If VMCG is limiting, how would you increase aircraft weight?

[john_tullamarine]

Sir Donald ..

My previous post serendipitously was the answer to your question. ... the reference in your subsequent post to reducing thrust MUST be read to mean using a derate(d) thrust (equivalent to a lower rated thrust engine) and NOT reduced or flexible thrust (where the rated thrust, which determines Vmcg, remains the same regardless of the flex values) ... one can, of course, use both simultaneously .. but only the derate situation is relevant to your question.

I'm guessing from your profile that you don't have a heavy aircraft background yet so, with a presumed small aeroplane outlook, your confusion is quite understandable.

For the typical bugsmasher operation, one is only concerned with TODR and TODA .... for that sort of operation mindset, your observation is quite valid, especially if we are looking at the combination of a reduced V1 (which is not relevant to small aircraft) and reduced rated thrust. The secret to resolving the confusion lies in the fact that heavies require consideration of a bunch of different calculations for determining the (most limiting) RTOW..... ie not just a case of looking at the TOD consideration ..

Going on from Tinny's previous post, the main concerns in the low weight/short runway regime are

ASDA and ASDR
TODA and TODR
TORA and TORR

Making up some numbers to illustrate his point .. and ignoring things like contamination and lineup (which just modify the numbers) ... unfortunately, I don't have any suitable AFMs with this sort of problem fix to hand so make-believe numbers will have to do .... doesn't affect the philosophy at all ...

ASDA = 2000 m
TODA = 2300 m
TORA = 1900 m

For the higher rated thrust case ...

RTOW = 80,000 kg
V1min = 100 kt
ASDR = 2000 m
TODR = 1600 m
TORR = 1200 m

Clearly, V1min is causing a problem by keeping the V1 higher than we need for the runway distances in question

For a lower rated thrust case ....

RTOW = 85,000 kg
V1min = 95 kt
ASDR = 2000 m
TODR = 2200 m
TORR = 1700 m

By reducing the rated thrust, which reduces Vmcg and V1min (or minV1 .. whichever you prefer) we can push up the ASDA-limited RTOW. Provided that the increased TODR/TORR don't cause either of them to become more limiting than the ASDA case, then the end result is a modest increase in RTOW. All a matter of juggling the oranges and lemons ...

If Mutt sees this thread he can provide all sorts of real world examples as his big birds routinely have this problem ....