And when you see the chart I posted it could save 20 minutes and over 20 gallons of fuel if you get it right.
Exploting a massive tailwind – I agree. But this is obvious to any pilot. All he needs to do is go to windy.com and see the likely wind, and if it is a tailwind then he should know that climbing high is well worth doing. Unfortunately in Europe you usually need an IR to make use of that.
It is also fairly unusual to have calm air at 3000ft and 40kt at 17k.
But in still air conditions, the calculation is much more marginal. If there is no wx factor, then I cruise at FL90-110. It takes about 15 mins to get there, distance travelled is about 30nm, fuel burnt is ~6 USG over and above flying 30nm in cruise. So any fuel saving by doing it more precisely will be some fraction of that 6 USG. And if I am avoiding wx, I will do whatever is needed to do that, usually a climb at best power to maybe FL190, and then I see what the computed LFOB is, and decide what to do…
And more often than average one has a headwind.
Peter wrote:
For example, assuming a constant engine efficiency, the best-MPG speed is Vbg. In practice it is higher than Vbg.
This is the critical point. Breguet’s range formula shows that, at a given L/D, range/fuel doesn’t depend on altitude . But at low altitudes, we tend to fly at lower L/D, because the speeds at optimum L/D are embarrassingly slow. At the absolute ceiling, you have no choice but to fly at optimum L/D. Accepting the “penalty” for a climb to 8000 ft does not increase your range if you fly at optimum L/D, but it does get you there faster.
A flight I’m planning for Tuesday. Personally, I always get the flight planning software give me the best solution. :)
Except, the software is not giving the whole picture. You will have little tailwind until Spain 
Indeed. You may have noticed I’m playing with Foreflight here. Ordinarily I use Garmin Pilot and I’ve found their wind modelling to be exceptional. It will be interesting to compare the two over the next week.
JasonC wrote:
JasonC
tschnell wrote:
My POH does not give ROD’s for given airspeeds and powersettings, so I would have to guess or try it out.
Without that it is impossible to calculate precisely.
A friend thought me the following:
Say that you fly at 10000’ and need to decent to 1000’, that is a 9000’ descent. Take the ‘0’ out and multiply by 3, that is 9×3 which gives 27, this is your distance in miles from target when one has to start the descent. Assume 120kts ground speed which is 2nm/min. 27/2=13.5min flying time to target, during this time we need to lose 9000’. 9000/13.5=666.66’/min decent rate. If you want to descent at 140nm then:
140/13.5=10.3min.
9000/10.3=873’/min.
on the other hand, if you want to decent at 500’/min then:
9*2=18min to descent from 10000’ to 1000’.
If descending at 120kt there is a need to start the descent at 36nm from target, if descending at 140kt then the descent has to start at a distance of 42nm from target (as you cover 2.33nm/min).
Edited for spelling.
Dave_Phillips wrote:
Indeed. You may have noticed I’m playing with Foreflight here. Ordinarily I use Garmin Pilot and I’ve found their wind modelling to be exceptional. It will be interesting to compare the two over the next week.
Are you using their performance models with Performance Plus?
Yes but it’s a little irritating that you cannot get to see too much detail of their models. I’ll let you know how things go and, if I get a chance, I’ll play comparison with GP.
Dave_Phillips wrote:
Yes but it’s a little irritating that you cannot get to see too much detail of their models. I’ll let you know how things go and, if I get a chance, I’ll play comparison with GP.
It worked brilliantly with the Mustang but the owner has one so you would expect it would. But i agree, you have to check and then adjust the slider.
boscomantico wrote:
Discounting the fact that one can almost everywhere in Europe also cruise IFR below the airway MEAs,
…which you really shouldn’t as that possibility is very important if you’re not FIKI.
