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Extra margins with assume thrust

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Extra margins with assume thrust

29th Oct 2014, 23:54

Join Date: Apr 2012
Location: Europe
Posts: 29
Extra margins with assume thrust

Hello you all! I am flying the 737NG:

I was told that if you use assume thrust, you will have the additional margin for take off between the density of the assume temp and the colder OAT. If i understand it correctly, if you have flat rated engines at 30 degrees C, the engines will adjust N1 and EGT to give a flat rate whenever it is colder than 30 degrees outside. Therefore you cannot select a colder assume temp than 30. But in my mind then, and here is the real question; you will only have the density margin from the assume temp down to 30 degrees then, and not all the way down to lets say 4 degrees C?

I hope this makes sense, here the mathemathical way of asking the same:

With assume thrust of 40 degrees, and OAT 4 degrees, flat rated engines at 30 degrees, Is the density margin then:
A: 40-30=10 degrees colder with the better density that gives, or
B: 40-4=36 degrees colder with the better density that gives?
29th Oct 2014, 23:59

Join Date: Dec 2002
Location: Where the Quaboag River flows, USA
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The air, on the wing and in the inlet, are at 4C in your example; not at the assumed temperature. You will always have that density difference on your side.
30th Oct 2014, 08:53

Join Date: Apr 2012
Location: Europe
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Yes, but my question is that if i understand it correctly with a flat rated engine, it will compensate and reduce EGT if temp is below it's flat rated temp. So will you then have the margins all the way down to 4 degrees? Only talking about the engines now, not the wings..
30th Oct 2014, 09:51

Join Date: Jul 2007
Location: uk
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Life on top:

Think of it this way. Up to the flat rated temperature, the engine is able to produce its stated rated thrust.
Now, a TOGA departure at 4C will achieve the same thrust as one at 30C
HOWEVER the engine will be using less fuel to achieve that same thrust. In other words there will be no performance difference - simply a lower EGT at the lower temperature.

When using assumed temp. methods then there will be a "density margin" as you point out. So, by my reasoning that will only apply for the range above the flat rated temperature.

Note: we are only talking about engine performance here!

As a historical footnote - on the B707 with JT3D (no FADEC) engines we had approval to operate the engines at HIGHER than rated thrust in limited circumstances. ie. the F/E over EPR'd the engines to achieve extra thrust. This was at the expense of engine life. Only used departing Nairobi.
30th Oct 2014, 10:22

Join Date: Sep 1998
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Normally the term Margin refers to engine temperature. The difference between the ITT/EGT achieved when setting thrust and the limit for that thrust.

The greater the difference between actual and assumed temperature, the greater the margin.

This also applies to any actual temperatures below the flat rated temp. A take off with full thrust at 4°C will have a lower ITT than a take off at 30°C.

So the flat rated temperature of an engine does not define the Margin when using assumed thrust. It's the difference between actual and assumed which defines the margin.

Now, if it's density you are interested in then we can say that we gain uncorrected benefits in terms of Lift and TAS when we use an assumed temperature compared to taking off at the equivalent actual temperature. This performance gain is also proportional to the difference between actual and assumed temps.

The only real significance the flat rated temperature has is to define the minimum acceptable assumed temperature.

Setting an assumed temperature of 25°C on an engine which is flat rated to 30°C is simply asking for max take off thrust. i.e. no reduction. If you could do this you would find that the N1 at assumed temperature would be higher than the N1 at actual temperature. You would then correct this N1 for the difference in density between actual and assumed and if you did the sums correctly you would get the same N1 as for your actual temperature.

Because the thrust scheduled at actual and assumed is the same and therefore the N1 required to achieve that thrust is the same.

No difference in thrust, N1 or ITT.

Bit of a ramble but hope it helps.
31st Oct 2014, 10:46

Join Date: Jun 2002
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Hi guys.

I'm just wondering if I could get your best definitions of a 'flat-rated' engine.
31st Oct 2014, 11:58

Join Date: Sep 1998
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An Engine who's rated thrust is fixed up to a published temperature.
Above this temperature the rated thrust will reduce as temperature increases.
31st Oct 2014, 13:27

Join Date: Jun 2002
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Ok thanks man; I appreciate it.
31st Oct 2014, 16:26

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@vilas
I think you may be mistaken.
1st Nov 2014, 11:28

Join Date: Jun 2007
Location: Wanderlust
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Disregard my post and read below, FROM FLIGHT SAFETY FOUNDATION
Effect of True Airspeed

Pilots who are skeptical about reduced-thrust takeoffs often sense that something very important is being taken away. However, there is absolutely no loss of any necessary performance margins involving field length, screen height,1 climb or obstacle clearance. If the airplane’s weight and power setting satisfied the certification standards at the higher temperature, then they certainly will do so at the lower temperature.
Although the takeoff speeds used by the flight crew are indicated airspeeds, actual performance is determined by true airspeed, which is a function of air density. Because we are operating at an actual temperature that is lower than the assumed maximum, true airspeed likewise will be lower.
Because of this true-airspeed effect, we enjoy a great deal of cushion between what the airplane must do and what it actually is doing. We are, in reality, using less runway and achieving a higher climb gradient, or obstacle-clearance margin, than if the ambient temperature was at the maximum for that same weight. Depending on conditions, the effect can be considerable — on the order of several hundred feet in field length. The benefit increases as the difference between the actual and the assumed temperatures increases.
1st Nov 2014, 12:07

Join Date: Sep 1998
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As stated.

One European airline I know uses a take-off software which shows the crew the TAS margin when assumed temp calculations are done. It also gives them the choice of V1s when a range is available.

Not sure it's necessary but some of the crews understand it. Others just shrug shoulders and go.
1st Nov 2014, 12:14

Join Date: Jul 2007
Location: uk
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vilas: in post 3 you will see that the poster is asking only about the consequences for the engine thrust. He is not asking about the TAS benefits to which your posting refers.
1st Nov 2014, 13:12

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1% /5c away from IsA..easy.
1st Nov 2014, 13:28

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Life on top
The margin you are talking about comes from difference between take off speeds in IAS and the TAS at which the aircraft is travelling because of higher density due to lower temperature but the example you have chosen is not correct. If you choose ass. temp. of 50 degrees for OAT of 35 degrees( which has to be higher than Tref). You will have performance margin as compared to the take off done at actual OAT of 50 degrees. Reproducing from my earlier post:
Because we are operating at an actual temperature that is lower than the assumed maximum, true airspeed likewise will be lower.
Because of this true-airspeed effect, we enjoy a great deal of cushion between what the airplane must do and what it actually is doing. We are, in reality, using less runway and achieving a higher climb gradient, or obstacle-clearance margin, than if the ambient temperature was at the maximum for that same weight. Depending on conditions, the effect can be considerable — on the order of several hundred feet in field length. The benefit increases as the difference between the actual and the assumed temperatures increases.
1st Nov 2014, 15:17

Join Date: Feb 2009
Location: Virginia
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As SLF, I probably have this entirely wrong, but let me try to put this down the way I understand it.

As far as the airframe itself is concerned, what matters is IAS, not TAS. Air density has the same effect as the wings as it does on the pitot tube, so drag, lift, stall speed, etc. will be the same at a given IAS, regardless of the density and corresponding TAS.

But when you're talking about the amount of runway needed for takeoff, what counts is the required ground speed, which corresponds to TAS (assuming calm winds). At lower density, an aircraft needs to reach a higher TAS to obtain the IAS required for takeoff. Reaching that higher TAS requires more acceleration and therefore more runway.
1st Nov 2014, 17:37

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Why does nobody bother to read what the poster is ACTUALLY asking!!!!
1st Nov 2014, 17:55

Join Date: Sep 1998
Location: wherever
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Why does nobody bother to read what the poster is ACTUALLY asking!!!!
There is no such thing as density margin in relation to thrust.

When calculating thrust for assumed temperature take off method. The N1 is corrected for the difference between the actual and assumed temperature. This means you get exactly the same level of thrust as if it were the assumed temperature.

The density is corrected out.

The margin appears as lower ITT/TGT. This is entirely dependent on the difference between actual and assumed temperature and the flat rated temp makes no difference.
1st Nov 2014, 18:02

Join Date: Sep 1998
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@Chu Chu

Lower TAS for same lift = higher climb gradient. They tend to be rather important for take-off performance.

So you get better stop and better go performance.
1st Nov 2014, 21:24

Join Date: Jul 2007
Location: uk
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FE Hoppy: Here is my take on what I think is being asked....

The OP correctly identifies that with a TOGA takeoff at 4C and another at 30C then the thrust used will be the same albeit with a lower EGT due to the greater air density and lower fuel flow at the lower temperature.

Now, step to the FLEX case. At 40C the FADEC computes the N1 setting as if the air temp. was 40C. OK so far?

Ignore the TAS issue.

He wants to know if the FADEC calculated N1 for 40C will benefit more from the 4C air that is being taken in (ie. more dense) or just from the 30C flat rated air density.

My question to you is this: when using a FLEX setting does the FADEC actually compensate for the air density differences between the ambient or is it simply programmed to work using the top end of the flat rated values?

This, I think is what is being asked, not the margins inherent with the IAS/TAS margins.
1st Nov 2014, 21:37