V1 speeds below Vmcg

[westhawk]

A recent discussion with some colleagues regarding the HS125-800XP business jet raised the issue of the scheduled V1 being below the published Vmcg for the airplane. (or aeroplane!)

A review of FAA CFR 14 part 25 (amendment 42) and AC 25-7A (part 25 certification flight test guide) seems to indicate that V1 must always be greater than Vmcg by a small margin. (5% perhaps?) The problem is that when following the instructions for use of the AFM performance charts to be used in determining V1, Vr and V2, the resultant speeds are well less than Vmcg. This is particularly so at higher field elevations and temperatures. In some cases, the scheduled V2 is below Vmca. In all cases, the v-speeds derived are at least equal to the "minimum" V1, Vr and V2 curves depicted on the chart.

So my question is whether such a circumstance where these performance speeds being below the associated minimum control speeds was allowable under the original certification criteria. It's understood that the original certification of the 125 series back in 1962 or so did not take place under the current certification regulations. Was this formerly allowed? And did the 800XP have to meet later amendments to the regulation than earlier models?

Thanks in advance.

Best regards,

Westhawk

[No_Speed_Restriction]

doesnt your QRH contain a V1mcg table to cater for low weight scenarios?

[5 RINGS]

Westhawk,
I'm not rated on the HS, but sounds to me that you should raise V1/Vr/V2 figures computed from your tables so as to comply with Vmcg/Vmca criteria.
Doing this you may end up with a reduced Max allowable TOW and/or realize your HS is so performance limited that you may not be legal to operate from particular fields under such and such conditions.
And definately, the 800XP complies with FAR/JAR25 requirements, but once again cannot fly to/from every airfields under all circumstances.

[411A]

Even on types certificated to CAR4b, the Vmcg/Vmca issues raised needed to be considered...IE: increase V1 so that it is equal to or greater than Vmcg etc.
Otherwise, 'taint legal.

[Mad (Flt) Scientist]

The problem is that when following the instructions for use of the AFM performance charts to be used in determining V1, Vr and V2, the resultant speeds are well less than Vmcg. This is particularly so at higher field elevations and temperatures. In some cases, the scheduled V2 is below Vmca. In all cases, the v-speeds derived are at least equal to the "minimum" V1, Vr and V2 curves depicted on the chart.

Are you comparing the v-speeds to a single-valued Vmcg and Vmca? Or to a chart depicting a variable Vmcg/Vmca?

Because it's possible that the manufacturer has scheduled Vmca/g with temperature and altitude, to reflect the reduction in engine thrust at higher altitudes and temps, and is basing the V1/Vr/V2 on meeting the regulatory rules for the relationships between those speeds and the various VMC speeds for the specific conditions.

Whereas they may be simply providing the ISA/SL values of Vmca and Vmcg as a single value. So you're not comparing apples and apples.

Technically, if they are doing that then they should be providing the VMC data in the altitude/temperature scheduled form, but maybe the alt scheduling was post-cert, and it simply didn't get presented fully?

Its a possible explanation...

[Keith.Williams.]

The answer to this conundrum may lie in some of the terms that you have used.

Pesumably the term "scheduled V1 " means v1 figures derived from tables/graphs.

But in using the term "published Vmcg" do you mean a single value for Vmcg that is stated in some part of the manual? If this is the case then your Vmcg value is probably set against ISA msl or some similar fixed standard.

The actual value of Vmcg and Vmca will decrease with increasing altitude so a fixed figure cannot be valid for all altitudes.

If you are taking values of V1 Vr and V2 that have been corrected for altitude and comparing them with a value for Vmcg that has not been corrected, then are likely to find the kind of problems that you describe.

Is it perhaps the case that for the aircraft in question the "minimum V1, Vr and V2 curves depicted on the chart" are actually based on the altitude-corrected vaulues of Vmcg and Vmca?

Your statement that "......In all cases, the v-speeds derived are at least equal to the "minimum" V1, Vr and V2 curves depicted on the chart." suggests that this is the case.

[Old Smokey]

I think that Mad (Flt) Scientist and Keith.Williams have "nailed it" quite adequately, with MFS emboldening the original clue contained within your own post (This is particularly so at higher field elevations and temperatures).

In a typical certification scenario, the worst case Vmcg and Vmca are established, and, if after applying the required margins, the required V1, Vr and V2 are not affected, the process is complete. If, however, Vmcg (or Vmca) is a problem, the regulations do allow the manufacturer to make application to the regulatory authority to consider the lower Vmcg and Vmca at higher altitudes.

In the latter case, the revised V speeds can be presented in a fairly complex form, or alternatively, by simply adding the words "If V1 calculated is below Vmcg, V1 must be increased to Vmcg".

It is far easier to add the "If V1 calculated is below Vmcg, V1 must be increased to Vmcg" caveat than to prepare and present a more complex V-speeds table or graph. This does not mean for a moment that the Field Length and Obstacle requirements are sacrificed in favour of a controllability problem. Within the Field Length and Obstacle calculations it HAS been considered, and a fairly standard V-speeds table presented in association with the caveat mentioned.

This has nothing to do with the "grandfather clause" suggested for a 1962 aircraft, the simplified system of adding the caveat continues to be applied on more recently certified aircraft such as the B777.

In westhawk's original post mention was made of requisite margins above VMC -

V1 must be equal to or greater than Vmcg,
VR must be equal to or greater than 1.05 Vmca, and
V2 must be equal to or greater than 1.10 Vmca.

Regards,

Old Smokey

[westhawk]

Thank you very much for the responses gentlemen. :ok:

I believe it is very likely the case that the published Vmcg value in the AFM is indeed a "single Vmcg value" established to meet a regulatory requirement to publish such a number and may not necessarily reflect the actual Vmcg under all conditions of weight, altitude and temperature. It seems likely that the published Vmcg value of 115.5 KIAS could have simply been the highest value obtained during testing.

The very fact that a "minimum V1 and Vr" line exists on the chart at all appears to imply that the certification option of establishing a Vmcg value for each condition of WAT which might produce a V1 below the "single-valued" published Vmcg of 115.5 KIAS was exercised. To raise the V1 to a value equal to or greater than 115.5 KIAS would certainly have increased the field length required at higher altitude and temperature field conditions. In all cases where V1 is less than 116 KIAS, it appears to occur at temperature/altitude combinations which would be beyond the 731-5BR engine thrust flat rating alt/temp. And of course, only at lighter weights which would allow for these values to fit within field length and takeoff path requirements. I believe we've arrived at an explanation!

Unfortunately I am so far unable to locate any reference to the option of a "variable Vmcg" which considers reduced engine performance at higher altitudes/temperatures in part 25 or in AC 25-7A. Any idea where else I might look?

Anyway, thanks again for steering my thinking in the right direction by way of your posts. I knew I could count on you. And I expect that my colleagues will appreciate the input as much as I do. Proper credit will of course be attributed! :)

Best regards,

[Mad (Flt) Scientist]

Unfortunately I am so far unable to locate any reference to the option of a "variable Vmcg" which considers reduced engine performance at higher altitudes/temperatures in part 25 or in AC 25-7A. Any idea where else I might look?

AC25-7A contains, in section 23(b)(2)(ii)(B) the following text buried in the paragraph:

If, at the option of the applicant, the AFM value of VMCA is to vary with pressure altitude and temperature, ....

Now, there's no explicit discussion of a similar nature in the VMCG section of AC25-7A, but the actual regulation with respect to engine thrust is essentially the same for both 25.149(b)(1) and (e)(2) i.e.

Maximum available takeoff power or thrust on the [operating] engines

So that would tend to suggest that scheduling thrust with altitude is just as valid for Vmcg. Certainly the regulations do not prohibit doing so.

There is however a further issue that Vmcg is not a manoeuvre that is generally established by analysis, but rather by demonstration. So there might be some arguments about how to establish the Vmcg values at the edges of the alt/temp boundary, without test. Corrections for engine thrust in Vmcg tests are generally rather crude when used to correct test conditions to the "target thrusts". But a similar technique would be possible, and could even be applied conservatively (since even partial credit for the thrust lapse is better than none).

[Basil]

Vmcg is not a manoeuvre that is generally established by analysis, but rather by demonstration
Going back a few years I was flying 757s with slightly different engines.
Oddly, the one with the lower powered donks had higher Vmcg.
Don't think the fin/rudder/engine location was any different.
We'd a bit of headscratching and the best anyone could come up with was as quoted.

[Alex Whittingham]

Could it be different certification standards? The British BCARs allowed 7kt of adverse crosswind whereas FARs and JARs do not. An older 757 with less powerful engines but certified under BCARs could easily have a higher Vmcg than a later one certified under JARs.