Il75 From Argentina, joined May 2001, 263 posts, RR: 0 Posted (12 years 4 weeks 1 day 13 hours ago) and read 6044 times:
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?
Rick767 From United Kingdom, joined Jan 2000, 2662 posts, RR: 51
Reply 1, posted (12 years 4 weeks 1 day 11 hours ago) and read 6041 times:
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 From Indonesia, joined Jan 2001, 117 posts, RR: 9
Reply 2, posted (12 years 4 weeks 1 day 10 hours ago) and read 6002 times:
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?
Cdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 4, posted (12 years 4 weeks 1 day 5 hours ago) and read 5945 times:
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 From United States of America, joined Jan 2000, 3474 posts, RR: 46
Reply 5, posted (12 years 4 weeks 1 day 2 hours ago) and read 5904 times:
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.
*NO CARRIER* -- A Naval Aviator's worst nightmare!
B747skipper From , joined Dec 1969, posts, RR:
Reply 9, posted (12 years 4 weeks 14 hours ago) and read 5844 times:
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
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...
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...
Gt1 From United States of America, joined Feb 2001, 133 posts, RR: 0
Reply 10, posted (12 years 4 weeks 7 hours ago) and read 5769 times:
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.