Vmca vs Vmcg, ATR-72

[SmolaTheMedevacGuy]

Hi!

Which of those speeds should be greater? As far as I've been told, the difference between Vmcg and Vmca is that Vmca is determined with 5deg bank, whereas Vmcg with wings level. Therefore the latter should be greater. However, The ATR 72 FCOM shows that for given conditions the VMCA is slightly higher than VMCG. The difference is about 1-2 kts, no big deal, but it makes the above statement false. Now, who is wrong? Me, or the manual? :} Can anybody help?

Greetings
Smola

[Keith.Williams.]

There is no law (physical or otherwise) that requires VMCA and VMCG to be in any particular order. In some cases VMCA will be the greater of the two and in others VMCG will be the greater.

It is true that the definition of VMCA imposes a fairly tight bank angle limit, but then VMCG imposes an equally demanding requirement to stay within a certain distance of the runway centre line. The definition of VMCG also excludes the use of nose wheel steering. This exclusion is intended to ensure that control can be maintained even on a very slippery runway.

[solone]

as Keith says there is no law...

there are indeed a number of factors, some of which are listed by john_tullamarine in this previous thread (http://www.pprune.org/forums/showthread.php?s=&threadid=98737&highlight=vmcg).

In the case of the ATR, VMCG could be lower because of the effect of gear geometry and tires side force (just guessing...).