Headwind question

[SLT]

Was asked this in a recent interview..............

When flying into headwinds of greater than 100 kts, TAS should be decreased in order to improve Ground NM per gallon.

True
False
Makes no difference

Whaddaya reckon.................:) I know what I said, but just interested............

[quid]

False....in general. But I can make a case where it would be True.

[*Lancer*]

If you're above max range speed (TAS), then True. If you're at or below it, then false!

[john_tullamarine]

Consider

TAS = 100 kt
HWC = 100 kt

SGR = (100-100)/FF = 0

Moving the levers forward increases TAS (and FF) but

SGR = ((100+D1)-100)/(FF+D2) > 0

Similarly, a modest increase in SGR may attend a reduction in TAS in a strong tailwind, although this is normally not done as we would be somewhere near the max SAR speed anyway if looking for maximum range.

Actual numbers and whether it is worth worrying about changing speeds will depend on the aircraft characteristics and the wind on the day. Often, in a strong wind (eg jetstream), the sensible option is to go lower and get out of it (and the often associated turbulence).

We ignore the generally spurious case of flying at speeds less than max SAR for the range case, of course.

[Notso Fantastic]

Don't agree Lancer-I think you fly faster into strong headwinds period. As pointed out, flying slower, you approach the speed of the wind leading to terrible range, so flying faster helps compensate for the higher fuel flow, and it minimises your exposure to that wind.

[Intruder]

When flying into headwinds of greater than 100 kts, TAS should be decreased in order to improve Ground NM per gallon.

Depends on current TAS!

IF you are currently at no-wind max range airspeed, then you increase airspeed in a headwind to increase range.

The limit will differ with airplanes, depending on where the "knee in the curve" of fuel flow vs IAS is.

For example, in the 744, you will seldom go above 0.86 IMN because fuel flow increases drastically above that, even though Mmo is 0.92.

A rule of thumb is to increase TAS by about 40-50% of headwind component. Again, YMMV between airplane types.

[SLT]

Thanks for your input guys - I agree with all of the above! Bit of a simplistic question I thought, seeing as there are so many things whch can make a difference in real operations. The 330 is similar to the 744 then - the FMGS will order a faster speed when flying into a headwind up to a point, as fuel usage increases dramatically above .83.
Oh well - no point in worrying about it now - just sit back and wait for the yea or nay!!!!

Cheers all :O