737 elevator

[allflight57]

Can someone explain how on the 737NG, while the aircraft is in a cold and dark state, the elevators remain in the neutral position, how is this done if the hydrauics are (obviosly) depressurized? Im assuming it goes into manual reversion after hydraulic pressure is no longer available, so wouldnt the elevator droop downwards kind of like ERJs? Also one more question, how are the flight controls proteced from wind gusts if the hydrauics are depressurized?

Thanks,
Tyler

[Dogma]

T -

Has to do with an accumulator that (should) maintain the position of the flight controls while on a turn around. Does not always work in gusty conditions or after a significant period of time. In adition the elevator is more neutrally balanced.

[Rainboe]

Without hydraulic pressure, the elevator control can move backwards and forwards in a tailwind quite slowly, but strongly- annoying if you are sitting there, so one normally gets an electric pump on quickly to lock it at neutral. Apparently Boeing assure that no gust locks are needed and no damage will be caused.

[Rainboe]

I don't recall an accumulator in the 737 elevator circuit. I thought it was all to do with control springs and feel unit mechanisms.

[BOAC]

Hmm! Nice.

allflight - I believe the elevator is mass-balanced and therefore gravity should not move it?

[john_tullamarine]

Intemperate use of naughty words is not appropriate to this forum .. and probably not the wider site .. Dogma is having a short R&R in the sin bin for the purposes of navel contemplation ... the other posts are not relevant with Dogma's posts suppressed so have been removed to keep some semblance of thread flow.

[Swedish Steve]

The B737 Elevator (and ailerons) are connected with cables to the control column. With no hyd power they will droop an inch or so until the stops in the PCU are contacted. They will then stop unless wind pushes them around.
Nearly every other airliner has fully powered controls. These will droop right down under gravity as the hyd pressure dissapates, this may take an hour or two, or with a small leak in the PCU will take a few minutes.
The B737 is showing its connection to its grandfather, with steel cables and manual reversion.

[allflight57]

Hey guys thanks a lot for the replies. One more question though, how do the PCU stops only let the flight controls droop about an inch or so, when the actual amount of deflection is much more during normal operation with the hydraulics pressurized? Anyone have a close up pic of the PCU or a detailed diagram so I can see a little better how it works.

thanks again,
Tyler

[Rainboe]

Try yourself at b737.org.uk

[IFixPlanes]

... how do the PCU stops only let the flight controls droop about an inch or so, when the actual amount of deflection is much more during normal operation with the hydraulics pressurized? Anyone have a close up pic of the PCU or a detailed diagram so I can see a little better how it works.

http://img295.imageshack.us/img295/227/737ngelevator01xg4.th.jpg (http://img295.imageshack.us/my.php?image=737ngelevator01xg4.jpg) http://img295.imageshack.us/img295/2761/737ngelevator02te6.th.jpg (http://img295.imageshack.us/my.php?image=737ngelevator02te6.jpg)

Wiht hydraulic power available, the complete PCU moves to actuate the elevator.
Without hydraulic power you actuate the (weight balanced) elevator mechanically, but you have a kind of "lost motion" until the primary input crank hit the mechanical stops.
So on ground, when you switch of the hydraulic, the elevator drop a little bit.