Speed reduction in cruise?

[Jumbo fodder]

Is it true that the TAS decreases as the flight progresses? (Can't find my flight planning notes) If it does, is it due to the CofG moving forwards with fuel burn requiring an increased down load on the tailplane thereby increasing the induced drag???

Or is it just because the aircraft is lighter and therefore requires less thrust to maintain Vmid, so the aircraft is allowed to slow?

Cheers
JF

[expedite_climb]

Not sure how much difference it will actually make, depends if the fmc allows for this when calculating an econ speed.

If you are flying at a fixed mach no, then your tas should remain the same (all else remaining equal).

Also depends on the a/c. On the 757 the centre tank is slightly forward of the wing tanks (due sweep), so if you have more than 13 tonnes CofG when you burn fuel your C of G moves backwards. Once you have burnt all centre fuel and start on wing fuel then the C of G will move forward.

[alatriste]

It depends on your cruise regime:

LRC: Yes, your TAS and M decrease as you burn fuel.

Constant M number: Tas remain the same regardless of GW.

Optimium/ Economic M number: M and TAS will be reduced as GW decreases.

Constant N1 (not used for jets): As fuel is burned TAS will increase, TAS will also change depending on CG shift. CG moves forward TAS decrease and viceversa.

[Jumbo fodder]

Thanks for the responses, all the information is in handling the big jets but it's hard to piece it all together sometimes.

Cheers
JF

[GlueBall]

Above FL 290 we normally cruise at M.84 or M.85, irrespective of gross weight or Center of Gravity changes. At and below FL290 we normally cruise, climb, and descent at 320 KIAS. The FMS in "Thrust Management" mode commands the Auto Throttles to fly at the set Mach or KIAS.

[Keith.Williams.]

The maximum range cruise speed (VMCR) for a jet is (in theory) where a tangent drawn from the origin touches the drag curve. It turns out to be at about 1.32 VMD. VMD is proportional to the square root of weight. So as a flight progresses, the consumption of fuel gradually reduces weight, VMD and VMCR.

[livingthedream]

Long Range Cruise is based on a specific angle of attack therefore as fuel is burned and the aircraft becomes lighter the speed is reduced to meet the reduced lift requirement.

[softpop]

In case of LRC and Econ Crz the spd decreases as ther flt progresses to account for the reduces a/c wt