Airline Pilot Central Forums - View Single Post

02-08-2015 | 06:35 AM

#3

Adlerdriver

Gets Weekends Off

Joined: Jul 2007

Posts: 4,064

Likes: 37

From: 767 Captain

You have some correct information and you're mixing it in with some not so correct.

For every takeoff, performance data is calculated that includes the highest temperature the aircraft can safely takeoff on a specific runway under the current conditions. If that temperature is higher than the actual OAT, it becomes the assumed temperature. Sometimes it's very high, like the 50C you mentioned, other times it's lower (but still more than actual).

Jet engines produce reduced thrust as the ambient air temperature increases. The flex temp is the highest air temperature at which the engines would produce the required takeoff thrust. The crew enters it into the flight management system (FMS) — which, as you said, tells the aircraft to assume the specified air temperature instead of the actual temperature. When flex (FLX) thrust is selected during takeoff, the engine controller produces maximum thrust for the assumed (flex) temperature.

Quote:

Originally Posted by AF330

*
Basically, as air get's cold, more it get's denser (more air molecules). Yes. So if air is hot, less you have air molecules so you won't be able to get the required thrust (so the turbine will not turn). If the engine is running, the turbine is definitely turning. But, yes, at a higher temperature, you will get less thrust at the same RPM.

So you basically tell the plane that the outside temperature is very hot (not true), so the plane will think that as the outside temperature is already very hot, Yes
if you add to much fuel (and compress a lot), the temperature will get to high and the air (+ fuel) won't be able to turn the turbine to get thrust. No, the FADEC will take care of fuel control, so basic engine operation at high temperatures isn't going be a problem. You will just have less thrust. Again, the turbine will still turn. If the turbine stops turning, you have much bigger problems than just high OAT.

FLEX TEMP = TOGA with the selected temperature. Yes
So if you enter 50°, it means that if the outside temperature would have really been of 50° and if you would have used TOGA, then you would have got the Flex temp.* (Don't understand what you're trying to say)

At a certain temperature, air + fuel won't be able to produce thrust (mostly no air molecules). If the engine is running, it can produce thrust. True, at high temperature (low density) the jet engine will produce less thrust at a given setting.
If the outside air temperature is very cold, we will be able to use more fuel and get a higher RPM. At a specific RPM setting, the engine will use more fuel at a colder temperature. The air will be denser and therefore, more fuel will be mixed to maintain the correct fuel-air ratio. Simply decreasing the temperature will not produce a higher RPM. I suppose it's possible that at extremely high OAT, a jet engine may not be able to attain it's highest rated RPM due to decreased air density.
*
1) Am I right? (about some stuff, yes)
*
2) So with TOGA, you won't get 0,001% thrust more. More than what? So does FADEC use the outside temperature to calculate the required amount of fuel to turn the turbine/fan at the highest RPM possible? It uses the OAT to calculate Takeoff power (Max power or TOGA - many names for it). That takeoff power is determined by a specific RPM. FADEC (along with other engine components) will control fuel flow to produce that RPM. It's not the highest RPM possible. It's what's needed to produce the Max power at that temperature. So the temperature and the weight of the plane is very important...am I right? Yes.

Thanks a lot!

Reply

0