Min fuel ops - why not shut down one engine?

[natedog74]

wouldn’t that be common sense in a pickle of a situation when making the apt on remaining qty seems doubtful? Considering that even @ idle fuel continues to be burned in exchange for a little in the way of thrust, wouldn’t conserving some of it be a fair trade for an intentional OEI ldg?
What about 3&4 engined apl's, is that part of the min fuel non-normal cklist?
P-3’s and other patrol a/c shut down 2 engines routinely to remain aloft for 18 hrs…
Which two would go, in- or outboard?

[411A]

Can only reply about two specific types...B707 and L1011.

For both (using round numbers), it requires approximately 500kg/hr more to cruise with one engine shutdown.

Sometime in the early seventies, DanAir, who at the time were operating the Comet4, asked the ARB if it would be OK to shutdown one engine in cruise...as on this specific type (according to them) fuel savings could be had.
DanAir was politely told....no. This was reported by Flight International at the time.
Others, who indeed have operated the Comet (or the Nimrod) may have additional comments.

[yardman]

The Airbus 330 FCOM Vol 3 states that the aircraft will, on average, use 30% more fuel flying on one engine as opposed to two.This is of course dependent on speed etc. But it appears that two engines are always better than one

[keithl]

Someone quoted the Nimrod, so I can add that we shut down one or two engines (depending on weight) for endurance not range . So it would depend on what you wanted to achieve - loitering until the weather cleared, or covering the distance to destination. I can't quite recall the equation just now, but for endurance you fly just above Min Drag Speed, and that's what makes the difference.

[ferrydude]

Used to happen quite often on the diesel 8

[Speedbird48]

In order to do a three engine ferry on a Comet (RR Avons) the oil system had to be filled to overflowing to allow for oil loss. The engine would still be rotating but would have no air pressure to seal the labryinth seals and the oil would run out.

This would be the same on many other engines although some, (VC-10) had a way of fitting a kit or a long piece of wood to prevent rotation and prevent loss of oil.

Remember that these are straight jets, or minimal by-pass (Conway) which do not have the drag of a big fan so fuel was not so much of an issue.

How the Speys in the Nimrod do it, for endurance, I have no clue but they must have a different or bigger oil system.

Hope this helps.

[keithl]

Speedbird, I'm an idiot for displaying my ignorance like this but I seem to remember the Speys, windmilling, generated enough air to seal the labyrinths. But I never was very good at understanding the oily bits...
We need an engineer to set us right. FEHoppy come in, please!

[compressor stall]

late at night and trying to get my body clock back in order and having trouble thinking of the answer to the following.

At what point is the changeover in that most(?) small (GA) aircraft can increase range and endurance with OEI but your replies seem to indicate that the big toys cannot.

Where and why is this changeover?

[alatriste]

If for any unforseen reason you have to fly very well below your optimum flight level, i.e. depressurization, it might be interesting to consider to shut down one engine (twins), only if you are dangerously running very low of fuel to reach any airport. It your flight planning was serious enough it would never happen.
Specific range for a jet aircraft depends not only on Fl, GW , etc but also on engine efficiency, if you HAVE TO fly at FL100 at long range, corresponding LRC speed will be so low that both engines will run almost at 70% at this rating both engines will be very ineficcient, in this scenario you may retard one throtel and check for new specific range, if dessesperated even shut it down.
For instance a MD80 flying at FL 130 and GW 48 T, specific range for both engine cruise is 122 NM/ ton of fuel, while cruising with just one engine is 130 NM / ton.
It is really not much difference for a real scenario for killing one engine, but at least you can go a little further. Anyhow, Would be we able to fly it precisely? What about the trim drag?
Too much risk for such a low gain.
REGARDS

[cortilla]

Just a quick thought for this one in a 2 engined a/c. Never flown jets so not exactly sure about this. Say you've shut down one engine on a twin to save fuel or whetever. What happens if the other one fails. Aren't you a bit up sh1t creek then, or can you restart t'other in time to save your bacon and not scare the pax so they don't report you to the press

[ShyTorque]

I think it's true to say that throttling one turbine engine back is a waste of time/fuel and much worse than shutting it down! The fuel burn consumed to run an engine providing very little thrust or torque makes it so.

Turbine powered helicopters are very badly affected. In the OEI case, the main gearbox is usually torque limited due to assymetric loading and the maximum speed may therefore be as little as 60% of the twin engined case. This can result in the need for an early diversion for lack of fuel, depending on wind velocity.

Keithl has given the big clue, shutting down a turbine and operating the remaining engine/s at a higher output (more fuel efficient regime) gives a greater ENDURANCE but not necessarily greater RANGE.

It's those (Air) miles per pound that count, not time flown per pound when it comes to getting where you need to go (rather than loitering where you are for longer). :ok:

[FJJP]

If you want to go for range, a significant factor is getting the ac to fly straight (I'm serious). By keeping the wings dead level and watching the heading, if you get any creep, then you've got a twisted airframe or the rudder trim is not set right. Set it all up and watch the speed creep up.

The Nimrod endurance technique called for shutting down outboard engines; the crew checked the fuel flow at the endurance speed on all 4 engines. One engine was throttled back and the 3 remaining spooled up to maintain the speed. The fuel flow was then compared and if it was less than the 4-eng case, the engine was shut down. The same procedure was repeated some time later to check if 3/2 was more economical. This technique considerably increased the time on task.

Clearly, drag is a serious factor - it is obviously more significant where the engines are wing mounted, as opposed to buried in the wing root.

[FE Hoppy]

Ahhh Keithl my old chap! How are you? and hello from the wrong side of the pond.

First as has been stated at altitude shutting down an engine is not good. More drag less thrust most likely you will have to descend. I suggest requesting higher from ATC(if you still have some way to go) or flying at the correct LRC from your AFM or AOM. Now if we are talking low level then for best range or endurance at some stage 3 is better than four, then 2 better than 3. Theoretically 1 will be better than 2 also but I wouldn't risk it considering all the other implications. Why? at range speed (@1.3 vmd) at low level your 4 engines will be operating well below their optimum band. Hence shutting one down and allowing the others to operate at a slightly higher rpm will, below a critical weight, reduce your overall gross fuel consumption (kg/hr) and thus increase your specific air range(nm/kg). The same applies at endurance speed(@1.1vmd) where Vmd is best Cl/Cd.

As for the speys leaking oil when wind milling. Yes they did but not much. It was a partial loss system at the best of times and the max endurance with an AAR probe fitted is limited eventually by a. Engine oil or b. Crew sanity.

[ROB-x38]

Say you've shut down one engine on a twin to save fuel or whetever. What happens if the other one fails. Aren't you a bit up sh1t creek then, or can you restart t'other in time to save your bacon and not scare the pax so they don't report you to the press

From my systems studies engine relight depends on 2 main factors: altitude and airspeed. How long it takes would vary from engine to engine i'd say. eg: the KLM 747 that lost all four flying through volcanic ash got them going again a few thousand feet later :oh:

And the oil supply for the shutdown engine reminded me of something - i've heard that carrying a 5th engine on a ferry flight needs a little fuel to be supplied to it while it's windmilling. Can anyone confirm or explain why/why not?

Cheers, Rob.

[Capt Claret]

With an ALF502/LF507 the engine will eventually be stuffed after windmilling with attendant low oil pressure.

Very expensive when compared to a Tech stop.

[tcr2]

Rob, I think it was a BA 747 that flew through all the volcanic ash, causing all 4 engines to fail. (Ask a certain Cpt Moody to confirm this). I think the only reason they were able to re-start the engines is because they flew out of (or below) the level of the ash plume.

[ROB-x38]

There were 2 747 incidents with volcanic ash. A KLM 747 http://aviation-safety.net/database/1989/891215-1.htm and a BA 747 http://aviation-safety.net/database/1982/820624-0.htm

[JohnR42]

On the 747, the 5th pod has a cover on the front to reduce drag and stop the engine from turning. I think the engineers do something to stop it turning as well. So - no fuel or oil supply is required.

[G-SPOTs Lost]

My Apologies but could somebody enlighten me with regard to this 5th Pod or carrying a 5th Engine on a 747?
:confused:

[Max Angle]

G-spot. A 5th engine can be placed on the 747 wing inboard of the No. 2 engine. (ie. on the left hand side beside the fuselage). It used to be quite common some years ago when engines were less reliable than they are today, a tech. engine could be transported back to base or a replacement ferried to an airport without a special cargo charter. The only web reference I can find with details is the report into the Air India 747 that crashed into the Atlantic in June 1985. The aircraft was carrying a spare engine back to base and the article gives some operational details.

http://www.airdisaster.com/special/special-ai182.shtml

Don't know how often in happens now but I have never seen it, I believe the 707 could do this trick as well.

[av8boy]

Ah, like the old story…

747 at night going across the pond. A couple hours out of JFK, the Captain makes an announcement to the pax:

“Ladies and Gentlemen, this is your Captain speaking. We’ve had a little mechanical trouble with one of our engines and we’ve gone ahead and shut it down just as a precaution. We’ve still got three good engines and that’s more than enough to safely get us to our destination, so there’ s nothing to worry about. However, not having that engine means that we won’t make the same kind of time we’d have made with all four, so our arrival is going to be a little later than planned. It’s looking like we’ll be about an hour behind schedule when we land. Hope this doesn’t cause too much trouble for you. Thanks.“

An hour later, the Captain makes another announcement:

“Ladies and Gentlemen, this is your Captain speaking. We’ve had some trouble with another engine and had to shut it down as well. Again, nothing to worry about. On this aircraft, two engines provide more than enough power to safely get us where we’re going. The bad news is that this is going to slow us down even more, and we’re going to be almost three hours late on our arrival. We’re really sorry if this has an impact on your plans. Thank you.”

An hour later, the Captain makes still another announcement:

“Ladies and Gentlemen, this is your Captain again. Well, we’ve had a little difficulty with a third engine and had to shut it down as well. Rest assured that this aircraft can safely fly anywhere in the world on a single engine, so there is absolutely no danger. However, it now looks as though we’re going to be right around six hours late on arrival. We’re terribly sorry about this. If there’s anything we can do to make your flight more pleasant, don’t hesitate to let one of the flight attendants know.”

Upon which one passenger turns to another and says, “Good Lord!. If we loose one more engine we’ll be up here all night!”



Dave :D

PS Lots of flexibility on the punch line. Like, “Upon which your mother turns to me and says…”

[ROB-x38]

g-spot here's a photo of a Qantas 744 carrying a 5th pod to Singapore. http://www.airliners.net/open.file/289429/L/

[411A]

Indeed the B707 (some models) were fitted for 5th pod engine ferry.
TriStars as well. Seem to recall that the BA machines fell into this category.

A useful alternative, if needed.

[Kotare]

P3s routinely shut down one or more engines to extend endurance generally because they were also operating at a relatively low altitude, say 15,000' and below. They also shut them down for range when forced to remain at low altitude. For example, patrol work was routinely carried out at say 9,000' and below and you were there for eight hours or more. Shutting one down in those circumstances saved a lot of gas. The jets I later operated only had one engine so options were limited in this area.
Cheers

[G-SPOTs Lost]

Thankyou gents

Most interesting, with the engine not spinning I would of thought that a lot of drag would have been created.

reading the report it seems that the cruise speed was only reduced to .81

Thanks for the explanations.

[Spodman]

Funny thing about P-3's, I've had a couple unable to restart one of the engines at the end of a patrol. Despite having the thing dead on the wing for several hours they call PAN when they have to land that way:8

[GotTheTshirt]

Rob,

Big engines can be gagged if inop before departure ( But not In Flight:D )
On RB 211 the fan is gagged by legs going through the fan blades and locking them to the fixed stator behind. This stops the fan turning, then a blank ( two semi circles) is fitted over the core engine to stop that rotating.

Podded engine are normally gagged so no fuel/oil requirement.

The original Big fans ( JT9, CF6 and RB211) were a pain to move as the the engine was basically one big lump with this big 12-15 ft diamter fan on the front. This needed a freighter aircraft to ship it hence the need for 4th ( L1011) and 5th ( B747) pods.
Later engines like the Trent ( and I'm sure the other lessor known makes) can be removed leaving the fan on wing and the rest just fits in a suitcase ;) ;)
and goes in the cargo bay

[ROB-x38]

Ah OK, cheers Tshirt. Just a quick one....is the blank the white part visible in the photo i linked above?