TOT and altitude

[Robbo Jock]

Talkturn said on the 'Jetranger at 20000 ft' thread:

TOT (turbine outlet temperature) is the limiting factor.

It's been a while since I did "Engines". Why does the TOT increase with altitude ?

[Head Turner]

Ratio of air to fuel. Think of it as a weak mixture scenaro. The FCU will adjust fuel flow for normal operations, anywhere near limits the FCU will not compensate for those atmospheric variations. Add this ti compressor and power turbine blade performance criteria and there you have it. You'll never get an engine that is all things to all men. They are designed within a tight set of requirements and this will post a set of limitations.
Or were you seeking a more scientific reply?

[Shawn Coyle]

Typically, the TOT will be the limiting factor in hotter than standard conditions. It will depend on the air temperature at the altitude. Standard temperature at 20,000' is (following some quick calculations on the included calculator) should be about -25C. If the temperature is much warmer than that, the engine will be running closer to the TOT limit than if it were standard.
Personally, I would suspect that the N1 limit were probably the limiting factor for this altitude. N1 goes up quite markedly in that series of engines at high altitude.
The fuel control should be maintaining the fuel to air ratio quite nicely - the compressor just has to turn faster to cram all the air it needs.

[Robbo Jock]

Thanks chaps.

Nothing more scientific than that was required.