744 Vmcg increase with HDP inop

[Luftis]

Would anyone be so kind and help me properly understand the following 744 issue?

The 744 MEL requires a 5 knot increase to the Vmcg with the No. 1 or No. 4 Hydraulic Demand Pumps inoperative. The scenario implies, of course, that you lose the associated engine on takeoff and thereby experience a complete loss of hydraulic system 1 or 4.

Yes, with the No.1 system inoperative, you lose nose and body-gear steering. But Vmcg certification is predicated on the use of aerodynamic forces alone, so I can't see that being the reason.

Yes, the Lower and Upper rudders are powered by systems 4 and 1 respectively, but with redundancy back-up from systems 2 and 3. So even with a complete loss of Systems 1 and 4, you've still got the full 3000PSI powering the rudder.

That's where the trail goes cold for me.

Any other ideas would be appreciated?

[By George]

Had this myself recently and my understanding is that if, as in my case, the number 4 demand pump was U/S the take-off performance must be calculated on the possibility of a failure of the number 4 engine. If this happened and with no number 4 system available the wing gear would stay down. The effect of this in the second segment is obvious with the climb performance the most limiting. ( I had a long runway). I assume the change in VMCA is an aerodynamic one because of the dangling bits in the second segment. I ended up off-loading 10 tons. I havn't got the manuals handy at the moment but I think there is a adjustment to the VMCA with the brakes de-activated when not using the de-activation tool. With capped lines the gear has to be left down for a few minutes to wait for them to stop spinning prior to tucking them away again having a large effect on performance.

Looking at your post again you mention VMCG? Are you sure it's 'G' and not 'A' If it's the ground one a reduction in roll or rudder performance might be the answer but to be honest with all the back-up I cannot see why.

[Luftis]

Thanks for the reply, By George.

Yes, then manual definitively says Vmcg. I suspect the reason has to do with either a)the loss of NWS, or b) a possible reduction in PSI or authority over the rudder. However, my knowledge leaves me short, and so does my manuals.

As for Vmca, yes, there certainly are issues associated with the 2nd segment climb. But the MEL does not specifically mention the Vmca associated with this item.

[Mariner]

Luftis is right George,

The MEL says:

29-11-02-01 No. 1 or No. 4 Demand
Pumps

One may be inoperative deactivated
provided:
a) VMCG is increased by 5 kts.,
b) Gear down climb limited take-off weight
limitation is observed.
c) Runway length/obstacle limited weight
must be reduced by 50,000 kg, and
d) Demand pumps 2 and 3 operate
normally.

So it is VmgGround.

As to why, I don't know either.
Bodygear steering cuts out above 20 kts, so that can hardly be a factor.
If credit were given for nosewheel steering, it would only apply to Hyd dem pump 1, the hydr system powering the nosegear steering.

[By George]

Thanks everybody, the trouble today is the manuals lack any depth. We all know what drives what and when but lack the details. The section on the APU is only about five lines. I know its down the back and makes a noise but that's about it. Ah the good old days, I remember the F27 with two days just on the prop. (still didn't understand it though).

[Luftis]

Anybody else want to give it a shot?

Looks like we're none the wiser.

[Mad (Flt) Scientist]

My guess would be its the rudders, and that either the backups aren't as capable (not as much flow rate, or don't pick up the demand instantly when 1 or 4 fail) or have never been demonstrated and so Boeing are conservatively assuming there may be some degradation in capability and adjusting VMCG accordingly.

I very much doubt they repeated VMCG for an MEL dispatch case, so chances are it is an analytical penalty, and when you do it that way the tendency is to be a bit more conservative so as to err on the side of safety.

[Luftis]

That makes sense, Mr Scientist.

Perhaps in the transition between 1 and 3 (or 2 and 4) there will be some loss of Rudder Authority, and at such a critical moment (exactly when you lose the engine), any momentary loss would could cause loss of directional control.

Thanks for that.

[By George]

Luftis, I spoke to a performance engineer yesterday and it does seem to be rudder authority during the degrading of the system as 'MS' mentions. I must admit I cannot see it with the system knowledge available in the manual. It must be one of those "you don't need to know" things.

[3holelover]

Give it a try on the ground... Kill one of those demand pumps, cycle elevator, aileron, and rudder, while your flaps are coming up/extending... watch what happens to all hydraulic pressures as you continue.... and the delay that builds in action vs motion...

[Luftis]

Thanks for that, Gents.

What an amazing resource PPRuNe is.