737 question
 

Hello ladies and gentleman,
I have a question regarding the b737/ classic and NG.

Please explain this to me like I was a five year old:

The ailerons and elevators have "manual reversion" available in the event of a hydraulic system A and B failure.

What is manual reversion and how does it work?

Dankeschön

I do not fly the aircraft in question, so feel free to correct me if I'm wrong.

I would liken manual reversion, to driving your car, without the power steering working. In normal operation, cables link the control column, and rudder pedals to various hydraulic power control units, which actuate the control surfaces.

If both hydraulic systems were to fail, then it is possible to move the control surfaces by a direct link between the cables and the flight controls. To assist in the huge control forces that are required to move the various surfaces, tabs and aerodynamic balance panels are used to create a pressure differential, and therefore lighten the physical effort required to control the aircraft.

TotalBegginer - You are correct, but the rudder on the 737 is hydraulically powered only, there are no cables or mechanical linkage. However, you have the standby hydraulic system when A and B are lost which along with the leading edge devices and thrust reversers powers the rudder. The stab trim is electrically operated (and if that fails manually operated as well) so hopefully during manual reversion you can trim out most of the elevator loads. With no hydraulics the differential flight spoilers are also lost.

Thanks alot guys!:ok:

This thread got me thinking the other day at work. I was watching the wind push the elevators of a 737 up and down, and I wondered if due to the manual reversion link, would the control column be forced fore and aft?

Yes - and it hurts!

NG
 

I also forget to mention - On the 737 NG, provided both Sys A & B switches are in Standby Rud the Yaw damper can be switched ON again. One of the reasons for this is to assist turning during Manual Reversion.

just a short one,
regarding Classic Differences........

if you lose hyd. Sys B, it's bye bye Yaw Damper, right?

Correct you loose the Yaw Damper on the classic. For info........ when Sys B is lost the Yaw damper switch remains on and NO amber warning light despite the Yaw Damper being inop. When the switch is moved to off the amber light then illuminates and the Yaw Damper will not re-enguage.

Manual reversion on the 737 is more than just the equivalent of losing power steering on a car, though that is a good analogy. It's a major emergency, and the impression I always had was it was a situation where survival is certainly not assumed. I am physically strong, but it would wear me out. It takes 2 pilots co-ordinated on the controls. Any sort of natural 'feel' is completely absent, and following a planned flight path is extremely difficult. The controls are VERY, VERY heavy and feel completely dead. The obvious critical time is contacting the ground, and flaring and any power changes make it a nightmare scenario. I find it hard to describe (apart from 'damn difficult'). The loss of the yaw damper is a triffle that one would hardly notice in comparison! It's the one feature of the 737 I never missed going onto something bigger with a nice round number of engines (4). If you ended up on manual reversion on that, your day was spoilt- there is not even any manual reversion!

Rainboe is right.

I did about 5 minutes in manual reversion, on an air test prior to deep maintenance to get some trim measurements.

The sim was heavy in manual - but the aircraft is much, much heavier, probably 3 times the control loads. At 250 kt IAS, both pilots trying hard to roll (in the same direction, yes!) we managed about 5 or 10 deg/sec roll rate. I would not like to try an approach to land like that.

This question is based purely on old rumours, but was it not the case that the 737 was originally designed not to have hydraulically powered flight controls at all?

I believe that the hydraulics were originally only installed for rudder, spoiler and undercarriage actuation, with elevator and aileron power derived from servo tabs, a la 707.

The design solution of having the PFCU's either in the wheel well for aileron and aft fuse for elevator would seem to suggest a retrofit of the system, when the control loads were found to be a 'little' heavy.

Every other type with power controls has the actuators in the wing or stab itself...or am I totally wrong?

thanks again LAX!

interesting discussion about the manual reversion,
guess it´s back to the gym for more iron pumping...........:{

I am not Iron man but I had manual reversion twice on an air test as well and yes difficult to say the least but very similar with the sim

and yes I wouldnt want to try an approach like that either

I have done about 50 737-300 airtests involving manual reversion in the last year alone. It is very heavy but not by any means impossible – even for a weakling like me!

My top tips are as follows:
1. The elevators have quite a dead band at neutral where nothing happens, outside this the elevators are heavy; this can be difficult to get used to.
2. Make sure that you are always in trim (specifically stab trim), otherwise it will get away from you every time you look away and recovery to S&L can be a handful. Remember to retrim for every change of speed, power & configuration.
3. Aileron deflection is heavy so use a little squeeze of rudder as you enter & exit a turn. Once established in the turn it should require no further effort as the ailerons will be neutral. Keep the bank angle small so that you don’t require much back pressure.
4. You wont have autopilot available so take turns between both pilots to give yourself a rest, it is very tiring!
5. If you have to land in this configuration, put out a PAN, use flap 15 to minimise trim changes on finals & if you have to go-around (quite likely). Take a very long final and configure early to get sorted out & allow for long radius of turn.
6. On finals make only very small adjustments to power as thrust gives large trim changes. Also keep an eye on the wind/drift on the way down so that you can set a heading and don’t have to chase it. Oddly it is easy to overcontrol & overcorrect, be economical with your inputs.

The main message is to keep in trim and only make small inputs and thrust changes.