View Full Version : 737NG - Derate & Assumed temp take off
The Puzzler
8th Jul 2003, 14:15
Puzzle me this....
At our company we can elect to derate and/or use reduced thrust take offs. It is my understanding that we are trying to keep EGTs to minimum to assist with prolonged engine life etc. However, is it not more beneficial to then use the lowest assumed temperature, which may then be the higher thrust rating? We have the 20k/18.5k derate fit and a typical scenario is a 20k take off with an assumed temp of say 40 degrees vs. an 18.5k derate with an assumed temp of 34 degrees with the corresponding N1 being higher on the lower thrust rating. Which is the better option? :confused:
Noise Unit
8th Jul 2003, 15:51
My understanding is that for a given OAT, aircraft weight and runway conditions, there is a minimum thrust level that will get you out of the airfield safely and should not matter whether it is achieved with derate and ATM or all ATM.
How does the EGT vary between these two options as this is the principal driver in terms of minimising damage to the engines?
Engine speed is also an issue. Can you confirm that when you say N1, is the unit RPM or %?
If it is the latter, does the datum 100% condition change with the derate level used?
Hope this helps.
The Puzzler
9th Jul 2003, 00:26
Puzzle me this...
Noise Unit, thanks for the response. Re: EGT the problem is unless you actually set the thrust you dont know which is producing the highest temp. Hence my supposition about %N1. Good point about engine speed at 100%, I'll speak to our engineers and report back.:cool:
Derates, are more cost effective and also offer greater savings, these are then passed to the opperator by the eng lease co, generally in a lower cost per hour, u can work with them on this. Reduced thrust (assumed temp, flex) also saves money, but you dont tend to see the savings untill major inspections.
To give you some idea ive got a letter from snecma. ''from experience a 9% de-rate on t/o thrust would cirtainly lead to a minimum of 5% reduction in maintenance coats and up to 10%, depending on flt profiles''
Rgds
Option 2.........
Fixed Derate with the Assumed Temperature.......
Mutt......
The Puzzler
13th Jul 2003, 17:22
Puzzle me this....
According to our engineers, the datum for 100% N1 does change with the selection of a derate. They have also compared figures for various assumed temp and derate configurations and found that it is 6 of one and half a dozen of the other, unless operating in temperature extremes.
Cheers :cool: