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

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  • $\begingroup$ No, it does not. Still looking for an actual source though. $\endgroup$ – DeepSpace Aug 2 '17 at 8:37
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    $\begingroup$ @DeepSpace I found some discussion on airliners.net which cite the mechanical linkage as being enough to control the plane without hydrolics $\endgroup$ – ratchet freak Aug 2 '17 at 9:07
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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 is pretty similar 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.

enter image description hereImage source

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    $\begingroup$ I'd be pretty nervous to fly on a plane with schematics labeled in Comic Sans. $\endgroup$ – Undo Aug 3 '17 at 2:10
  • $\begingroup$ I'm pretty sure the battery only says BATTERY. $\endgroup$ – Koyovis Aug 3 '17 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 '17 at 11:57
  • $\begingroup$ @RalphJ Thx, have included into answer. $\endgroup$ – Koyovis Aug 5 '17 at 5:30
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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 '17 at 18:44

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