  • $\begingroup$ I'm pretty sure the battery only says BATTERY. $\endgroup$
    – Koyovis
    Aug 3, 2017 at 5:02
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    $\begingroup$ Small point: with no generators & no APU, the battery/batteries do NOT power the standby hydraulic pump. That'd be way too much load. If it's only loss of all generators, you have the EDP's to power hydraulics; if both engines have failed with the APU deferred (boy are you having a bad day) you'd still have some hydraulic pressure from the fans windmilling. And, bright side, no asymmetric thrust to counter with the rudder in that scenario! $\endgroup$
    – Ralph J
    Aug 3, 2017 at 11:57
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    $\begingroup$ Interesting... there have been a few airliner crashes resulting from loss of all hydraulic fluid, and consequent loss of control. They could have used a manual reversion system. A bit of control is better than no control. Quite a few A10's made it home on manual reversion when their hydraulics were shot out. $\endgroup$
    – tj1000
    Aug 5, 2017 at 17:17
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    $\begingroup$ @tj1000 Yes UA232. B737 would be about the largest aircraft where manual reversion is an option. $\endgroup$
    – Koyovis
    Aug 5, 2017 at 18:44

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