View Full Version : Spool up...

south coast
13th Nov 2004, 14:02
Could someone please give me the definition of spool up.

Is it when the engine is accelerated from an idle power setting to a power setting where thrust is being developed?

And, if you are in the cruise at a particular epr setting and need to set max thrust, which is say 0.4 epr above the cruise, (made up values) could that acceleration of the engine be considered 'spool up'?


13th Nov 2004, 16:28

My recollection is that from ground idle to max power can take between 5 and 7 seconds, a little less from flight idle.

The spool up time is from idle to a point at which max thrust is available virtually instantaneously

Remember the AF A320 fly-by that ended up in the trees? Well, they set max thrust shortly before they reached the trees, and you can still here the thrust increasing as they hit the trees.

On the Airbus, approach idle is commanded by the FADEC system whenever the flaps are awat from 0, facilitating 'rapid' achievement of TO/GA thrust; however in the AF case they had overridden the autothrust and were at a height above ground where the logic said they were landing, so some of the autoflight functions were disabled.

13th Nov 2004, 22:41
"Spool up" is a generic term which refers to engine acceleration characteristics on a turbine.

So, one might refer to "spool up" from idle to a minimum nominated thrust level at some point during the approach or, indeed, as has been suggested, the time to "spool up" from idle to max thrust.

14th Nov 2004, 04:23
well in years gone past, with older turbojet designs (IE: not turbofans) such as the JT4A, used on heavier models of the 707, you could count on 12 seconds minimum, from idle to rated thrust.

Get low and slow on final...prepare to find brown stains on the seat after landing.

A wakeup call, to be sure.

Old Smokey
14th Nov 2004, 14:22
Damn you John Tullamarine, I was going to say that, been away for too long counting lumpy bits in Takeoff splays.

Yep, "Spool up" is a generic term for jet engine acceleration all the way from idle to the maximum. To add just a little bit.......

"Spooled Up" generally refers to already being at an engine speed where further thrust required is available almost instantly, for all practical purposes.

15th Nov 2004, 00:12
... ahhh ... recognition ...

15th Nov 2004, 08:49
You are aware (of course) that there is an engine certification requirement for slam acceleration response. JAR-E 745, IIRC, :

For all Engines, an increase can be made from 15% of the rated Take-off power or thrust, to 95% of the rated Take-off power or thrust in a time not greater than 5 seconds. A longer time may be accepted if properly justified. This power or thrust response must occur from a stabilised static condition using only the bleed air and accessories loads necessary to run the Engine

The requirement is for a sea level test, and "most" modern turbofans meet it.

As operation altitude increases, the speed of response does reduce (to prevent stall / surge).

Hope this helps...