Fuel consumption seal level vs. high altitude.

[AdamFrisch]

I've been trying to find a rough ballpark percentage figure for how much a turbines fuel consumption will reduce with height. Basically, what on average is your pph at low levels compared to on top for similar thrust settings? Thanks.

[Intruder]

Fuel flow is relatively constant as a function of Indicated Airspeed at a given gross weight. However, 300 KIAS at sea level is more like 500 KTAS at 30,000'.

For a constant TAS, 2% reduction in IAS per 1000' altitude is a quick rule of thumb.

[eckhard]

Hi Adam,

Intruder is correct; Fuel flow and IAS are fairly well linked at any altitude.

Your initial question is worded in an interesting way:

Basically, what on average is your pph at low levels compared to on top for similar thrust settings?

The key to understanding jet engine performance is to realise that Fuel flow is proportional to the thrust.
(For a piston engine, Fuel flow is proportional to the power which is thrust x TAS {or drag x TAS} )

So, to come back to your question, you can see that the fuel flow at sea level will be very similar to the fuel flow at high level for a similar thrust setting. Actually, due to engine efficiency at higher RPM, fuel flow will be slightly less at higher altitude at the same thrust.

The trick is, of course, that at high level the thrust setting required for a given TAS is much less than at sea level, due to the decrease in drag caused by the lower air density. That's why jets like to fly high! (Pistons, on the other hand, have to produce so much power at high level to achieve the higher TAS that the fuel flow increases to an uneconomical level {OK, grossly simplified, ignoring supercharging, etc, but you get the point}).

Some examples:

1. CitationJet CJ1+ at sea level. 180 kts IAS. N1 70% FF 720pph. TAS 180 kts
Same machine at FL410. 180 kts IAS. N1 100% FF 650pph. TAS 370 kts

2. B747-400 at sea level. Max EPR 1.72. Thrust 60,000lbs. FF 8,000kg/hr. (each engine)
Same machine at FL410. Max EPR 1.72. Thrust 12,000lbs. FF 2,000kg/hr. (each engine)

All above figures are from memory and after 1/2 a bottle of French red wine, so not to be taken too literally. But I think they illustrate the way that high altitude drastically affects the thrust and therefore the fuel flow.

An old chestnut often crops up along the lines of: "If we have to hold, we'd better stay high because our fuel flow at lower levels will be astronomical."

Actually, the fuel flow at a given holding IAS will be nearly the same at any altitude, as explained above. There will be a slightly better fuel flow at high level, due to the engine RPM being higher and more efficient, but it will only be 5-10% better than at low level.

Now if you plan on diverting a long way after holding, or if there is icing at lower levels, that's a different kettle of fish entirely, and in those cases you would be better off staying high.

Hope that helps. (Now, where's that bottle.....?)

[Intruder]

All above figures are from memory and after 1/2 a bottle of French red wine, so not to be taken too literally.
If you had drunk Washington wine, maybe you'd be able to remember the 747 numbers better... ;)

[AdamFrisch]

Interesting. I didn't know that.

So explain again why the TAS gets higher when you go high? Is it less wind resistance in the thin air, or the coldness or what? And is the same true for Turboprops and Pistons - the higher you go, the higher the TAS?

[Intruder]

The airspeed indicator measures ram air pressure, and is calibrated to standard sea-level atmosphere. Thinner air has less pressure, so the airspeed indicator will read low at higher altitudes. Thus a correction for altitude and temperature has to be made to get True Airspeed (TAS).

Jet engines can still operate efficiently at high altitudes where drag from air resistance is significantly reduced. Piston engines dot not, due the the combination of propeller inefficiency at high altitude and the more limited power output. Supercharged piston engines and turboprop designs each mitigate some of the limitations, thus work best at moderate altitudes.

[BelArgUSA]

Long ago, I used to teach pilots transitioning into 727 airliners.
They were out of a Cessna or Piper airplanes, even Citations.
Was in USA, so then, I used pounds per hour fuel burn.
So, I used to lecture in the classroom, like this -
xxx
The 727 at sea level burns 2200 lbs FF per engine to fly at 220 knots (IAS).
A certain fuel flow gives you a certain IAS... we just deduct a 0...
At sea level, as mentioned above, IAS equals TAS.
At 35,000 feet, a fuel flow of 2700/engine still gives you 270 knots IAS.
But up there, 270 KIAS becomes some 470 KTAS.
xxx
If of any interest to you - not trying to confuse the issue here...
A 727 has 3 engines, so flying at 250 KIAS, sea level, burns a total of 7500 lbs.
Let us fail an engine...
So, how much fuel flow needed on 2 remaining engines to fly at 250 KIAS...?
Same total fuel flow... that is 7500 lbs total burn.
But this time, with 2 engines, it is 3750 FF per engine to get 7500 FF total.
xxx
See, you have to be a brain surgeon or rocket scientist to be airline pilot.
Of course, with the JAR fATPL exams, takes a week to learn the above science.
These guys would correct me to read that it takes 3751.5 lbs FF/engine.
And advise me that "their FMS this and that..." -
xxx
Hey, Intruder, is the wine a Haut Médoc, or a St. Emilion 1971...?
:8
Happy contrails

[Intruder]

A good Syrah or Zinfandel from Walla Walla is all you need!