The effect on a Lockheed TriStar at heavy weights would be rather severe, if a much slower climb speed is used. Normally, the L10 is climbed at 250/320/.82 at mid-weights and 250/340/.84 at very heavy weights, to achieve best time to climb performance. Yes, you could use a slower climb speed initially to FL200, to perhaps meet a close in climb restriction, but then would have to accelerate, as ROC would drop off significantly thereafter, at the slower speed. The TriStar was designed as a high speed cruiser, and the wing does not tolerate rather slow climb speeds at heavy weights. With -524B4 engines, the ROC is much better, due to the higher thrust available, but still the higher climb speeds are beneficial for enhanced climb performance. Further, if a slower climb speed was used regardless (at very heavy weights), expect a longer time for the climb, and increased fuel burn. Not a happy scenario.
On the -400, if you accelerated straight to 340 KIAS right after cleaning up the airplane, you are probably looking at savings of ~ 1500 lbs to Top of Climb, versus if you climbed at 250 kts/10,000 ft before accelerating. It's got to do with jet engine efficiency increasing with airspeed although i can't give you any specific numbers off hand. Also swept wing airplanes are a lot happier at those higher airspeeds.
Two books for reference come to mind - "Mechanics of Flight" by A.C. Kermode & "Aerodynamics for Naval Aviators" - if you're the numbers type
A high speed -400 climb would be very restricting on maximum altitude, as would a low speed climb. What I think the question is getting at is best angle climb or best rate? It has to be best rate- sometimes it is all you can do to get to the highest cruise altitude. 250kts for a heavy jet is on the lower part of the drag curve and not minimum drag. The most efficient is CI 0 and then resetting in cruise to the cruise CI balancing max fuel saving against aircraft costs in operation.
At any time that ANY aircraft conducts a climb at less than, or more than, optimum range climb speed you will consume more fuel. This speed is typically slightly higher than best Rate of climb speed, and significantly higher than the best Gradient speed.
The best Angle of Climb occurs at a speed where there is Maximum Excess Thrust. This speed is, for PRACTICAL purposes, Vmd. Best Angle speed is somewhat less than the normal En-Route Climb speed.
The best Rate of Climb occurs at a speed where there is Maximum Excess Power. As jet engines directly produce Thrust, not Power, it is necessary to consider Thrust multiplied by speed (Power = Force X Velocity). Thus, for a given Thrust setting, Power increases as TAS increases. Thrust actually ‘dips’ as speed increases, but then there is significant Ram recovery at higher Mach Numbers, thus further increasing Power at higher speeds.
If we examine a Power Required and Power Available graph for a jet aircraft (about the only time that we’re interested in Power for a jet), it is observed that at the higher speeds, Power Available Vs Power Required (thus excess Power) is close to parallel with minimal divergence over a fairly wide speed range. Speed variation of the order of about plus and minus 20 to 30 knots from the optimum, shows only a slight reduction in excess Power with this speed variation.
For a fairly heavy jet aircraft such as the B777, a fairly typical best Angle speed is 250 KIAS, best Rate 300 KIAS, and a typical Economy climb speed is 320 KIAS.
Best Angle speed is at a somewhat lesser Rate than best Rate speed, so, if the two are compared, the low speed climb will take longer and consume more fuel to Top of Climb. Then, for a climb time of, say, 30 minutes at a difference of 50 KIAS (about 80 KTAS), there will be an incremental cruise of 40 nm required to just achieve the point where best Rate of Climb would have ended. Therefore, in the comparison between climb at best Angle and best Rate, the lower speed will cost you both time and fuel.
The best Rate of climb can be refined further to optimise time and fuel. Remembering that up to 20 to 30 Knots ‘Off Optimum’ have only minimal effect upon excess Power, an Econ Climb speed a little above best Rate (+20 KIAS for the B777) will have a negligible effect upon Rate of Climb, but put the aircraft 30 miles or so further down track at Top of Climb. The very slight increase in climb Time and Fuel is more than off-set by the extra distance covered in the ‘fuel expensive’ climb, and elimination of the incremental cruise.
If we consider operations for a Maximum Range profile (absolutely minimum fuel), it’s not at all uncommon to see a Climb speed in EXCESS of the initial Cruise speed.
As I’ve mentioned in other posts, where local legislation allows it, I frequently accelerate above the nominal low altitude speed limit of 250 KIAS from 5000 feet instead of the ‘standard’ 10000 feet. For the B777, this repeatedly saves 400 Kg of fuel per sector (and that’s only over 5000 feet).
Obviously, if the operational situation requires maximum gradient, put the economics aside until achieving the altitude constraint, and then, go like the clappers.
Great explanation, thank you so much for confirming what I understand to be the case. My company SOP is to climb at 250/280/.06 however there are several people who consider climbing @ 250 kts all the way to TOC ,or even slower, generally FL230 on the sectors we fly, is a better option. The general belief is that we will get there quicker and thus benefit from a tailwind longer. I have been scratching the brain with this theory and dont believe it to be true. Always look forward to your posts thanks
I have to go along with mutt in asking where does F/L 230 come into the picture?
One of the considerations that we must make is the altitude where, if a 2 speed climb profile is to be used (e.g. 250/280), the acceleration from 250 to 280 is fuel expensive. At lower levels (e.g. 10000 feet), there's plenty of excess thrust, and this acceleration can be accomplished fairly quickly. At higher levels, like F/L 230, excess thrust is much less, and the excess thrust (and excess Fuel Flow) must be used for much longer for the acceleration, at the expense of climb performance, thus penalising your climb economics.
Also, it's in the approximate band 20000 to 30000 feet where Mach Number (generally anything above M0.5) is increasing to those values where Ram recovery is of greatest benefit, increasing Thrust and Power. In this same height band, also, TAS is at it's highest just prior to Mach changeover, and Ground miles per unit of fuel benefits greatly from the higher speed.
Speaking of Mach changeover, I think that you made a typo, M 0.6 does seem to be a tad slow, M0.76 perhaps?
We climb @ 250Kt to FL080(Bird impact limitation)(FL100 in class A&B Airspace) then accelerate to 280Kts and continue@ this speed until we reach M0.6 usually around FL190. On a sector distance of 250nm we fly @ FL230.
Climbing in this way with a tailwind generally results in a parabolic situation where the TOC = TOD, or very close to it, therefore giving the best possible fuel economy for the overall flight. So now given this set of data how would you manage the climb in relation to speeds/performance and why.?
I think that the situation that you describe is one that best justifies the High Speed Climb.
Consider this, En-Route performance is essentially 3 phases, Climb, Cruise, and Descent. The Climb is far and away the most 'fuel expensive' phase of the flight, and every effort must be made to optimise fuel use per mile in this phase.
This will be particularly so on short sectors, such as you describe, when we have eliminated the cruise phase, and the climb phase remains the most critical in terms of good economics management. It is absolutely true that, for short sectors, the 'slingshot' profile (climb to descent point) is the most efficient. Climbing at best Rate or best Econ speed is at a somewhat shallower angle, thus the apogee of your 'slingshot' profile will be at a lower altitude, but far less fuel in a reduced time to that lower level is the result of the higher speed climb.
You mentioned that some of your colleagues were opting for the lower speed (230 KIAS) climb to allegedly reach higher levels with the tailwinds sooner. At your CAS/Mach changeover height for 280/M0.6 at 19167 feet, TAS at ISA is 370 Kt. At 230 KIAS at the same level, the TAS is 306 Kt, some 64 Kt slower at the same Fuel Flow. Your 'slow flying' colleagues will need to find 64 Kt of Tailwind that you don't have, just to remain equal, I think an unlikely scenario. In the reverse direction, flying into a Headwind, the faster climb wins again, as a given Headwind component has a lesser percentage penalty upon GNM per Unit of Fuel at high speed, than at low speed.
As an afterthought, even long distance operations are optimised by the 'slingshot' profile, that is, climb to optimum level, and then Cruise Climb at the steadily increasing Optimum Altitude until co-inciding with Top of Descent. Infrequently possible due to ATC constraints.
An interesting speed profile, 280 / M 0.60, what type of aircraft is this?
We operate the Bae 146-200. I have set up a spredsheet in excel to record data derived from each 5000 ft of climb in various climb profiles. It will be interesting to compare the results. I am confident that what you have explained in theory will be backed up in practice. Thanks again for your input , its rather like having a theory instructor at the end of the line.