CFM 56 engines N1 versus OAT FMC reading

[Tee Emm]

Now I know there is an obvious answer to this question but it evades me at present but here goes:

Let's assume an engine bleeds on take off with OAT 14 C. For the 20K engine the FMC gives 90.7 N1. Now enter an assumed temperature (ATM) of 45C and the FMC drops back to 87.2 N1.

However let's forget any ATM for a moment and assume the actual OAT is 45C then the FMC shows 91.8 which ties in with the QRH figure for 45C.

Question: If the assumed temperature method of 45C gives an N1 of 87.2 N1, then how come the actual OAT at 45C gives a much higher N1 figure of 91.8 N1?

After all, the whole purpose of the ATM is increasing engine life hence plugging in 45C to the ATM box gives the lower figure of 87.2 N1 and subsequent reduction in EGT.

I just know the above is a stupid question but I am having one of those senior moments when my brain simply fails to see the obvious. :confused:

[Sciolistes]

I understand that you're telling the FMC to give you an N1% that works out as the thrust equivalent for the current temperature (14degC) as if you were taking off in actual conditions of 45degC with an N1% of 90.7.

Whereas the N1% of 91.8 is to achieve the same thrust (20K in this case) in 45degC conditions as 90.7 would achieve in 14degC.

Clear as mud?

[root]

Flat rating on the CFM56 occurs at ISA+15
The above graph would only be valid on a standard day at sea level.

It is important to note that What you see on the N1 Gauge is not an exact representation of what thrust the engine is producing. The "Gauge N1" indication is usually a true measure of the low pressure turbine tachometer based upon a default value. Rolls Roys uses 3900RPM on some of their engine as representing 100% N1.

This explains the concept of flat rating. You will see that Gauge N1 in the FMC increasing with increasing OAT because the engine EPR remains unchanged while in the flat rated area up to ISA+15.

The Corrected N1 which is not visible in the flight deck is the true measure of engine thrust. In Boeing airplanes corrected N1 equals Gauge N1 divided by temperature raised to the power of an engineering constant provided by Boeing.

The procedure to derive Gauge N1 for ATM goes along these lines:

1. Find the Gauge N1 at the assumed temperature.
2. Derive the corrected N1 through the formula described above.
3. Now recalculate a Gauge N1, using the corrected N1 from point 2 AND using the actual temperature instead of the assumed temperature.

The result in point 3 is what the FMC actually shows you.

A long-winded explanation, from dusty memories of performance courses long past, to explain why the same value of +45C is actually two completely different scenarios for purpose of thrust calculation.

[Tee Emm]

Many thanks for the replies. I will don my thinking cap and work through them.
TM

[Vaneev]

http://aviacom.ucoz.ru/img746.jpg

[Vaneev]

I have specified FMC N1 calculations and have changed graph. Sorry for mistake. :)

[FR8TDOGii]

Ans: 87.2 N1 with the denser (14C) air equals the same thrust as the less dense (45C) air at 91.8 N1. The engine actually has to work less for the same thrust.

[isaneng]

Hmm, agree with last post. In the OPs initial enquiry, hasn't his assumed temp been used to produce a factored/reduced thrust T/O figure, using greater runway length as the payback?