Descent Efficiency

[kangaroota]

Can you settle an argument?
In a turbo prop, what is more fuel efficient - A steep descent with power off or a shallow descent with power on.
I maintain the former.

[low n' slow]

I would argue that by staying high as long as possible and then descending with the least fuel flow setting (basically power off glide), you would get the best fuel efficiency. What is interesting though (and something we have discussed in my company) is weather the extra time this will cost you, reduces the savings margin. By going in as fast as possible for a longer period of time, you save time and thus there is less time to burn fuel, although, you will burn a higher percentage at lower levels where fuel economy is poor.

I'm sorry, I can't answer your question based upon facts and spectacular graphs, but I'm inclined to agree with you. I think the reason most of us plan our descent to maintain the highest speed possible is that this minimizes the time in the air. What is more expensive than fuel is maintenance costs and by getting in as many legs per flown hour between overhauls you optimize the aircraft's production.

/LnS

[FE Hoppy]

Stay as high as possible for as long as possible then a dive as fast as you can. With a TP you can bleed of the speed easily once you get down.

[john_tullamarine]

Better to do an analysis for the given airframe/engine combination. I would be expecting to see an initial descent with some power, then a FI descent for the lower levels ..

[411A]

Better to do an analysis for the given airframe/engine combination. I would be expecting to see an initial descent with some power, then a FI descent for the lower levels ..

I would agree.
It should also be remembered that on some engine designs, idle descents are to be absolutely avoided, the RR Dart is a classic example.

It is also interesting to note that on some jet transports, complete idle descents are not the most economical, contrary to popular belief.

[OzExpat]

I find myself in basic agreement with john_t but I think the analysis of engine/airframe combination needs to consider the fuel used in the climb, in the first instance. The question posed in the opening post is probably the best aviation-related example of the age-old question :- which came first, the chicken or the egg? :uhoh:

I also think it's a case of "horses for courses". Sure, we all try for the simplest, most fuel-efficient descent that doesn't upset the residents under the approach path too much. Where there's no obstacle/airspace limitations, I've always gone for a standard 3-times descent profile in a TP with P&W engines - this profile requires some power but, as long as there's no ATC hold-ups, I've been able to maintain the profile in the lower levels, through configuration changes that result in a bit more power being used.

The only time I ever executed a full FI descent was one time when ATC held me up (under procedural control) to 10,000 feet at 10 DME! Even then, the thing in my favour was a fairly strong headwind. :ok: Had it not been for that, fuel savings would've been history - and, of course, being held at 10,000 feet didn't actually help the fuel economy very much so I doubt that, overall, any real savings eventuated. :sad:

[a-320crew]

what about fuel and time?

[Pontius's Copilot]

Consider the relative merits (and reasons for) a normal - high speed - flight profile and a long range (diversion) - more fuel efficient - flight profile. Certainly on my heavy-ish turboprop, the more fuel efficient profile requires a lower speed climb for better gradient up to efficient cruise levels, and low-speed/low-power descents for later descent from cruise levels.

[john_tullamarine]

...10,000 feet at 10 DME

similar to being held at 11000 to 10 on the Electra ... a couple of little S-turns on the way down and Bob's yer uncle .... the best was going northbound into SYD in the wee hours for a landing into the south. So long ago now I can't recall the max cruise (300 ?) but, being empty, that's where we were .. just for the exercise .. started descent abeam the airfield ... causing ATC great consternation .. and even ended up having to put power on from mid final .. but, then again, the Goose was a real aeroplane ..

[Admiral346]

The most efficent descent would be not the steepest, but the one with least resistance. You should fly idle and your minimumclean speed. If you maintain altitude and then fly a steep descent, you would be increasing your speed, and doubling speed quadrouples resisting forces...

Nic

[CJ Driver]

The most efficient flight from a fuel consumption point of view is actually a very slow flight - you should fly the whole flight - climb, cruise, descent - at best L/D speed, which will start slow and will actually get slower as you burn off fuel. In the original question choices that would recommend a steep descent with low power, but would also recommend a tediously slow cruise.

But in all the aircraft that I am familiar with it is not the fuel flow that is the dominant cost of flying - the dominant cost is the hour meter (which determines the maintenance costs and the airframe depreciation).
In other words, if by "efficient" you mean least total cost, then you should always go as fast as you can. That still means staying high as long as you can (to get the benefit of favourable TAS/IAS ratios) but then descending at red line, with as much power as you need to stay at that speed. So - neither of your two original choices!

[SpootNICK]

I would say the best descent and approach is one flown with the power levers at idle, speed at a constant Vman clean until FL100, thereafter 250Kts all the way to your first configuration point. Once you advance the props for landing, the aircraft looses speed in a jiffy, so what ever you can manage.

Its a fine balance between fuel efficiency, passenger comfort and personal ability. Find the balance, and you will always be on the safe side.

Sn

[Re-entry]

No doubt the former.

You will travel the required miles at least specific fuel consumption followed by a descent (at the appropriate point) with minimum fuel burn at idle thrust and vmd ias.

That is why we cruise at altitude after all, for the best SFC.

All subject to ATC restrictions of course, which are rife nowadays.

[ssg]

This question is moot..

If you carry people, people that sign your paycheck, they will not appreciate a dive bomb approach, steep deck angles, rapid decents, ect...fly like that too much, and your looking for a new job pretty soon..

That said...you can fly around with 50% power all day long and call it 'efficient' because your saving fuel...others will counter that your burning up engine time...everyone will have thier calculators out, then start arguing seat per nautical mile, compare the cost per trips on the airlines...and I have known guys that can work the numbers right to get anyone to consider a GIV by the time they are done...

Last I remember we are all trying to get from Point A to Point B. safely..he who get's there the fastest wins...he who get's there the fastest and most comfortable really wins...and he who get's there the fastest, most comfortable and for the least amount of money, really, super duper wins...

So in your plane, which type of decent profile, will obtain all these objectives...

Usually a nice normal decent, as fast as you can go, in such a way that's safe, and comfortable for the passengers...

[kangaroota]

Thanks for your inputs everyone.
I guess that I was a bit broad in using the term "efficiency".
What I am keen to establish is what technique uses the least amount of fuel between top of descent and touchdown - stay high and descend fast as possible with power off or descend early with power on. Other criteria don't matter for the purpose of this exercise.

[CJ Driver]

Well, kangaroota, since you have been specific about your question, and said all other considerations can be ignored, the least amount of fuel you can consume is clearly "none".

Calculate the engine-out glide angle for your chosen aircraft. Project that glide angle backwards to the point where it intersects your route at cruising altitude, which we will call Top Of Descent. When you reach TOD, shut down all the engines. You are now assured of using the minimum possible fuel from TOD to the airport. Subject to traffic you are not actually assured of reaching the airport but that, as you say, doesn't matter for the purpose of this exercise. :hmm:

But slightly more seriously, this technique works quite well for jet aircraft, because instead of shutdown they can select flight idle and broadly speaking follow the instructions given above. And the larger transport types, when it is offered to them, do sometimes actually follow a profile close to that ideal.

The calculation for a turboprop is less obvious, because for the "ideal" profile with zero fuel consumption, you'd really need to feather the props when you shut down the engines. On the assumption that reaching the airport with a high degree of certainty DOES matter for the purpose of this exercise, and you want to keep the propellors going round, you've got a drag versus fuel consumption problem to solve. You'll need to keep the engines spooled up in order to keep driving the propellors, so you are not really at idle. So you would actually need a quite in-depth understanding of the propellor drag curve versus fuel burn to know where the "sweet spot" was for the nearest equivalent of the feathered-prop glide approach.

But to go back to my original answer, and to generalise, all aircraft fly faster than their optimum speed almost all the time. So any time you want to save fuel, slow down. Just remember that by slowing down you may be saving fuel, but you might not be saving money.

[rubik101]

Since it has been mentioned relating to Turbo-props, losing 10.000 in 10 miles is readily achieved in a B737. Fully configured at 40 Flap with the gear down and speed of 140kts. the 737 will lose one thousand feet a mile in a 10kt headwind. (No speedbrake)
I would think the same might well be possible in other jet aircraft but I don't know from experience. However, long gone (pre GPWS etc.) are the days when it was considered 'slick' to do power-off descents to touch down.
Ah, happy days!?!? perhaps!!

[Piltdown Man]

Somewhere approaching 10-15% torque will give a zero thrust descent. Any less and you start to create drag with the props and that would be a waste. The planned descent should be in the order of 3nm per thousand feet. Rocket science not required.

PM

[john_tullamarine]

losing 10.000 in 10 miles is readily achieved in a B737

used to do that regularly coming into Cairns from the west for a circuit if conditions were turbulent .. over the upwind end (plus a bit) at 10-11,000 and then down she went .. although I preferred to use 25 and a bit more speed so that we could comply with SOP re spin up ... gave the punters a much more comfortable ride than hammering into the circuit normally ... and, just as importantly .. gave us a more comfortable ride.

[rubik101]

Nostalgia is not what it used to be John!
I have to smile when I think of the antics we got up to 25 years ago which are now, quite rightly, considered absolute no-nos. Power off arrivals from TOD until turning off the runway were considered the norm for many 'island' arrivals.
FLIDRAS, GPWS, etc. have put paid to such antics. But that is for another thread!

[Slasher]

Yeh John I remember hearin when you Wombats used flap 40
at flt idle youd drop outa the sky like a lump of lead-laced sh!t.
The ex-DC9 blokes with Ta-Ta's had some storys about the
Goose exploits in Tassey. :ok: