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Pack2
1st May 2005, 18:53
Can anyone explain why more fuel is used when flex thrust takeoffs are used...Smokey?

oldebloke
1st May 2005, 19:01
Does it use MORE??only thing I can think of is Longer time at reduced thrust until 'climb'selected at 1500':ok:

machone
1st May 2005, 20:12
How do you mean more fuel?
Yes it will take longer to reach a specific height therefore more fuel will be used.
But at a lower power setting less fuel should be used,than at a higher power setting.
Flex take off reduces engine wear and hence overhaul cost

Medwin
1st May 2005, 20:14
I think his question was "why de-rated thrust uses more trip fuel"?

Easy, de-rated thrust will take you longer to climb to your optimal altitude (best SFC). As with a de-rated thrust climb, the a/c has a lower initial rate of ascent and therefore takes longer to reach its transition to its enroute climb profile and then its cruise altitude. Consequently, it spends less time at its optimal cruise altitude and therefore uses more trip fuel.

Bristow p.208

hawk37
2nd May 2005, 00:30
"I think his question was "why de-rated thrust uses more trip fuel"?"

I think this is a very good question. Can anyone confirm that this is even true? Consider that the best specific fuel consumption (SFC), which is thrust produced per pound of fuel consumed, is not at max N1, but in a range of 93 to 97 %, depending on the engine. Ergo, one can expect less fuel consumed.

Which leads to the further thought; Does max rate of climb mean minimum fuel burned?

Hawk

Intruder
2nd May 2005, 01:27
Higher altitude => less drag.
Less drag => less thrust required.

The sooner you get to higher altitude, the sooner you start burning less fuel.

In general, best trip fuel consumption is achieved with max rate climb to optimum alitude, then idle thrust to landing. Reduced thrust climb gives slightly higher overall fuel consumption, traded for longer engine life. Total life cycle economics of the airplane and engine are considered.

BOAC
2nd May 2005, 11:35
My Euro's-worth is that a flex TAKE OFF in itself does not use 'more fuel', or at least it is not measurable. If, however, you then climb away at flex (reduced) climb power, it will - for the reasons above.

Gary Lager
2nd May 2005, 11:39
Additionally, isn't the specific fuel consumption (ie kg/hr per N of thrust) improved at higher engine temperatures?

One of the main reasons jet engines have become more and more efficient over the last 60 years is due to the development of materials which can withstand higher EGTs, thus leading to improved thermodynamic efficiency of the engine.

Therefore, if you aren't running an engine as hot (ie high EGT/high thrust) as you can, you are sacrificing a degree of fuel efficiency, even though it it might actually be only a small amount of fuel wasted.

Anyone else comment on the validity or otherwise of that lot? University was a long time ago...

Of course, other factors do come into play in the real world: engine maintenance costs being the major one.

barit1
2nd May 2005, 13:18
Rule of thumb: 20 C decrease in hot section temp yields double the parts life. That is the driving logic for flex thrust (or alternate rating, or any other means of turning down the wick).

And even if the SFC is marginally better at reduced thrust (*), the lower aircraft performance means you're at the lower thrust & altitude for a longer time period. Thus, higher trip fuel burn.

(*) Whether or not reduced thrust improves takeoff SFC is a function of the individual engine model. Your mileage may vary, but usually there's a slight "hook" in the curve.

Old Smokey
2nd May 2005, 13:56
Pack2,

Can anyone explain why more fuel is used when flex thrust takeoffs are used ? - IT DEPENDS.

Like some, I'm not sure if you're addressing the Takeoff in isolation, Takeoff and Climb, or Sector Fuel.

Any discussion relating to fuel saving or penalty in making comparisons between optional thrust settings must consider optimum Thrust Specific Fuel Consumption (TSFC), with the optimum typically ocurring in the vicinity of Climb Thrust, around about 93%. Any more or less than optimum TSFC RPM and you burn more fuel per unit of thrust produced. It must also be borne in mind at all times, that the primary goal in using Thrust Reduction for Takeoff (and Climb) is economics, i.e. increased engine life at the lesser expense of time and fuel. Safety, is of course, a huge factor as Reduced Thrust is much less likely to precipitate engine failure.

Takeoff - Full Takeoff thrust is at an engine speed somewhat above optimum TSFC, thus a Full Thrust Takeoff is "fuel expensive". At the lowest typical Flex / Reduced Thrust, particularly if the Flex is applied to a lower RATING, the RPM is well below the optimum, and is again "fuel expensive". If fuel saving during the Takeoff was a consideration (It is NOT), the Takeoff would be best conducted at a modest Thrust reduction from Maximum, where the N1 was about 93%. As I've said on previous occasions on this same topic, you have to be a bit of a nit picker to find it. Normally, Takeoff thrust to a nominal 1500' is for only about 2 to 2.5 minutes, such a short period of time that any 'off optimum' TSFC would barely be noticeable. If you're an operator that usually uses modest thrust reductions (medium / long haul) you MAY notice a small improvement in fuel used. If, however, you are typically operating Very Long Haul (Full Takeoff thrust) or short haul (large Thrust reductions) you may well notice a small fuel penalty.

Climb - As optimum TSFC is typically close to Climb Thrust, any Climb reduced thrust will suffer fuel penalties. This is where fuel penalty becomes noticeable, because the 'off optimum' TSFC is applied for a MUCH longer time. As an example, the B777 with RR Trent has 3 Takeoff thrust ratings, TO, TO-1, and TO-2, with 3 associated Climb thrust ratings, namely CLB, CLB-1, and CLB-2. Climb at CLB-1 or CLB-2 costs additional fuel, but saves on engine maintenance. Above 10,000 feet, CLB-1 and CLB-2 revert to full Climb thrust (we can take the pain no longer).

Having said that, the climb speed chosen for climb is probably of far greater significance. The fastest way to 'kill' your sector fuel is to climb at a too low speed. Many jurisdictions do not have the 250 KIAS limit below 10,000 feet, and upon passing 5,000 feet (my company's 250 KIAS limit) immediately accelerate to normal climb speed, about 320 KIAS. (I can sense Canuck Birdstrike about to swoop). At the first Cruise Waypoint, I regularly note Fuel is UP by about 500 Kg, whereas it is Zero gain if the 250 KIAS is maintained to 10,000 feet. Sector fuel is all about fuel consumed per mile, NOT the lowest fuel flow, or the fuel consumed for the climb itself without considering the distance covered.

Optimum Sector Fuel, will require a climb a little faster than the best Rate of Climb speed. Rate of Climb depends upon excess Power (NOT Thrust), and, in the vicinity of the greatest delta between Power Required and Power Available, the PA and PR curves have only slight convergance until Mcrit is reached. The upshot of this is that, FOR THE JET, a change of climb speed of (typically) up to 20 to 30 knots has only a small impact on rate of Climb, whilst the ground covered for the same Fuel burned will vary almost directly with the speed. Typical figures for the B777 are Best Rate of Climb = 300 KIAS, Best Econ Climb = 320 KIAS. At 320 KIAS, Rate of Climb decreases by only 2%, whereas ground distance covered increases by about 7%. If the B777 were to climb at Best Rate speed instead of 320 KIAS, it would reach TOC less than 1 minute earlier, and some 25 miles sooner. Now, it will have to cruise those 25 miles to the point it would have reached TOC at 320 KIAS, and have burned more fuel to that point (a little saved on the climb, a lot more wasted for the incremental cruise).

If you want to put the windy statement above into simpler terms, Climb is the most sustained high fuel flow regime of our flight, so, if you're going to suffer a high fuel flow, cover as much ground as possible during the climb. Tolerate a very small ROC penalty, and cover a lot more ground for the fuel used.

Regards,

Old Smokey

Gary Lager
2nd May 2005, 16:25
barit1 -

I was trying to say that I thought reduced thrust TO/CLB gives a worse TSFC, due to lower engine operating temperatures, though I agree with Old Smokey that for the TO phase the difference must be close to b****r all.

But I agree with you both that the time spent at lower altitude in reduced thrust TOs/Climbs is the significant cause of increased trip fuel for these departures.

Interesting point about climb speed too.

Pack2
3rd May 2005, 14:05
Sorry people I have been away but thank you all very much for your answers.

Old Smokey
Yes I was asking about total trip fuel. As usual your post was excellent and and I have saved it for future reference thanks.

Shamrock 602
3rd May 2005, 16:45
Is this a bad time (or place) to ask for a definition of FLEX?

Clearly it's a reduced thrust setting for take off, but where does the name come from?

Thanks in advance...

Shamrock 602

Old Smokey
4th May 2005, 09:29
FLEX = FLEXIBLE

At least on the Eastern side of the Atlantic. They're not so flexible on the Western side, where they ASSUME a lot.

Regards,

Old Smokey

barit1
4th May 2005, 11:15
Flex or assumed temp TO - In its simplest form there are two elements. For a given runway, increasing ambient temp. hurts your TOGW capability for two reasons:

1) Higher TAS (inertial speed) is required to achieve V1, Vr etc., which are IAS values.

2) Engine thrust available is less at higher ambient temp.

Playing this game backwards, if you are planning a takeoff on a relatively cool day, at a lesser TOGW, you can ask "What is the hottest ambient I can stand and still be legal at this TOGW?"

Once this temperature is known, you can set thrust equivalent to what the engines could produce on the hot day (i.e. reduced thrust), and have assured performance no worse than the certified hot day value. (Remember, the wing is still flying in cooler air)

OK?

Shamrock 602
4th May 2005, 16:11
Smokey and barit1,

Thanks for the helpful replies. Amazingly, I had actually worked out the rest of the word (FLEXible) for myself ;). (Maybe it's because I learned English somewhere in the eastern Atlantic Ocean...)

The question was prompted because I'd been thinking, "If they call this flexible rather than reduced thrust, maybe there's something fancy going on with the autothrottle/autothrust during the acceleration for take off." :confused:

Sometimes the truth is actually simpler than we imagine. Important to remember when being drawn in by a conspiracy theory...

It helped...

Shamrock

GearDown&Locked
13th Jul 2005, 13:59
Playing this game backwards, if you are planning a takeoff on a relatively cool day, at a lesser TOGW, you can ask "What is the hottest ambient I can stand and still be legal at this TOGW?"
Once this temperature is known, you can set thrust equivalent to what the engines could produce on the hot day (i.e. reduced thrust), and have assured performance no worse than the certified hot day value. (Remember, the wing is still flying in cooler air)

So how is this Assumed Temp calculated?

GD&L

Old Smokey
13th Jul 2005, 14:51
GearDown&Locked,

Essentially Flex/Assumed thrust is calculated by examining the temperature at which, for a particular weight and set of environmental conditions, the aircraft would be at it's Maximum Takeoff Weight, and then operating the engine to the lower Pressure / EGT / RPM limits associated with the higher environmental temperature.

To address the twice repeated remark - "assured performance no worse than the certified hot day value", actually performance is BETTER than if operating at the actual higher temperature. Whilst the engines are producing the same thrust, no credit is taken for the lower TAS associated with the same V1/Vr/V2 on the cooler day. For example, on a 20°C day with an assumed temperature of 50°C at Sea Level, and a V1 of 140 KIAS, TAS at V1 is actually 141.2 Kt Vs 148.3 Kt at the higher temperature. Kinetic energy to be dissipated in a rejected Takeoff from V1 is thus only 90.7% of that if the actual temperature was 50°C.

Regards,

Old Smokey