Cruise altitude

[B767Driver]

Originally Posted by NJA_Capt

I don't have a book at home, but I ran it through the FMS software. For a 2 hour flight, the FMS calculates an "optimal" cruise at FL280. Running a "What If" calculation for FL370, the leg time decreases by 1 min and 1000# fuel. For a 3 hour leg it was also 1 min quicker / 2000# less fuel.

They probably use an LRC thrust setting, which means you are producing more thrust at higher altitudes. Sacrificing some efficiency for better speed.

If the computer used identical thrust settings for each altitude...you'd go faster at FL280.

So I doubt the computer is using comparable power settings to make a valid comparison.

Plug the numbers into an E6B or other flight computer. See what the numbers come out to be.

i.e. See what TAS values are for 300 KIAS at FL200, FL220 up to FL280...then TAS values for Mach .78 up to FL420. This should prove a more valid comparison.

[NJA_Capt]

Originally Posted by B767Driver

They probably use an LRC thrust setting, which means you are producing more thrust at higher altitudes.

If the computer used identical thrust settings for each altitude...you'd go faster at FL280.

So I doubt the computer is using comparable power settings to make a valid comparison.

Both simulations were run on identical flight plans and both at MCT. The only difference was the first allowed the FMS to choose the optimal altitude and the second I chose FL370.

[staplegun]

Originally Posted by tgrayson

Interesting. On page 5 he says:

As you can see in the graphs to the left there should be no change in the maximum endurance of the aircraft with altitude.

You have to keep reading that whole section to get his drift:

As you can see in the graphs to the left there should be no change in the maximum endurance of the aircraft with altitude. However, the required endurance speed will increase. As before maximum endurance always occurs at L/Dmax. (i.e. always the same angle of attack.)

On web page 8 we will discuss the effect of engine and propeller efficiency.


Then he goes on to say on the top of the Range and Endurance Summary page 9:

Previously we made a preliminary performance summary, which did not include consideration of engine efficiency. Based on the preceding web page we can now make a final summary of range and endurance performance for different types of aircraft.



Then under Jet Aircraft heading states:

The TSFC of the jet engine improves up to the altitude for the coldest air temperature. In the ISA this is the tropopause. (TSFC holds constant in the stratosphere.)

1. Endurance will increase with altitude as long as temperature decreases with altitude. Maximum endurance will therefore occur at the tropopause.



Taken together, this would indicate that Max Endurance in a turbojet aircraft does in fact change with a change in altitude.


I concur with this, and have observed it to be true while flying turbojets, hence my puzzlement at your initial statement...



Kevin

[tgrayson]

Originally Posted by staplegun

Taken together, this would indicate that Max Endurance in a turbojet aircraft does in fact change with a change in altitude.

No, I concur with it too. His statement that minimum FF is altitude independent is not true.

[B767Driver]

Originally Posted by NJA_Capt

I don't have a book at home, but I ran it through the FMS software. For a 2 hour flight, the FMS calculates an "optimal" cruise at FL280. Running a "What If" calculation for FL370, the leg time decreases by 1 min and 1000# fuel. For a 3 hour leg it was also 1 min quicker / 2000# less fuel.

I'm assuming wind was not involved in this calculation. The above doesn't make sense to me.

I ran through some performance charts for Mach .80 cruise for the 767. From FL260 to FL430, the TAS decreased linearly above FL 280. So did the fuel flow.

I wonder why the FMS calculated FL280 as optimum? It is showing higher altitudes going faster and burning less gas.

The only other explanation would be a buffet boundary/stall protection issue...but there is no way at FL280 that would be an issue at even the heaviest of weights...and least I would assume.

That makes me wonder if some winds were programmed in the scenario?

[seagull]

I didn't look at this whole thread, but what immediately comes to mind is to inquire what the "cost index" used was. That can make a huge difference, due the the cross-over between the value of time and fuel.

There can be other factors as well. Not sure what type aircraft, but the FMS is not the most sophisticated box out there....