Fuel consumption and SAT

[V/S]

Hi Chaps,

I`m not sure about this one, could any of you help me out?
If flying at a certain altitude, with an SAT quite above the standard, thus warmer than usual, will fuel consumption be higher or lower?

Cheers

[DFC]

When the temperatre is above standard the fuel consumption (Kg/hr) will be higher than standard so the fuel used in a given time period will be increased.

When the temperature is above standard, TAS will be higher than standard so trip time will be reduced.

The various allowances will be in the fuel planning.

Regards,

DFC

[hawk37]

V/S, it really depends what you’re asking. If you’re at high level, and choose to fly constant mach which is above the max range mach, then flying into warmer, or colder air, will have almost zero effect on the specific range, ie the distance per weight of fuel burned. Looking closely at two business jet's fuel figures, it does seem to be VERY slightly more specific range at the colder temperatures though.
Hawk

[Old Smokey]

I have to agree with Hawk37 that a slight increase in fuel use will occur. Fuel flow is increased, but TAS is also increased. Typically, the Fuel Flow increase as a percentage is slightly higher than the TAS increase.

In cruise it's generally not a significant factor, however, additional climb fuel in hotter than standard conditions will make a significant difference to the overall sector fuel use.

Regards,

Old Smokey

[Elroy Jettson]

Fuel flow will increase, there is another consideration too. The temp rise will change your optimum altitude. If the temp rise puts you above your optimum altitude for your gross weight, fuel flow will increase signicantly.

[jonny dangerous]

The temp rise will change your optimum altitude. If the temp rise puts you above your optimum altitude for your gross weight, fuel flow will increase signicantly.

Elroy, not sure where the following info fits into the argument, however:
On the 737NG for instance, a change of OAT on the Perf Init page of the FMS (The FMS doesn't auto update this...), will lead to a recalculation of the Max Alt, yet not the Opt Alt.

As well, there are cruise performance charts (containing both max and opt altitudes) for ISA and below, ISA + 10, and ISA + 15. These charts, especially at heavier weights and higher temps, show a progressive change in the Max Altitudes, but the Opt Altitudes remaining the same. (Indeed, on the 26K-engined 800NG, at high weight and ISA + 15 degrees, the Max (1.3G) Altitude in some cases is LOWER than the Optimum Altitude. Weird.

Not sure why temperature doesn't affect the Opt Altitude calculation. It would seem intuitive that the increased climb duration in higher temps would lead initially to a lower cruise, or more appropriately, Optimum Altitude.

[hawk37]

Optimimum altitude charts (best specific range) I've seen are for a set mach number. For a 737 NG or A320, as I recall shows that optimum altitude (based on weight) does not depend on isa deviation. The line on the graph has always been well below the maximum altitude, although certainly, the max altitude decreases as temperature increases. Can't say I saw a line for ISA + 30 though
Hawk

[jonny dangerous]

I should clarify that the Max Alt indicated on the FMS is the lesser of the buffet limited Max Alt or the thrust limited Max Alt. With increasing SAT, it is the thrust limited Max Alt that is decreasing.

(Trying to avoid thread creep.)