ISA vs Altitude

[ElNull]

Can someone please explain how ISA deviation affect optimum and maximum altitude?

[Skyjob]

The long version:
The FMC generates optimum and maximum altitude data to assist the flight crew in choosing the best cruise altitude for a given trip. The calculated maximum altitude is used to ensure that the predicted path of the airplane remains within its performance capabilities. This maximum altitude is the lower of the thrust-limited altitude and the limits of the VNAV operational envelope.

The computation of optimum altitude includes an estimate of the climb fuel burn between the current altitude and optimum altitude, unless the airplane is above the optimum altitude. If the current altitude is above the optimum altitude, the computations are performed at the current gross weight.

The thrust- limited altitude is shown for temperatures of ISA, ISA + 10o
C, ISA + 15o C, ISA + 20o C, and has been computed assuming maximum cruise thrust and a residual R/C margin of 100 FPM.

These thrust rating and residual R/C margins are the standard FMC defaults used to calculate the thrust-limited altitude during the climb flight phase.

Both assume standard day temperature conditions. The thrust-limited altitudes determined may be corrected for other temperature conditions and residual rate of climb values.

[FE Hoppy]

Forget the FMC.

Density is your answer.

[ElNull]

Does this mean in case of weather colder than standard (ISA minus) there will be higher density and optimum altitude will increase?


Also is that's why during a temperature inversion optimum altitude equals maximum altitude?

[underfire]

Perhaps it would be best if you went back to the ISA definition, and go from there, 0 MSL, 15 degrees C, and 1013.25 hPa (1 atm)

[barit1]

When OAT is significantly above ISA: http://www.pprune.org/rumours-news/148490-pinnacle-airlines-aircraft-incident-2.html#post1571598

[Microburst2002]

Density aside, temperature itself affects greatly engine performance. The maximum level you can achieve is therefore affected by temperature. It is more convenient to express this effect in terms of ISA dev than in specific temperatures.

[FE Hoppy]

M2002.

The reason temperature affects engine performance is due to it's relationship with density! You can't put it aside. It is the reason.

http://en.wikipedia.org/wiki/Density_altitude

[barit1]

The engine, just like the wing, works on the mass of its working fluid (i.e. air). Mass per unit volume is (TaTaaa!) DENSITY.

So both the airplane and, independently, its engine lose performance as density decreases with altitude, and further decreases with increased temperature.

But you knew all this in your PPL written exam, right? :8

[Gysbreght]

For turbine engines, density is one effect, temperature is another, particularly near the engine operating limit, which is what we are discussing here. That operating limit is defined by the internal temperatures within the engine, and engine performance is then a function of the ratio between gas temperature limit and ambient temperature. Therefore turbine engine performance parameters are usually normalized with ambient pressure and temperature, not just density.

[FE Hoppy]

Gysbreght

Read the OP. Effect of ISA deviation on optimum and maximum altitude.

ans. Density.

[ElNull]

Thanks everyone. FE Hoppy probably that explains why during temperture inversion or when OAT above ISA the maximum altitude decreases and sometimes they become same as optimum altitude on FMC.

[Microburst2002]

Gysbreght, you know your staff

For turbine engines, density is one effect, temperature is another, particularly near the engine operating limit, which is what we are discussing here. That operating limit is defined by the internal temperatures within the engine, and engine performance is then a function of the ratio between gas temperature limit and ambient temperature. Therefore turbine engine performance parameters are usually normalized with ambient pressure and temperature, not just density.

Floppy, when I say
Density aside,…
I mean apart from densite, or in addition to the density factor.
English is not my mother tongue, anyway

I know that density has an effect on engine performance, but temperature by itself also has an effect on engine performance. however Gysbreght explained it much better

Barit1, mass flow is mass per unit time, not volume

[barit1]

Barit1, mass flow is mass per unit time, not volume

Correct, Microburst2002, but my statement was:

Mass per unit volume is (TaTaaa!) DENSITY.

[FE Hoppy]

Again I will just comment that the OP is talking about optimum and maximum altitude. At optimum altitude you are no where near the engines TET limit due to the ambient being minus bejesus. The answer to the OP is density.

[deltahotel]

Blimey. Don't over complicate it. If in doubt, listen to Hoppy. IT'S ALL ABOUT DENSITY.

[Owain Glyndwr]

But the OP asked about optimum AND maximum altitude. By definition if you are talking about maximum altitude you are talking about operating at maximum cruise power and the TET limit that goes with that.
Although you can say that thrust varies with density that is not a complete statement. A more accurate statement would be that at a given rpm thrust varies with density.
As temperature above ISA increases the rpm has to be reduced to keep inside the TET limit,
So a full answer to the OP is not as simple as "density"

[Microburst2002]

Also, reduced density has some benefits for Optimum level.
The less the density or the more the temperature the more the TAS for a given Mach, which gives better specific range.

Density is very "good" at low level, and yet the optimum levels are very high, because of specific range. Given that fuel flow is approximately proportional to thrust, for the same EAS you have more TAS at altitude while having the same drag, which needs the same thrust. We get more miles for the same money.

However, for a constant mach number EAS also increases for with increasing temperature, which will increase drag, I suppose, plus compressibility effects compared to same EAS at very low level. Some more thrust is needed with increasing temps, I guess.

It seems to me that there is a direct relationship between MAX and OPT levels, like they depend on the same things. When I fly at MAX, I have noticed that my engines are giving me already maximum thrust. I have literally advanced the thrust levers to TOGA and the N1 did not increase a bit of a bit. I don't know if engine temperature limits play a part in that, or if its something else like rpms, but probably the engine at that condition has more thrust specific fuel consumption than at the optimum level, to the extent that it pays flying at that level instead.

[FE Hoppy]

Also, reduced density has some benefits for Optimum level.:=

You probably need to rethink this!

[Goldenrivett]

Hi Microburst2002,
When I fly at MAX, I have noticed that my engines are giving me already maximum thrust. I have literally advanced the thrust levers to TOGA and the N1 did not increase a bit of a bit.
Why on earth did you feel the need to select TOGA?

If autothrust was engaged, see FCOM, Performance, THR Thrust ratings, MCR Maximum Cruise DEFINITION

"It is the maximum thrust approved for normal cruise.
There is no thrust lever position corresponding to this thrust rating.
It is not displayed to the pilot, and the N1 limit which is displayed in cruise is the maximum climb N1.

The FMGS uses the maximum cruise N1 to compute the aircraft maximum speed.

In manual thrust setting, in cruise, the pilot should limit N1 to the maximum cruise N1 that is equal to the displayed maximum climb N1 minus ​1.9 %."

[Microburst2002]

"And yet it moves" as Galileo put it.

We were in cruise, very heavy, MAX level, very narrow margin between VLS and VMAX. It took us all the morning to climb there! The N1 and the limit lines all together. The captain tells me to advance the lever. I say "no way", then says go ahead, my responsibility I want to show you something. This dude really knows the airplane, so I say "OK, you do it, it's your ass" (it would be mine, too anyway, but…) So he does and to my surprise, absolutely nothing moves. Not a decimal of N1, nor EGTº, nor N2. Nothing!

Of course, if you do this being light it would be different, I'm just saying that it is possible to fly at an altitude and conditions such that this experiment can be done. Don't do it at home, though!

I was impressed, and tt made me rethink the turbulent penetrating speed. In those conditions accelerating a knot is very difficult, but decelerating 10 is very easy...