il75
Posts: 257
Joined: Tue May 08, 2001 7:35 pm

Full Thrust Take Off

Fri Aug 23, 2002 6:43 pm

Hello,

In an another thread I read the following:

"I think the full thrust takeoff check must be done every 10 days in the US, here in the UK I think it is every 5 days. We must prove to the authorities that it can still be done. If a takeoff is operated with full thrust anyway, the check is then not needed for a further 5 days.

Sometimes we fly relatively long (4 hr) flights with full thrust when a check is needed right..."

Would someone please shortly explain to me what these checks are and what its purpose is?

Thanks
Erico
 
Rick767
Posts: 2613
Joined: Sun Jan 30, 2000 8:11 pm

RE: Full Thrust Take Off

Fri Aug 23, 2002 8:56 pm

Just an overview first for those who may not be aware.

Generally speaking, twinjet aircraft are generously overpowered. This is to cover the scenario of engine failure on takeoff, where the aircraft have to be able to climb away safely on one engine and clear the obstacles on a departure course by a designated margin.

Because the engines are so powerful, we generally do not need to use full thrust on every takeoff. What we use is a "derated" thrust setting. This is accomplished by "fooling" the engine into thinking it is hotter outside than it actually is. So if we do a departure from Manchester (10,000ft runway) with ISA conditions in calm wind with a given load the tables may say that we can use a takeoff derate of say 59 degrees C.

The engines will then give us the power we would expect to get from them taking off at 59 degrees C ambient temparature (much less thrust than the actual maximum they could produce).

This derated thrust technique helps reduce engine wear and tear, reducing maintenance costs, and prolonging engine life. It also reduces fuel consumption slightly for a given trip.

Now we could almost use a thrust derate on every single takeoff we do (providing runway length is sufficient). The problem with that is, how does anyone know the engine is capable of producing full thrust anymore, if it is never tested? That is the thinking behind the CAA regulation I imagine.

Apart from shorter runways, the only times we are not permitted to do a derated thurst takeoff are if the runway is contaminated (wet / slush / snow) or if it is specifically enforced that we must do a full thrust takeoff by the airport or company (LPMA springs to mind).
I used to love the smell of Jet-A in the morning...
 
ismangun
Posts: 115
Joined: Sun Jan 07, 2001 6:35 pm

RE: Full Thrust Take Off

Fri Aug 23, 2002 9:15 pm

If somehow on short final we discovered a windshear and have to power the engines beyond it's GA thrust (e.g. firewall it) but still within its limitation, will the engines have to do a 'stripdown' inspection, or might just some simple 'hot item' checks?

regards,
ismangun.
If it's an Airbus, I'll take the bus
 
FDXmech
Posts: 3219
Joined: Sun Mar 19, 2000 9:48 pm

RE: Full Thrust Take Off

Fri Aug 23, 2002 10:30 pm

Imteresting bit of information: Approx. 80% of engine failures occur during max power take-offs.

I've never heard of a max power regulation though.

You're only as good as your last departure.
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: Full Thrust Take Off

Sat Aug 24, 2002 2:19 am

Because the engines are so powerful, we generally do not need to use full thrust on every takeoff. What we use is a "derated" thrust setting. This is accomplished by "fooling" the engine into thinking it is hotter outside than it actually is.

Isn't it the other way around. Jet engines don't perform as well in hot weather as they do in cold weather. If u were going to derate it (less thrust), wouldn't u want to fool it with a colder temp than a hotter. If the engine thinks it's hotter out, wouldn't it need more thrust?
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Full Thrust Take Off

Sat Aug 24, 2002 5:56 am

Almost always dependent upon the airlines approved maintenance procedures, a full power takeoff may/may not be required to maintain government approval and/or manufacturer warranty.

AA's MD80 fleet used to (when I flew 'em) require a "max thrust check" takeoff at least once every 10 takeoffs. This was an engine warranty requirement. All other AA aircraft I've flown had significant automatic monitoring (most are "real-time" via ACARS) of all engine parameters thereby negating any checks based upon cycles or hours. When something starts to show signs of inefficiency, the plane is routed for a more detailed check by maintenance folks. The only exception to this was the MD90, but then again, we did max thrust takeoffs 4-6 times per day.  Big thumbs up
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
Guest

RE: Full Thrust Take Off

Sat Aug 24, 2002 10:05 am

This airline policy, and Pratt & Whitney recommendation...
Full thrust takeoff, either every 10 days, or 10 take offs... whichever comes first...
(s) Skipper
 
concorde1518
Posts: 723
Joined: Thu May 17, 2001 12:02 pm

RE: Full Thrust Take Off

Sat Aug 24, 2002 10:09 am

What is a max thrust takeoff? Is it just where the takeoff power is not derated, or do you literally put the throttles all the way forward, making the N1, N2, etc.. lights come on?



 Smile
 
jsuen
Posts: 204
Joined: Tue Aug 14, 2001 2:36 pm

RE: Full Thrust Take Off

Sat Aug 24, 2002 3:35 pm

The requirement is 30 days / 150 takeoffs for US Air.
 
Guest

RE: Full Thrust Take Off

Sat Aug 24, 2002 5:41 pm

Dear Jsuen -
Full power is a number derived from charts, based on temperatures or pressure which give you an EPR and/or N1 value to be set for takeoff, to see if the engine is capable of producing its rated thrust... Some engines occasionally might fail to reach that value, and if "reduced power" was constantly used, nobody would be aware of it... On our 747s with have an FFRATS sustem, (Full Flight Regime Auto Throttle System) which we engage at 80 kts or so on takeoff roll, the FFRATS selects the maximum trust applicable however we do verify that number prior to flight
xxx
If you "litterally" move the thrust levers "forward" to the stops, chances are that you will overboost the engine, exceed its limits, such as N1 or EGT limits, and at the same time, the VMCG or minimum V1 speed would become invalid, should an engine fail "on the other side" of the overboosted engine, you might not be able to control the airplane directionally...
xxx
The Pratt & Whitneys (JT9s) are rather tolerant about being overboosted, the
General Electric (CF-6s) not so much, and the Boeing Pilot Training Manual (747) states that the Rolls Royce (RB-211s) is the least tolerant...
RR lovers, dont criticize me, it is Mr. Boeing who says so...  Wink/being sarcastic
(s) Skipper

 
gt1
Posts: 124
Joined: Wed Feb 21, 2001 10:30 am

RE: Full Thrust Take Off

Sun Aug 25, 2002 12:44 am

cdfmxtech, you're statement regarding jet engine performance in hot weather being worse than in cold weather is correct, however Rick767's entire post is correct (and quite a good one too).

Modern engines, as well as thrust management computers (or equivalent), figure out take-off power. Engine power is directly related to fuel flow. If the engine is told the temperature is higher than it actually is, the engine thinks it can make less power than it actually can. Therefore, when it reaches that reduced level of power, it stops increasing fuel flow, which is a lower fuel flow than it would have with full power, which results in "de-rated take-off power".

The limiting factor in most engines is "over-boost", which is total pressure at the outlet of the high pressure compressor. This is a "do not exceed" number. The max pressure for the PW4060 is 490 psi and for the CF6-80C2 it is 450 psi, if memory serves. Both of these engines EEC will prevent these values from being exceeded, in Normal mode at least. This pressure is directly related to fuel flow.

Hope this answers some questions.
 
il75
Posts: 257
Joined: Tue May 08, 2001 7:35 pm

RE: Full Thrust Take Off

Sun Aug 25, 2002 6:44 pm

Thank you - all of you - for the explanations.

Erico

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