B737-200 JT8D -15 and 17 Reduced thrust N1

[Centaurus]

Normal take off EPR is 2.10 bleeds on for 20C. The operator uses 1.99 EPR as a reduced thrust figure for an assumed temp of 40C. Actual OAT 20C. The QRH N1 for EPR 1.99 for 20C is 94 percent and for 40C is 97 percent. When calculating the EPR versus N1 crosscheck on the take off data card, which N1 figure is the valid one?

My understanding is that the N1 for the actual OAT is the correct one to use. However the operators uses the N1 for the assumed temp of 40C. This seems a bit dodgy to me.

[Guppy Driver]

Nope, don't think it's dodgy - my understanding is that it is actually the right thing to do. Remember that what you are doing with the N1 / EPR comparison is ensuring that you are producing the amount of power that you AUGHT to be doing, ie ensuring that the PT2 probe is not blocked, giving you a higher EPR than that which should be associated with a given N1 value.

So, you have already decided through the use of reduced thrust that available runway is consistent with with the actual TOW and that this can be achieved with a reduced EPR (assumed higher OAT). You have hence decided to advance the thrust levers only far enough forward to produce this REDUCED EPR, and therefore the comparison should be with the reduced EPR and not a higher EPR setting (actual OAT) that you have decided to only use in the event of loss of engine thrust on one engine.


Sorry - this is probably clumsily formulated, but the crosscheck is to make sure that you are getting the thrust out of the engine that you have computed to get with the power seetting ACTUALLY used for takeoff. The N1 crosscheck serves to validate the EPR you have chosen to use.

Hope that helps.

[mutt]

The QRH N1 for EPR 1.99 for 20C is 94 percent and for 40C is 97 percent

The concept is that you must use the assumed temperature N1, however you cannot use the normal N1 chart to provide the value. The N1 values in the QRH are only valid for actual temperatures.

Mutt

[Guppy Driver]

Hmm, thanks Mutt. Re-reading the original question I think I mis-understood. Centaurus was not questioning the use of full or reduced EPR-value, but merely the difference between the N1 values at the same EPR but different OAT. So my attempt at answering was a bit gibberish'ish (if that's a word).

The N1 vs. EPR chart in our -200 QRH has a note: "Use actual OAT only", so we enter the chart using the reduced EPR from an assumed temperature, but use actual OAT.