Hydraulic oil quantity

[Ton van Bochove]

Engine 4 has a low quantity of hydraulic oil (72 versus mid 90 for the other engines ) Until now there are no problems, pressure and temperature are OK. I read the hydraulics chapter of the manual but nowhere a discription how to top up the oil quantity. Where can I find it?

[Avi]

You are in cruise, right?
Check it again with gear down.

However, I must say, I expect to see more fluid in the reservoirs of 1 and 4 with gear UP (actuators in) and not DOWN (actuators out).
In normal condition actuated doors are close in both cases.

Cheers,

[Ton van Bochove]

Hi Avi, I was in cruise! On the ground all 4 tanks are 1.0+ What causes this quantity differences? something o with the AUX?

[Avi]

Hi,

I said, it is the landing gear. Think of the actuators that move the gear. There is a big difference in their inside volume between extended (rod out) and retracted (rod in).

As I said, it is interesting why we see it only in #4 (actuates the body gear) and not also in #1 (actuates the nose and wing gears) and why there is more fluid in the reservoir when it should be less (when the gear is down). I think this question was asked before.

Hyd #1 and #4 also actuate the flap power packages but, in this case, it is only an hydraulic motors so there is a use of a lot of fluid (this is why #1 and #4 demand pumps run when flaps are out of up or 1, or when in transit – an airline option) but there is no real change to the volume.

Note that the quantity display shows a relative number, not a volume. There is more fluid in reservoirs 1 and 4 than 2 and 3.

Cheers,

[John H Watson]

I expect to see more fluid in the reservoirs of 1 and 4 with gear UP (actuators in) and not DOWN (actuators out).

I'm not sure the reservoirs would have the capacity to accept all the fluid displaced if you looked at piston size only. The fluid may be hidden elsewhere.

Also, regarding system 4, the wing gear pistons extend for retraction. The wing gear tilt actuators also extend, as do the wing gear doors. The wing gear downlock actuator also seems to extend for retraction.

On system 1, the nose gear piston extends for retraction. The small nose gear unlock actuator appears to retract.

[John H Watson]

it is interesting why we see it only in #4 (actuates the body gear) and not also in #1 (actuates the nose and wing gears)

Isn't it nose and body on the #1 system?

Note that I had to make some amendments to my previous message (my memory is getting a little rusty)

[Avi]

Also, regarding system 4, the wing gear pistons extend for retraction. The wing gear tilt actuators also extend, as do the wing gear doors. The wing gear downlock actuator also seems to extend for retraction.

On system 1, the nose gear piston extends for retraction. The small nose gear unlock actuator appears to retract.

That's explain the "direction" of #4 (I had a vague memory we talked about it in the past).

Thanks,

[Ton van Bochove]

Thanks guys!

[Jeroen Hoppenbrouwers]

I don't know what kind of actuator is used on the 747, but in normal life, the typical hydraulic cylinder does not significantly grow or shrink the required volume of the fluid when it is moved. These cylinders have fluid on either side of the piston and therefore can both push and pull with the same force. When you push, you get fluid back from the other side.

If a 747 uses plunger type of cylinders, that have only fluid on one side of the piston and can only push, not pull, then what force is doing the extension? Plain gravity plus air loads only? If so this would be truly a safe mechanism, although I wonder whether it would be fast enough for normal operations. Doesn't alternate extension take longer than normal extension? Of was that only for flaps which is a completely different mechanism?


Hoppie

[John H Watson]

These cylinders have fluid on either side of the piston and therefore can both push and pull with the same force. When you push, you get fluid back from the other side.

Exactly. Even gear (bogey) tilt, which pushes the rams only in one direction, has fluid on both sides of the actuator.

Doesn't alternate extension take longer than normal extension?

Yes. Gravity is weaker than hydraulic pressure.

So there is still a mystery. There may be momentary imbalances, but overall you would think the levels would remain the same. I'm assuming Hardy programmed this stuff on cockpit observations.

[Hardy Heinlin]

Yes, Peter observed this stuff several times in the BA sims (January 2018).
He mailed me these EICAS quantity indications:

Code: [Select]


#1 #2 #3 #4

92 92 92 92   (all gear extended)
94 92 92 69   (all gear retracted)


As you can see:
#1 increases by 2 units
#4 decreases by 23 units


I implemented the effect in PSX 10.16 (16 January 2018):

16.03. Hydraulics: System 1 & 4 quantities now vary with gear extension states (source: BA sim).

Regards,

|-|ardy