krisyyz
Posts: 1266
Joined: Thu Nov 25, 2004 11:04 pm

Engine De-rating And Outside Temperature

Fri Jul 30, 2010 12:33 am

Hello!

I have been looking at information on the net and on this forum about engine de-rating. However, I still have one question that I haven’t been able to find the answer to anywhere. I know that the ability to de-rate an engine depends on the weight of the aircraft, the wind and the length of the runway. But how does the outside temperature affect engine de-rating?

I understand how hot and high conditions affect the aerodynamic properties of an aircraft wing and its performance, but I still don’t understand how hot and cold weather affect aircraft engine performance.

Is colder weather better for an aircraft engine? Can you de-rate an engine to a greater extent when the outside temperature is colder?

How hot is too hot to de-rate an engine on take-off? Or does that depend more on the runway in use and the type of aircraft?

Lets say that a B763ER weighing 280,000lbs is taking off on YYZ’s Rw 24R (9697ft), what would the de-rating setting be when it is +25C and -25C ?

I hope I have explained my question properly. Many thanks in advance for any information!


KrisYYZ
 
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Starlionblue
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RE: Engine De-rating And Outside Temperature

Fri Jul 30, 2010 12:37 am

Quoting KrisYYZ (Thread starter):
I understand how hot and high conditions affect the aerodynamic properties of an aircraft wing and its performance, but I still don’t understand how hot and cold weather affect aircraft engine performance.

Is colder weather better for an aircraft engine? Can you de-rate an engine to a greater extent when the outside temperature is colder?

Colder air has higher density, and therefore you get higher mass flowing through the engine. Thus colder air -> more thrust, all else being equal.

The extreme of this was water injection, where water was sprayed into the engine to increase air density and give extra oomph.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
kingairta
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RE: Engine De-rating And Outside Temperature

Fri Jul 30, 2010 1:02 am

Denser air means more oxygen. More oxygen means bigger bang.
 
tdscanuck
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RE: Engine De-rating And Outside Temperature

Fri Jul 30, 2010 1:29 am

Quoting KrisYYZ (Thread starter):
But how does the outside temperature affect engine de-rating?

We need to distinguish between derate and assumed temperature method (ATM)...they're not the same thing. Outside temperature has no impact on the engine's ability to derate, although it has a big impact on whether you actually want to derate.

Derate is just telling the engine to run as if it had a lower thrust rating than full. ATM is telling the engine that it's hotter outside than it really is, which causes a thrust reduction.

Quoting KrisYYZ (Thread starter):
but I still don’t understand how hot and cold weather affect aircraft engine performance.

Density is the big factor...cold air is denser and therefore allows the engine to generate more thrust. The important other factor is exhaust gas temperature (EGT) margin...if the air coming in is hotter, it's going to be hotter exiting the combustor (for the same fuel/air ratio) and, at some point, you'll hit the limit the turbine can withstand. You hit the limit earlier on hot days.

Quoting KrisYYZ (Thread starter):
Is colder weather better for an aircraft engine?

Yes.

Quoting KrisYYZ (Thread starter):
Can you de-rate an engine to a greater extent when the outside temperature is colder?

From a pure engine function point of view, no. But you can apply a bigger derate and get off the runway safely on a cold day than a hot day.

Quoting KrisYYZ (Thread starter):
How hot is too hot to de-rate an engine on take-off? Or does that depend more on the runway in use and the type of aircraft?

It depends on a lot of things. The big hitters are the length of the runway, flap setting, weight, altitude, and wind. Very high temperatures may actually force you to do a derate due to the EGT margin problem discussed above.

Quoting KrisYYZ (Thread starter):
Lets say that a B763ER weighing 280,000lbs is taking off on YYZ’s Rw 24R (9697ft), what would the de-rating setting be when it is +25C and -25C ?

Not enough information...you also need to know the wind, flap, and whether or not you're using ATM on top of a derate.

Tom.
 
krisyyz
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Joined: Thu Nov 25, 2004 11:04 pm

RE: Engine De-rating And Outside Temperature

Fri Jul 30, 2010 6:36 pm

Quoting Starlionblue (Reply 1):
Quoting KingairTA (Reply 2):
Quoting tdscanuck (Reply 3):

Thank you!!

Quoting tdscanuck (Reply 3):
We need to distinguish between derate and assumed temperature method (ATM)...they're not the same thing. Outside temperature has no impact on the engine's ability to derate, although it has a big impact on whether you actually want to derate.

Derate is just telling the engine to run as if it had a lower thrust rating than full. ATM is telling the engine that it's hotter outside than it really is, which causes a thrust reduction.

Thank you very much for all the information!!

So, lets say on a B767, the pilot enters +36C on the TO ref page, that is the ATM? Doesn't entering an assumed temp in the FMC automatically de-rate the engine as well?

TO/CLB 1 and 2 de-rate the engine by limiting the N1, while entering an assumed temp limits the thrust output, am I correct?

Thank you!

KrisYYZ
 
tdscanuck
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RE: Engine De-rating And Outside Temperature

Sat Jul 31, 2010 3:59 am

Quoting KrisYYZ (Reply 4):
So, lets say on a B767, the pilot enters +36C on the TO ref page, that is the ATM?

Yes.

Quoting KrisYYZ (Reply 4):
Doesn't entering an assumed temp in the FMC automatically de-rate the engine as well?

No. De-rate and ATM are separate methods to achieve (basically) the same thing. You can use either, or both at the same time.

Quoting KrisYYZ (Reply 4):
TO/CLB 1 and 2 de-rate the engine by limiting the N1, while entering an assumed temp limits the thrust output, am I correct?

Not exactly...TO/CLB 1 and 2 may derate by limiting N1 (fan speed) or EPR (basically thrust).

It's usually easier to explain this stuff by example. Suppose you have an airplane with an engine rated for 100k pounds of thrust (imaginary, but round numbers are convenient).

Assuming it's not so hot out that you're above the flat rated temperature, that engine would produce 100k in TO mode. It would provide some lower numbers for CLB1 and CLB2 (say 90% and 80% of TO, respectively). So the thrusts would be:
TO: 100k
CLB1: 90k
CLB2: 80k

You could also have a derate, TO-1. For convenience, let's say that's 10%...in that case, TO-1 would be 90k.

Selecting TO-1 is saying to the engine "act like you're a 90k engine".

Now, if it's really hot out (above the flat rated temperature) the engine isn't physically capable of producing 100k anymore. Let's suppose, at 50 degC, the engine can only produce 80k. ATM would be telling your TO engine that it's 50 degC out (denoted "TO 50C")...it's still running as a 100k engine, but you're telling the engine it's way too hot out so it's pulling back on thrust.

However, if you take a derated engine (TO-1, equivalent to 90k at flat rated temperature) and take it out at 50 degC, it's also going to be limited. If you tell your TO-1 ("90k engine") that it's 50C out, it's going to run even lower thrust...say 75k.

So, on any one day, you could do TO (100k), TO-1 (90k), TO 50C (80k) or TO-1 50C (75k). You can layer ATM on top of any derate. These are all made up numbers to illustrate what's going on, but hopefully that helps visualize it.

The easiest way to think about it, I find is:
derate = temporarily changing the engine's rated thrust
ATM = temporarily lying to the engine about how hot it is outside

Tom.
 
krisyyz
Posts: 1266
Joined: Thu Nov 25, 2004 11:04 pm

RE: Engine De-rating And Outside Temperature

Sat Jul 31, 2010 3:03 pm

Quoting tdscanuck (Reply 5):
So, on any one day, you could do TO (100k), TO-1 (90k), TO 50C (80k) or TO-1 50C (75k). You can layer ATM on top of any derate. These are all made up numbers to illustrate what's going on, but hopefully that helps visualize it.

The easiest way to think about it, I find is:
derate = temporarily changing the engine's rated thrust
ATM = temporarily lying to the engine about how hot it is outside



Thank you! I understand it now.

I really appreciate you taking the time to answer my question with such detail.

KrisYYZ

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