747 Classic Elevator Feel

[Hangin' on]

Anyone got any idea what the allowable leak rate between sys 2 and 3 is (if any)?:8

[18-Wheeler]

Not sure if this is what you want, but if there's more than 25% difference in pressure between systems 2 & 3 then the ELEVATOR FEEL annuciator light on the front panel comes on.

[Flight Detent]

Hi, 18-Wheeler,

To add, for which there is no specific procedure, just carry on!

I think I can remember back that far, in the 737 world these days!

Cheers, FD

:cool:

[Earl]

18 Wheeler is correct.
An amber ELEV FEEL light on the center instrument panel illuminates if their is a difference in system 2 or 3 pressure greater than 25% or if the feel system is powered by only one hydraulic system.
Their is no leak rate between the systems as these are all independent of each other.
The only way for one system can transfer to another is through the brake system.
Being systems 1, 4, and 2.
These has to be done manually.
A technic that maint uses sometimes on the ground to transfer fluid between the reservoirs.

[spannersatcx]

You can get internal leakeage in a hydraulic system, for systems 1 and 4 max gross internal leakeage is 5 gpm 2 and 3 is 3 gpm. But this is different to leakeage from one sys to the other, this is internal leak rates within actuators and motors etc.

As PCU's are normally powered by dual hydraulic systems I guess it is possible that you could get an internal leakeage from one side to the other, but highly unlikely, can't remember ever having seen it, but it would more likely be picked up in a shop rather than in day to day use.

As Earl says the main cross over of Hyd systems on the 747 is the brake systems, normal brake source being sys 4 secondary source is sys 1 and reserve is by sys 2. Quite often we will see a disparity on the ramp between sys 1 and 4 and it is quite easy to transfer back between the two systems. Depending which way you want to transfer it, you pressurise the system with the greater qty in apply the park brake, depressurise the system, once that has dissipated then pressurise the other sys and release the park brake, due to the nature of the system shuttle valve fluid will now transfer from one sys to the other, you repeat this a few times until the systems are balanced qty wise.