3 ms·

Can't really tell without knowing their weight and available cruising levels. But in general you should not run a commercial jet engine at lower thrust settings

Can't really tell without knowing their weight and available cruising levels. But in general you should not run a commercial jet engine at lower thrust settings. The optimum altitude as calculated by the flight management system is essentially the altitude at which 90% thrust gets you the best cruise speed. That's also why flying low is really bad for fuel consumption, you're going to have to decrease thrust and you'll be aerodynamically less efficient at the same time.

WalterBright 7y ago
At lower altitude the jet has to push aside a lot more air.
- NikolaeVarius 7y ago
  Yes, but to the previous posters point, turbofans operate much less efficiently at lower altitudes. They are optimized to run the majority of cruise at a very specific set of altitude and power output settings. This is compounded by the fact that fuel load also assumes a set of assumptions of the flight plan
dnhz 7y ago
This sounds wrong to me because I've read that the plane uses 90% thrust or something around that to take off, and at cruise, less thrust is needed.
- VBprogrammer 7y ago
  I'm not sure where the 90% comes from. What is it refering to? Rotor stage RPM, static thrust, fuel flow? In general turbine engines are limited mostly by the temperature at the first stage of turbine blades after the combustion stage. It's hard to get reliable instrumentation in this zone so a proxy temperature is often taken from a later stage. In older airliners there was a table of max ITT values varying with altitude and outside air temperature. In modern FADEC engines the computer does the same calculations. It's not unusual for smaller jets to be set at their max continuous thrust setting shortly after takeoff and be left there until the beginning of the descent. Modern airliners will be flying a cost optimised Mach number. That Mach number is entered on their flight plan and is used as a basis for ensuring they are separated from other flights on the north Atlantic tracks so they wouldn't be able to change it readily. The interesting part of all of this is that they would have been assigned an altitude for the crossing, at high altitudes maximum speeds drop (because flutter margins are proportional to velocity rather than effective pressure) but stall speeds increase so the range of viable flying speeds is actually quite limited at cruise altitudes. They probably couldn't have slowed down if they wanted too.
  - dnhz 7y ago
    I was referring to the N1 and N2 values. But I realized I was wrong and that jet engines do operate at say 90% N1 during cruise.

WalterBright 7y ago

At lower altitude the jet has to push aside a lot more air.

NikolaeVarius 7y ago
Yes, but to the previous posters point, turbofans operate much less efficiently at lower altitudes. They are optimized to run the majority of cruise at a very specific set of altitude and power output settings. This is compounded by the fact that fuel load also assumes a set of assumptions of the flight plan

dnhz 7y ago

This sounds wrong to me because I've read that the plane uses 90% thrust or something around that to take off, and at cruise, less thrust is needed.

VBprogrammer 7y ago
I'm not sure where the 90% comes from. What is it refering to? Rotor stage RPM, static thrust, fuel flow? In general turbine engines are limited mostly by the temperature at the first stage of turbine blades after the combustion stage. It's hard to get reliable instrumentation in this zone so a proxy temperature is often taken from a later stage. In older airliners there was a table of max ITT values varying with altitude and outside air temperature. In modern FADEC engines the computer does the same calculations. It's not unusual for smaller jets to be set at their max continuous thrust setting shortly after takeoff and be left there until the beginning of the descent. Modern airliners will be flying a cost optimised Mach number. That Mach number is entered on their flight plan and is used as a basis for ensuring they are separated from other flights on the north Atlantic tracks so they wouldn't be able to change it readily. The interesting part of all of this is that they would have been assigned an altitude for the crossing, at high altitudes maximum speeds drop (because flutter margins are proportional to velocity rather than effective pressure) but stall speeds increase so the range of viable flying speeds is actually quite limited at cruise altitudes. They probably couldn't have slowed down if they wanted too.
- dnhz 7y ago
  I was referring to the N1 and N2 values. But I realized I was wrong and that jet engines do operate at say 90% N1 during cruise.