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Does a long continuous descent save fuel?

From here

Here’s an oldie but goodie – how a transatlantic Avanti pilot does everything wrong you possibly can do. But also some basic reminders for all, such as:

1. You don’t save fuel by shutting down one engine.
2. You don’t save fuel by doing a slow, gradual descent.

AOPA

Last Edited by AdamFrisch at 18 Mar 22:32

I don’t think there is a way to “save fuel” using fundamental knowledge, in practice, you need to see ground speed (GS) and fuel flow (FF) to be able to answer that question without theory, I guess he had these on-board…

But why shutting down one engine will not save fuel on twins?

I understand, a twin will be underpowered one one engine, inefficient yaw, and need a lot of pilot skills, but in theory, if that puts you on the right spot of the “power curve”* that may do to save distance/fuel? AFAIK, this is done on C130 in low cruise but that is done 2/4 engines with two engines working really hard to save fuel, but you may loose the alternator

*On a twin I guess you have a 3D “power surface” of two engines % of power vs IAS, why its minima can’t be on one engine?

I got a weird feeling flying C172 to its max range (roughly 2100RPM/85IAS), I almost lost patience to go faster (not slower) as it did not feel intuitive as “distance/fuel efficient” with a slow flight and lot of left rudder, so you need some TK

Last Edited by Ibra at 19 Mar 01:04
Paris/Essex, France/UK, United Kingdom

You don’t save fuel by doing a slow, gradual descent.

However, it must be more complex, because this is what passenger jets do all the time. Well, maybe not “slow” but certainly long.

I also think, from many observations of mine, that it saves fuel in pistons, provided you don’t allow the IAS to climb up in the descent.

Administrator
Shoreham EGKA, United Kingdom

Cover as much of the distance as you can at optimum altitude with minimum fuel consumption per NM, then glide down to the airfield at max L/D. Anything else is a loss.

Last Edited by loco at 19 Mar 07:38
LPFR, Poland

AdamFrisch wrote:

2. You don’t save fuel by doing a slow, gradual descent.

Someone is confusing fuel and time. Time is money, fuel is money, therefore they are the same thing? no.

The elephant is the circulation
ENVA ENOP ENMO, Norway

Minor factors like throttle position (“pumping losses”, thanks peter), specific consumption and proper leaning aside, there is no optimal long range altitude for piston engine propeller aircraft. Although there is an optimal speed (1,2 x vmin power|max endurance).

Last Edited by a_kraut at 19 Mar 09:58
Bremen (EDWQ), Germany

How is it for no wind scenarios?

If only fuel used counts?
If only time counts?
If both count?

always learning
LO__, Austria

a_kraut wrote:

there is no optimal long range altitude for piston engine propeller aircraft

Does not SL = 0ft achieve max range on SEPs? (or 500ft above AGL to keep it safe/legal )
True, the max range you will achieve at 0ft is almost similar to 6000ft, except you don’t have to climb…

Last Edited by Ibra at 19 Mar 09:11
Paris/Essex, France/UK, United Kingdom

Peter wrote:

it must be more complex, because this is what passenger jets do all the time

To abstract from this remark, based on research I did a decade ago using proprietary performance data for a FAR25 aeroplane there are still savings to be made by optimising the climb trajectory which is by far the more interesting case.

London, United Kingdom

Cover as much of the distance as you can at optimum altitude with minimum fuel consumption per NM, then glide down to the airfield at max L/D. Anything else is a loss.

I think that is self evidently true The Q is what is the optimum altitude.

For a non turbo piston, IO540 type of engine, certified, Vs=60kt, it is around 8-10k feet, and it doesn’t vary much. You just need to be flying with WOT (wide open throttle) for the minimum pumping losses, and peak / lean of peak on the combustion.

For turbo it will be higher up.

For turboprop and jet, it will be very different and usually a lot higher up.

BUT often the time you want to maximise range is when flying into a headwind, and then commencing the descent early will benefit you because you are descending into a reducing headwind. One could probably argue that a descent at best L/D to the lowest altitude at which you are still WOT is the optimal strategy there.

Against this you have the very common operational considerations e.g. once down and OCAS it may be hard to get back up again if you encounter bad wx / icing, but that’s a different debate. I have found myself in this situation quite a number of times (strong headwind + bad wx + climbing to FL150+ would get me above the wx but with a big headwind cost).

based on research I did a decade ago using proprietary performance data for a FAR25 aeroplane there are still savings to be made by optimising the climb trajectory which is by far the more interesting case.

I think the piston version of this is a climb at peak / LOP but you have to climb quite slowly – of the order of +300fpm to say FL100 – to keep the CHTs down.

Does not SL = 0ft achieve max range on SEPs?

It achieves the highest IAS and the highest TAS, but not the best range, because you are flying with a partially closed throttle (high pumping losses) and aren’t getting any TAS gain. If you go to full throttle, a TB20 does 165kt IAS and TAS down there, but burning 24 USG/hr because at full throttle you have to be very rich on the mixture!

Administrator
Shoreham EGKA, United Kingdom
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