V1 Additives

[navy85]

I'm in a "discussion" with some of my fellow flyers and was hoping someone out there mey be able to help us out. Most airplanes I have been associated with have and small (1 to 4 knot) correction to V1 and Vr at high altitude airports. Why is this? What does the increased speed achieve? Any thoughts would be appreciated.

[DCDriver]

I think the reason is thus:
V1 cannot be less than Vmcg. At high alt a/d's (NBO;HRE;JNB etc)due to density alt Vmcg is slightly higher than at sea level (basic gas laws)so V1 has to be increased correspondingly, and as Vr is a function of V1 that goes up too.
V2 is a function of Vmca, which also increases with altitude.Thus you also have higher V2's at high a/d's.
I remember having to add a few knots to the T/O speeds at airfields >3000 amsl on various types.
...not a particularly scientific explanation I know, but I hope it's helpful! DCD

[av8er]

Now, I would have guessed something a little different. Once you are above VMCG you are above VMCG, no problem.

However, regulations require one to pass the end of the runway at V2 at 35' (dry). At higher altitudes the thrust reduces and I suspect the V1 needs to be higher to achieve the V2 speed at the end of the runway.

[john_tullamarine]

I probably have to take some contrary views here ...

DCDriver

Vmcg is related to thrust output. As a result, other things remaining constant, the 'real' Vmcg will reduce for typical turbine powered aircraft as the pressure height and OAT increase. Scheduled min V1 relates to the standard published Vmcg determined for specified certification conditions. Looking at the 732 with -17 motors, for instance, the real world Vmcg varies from 105 kt at sea level down to 87 kt at 10,000 feet and 30C (ref QRH). The minimum scheduled V1, based on the certification data, remains constant. The handling considerations, however, vary considerably with the variation in real world Vmcg.

VR principally relates to V2 and is scheduled to provide V2 (or thereabouts) following a V1 failure as the aircraft completes the takeoff rotation flare at 35 feet.

V2min (as opposed to overspeed or improved performance V2) relates to both Vs and Vmca - the Vs limitation applying for higher weight and the Vmca limitation applying at the low end of the weight range. For normal takeoff heights, Vs is sensibly dependent only on weight while real world Vmca, in a manner similar to Vmcg, will reduce with decreasing thrust output. Scheduled data, however, will be predicated on certification data.


Saltydog,

The (I presume simplified) performance documents you have referred to have to be looked at in the context of their construction. The AFM will provide for a range of acceptable speed schedules. If a simplified set of schedules is prepared then the schedules will vary on the basis of how the basic takeoff chart used has been prepared. Although such an answer is not very helpful in the context of your question, one has to look at the specific documents to which you refer in order to offer a sensible engineering answer.


av8er,

I would urge considerable caution when operating in the vicinity of minV1. It is worth keeping in mind that Vmcg is not a cut and dried concept. The value published is for specific conditions and, if the conditions vary adversely, then the real world Vmcg can increase - putting an apparently legitimate takeoff at increased risk of control loss in the event of an engine failure. This consideration is mainly a concern in the situation where the aircraft takes off on a minV1 schedule with a high crosswind. Typically, the real world Vmcg will increase above the published figure (for US aircraft - based on nil wind) by around half of the crosswind component.

[ 14 October 2001: Message edited by: john_tullamarine ]