Derated take off

[Transafrik]

Hi,
I have read that, at Take off, one shouldn’t exceed the max EPR as per tables, but I want to know: is it mandatory or only a suggestion?
Thank you
Lorenzo

[Paul Golding]

Unless you'd want to try the 727 glider, or have big enough pockets to keep buying new engines............I'd treat it as mandatory.

Oh, and setting the power as per the EPR tables has nothing to do with derated settings, the EPR tables are for max settings. The fact that you could push the throttles far enough to exceed the settings in the table doesn't mean it should be done........much like in the old days, before cars had rev limiters, revving past the red line would have been a quick way to knacker the engine.

[Transafrik]

Thanks Paul, so how am I suppose to know how to execute a derated take off? I thought that the indications of the max allowable EPR were the ones to follow.

Lorenzo

[Michael E.]

I think you may be refering to a balanced thrust takeoff.

If I get a few minutes tomorrow, I will copy and post the information and method of computation.

Not sure why you'd do this in FS, as the procedure is rather lengthy to compute with all of the other computations required for a flight, plus there are restrictions on when this procedure may be used.

If you feel up to it though, I'll post the information.

[Paul Golding]

I'm not sure that's the same thing Michael. I'm assuming derated is as in reduced thrust i.e. quieter?

I looked through all my documentation last night and couldn't find a single thing on this; maybe in days gone by, nobody gave a rats arse about noisy take-offs?

Paul

[xxmikexx]

Paul,

People did give a darn -- that's why Eastern Airlines pioneered the "hush kit" -- the addition of some sound proofing in the engine nacelles, as I recall. (But don't hold me to this, my aging memory isn't always accurate, and I don't feel like monkeying with Google this morning.)

I believe that the issue with derated takeoffs is related to engine maintenance. If you overtemp the engines by more than X degrees for Y minutes then an engine hot section inspection will be required, necessitating what is basically going to be a complete engine teardown.

So a derated takeoff is probably one which ducks the whole issue by never overtemping the engines to begin with. (My guess, but an educated guess based on helicopter turbine engine maintenance philosophy with regard to engine hot starts.)

[Paul Golding]

All sounds reasonable, I suppose I'll have to Google it to find out more about this now

Paul

[Michael E.]

Well, the balanced thrust take-off essentially uses only the amount of thrust required for runway length and atmospheric conditions to safely take-off at a given gross weight.

For a long runway on a favorable day, you could use less thrust than for a normal EPR setting take-off.

As xxmikexx said, it does save wear and tear on the hot section, but at the same time saves fuel and reduces noise. They probably dreamed this up during the gas crunch of the early '70's.

I don't know why anyone would fool with it in FS as it's a PITA to calculate.

[Paul Golding]

Interesting read............... http://www.airliners.net/aviation-fo...d.main/155082/

Paul

[PhilipsCDRW]

So is derating the takeoff as simple as adding 15°C to the OAT when finding the max EPR for takeoff?

Might not be a good enough calculation for real life where you only get one overrun runway per career but should be fine in FS if mixed with a healthy dose of intuition re runway length...