QUESTION RE: B744 MEL ADP 1 or 4

[jose jimenez YYC]

Operational Requirement to increase VMCG by 5 knots if either ADP 1 or 4 is MEL-for ADP 2 and 3 does not have requirement. Can anyone tell me what the performance issue is. ENG 1 and ENG 4 can either be critical engine but still have rudder authority with loss of engine 1 or 4 if adp is u/s. Upper rudder systems 1/3 and lower rudder 2/4. Can't seem to figure out the issue of controlability. Any suggestions or ideas why required for ADP 1 and 4 and not for ADP 2 or 3. I know there is issue with gear after rotation for 1 and 4 but on ground? Thanks:ugh:

[600ft-lb]

Maybe because there is no ADP at pylon 2 and 3.

[main_dog]

Just my guess: perhaps a single hydraulic system 1 or 4 operating on the upper or lower rudder could be insufficient to guarantee full deflection of that rudder... hence the increase in VMCG.

This is required only for 1 and 4 because being outboard engines, their failure causes the most yaw; 2 and 3 would require would less rudder.

PS For 600ft-lb: not all 744s have electric demand pumps on 2 and 3, some have ADPs on systems 2 and 3 as well (just like the classic)

[Intruder]

Also, with Hyd 1 gone, no nosewheel steering. Check aileron & spoiler decrements, which will exacerbate any rudder deficit.

With either Hyd 1 or 4 failure, not all the gear will retract if you take it airborne, so climb performance will suffer.

[spannersatcx]

Maybe because there is no ADP at pylon 2 and 3.

There are on some, AC demand pump was an option or vv.

I don't know VMGC is exactly, obviously to do with speed, I would suggest flap operation is the key here.:eek:

Added from DDG:

With Demand Hydraulic Pumps No. 1 or No. 4 inoperative, an engine failure
at the same position (1 or 4) on take-off will cause a hydraulic system loss,
preventing retraction of some landing gear, and will require secondary mode
(electric) operation of trailing edge flaps. Take-off performance (including
obstacle clearance) is thus based on gear and flaps extended.

Time required for retraction of the trailing edge flaps after take-off will be much
longer due to the slower operation of the electric drive (36 seconds for hydraulic
operation vs. 275 seconds for electrical operation). Consequently, the
Operations Manual charts 'Maximum Level Off Height' and 'Third Segment
Distance' may not be valid for this situation. It must be assumed, for
performance calculations, that flap retraction is accomplished at Maximum
Continuous Thrust.

[main_dog]

Spanners, Vmcg is minimum control speed on the ground, in other words the minimum speed on the ground at which there is sufficient airflow over the rudders for them to have enough control authority to deal with the yaw following failure of the critical engine. As I recall nosewheel deflection is not taken into account. VMCG is thus strictly related to the takeoff roll, so gear and flap retraction hydraulic loads are not a factor.

As such JJNYC's question is a good one: why is VMCG increased with what is effectively a Hyd Sys 1 or 4 failure considering that the upper and lower rudder should also be redundantly powered by Hyd Sys 3 and 2 respectively?

We need a Boeing test pilot on this forum :ok:

[Intruder]

Why do we need a Boeing test pilot? You have all the answers!

[main_dog]

Why do we need a Boeing test pilot?

Well, I find it reassuring to keep one around, sort of like Linus' blanket! :}

No seriously, I don't think we've answered all of his question, but I'm probably missing something: why do Boeing recommend increasing VMCG by 5 kts in this case, considering that the rudders are powered by two hydraulic systems each?

It should have nothing to do with nosewheel steering (as VMCG does not take NSW into account) nor with aileron/spoiler authority (as we're still on the ground), so what's going on?

My guess is still that with only one remaining hydraulic system on one of the rudders, full deflection on that rudder is no longer assured, but does anyone know for sure?

MD

:confused:

[gearpins]

Hi Folks,
I stuck upon the same question recently and was comforted by this post ( I am not the only one )
One explanation given is the increased Vmcg will result in a corresponding increase in V1 Vr and V2 thus providing a greater margin to Vmca.
looking for a more convincing answer though.
cheers

[Intruder]

Increasing V1 does not necessarily increase Vr or V2.

The simple answer is the best. With an ADP inop, the hydraulic system will fail when the engine fails. With Hyd 1 or 4 inop, there will be reduced control effectiveness, including half the power to one rudder.

[gearpins]

Thanks intruder,
I find that the best available explanation.
considering the rudder upper / lower are powered by 2 hydraulic system a failure in one - will make it respond slower I suppose.
cheers