Flight level of twins

[Flight_Idle]

I seem to remember reading on this forum that twin engined airliners can fly an average of 4,000' higher than the four engined airliners.

Being a non pilot this puzzles me, why should the narrowing flight envelope between stall & undesirable mach effects be different on a twin?

How did the subsonic Victor manage to fly 'well above 60,000' & still keep flying? it must have been in a very narrow band at that altitude. I guess it must be something to do with the wing design? military aircraft not being subject to fuel cost concerns?

I would be grateful if anyone can shed any light on this.

Regards, Flight Idle.

[Akrapovic]

Not true I'm afraid (if we're talking about your typical airliners : Boeing / Airbus etc - can't really speak for the military types)

4-engined airliners are certainly capable of getting the these levels, but it's normally a question of weight which restricts them. When loaded up with fuel for a long flight, these aircraft are too heavy to reach their optimum levels, so as they burn off the fuel they 'step-climb' to the higher levels as they get lighter . . .

The smaller twins (737, A320 for example) can suffer from this also (mainly on the longer sectors), but the time to make the step climb is comparitively less than the heavier types.

[SNS3Guppy]

In the classic 747, we're 833,000 lbs at takeoff, with just over 200,000 lbs of thrust available. Conversely, a 737-800 has takeoff weight of 155,000 lbs, but with 56,000 lbs of thrust. We're roughly 4 lbs for each pound of thrust available, whereas the 737 has rougly 2.7 lbs of weight per pound of thrust. Each pound of thrust has to lift and move less weight with the 737...or in other words, it's got a higher thrust to weight ratio.

I can still fly the 747 at 41,000 on a ferry flight...but at the operating weights we typically use, we're flying more like 33,000. We are restricted not by "coffin corner," but by our weight.

We're concerned not so much with stall margins or buffet margins, because we don't come close to either one. We're concerned with the most efficient cruise altitude, and this is dictated by the temperature at altitude, and by our weight.

Too much is made of the area where theoretically one is bounded by the buffet margin (too fast) and the stall margin (too slow). Most civil aircraft don't have the capability to fly into coffin corner, and don't operate close to it. It's not that four engine aircraft can't fly as high as two engine aircraft, it's that the operating weight and conditions, leg length, and other factors combine to determine how high the aircraft will be flown in order to meet the needs of the operation in question. Other factors such as the jetstream are taken into consideration when picking an altitude at which the flight will be conducted.

We don't simply fly as high as we possibly can...there's more to the big picture.

[WHBM]

This raises a trivia question that has puzzled me over the years since it happened.

Pax in a Cathay A330 flying Hong Kong to Perth, Australia, using the information on the seatback moving map. We climbed up and eventually were cruising at 41,000, which was straightforward. A few hours into the flight and over Indonesia we climbed up to 43,000, which was notable to me as it was the highest I had seen. After about an hour at this height, and before approaching Australia we descended back to 41,000 and continued the remainder of the flight at this level.

Why descend again for the remainder of the cruise ? Some procedural aspect of ATC ? This is very sparsely-trafficked airspace here.

[Akrapovic]

Why descend again for the remainder of the cruise ?

Number of possible reasons:


ATC Request.
Reported turbulence at that level.
Military Traffic (the French love this one!).
Crossing Traffic.
Better winds (head or tail) at lower level.

[Rainboe]

WHBM- the reason most likely is the use of what is called semi-circular flight levels where your cuise altitude is determined by your direction east or west. If the general direction of flight due to airways changed, or the aeroplane passes to another ATC region, it is not rare for a change to be required. If you fly over states like China or Russia where cruise altitudes are in metres, you often have to make fairly large cruise altitude changes to conform to their procedures. Once up there at cruise, you can stay there. The only reason for descent would be ATC or weather.

[HeathrowAirport]

Here is an example from a BAW208 Real Flight plan.

PADUS/N0492F330 BR66V ZFP DCT LOUIZ/M085F350 A699 SUMRS M204 SOORY DCT 43N050W 48N040W/M085F370 52N030W 53N020W DCT MALOT/N0476F370 UL9 SLANY/N0466F330 UL9 STU/N0457F290 UP2 OKESI Y3 BEDEK


Goes up at LOUIZ, then further at 48N040W then down at SLANY then further at STU.

[SNS3Guppy]

The BAW208 fligtht plan snippet above doesn't come backdown until eastbound off the North Atlantic tracks. Commonly flight descend after coming off the tracks to fit into European space, and to descend into Europe. This is a little different than altitude changes while on an oceanic crossing.

[Rainboe]

It might be useful to point out that apart from route information, the requested flight plan flight levels are only planned. On the day, no more interest is taken in stated flight plan levels. The aeroplane flies at what level it requires and gets clearance for, and during the flight, nobody would make it descend 'because the flight plan said'. In practice, the flight usuall ends up well above the stated levels in the flight plan following efficient flight altitudes and actual winds. The information stated in the flight plan normally allows for a far heavier aeroplane as at that stage of flight plan application, the actual weights are unnown.

Shannon and London ATC , and on the other side Gander, Moncton and the US happily slot you in at best available flight levels without any reference to altitudes stated in the flight plan.

[Rainboe]

Guppy explained all that in Post 3. Hello! Keep up at the back there purrlease! Do you have to be led by the nose through this?

[HeathrowAirport]

Is that why sometimes the planes go further up than filed on some occasions?

[Rainboe]

Nobody cares what is filed. During climb, the pilots work out desired flight level and negotiate with ATC. Once up there, you are normally passed from one atc unit to another, and they will endeavour to keep you at a steady flight level. VERY rarely, you may be made to descend. Sometimes you can avoid this by climbing early at a higher weight than is efficient in order to prevent a descent. Nobody looks at the flight plan for altitudes once you are in the air.

[PappyJ]

VERY rarely, you may be made to descend. You've never had to overfly Shanghai, have you?

[Lancman]

Or, to simplify Guppy's reply even further, twin engined aircraft have more power installed than those with 4.

[SNS3Guppy]

Well, not necessarily more power installed, but more power per pound (or kg) of weight. It's one thing to push a 155,000 lb airplane to 37,000', and it's entirely something else to push an 833,000 lb airplane to the same height.

Conversely, I just ferried a B742 at FL430. The airplane was about 400,000 lbs as ferried, and shot right up to altitude with no problems. With less thrust per pound at gross, it's still a lot more sprightly at twice the twins gross weight, when the 4 engine airplane is operating light...and faster, too...and operates higher as well.

Conversely again, reduce the B737 back to an empty ferry weight with ferry fuel, to make the comparison even, and see what performance one gets.

[PappyJ]

I'm not the engineering guru that some other folks such as JT and Old Smokey, but I'm pretty sure that some of the differences can be attributed to Manufacturing Requirements, etc. I'll try to explain.

Everything designed and built requires some "give and take". With aircraft, if there wasn't any necessary consideration toward weight, payload, cost and efficiency, then aircraft would be very large, very fast, and carry immeasurable loads over infinite distances.

Unfortunately, we don't get everything. So, when it comes to the question of "...how much power do we give this new design...?", the question is typically answered with "What's the Minimum required?", and "...how much more can we allow for without a cost/weight penalty that our customers will not be interested in paying for..?"

There are several difference design/certification requirements around the planet, but a common idea is that the aircraft must be able to meet specified climb performance criteria with HALF of its installed engines inoperative.

If you have only two engines, rest assured that EACH of those engines is going to have some impressive power, as that aircraft needs to certify on ONE engine, where in this example the 4 engine bird, it certifies on two.

If you have four engines, then each of the engines would - at least in theory - need 1/2 of the Twin Engine Aircraft's requirements single engine requirement(assuming of course that the two aircraft had to same mass).

I'll agree that this is a piss-poor explanation, but I'm pretty sure that there is some validity to it.

Cheers

[Capt Pit Bull]

There are several difference design/certification requirements around the planet, but a common idea is that the aircraft must be able to meet specified climb performance criteria with HALF of its installed engines inoperative.

Well, I'm no performance guru, but that doesn't sound correct to me. Two engine performance on 4 engined aircraft is a serious problem, 1 engine performance on a 2 engined airliner isn't that big a deal, and 1 engine out on a 4 engined aircraft is almost a non event.

Is it not more accurate to say that performance is mostly about lifting off and cleaning up after the loss of one engine. Therefore if an aircraft need T ammount of thrust to do that satisfactorally, then:

A twin with all engines operating has 2T (T from each engine), loss of an engine leads to T so you are ok.

Whereas a 4 engine aircraft needs (4/3)T , loss of an engined leads to (3/3)T.

Therefore everything else being equal twins with all engines operating tend to have better specific excess thrust and therefore rate of climb than a 4 jet.

[PappyJ]

Therefore everything else being equal twins with all engines operating tend to have better specific excess thrust and therefore rate of climb than a 4 jet.

That's exactly the point I was making.

[Capt Pit Bull]

That's exactly the point I was making.

We had the same conclusion, but I don't believe it follows from N/2 whereas it does follow from N-1.

pb

[PappyJ]

I stand behind my disclaimer....

I'll agree that this is a piss-poor explanation, but I'm pretty sure that there is some validity to it.
;)

[SloppyJoe]

WHBM:

Must have been an error on the moving map. The 330s CX has are not certified to go up to FL430. Highest we go is FL410. The route through indonesia is not that busy but there are only a few airways for everyone going from AUS to HKG and BKK, SIN etc and there are a descent number of flights. Certain waypoints are where you either converge or diverge with traffic going to or coming from other cities so maybe in one flight you will have to change level for traffic on a couple of occasions. So you may go up then come back down. Also the airbus is slower than a boeing so you also change level if you get too close but it is normally the overtaking one that has to change level unless you are nice and can climb for them if they cant get above you yet.

This was not the case though as it would not have gone above FL410.

[PappyJ]

I think the Moving Map info may come from a GPS reference, which often reads several thousand feet different depending on the atmospheric conditions, etc

[Capt Pit Bull]

Well, I'm no performance guru

hiding behind mine as well ! ;)

[arearadar]

Hi,

Interesting to read all about flight level changes and the reasons why.

Some said that the flight plan was ignored when once in the air, but don`t thow it away........you might need to refer to it in the event of radio failure.

As retired ATCO I would have expected the aircraft to regain and follow the filed flight plan details !!

Dave:)

[chiglet]

Not to mention the
"Single Engine U2" at 65,000 feet...
"Twin Engine SR71" at 90,000 feet...and the
"Four Engine 'Concorde'" at 65,000 feet
Take your pick :ok:

[SNS3Guppy]

Some said that the flight plan was ignored when once in the air, but don`t thow it away........you might need to refer to it in the event of radio failure.

I can't speak for other operations, but we follow our flightplans and dispatch releases fairly religiously. Our fuel consumption, times, routing, overflight permits, etc, are all predicated on what's on that form. We're required to contact our scheduling to obtain a new release or updates if our altitude changes by more than 4,000', or if we have to deviate more than 100 nm, or if our weight differs by 10,000 lbs, and a few other considerations.

Our actual altitude requested will depend entirely on performance. While the company may have planned us for a higher altitude, perhaps we can't do it for the first couple of hours. Perhaps we find we can fly higher than forecast; we look at the forecast winds, our existing performance, our altitude capabilities, and make our requests with ATC accordingly.

Generally speaking, the only time our route of flight or altitude differs from the flight plan is when we're given a different clearance by Air Traffic Control.