29

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

300D7309EF17
  • 2,215
  • 9
  • 21
L.Halfpenny
  • 1,707
  • 4
  • 18
  • 27

1 Answers1

45

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.

enter image description hereImage source

Koyovis
  • 61,680
  • 11
  • 169
  • 289
  • 36
    I'd be pretty nervous to fly on a plane with schematics labeled in Comic Sans. –  Aug 03 '17 at 02:10
  • I'm pretty sure the battery only says BATTERY. – Koyovis Aug 03 '17 at 05:02
  • 4
    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! – Ralph J Aug 03 '17 at 11:57
  • @RalphJ Thx, have included into answer. – Koyovis Aug 05 '17 at 05:30
  • 2
    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. – tj1000 Aug 05 '17 at 17:17
  • 3
    @tj1000 Yes UA232. B737 would be about the largest aircraft where manual reversion is an option. – Koyovis Aug 05 '17 at 18:44
  • @Koyovis, lot of non comparable things. « The A320 flight control system is pretty similar except for the manual reversion »: no A320 has FBW flight controls so totally different even though requiring the hydraulics. « the A320 has no direct mechanical link to the elevators and ailerons, only to the rudder »: true but incomplete, in last resort (total electrical power loss) it has too a mechanical command to the stab trim, it operates directly the hydraulic valves and is very soft to use, to the contrary of the 737-800 which has very small trim wheels, very hard to drive the trim screw – user40476 Jun 11 '19 at 12:26
  • @user40476 Yes indeed, fly by wire. The hydraulic bit is very similar though, with its Green and Blue and Yellow hydraulic systems. Rudder and trim wheels to stab trim are almost identical. – Koyovis Jun 11 '19 at 13:49
  • @Koyovis. , no , the trim wheels operations are totally different: the 737 wheel should drive directly the trim screw which requires tremendous effort with the small wheel on the 800 compared to the 1st generation of the 737( same problem with the max). On the 320 the wheels have no handle because the wheel is not driving the screw, but a hydraulic valve block that’s why it is very soft. – user40476 Jun 11 '19 at 17:20
  • @user40476: ...which means that losing all hydraulics means you also lose stabber trim. – Vikki Sep 28 '19 at 01:49