Vmcg again......

[fivegreenlight]

Ferry flt. Perf calc gives the following; V1=112, Vr=113, v2=120.
Check the manual and Vmcg is 117.
What would you do?

V1=117, Vr=118, V2=125 ??

[john_tullamarine]

Whichever manual it is to which you refer should give you some appropriate guidance.

If there is no other guidance run the calcs again either

(a) for incremental increases in TOW until you get a speed schedule for which V1 is not less than Vmcg. For a ferry flight situation off a normal length runway, this will fix the problem very quickly. If you have room to spare, and there is no contrary AFM caveat, consider increasing the weight to increase the schedule further if you expect to have a crosswind.

(b) for incremental overspeed schedules until likewise

or

(c) if your runway is really short, use another one, or wait for a better wind, or offload some weight (in the hope that you might be able to achieve a suitable overspeed schedule at the lower weight

[mutt]

You have to read the manual, we have different aircraft built by the same company that deal with this quite differently. Sometimes just round up V1, other times round up V1,VR,V2, sometimes with a minimum runway length.

I would love to know what are these "overspeed schedules" that JT is talking about? Any ideas???? :):) (OS where are you?...... )

Mutt

[john_tullamarine]

Ah .. I hear the far off sound of the converted ...

[eckhard]

The manual may have the option of selecting a fixed lower thrust setting for take-off.

In the 747-400 (RR engines) you can select fixed thrust derates (TO-1 or TO-2) which will yield a lower Vmcg. I think that these can then be further reduced using the 'assumed temp' method. (Not exactly sure, as I've never had to use TO-1 or TO-2.) A comparison of Vmcg for Sea Level, OAT +15C gives 123, 117, 111 for full TO, TO-1 and TO-2, respectively.

There is a warning when using the 'fixed' derates to the effect that in the event of engine failure, advancing the thrust levers could result in a loss of directional control.:eek:

My understanding of 'overspeed' is that if you are climb limited (but not runway limited) you can get a better second segment gradient by using a higher V2. You also have to increase the V1 and Vr but as you have 'extra' runway, that should not be a problem. I think JT is suggesting that this technique would help by increasing the V1 to a higher value than the Vmcg.

Not trying to put words in your mouth JT, just giving my interpretation!

[galaxy flyer]

I think that these can then be further reduced using the 'assumed temp' method. (Not exactly sure, as I've never had to use TO-1 or TO-2.)

:=Can't lower Vmcg for a "reduced power" take-off, Vmcg has to be calculated on rated thrust. Which is why you can lower the V speeds on a derate. :ok:

[eckhard]

Thanks GF.

I meant to say that the thrust can be reduced further, not the Vmcg which, as you point out, has to be valid for the 'rated thrust'.

If we decided to use a full thrust take-off, we would see 'TO' annunciated as the active thrust limit.

Normally we use a reduced thrust, so we see 'D TO' annunciated as the thrust limit, i.e. 'Derated Take-Off' thrust.

If we decided to use TO-1 as our fixed derate (in order to get a lower Vmcg for example), the annunciation would be 'TO-1'. We could then apply the assumed temperature method to reduce further the (already derated) thrust. The annunciation would then be 'D TO-1'

Similarly we could see 'TO-2' if we decided to use TO-2 fixed derate, or 'D TO-2' if we decided to use a reduced fixed derate(!)

In all the above, the Vmcg would be appropriate to the 'rated thrust', i.e. Full thrust in the first example, TO-1 thrust in the second example and TO-2 thrust in the third.

OK, back to my glass of wine!

[fivegreenlight]

J.T said "Whichever manual it is to which you refer should give you some appropriate guidance.

If there is no other guidance run the calcs again either

(a) for incremental increases in TOW until you get a speed schedule for which V1 is not less than Vmcg. For a ferry flight situation off a normal length runway, this will fix the problem very quickly. "



Do you mean use artifically high V speeds by using a higher weight for the calculation, or do you mean increase actual TOW by actually loading more mass ie fuel ?

[RVF750]

There should never be a need to add additional real weight unless you need to for trim.

The procedures for increased V2 procedure, allow you to use a higher sped on the ground to give better climb performance margin in the event of a loss of one engine. All in the Perf A exam which I last took 11 years ago. My brain ain't that good!

My little Dash jumps into the air usually in the touchdown zone on most runways, so I don't think about such things much anymore....

[john_tullamarine]

Do you mean use artifically high V speeds by using a higher weight for the calculation

Make believe weight increase to give a sensibly calculated higher speed schedule. Caveat is that one needs to make sure that the AFM doesn't preclude such a procedure (which I would not be expecting to see).

As noted, the other concern with short ferry flights is that the low speed schedule brings you down into the area where the effect of cross wind on the real world Vmcg is worth considering .. pushing the schedule up to account for a notional delta might just avoid some sweaty palms if one goes quiet during the takeoff ...

so I don't think about such things much anymore....

Takes me back 40-odd years to the silly things we used to do in SuperCubs to impress the ordinary folk in Cessnas and other such mundane hacks ... a touch of head wind would see the bird leave the ground somewhere near the end of the piano keys ... all very silly viewed now with the conservatism of years .. but it was great fun at the time.

[Permafrost_ATPL]

Without specific guidance from manuals, would there be anything wrong with increasing V1 to Vmcg, Vr to the new V1 and keep V2? I am definitely NOT very knowledgeable about perf, so be gentle :ok:

P

[john_tullamarine]

Some thoughts ..

(a) increasing V1 to Vmcg,

.. but check for ASDR conflict (generally not likely to be a problem for a ferry)

(b) Vr to the new V1 and

ought not to be a problem

(c) keep V2?

Vr to V2 usually involves a time delay associated with the rotation flare speed increase ie it would be usual to see a V2 exceedance.

[fivegreenlight]

If V1 and Vr both equal a speed below Vmcg I assume there is no need to
do anything as Vmcg becomes irrelevant?

[john_tullamarine]

.. I assume there is no need to do anything as Vmcg becomes irrelevant?

Disregarding the rule book, that might be the case only if all the noise keeps time. Otherwise, Vmcg is much more relevant than for routine high TOW departures .. unless you are quick enough to change your gameplan from "go" to "stop" if the post-V1, pre-Vmcg failure track heads to the grass.

The generally not understood consideration is that the normal, gentle centreline deviation for higher speed failures ... becomes rapid, aggressive, and attention-grabbing as the speed approaches the real Vmcg for the day. This change is not spread over a wide speed band but cuts in very quickly as failure speed reduces .. a range of several knots can make the difference between a straightforward takeoff, a sweaty, eyebrows-raised takeoff .. or a walk in the grass ...

[Old Fella]

A couple of things come to mind. I have always understood that V1 could not be less than VMCG, regardless of any other factor, which brings my second point. How does the aircraft know whether or not it is on a ferry flight and, even if it did, how does that impact on the initial question?

[john_tullamarine]

V1 could not be less than VMCG

True. However, sometimes the paperwork gives you the non-limited calculation and expects you to apply the over-riding limit .. as in the original post

even if it did, how does that impact on the initial question?

The ferry bit is not the significance .. only that ferries usually involve low TOW (which is)... and low weights generally come up with low V1/VR/V2 schedules .. ie Vmcg/Vmca considerations are not a common problem because we normally are trying to make a buck by carrying payload.

[fivegreenlight]

Great stuff, thanks:ok:

[galaxy flyer]

J_T

Quick on-topic question-why do some airplanes, assuming dry runway, have V1 always equaling Vr; while others always have a split between the two speeds? Our GLEX, V1=Vr on dry runways, but the CL605, V1 is always less than Vr. But V1 is always equal to, or greater than Vmcg. ;)

GF

[john_tullamarine]

V1 always equaling Vr ... a split between the two speeds?

Depends on the aircraft (ASDR performance, stall schedules, etc) and how the AFM sums have been presented by the OEM (balanced/unbalanced, etc.)

Usual points to consider -

(a) V2 drives VR ... in that VR will be associated with the time delay (and associated speed increase) taken during the rotation to end up with V2 at screen. V2 generally is stall driven.

(b) V1 schedules will depend on how much detail the OEM chooses to put in the AFM and the ASDR characteristics of the aircraft.

That's probably not terribly helpful but, without a specific AFM to speak to, there is not much more we can do but generalise and waffle around the topic.