This might not be noticed my most people, however the high altitude the fuel flow is exactly the same at sea level at the same throttle settings, this might be what causes the fighters go supersonic this easily, at high altitude, the engine should only produce a portion amount of thrust as the plane climbs, for example, at FL 360 I should only produce 22% of the thrust at sea level, so the airplane should not be flying supersonic.
Generally you’ll find fuel flow at low altitude very similar to that at high altitude for a given IAS.
The 737 burns around 1200kgs per engine per hour at 250kts regardless of altitude.
But then the fuel to air mixture would be off. How do they fix it?
It’s a very complex answer for such a simple question. Basically there is an excess of thrust at lower altitudes so for the same IAS the engine RPM will be a lot lower, for the same fuel flow, and higher at high altitudes. Then throw in many factors like (but not limited to) air density, air temperature and lots of clever gizmos before the combustion chamber like variable stator vanes to control compression ratio.
You’re probably better of asking the internet if you want the perfect answer.
Would be very cool at airports such as Cali and Eagle/Vail.
Correct me if I’m wrong but this is why high altitude airports generally need a longer runway right?
Pretty much. To achieve take of thrust at high altitude airports you have to run the engine at a much higher temperature. Then you start getting temperature limited on the engine so in certain circumstances you won’t be able to achieve what you’d consider normal take off thrust.
my textbook says both fuel flow and thrust drops, but thrust drops more due to loss of pressure thrust.
Ok, does it say what fuel flow reduces in relation to?
yes the fuel flow drops.
Yes sure if you changed nothing else. What happens in reality is that engine RPM increases gradually (a function of the FMS), as you climb, to minimise thrust loss.
You’ll find that N1 and N2 will be higher at high altitude for a similar IAS. To maintain the higher engine RPM you need to increase fuel flow.
I mentioned that assuming same throttle settings. Cause in IF I tested on ground run-up and flying at that throttle setting. Good point though. My IAS is obviously different.
Ok, I missed the bit about same throttle settings.
In that case yes fuel flow would reduce.
This was a problem for me on Sat. 16 March. I was flying from Barrow to Anchorage (AS50) B738 and around 34,000 feet I was losing airspeed with the speed locked at 240 knots and dropped below 200 so I had to level out at 35,000 feet instead of my filed 37,000.