High Speed Climb

[ManagedNav]

I am trying to find documentation supporting the theory that a high speed climb will result in better fuel economy over the course of a trip.

Although I have never seen it in writing, I was taught that climbing a swept-wing jet at around 300kts to cruise altitude resulted in the a/c being further down-range and created an overall increase in performance of the wing post FL280.

I have proven this to myself numerous times on the DC-9, DC-8, A-320 and even on corporate jets with supercritical airfoils.

My problem is convincing people that in most cases a high-speed climb is more efficient in terms of fuel burn and time enroute.

Anyone know of a publication that can support this theory?

[Human Factor]

Our 777 flying manual suggests that, when permitted by ATC, you should exceed the 250kt limit at the earliest opportunity and climb at the appropriate ECON speed for the input Cost Index. A CI of zero equates to approx 280/0.82 and a CI of 100 gives approx 320/0.83.

Hope this helps. For those not familiar with a Cost Index, it's a figure input into the FMC which bases the most economic speed for the route on a number of factors such as cost and time. A cost index of 0 equates to minimum cost, a CI of 999 is minimum time (in theory).

Can't get you a copy though, sorry. Have a word with Boeing, they wrote the original.

[Right Way Up]

I was under the impression that low speed climb gave a faster flt time but higher fuel burn. I guess it would also depend on wind components at different levels.

[Intruder]

As always, "it depends"...

In general, however, the maximum rate of climb airspeed (Vy) will yield the best fuel economy. The reason is that the engine burns substantially less fuel at higher altitude, so the quicker you get to optimum altitude, the lower the burn rate will be.

Climb winds may have an effect on overall burn, but it is still advantageous to get to the optimum altitude -- considering winds -- as soon as possible. Any lower altitude will, by definition, be less than optimum.

[ManagedNav]

I agree mostly that the quicker you get to cruise altitude, the less fuel you will burn.

My contention is that a swept-back wing does not fare so well at slower airspeeds above the mid-20's. A higher airspeed will increase the performance of the wing in terms of lift and consequently result in a higher average rate of climb. Combine that with the fact that you will be further down-range at the TOC and the net result seems to be an earlier arrival and less fuel burned than flight-plan fuel.

Try flying it both ways; I think you'll notice that after the mid 20's, most large jets have a substantial decrease in climb performance, sometimes degrading to under 1000/fpm. At a higher speed/greater lift component, you can average around 1500/fpm all the way to cruise. (subject to variables in aircraft type, of course)

Human Factor - your illustration of cost index hits it right on! I'm assuming that a cost index of 100 is mo' betta than 0! We had them on the Airbus but I never messed with it in the FMS. I just manually set a higher airspeed for the climb if I felt like it would benefit me.

[Intruder]

Indeed, the transition from a Vy based on IAS to Vy based on IMN occurs in the mid-twenties (e.g., 360/.83 for a typical heavy 744). However, note that the IAS will decrease as the climb IMN is used...

Also note that in the case of the 744, Vy is very close to Vmo (365 KIAS).

[CI54]

Hello,

For a variety of reasons, it is more fuel efficient to accelerate to ECON speed as soon as practicable but one of the most logical reasoning is the engine performance is much better at lower altitude than at higher altitude. Therefore, the aircraft accelerates easier at lower altitude.

The other reason is air is much thinner at higher altitude (by 10000 ft, the atmosphere has 50% less pressure than at sea level), thus TAS becomes much higher at higher altitude. It will take you much more distance to accelerate the aircraft to your final climb speed than at lower altitude, so less efficiency.

A point to note, flying cost index zero is usually not flying the minimum cost. Operating cost is divided into two parts, fuel cost and time cost. Flying cost index zero just means fuel cost is so expensive, u need to conserve every single bit of it. If your company time cost is high, it stands to reason to fly faster so that time cost can be minimized. By burning extra fuel, you can save company money by saving engine and airframe maintenance cost, crew per hour flight cost etc... Cost index zero will mean flying maximum range (MRC). LRC is a speed that gives you 99% of MRC.

All this however, is not exhaustive...

[ManagedNav]

here are some excerpts from "Flight Theory for Pilots" byCharles E. Dole:

"When an aircraft is in steady flight, horizontally, the lift is equal to the weight of the aircraft. "...,shows that the lift is less than the weight if the aircraft is in a climb. this is possible because some of the thrust supports some of the weight of the aircraft. The steeper the climb, the less the lift supports the weight."

"The velocity for for maximum rate of climb cannot be determined by a simple examination of the Ta-Tr curves.. RC depends upon velocity and excess thrust, both of which are variables."

[jtr]

Isn't the -400 Vy @ MTOW going to be closer to 315/0.84?

Vref30+125 (+ some for mum)

Econ CI 250 will give a climb speed of 360kts @ heavy weights.

[Intruder]

(by 10000 ft, the atmosphere has 50% less pressure than at sea level)

C154:

Actually, 50% Sea Level Pressure occurs about 18,000' MSL.

Isn't the -400 Vy @ MTOW going to be closer to 315/0.84?

I just plugged it into the AeroSim program and got 340/.83 at CI=0.

Last week out of Taipei at MTOW I got 360/.8xx at CI=100.

[jtr]

Are you looking at ECON CLB speed, or max angle?

According to the books (can't remember which one) max rate comes from max angle plus 25, and max angle comes from ref30+100. So for a heavy t/o where v2 (and therefore Vref30) is say 180kts, Vy=280, and Vx=305. There is an additive for weights above 317t, but since it is not quantified, I think it is fair to assume it is not large.

For a Max wt t/o I would expect the numbers to be something like..

Max Angle 280
Max Rate 310
Econ Clb 360 (CI=250)

Posting after a long haul, so treat info with caution

[MasterGreen]

Climb Speeds for Economy (from "A Few Notes from my Notebook - a series of 1000")

For the B777 here are some actual figures - the same principle applies to other Boeing (all ?) FMS equipped aircraft.

The Best Rate of Climb (ROC) Speed is extracted from the FMC by using Cost Index (CI) 0. The VNAV1 (Climb) page will then show climb speeds to achieve Minimum Fuel = Best ROC Speed for the climb. This is weight and ISA Temperature sensitive so is subject to change as the climb progresses. However the ISA change variation on speed required is small and setting a TOC ISA in the First Waypoint after TOC in LEGS will achieve virtually perfect results without having to leave the FMC in CI = 0 throughout the climb segment (see below).

Obviously if there is an ATC climb restriction then the nominal 250 to 10K will have to be observed, but if possible accelerate to optimum climb speed ASAP after the Forth Segment (Clean A/C). Removing the standard Climb Restraint in VNAV1 will achieve this automatically.

From a theory point of view all of the above posts are just about on the money so no repeat is required.

The other Climb / Fuel issue is regarding Derated Climb. Derated climbs save engine life and not Fuel. Cancelling the Derate (which washes out by 13K anyway) will achieve a small saving in Trip Fuel also. (Obviously by allowing cruise altitude to be aquired ASAP).

From a practical viewpoint and also to avoid clashing with SOPs, it is best to query the CI = 0 Climb Speed before departure (after a valid ZFW/Fuel/AUW is inserted + a TOC ISA) and manually inserting that in the VNAV1 (Climb) page. Remember to set the CI back to the Company Approved value.

In real life a saving of 300-400 kgs can be made on a B777 using a CI=0 climb profile and another 200Kgs by cancelling the Climb Derate. If you are going to be on a "tight" sector these are obviously useful savings that can be "banked" from the TOC fuel check.

These are proven numbers (for me) and do result in savings on fuel at the cost of a small (1-2 minutes) increase in flight time. Obviously there are a lot of variables and savings do vary depending upon AUW. All other influences being factored out on the CFP - try it for yourself. I have always wondered why it isn't SOP particularly on "redispatch fuel" type flights. Maybe it's just too hard to understand.

MG

[5milesbaby]

Just trying to get my head around this, but is this why the A320/A321's appear to give 2000/2500+ ft/min upto FL250 and then dramatically reduce to 1000ft/min or less? We see this all too often in ATC, not that it causes many problems, just means our level planning is to never bust a gut for these, favouring the B737's etc.

[DCDriver]

5milesbaby,
As you are an ATCO & mention the A320 (my current type) and it's climb perf, or rather lack of, I offer my observances.
The V2500 engined 320's climb better at altitude than those with
the CFM56 engine
Climb performance is satisfactory up through the mid-levels, but >1000fpm is out of the question above FL 250. This is just below the point at which we transfer from flying IAS and go onto Mach.
I have tried high-speeding the thing onto the Mach No's as soon as poss (which usually works in older & heavier jets) and this can offer temporary respite up to approx FL300; from there on its a long flog up to "Optimum FL".

My own gut feeling is that the engines, particularly the CFM56, don't like high Mach Nos and lose their propulsive efficiency, but as my old prop notes from ATPL days are stored up in the roof, I can't offer any clever scientific reasoning for stating this!

Incidentally, I have experienced these characteristics once before in my career, on the early MD80's, which climbed well to FL250 and then dropped off markedly, much as the 320 does.

[5milesbaby]

Cheers DCDriver, its good to know this as it can come into play obviously for separations etc. You mention the 2 types of engine, any guesses for who uses which in the UK??

[DCDriver]

5miles,
I'd say that there are probably more CFM-engined 320's in the UK charter business than the V2500. This is mainly because the CFM has a better field performance than the V2500 (ie) one can get more weight out of a small greek island strip on a hot day thus can get back to any UK a/d without the need to techstop, as was sometimes the case I believe with the older V2500's (before my time on the 'bus).
I wouldn't like to guess who uses what as that would result in howls of protest from all quarters; suffice it to say however there are still plenty of V2500-engined 320's in the UK , and some might say that it is a far more refined engine, but that's another matter!

DCD

[Cuban_8]

Guy's

There seems to be a modicum of confusion here w.r.t. efficient operation of modern jet transport a/c. I think the question you have to ask is what are the operators/manufacturer's specifically trying to achieve - even someone as academically challenged as myself can work out "lowest overall cost"!

Operating a jet transport a/c from A to B for minimum fuel burn involves climbing at best lift/drag ratio speeds (Vy) into the rarefied atmosphere, where both airframe efficiency (IAS/TAS) and engine efficiency (SFC) is greatest. One would then remain there until the point at which a descent, again conducted at best lift/drag ratio speed (Vy), is required to arrive at destination. This is (or very close too) the profile that an FMS would furnish with Cost Index 0 (very high fuel cost).

However, is this what operators are trying to achieve - no. Other costs, such as crew, maintenance and a/c utilisation have a large part to play. Remember that fuel is relatively cheap! Thus, over all ECON speeds are a compromise between fuel burn and airborne time, and are defined by the Cost Index that we enter into the FMS. The result will be climb/descent speeds in excess of Vy. Try popping a CI of 999 in the FMS next time you fly, and look at the predicted speeds for climb/cruise/descent! Makes for a very attractive flight time, at the expense of a lot of fuel.

Of course, much of this is rather academic, as we have to vary our speeds to suite various restrictions/ATC. Reverting to the cost index 0 techniques is, however, a very handy trick to have up ones sleeve should you find yourself becoming rather tense about your fuel situation:oh:

Regards,

Cuban_8