Fuel burn below/above tropopause

[Walter Sobchak]

Hello,

does somebody know which of this theories apply(if at all) in terms of overall fuel consumption during cruise in a no wind situation?

1.

below tropopause

Alt incr.
Drag decr.

but

T decr.
TAS decr.
Flighttime & Fuelburn incr.

above tropopause

Alt incr.
Drag decr.
T no change
TAS no change
Flighttime no change
Fuelburn decr. because of drag benefit

the aim would be to fly higher than the tropopause to save fuel

2.

below tropopause

Alt incr.
T decr. up to tropopause
Efficiency incr. better fuel burn

above tropopause

no need to climb above because T remains and therefore no benefit


rgds Walter

[bookworm]

The key parameter, which is affected by both temperature and altitude, is density. The drag, and therefore the fuel energy required per unit distance travelled, decreases with decreasing density.

Whether above or below the tropopause, the density decreases with altitude. Thus if fuel economy is your objective, and all other things equal, higher is better, regardless of the temperature.

[Old Smokey]

For a given weight, there will be a most efficient speed (MRC, Maximum Range Cruise) for each and every altitude from Sea Level up to the maximum operating altitude of the aircraft. For a given weight, this will be a constant EAS (increasing IAS) up to the Pressure Height where Mcrit is reached, and MRC is in accordance with Mach Number thereafter.

If all of the possible Altitudes are examined, there will be one (and one only) where MRC speed is achievable at the optimum Thrust Specific Fuel Consumption RPM, this is the Optimum Altitude, and will steadily increase as fuel burns off, weight reduces, and the MRC speed decreases. Higher or Lower engine speed requirementss, i.e. Higher or Lower altitudes respectively, will be at "off optimum" engine speed, and range will suffer, with a 'too high' cruise suffering MORE than a 'too low' cruise.

As we climb through the Troposphere, Pressure reduces, Temperature reduces, and Density reduces. Density reduces at a slower rate than does Pressure, because there is a degree of "Temperature Recovery" due to the decreasing temperature, remembering that Density is a product of Pressure divided by the Absolute Temperature. When the Tropopause is passed, Temperature is constant, there is no "Temperature Recovery" of Density, and Density decreases much faster than below the Tropopause.

Air Density is a huge factor in the amount of thrust produced by a jet engine, thus, if we climb too high into a lower density region, engine speed must be increased to recover the required thrust. If this engine speed is above optimum TSFC, range and overall fuel burn will suffer. So, flight in a low density region may be desirable from the point of view of Drag, but terrible in terms of fuel burn required to produce the thrust to overcome the (lower) drag. A further major consideration is that the fuel burn for the step climb too soon will be excessively high.

When considering range, airframe drag cannot be considered in isolation, Airframe AND engine efficiency must be considered together.

Day to day operations for aircraft such as the B777 (with which I am most familiar), indicate a 2000 ft 'step' in Optimum Altitude every 700 to 800 miles when below the Tropopause. When the next step is above the Tropopause, it is common to see the next step in 1500 miles or so, or, more commonly, not at all. (Operations at light weights will see an early step there).

Flight above the Tropopause is viable, it happens every day, but a too early climb there will have dire consequences upon overall fuel burn (amongst other things). The Fuel Flow might look 'nice and low', but the speed will be 'nice and slow'. It's fuel per mile that matters, not the fuel flow in isolation.

P.S. Now doing the cowardly thing and going to work for 4 days, Flak jacket at the ready.

Regards,

Old Smokey

[Walter Sobchak]

thanks for the explanation.

Walter Sobchak