flexo
Posts: 344
Joined: Thu Mar 29, 2007 6:55 pm

Questions About Cruising Altitude

Mon Nov 19, 2007 9:37 am

I recently returned from a ~2000nm trip DUS-LPA on a B753 and noticed that our cruising altitude towards the end of the flight on both legs was 33,000 ft. On transatlantic routes the aircraft I'm on usually tend to end up at an altitude much higher (39-40,000 ft.)

So here are my questions:
- Is the optimal cruising altitude generally lower for short (or in my case medium) range flights? And if yes, why is that?
- On long range flights is higher usually better (= more efficient)?
- How come some business jets have a certified cruising altitude much higher than that of airliners (i.e. Gulfstream V: 51,000 ft)? Would that be desirable for airliners as well and why can't they do it?
 
oly720man
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RE: Questions About Cruising Altitude

Mon Nov 19, 2007 10:20 am

On the transatlantic routes there'll be planes probably from FL320 to FL380 flying in formation. The level will depend on the plane and its weight and where it came from and what else is going the same way. Over Manchester for example, there are planes going West from the UK, as far east as the Middle East and all points in between.

Quoting Flexo (Thread starter):
Would that be desirable for airliners as well and why can't they do it?

The benefits to be obtained from cruising at that altitude would probably be outweighed by the design requirements to get the plane that high - stronger fuselage, engine performance..... From what I've read on other posts, the manoeuvre envelope is small at high altitudes and will probably disappear for commercial aircraft much above FL450.

http://en.wikipedia.org/wiki/Coffin_corner_(aviation)

Quoting Flexo (Thread starter):
- Is the optimal cruising altitude generally lower for short (or in my case medium) range flights? And if yes, why is that?

Cruise altitude is dictated by what can the plane get to and what else is in the sky at the time. You may want FL350 or FL370 but if there are other aircraft already at that level.....
wheat and dairy can screw up your brain
 
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Jetlagged
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RE: Questions About Cruising Altitude

Mon Nov 19, 2007 11:31 am

Generally speaking, the higher the better. However you have to weigh the benefit of flying higher against the extra fuel it takes to climb up there. The heavier you are, the more this is. Also, there's not a lot of point in climbing to 39,000 ft if you have to descend immediately. The optimum cruise altitude will therefore increase as the aircraft burns fuel so ideally the aircraft would be allowed to climb slowly as it cruises. Concorde did this routinely because nothing else was at the altitudes it flew at, but conventional airliners must "step climb" in a more controlled manner. On shorter range flights in heavy traffic it's more likely you will be at the same cruising level throughout.

Leaving "Coffin Corner" limits aside (which apply to any aircraft), airliners must comply with certification rules about how quickly they must be able to get to lower altitudes in the event of a cabin depressurisation. A search of this forum will get you to the details.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
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Starlionblue
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RE: Questions About Cruising Altitude

Mon Nov 19, 2007 1:17 pm



Quoting Oly720man (Reply 1):
From what I've read on other posts, the manoeuvre envelope is small at high altitudes and will probably disappear for commercial aircraft much above FL450.

As Jetlagged mentions, this is called "coffin corner". A subsonic aircraft is speed limited at all altitudes by stall speed (lower limit) and mach buffet (upper limit). As the aircraft climbs, stall speed increases due to lower air density. At the same time, mach buffet speed decreases due to lower mach speed. At a certain altitude dependent on atmospherics and aircraft design, the two limits converge until further climb is impossible. Just before that point is "coffin corner", where the margin is very small. U-2s regularly fly in coffin corner, with only 10 knots between mach buffet and stall.

Of course you can design the plane differently, but that just brings other problems. While a U-2 can climb very high, the design is useless for commercial purposes.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
CosmicCruiser
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RE: Questions About Cruising Altitude

Mon Nov 19, 2007 1:40 pm

As Jetlagged said generally higher is better but many other factors come into play. Flying higher than optimum will cost you fuel. For us, since we are most often going against the tracks, we will be cleared at a lower alt for the crossing so we will descend prior to coast out and then climb again after coast in.
 
flexo
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RE: Questions About Cruising Altitude

Mon Nov 19, 2007 9:37 pm

Thanks for all your answers!

Quoting Oly720man (Reply 1):
The benefits to be obtained from cruising at that altitude would probably be outweighed by the design requirements to get the plane that high - stronger fuselage, engine performance..... From what I've read on other posts, the manoeuvre envelope is small at high altitudes and will probably disappear for commercial aircraft much above FL450

So I suppose since the Gulfstream V is certified to FL510 it means that it has a lower stall speed / higher mach buffet and also a stronger fuselage and engines (thrust / weight ratio) than airliners? Would be safe to say that if go long distance in that aircraft you would achieve the best performance by operating close to that ceiling altitude?
 
dw747400
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RE: Questions About Cruising Altitude

Mon Nov 19, 2007 11:38 pm



Quoting CosmicCruiser (Reply 4):
For us, since we are most often going against the tracks, we will be cleared at a lower alt for the crossing so we will descend prior to coast out and then climb again after coast in.

How much extra fuel does that burn? I know in flight plans I've done for GVs, etc, its normally more efficient to fly a longer route (sometimes up to an hour added to flight time) and avoid the tracks than to go under them (of course, in the right conditions, a GV can go over them and everyone is happy, but at high weights thats tough).
CFI--Certfied Freakin Idiot
 
CosmicCruiser
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RE: Questions About Cruising Altitude

Tue Nov 20, 2007 12:11 am

I wouldn't know how much extra burn is involved. We have figures for burns at higher and lower alt but it's not for just the NAT segment. I would guess 1500-2000 lbs but remember for us to go 1 hr. out of the way to save fuel can really mess up the arrival times/ sort/ connections of many other flights so that's not much of an option. Often high gross wgts would keep us down for awhile anyway. Also as I posted earlier going higher than opt. will cost you fuel as well.
 
pilotboi
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RE: Questions About Cruising Altitude

Tue Nov 20, 2007 12:33 am



Quoting Jetlagged (Reply 2):
Also, there's not a lot of point in climbing to 39,000 ft if you have to descend immediately.

Actually, this is optimum. I don't really want to get into the details right now, but just because it's a short flight - doesn't mean you should only climb for a proportionality small amount of time. If it's a short flight, the most optimum (fuel wise) flightpath is a 'pyramid', or basically a climb to the 'middle point', followed by an immediate descent to the destination. Yes, you burn some extra fuel to climb, but a higher altitude will always pay off.
 
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Starlionblue
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RE: Questions About Cruising Altitude

Tue Nov 20, 2007 1:52 am



Quoting Flexo (Reply 5):
So I suppose since the Gulfstream V is certified to FL510 it means that it has a lower stall speed / higher mach buffet and also a stronger fuselage and engines (thrust / weight ratio) than airliners?

I would speculate that the wing can handle higher speeds. This is probably more costly but since it's a rather posh business jet the customers can afford it.

Quoting Flexo (Reply 5):
Would be safe to say that if go long distance in that aircraft you would achieve the best performance by operating close to that ceiling altitude?

It depends on atmospheric conditions and aircraft weight. But the trend is probably there.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
PhilSquares
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RE: Questions About Cruising Altitude

Tue Nov 20, 2007 2:19 am



Quoting Pilotboi (Reply 8):
Yes, you burn some extra fuel to climb, but a higher altitude will always pay off.

As has been pointed out by CosmicCruiser and Jetlagged, higher is normally better. That is in an ideal world. For commercial operations there are several factors to be considered. On a short segment, flying at a lower altitude, while you will burn more fuel, you could also have a shorter block time. Shorter block time equals lower costs. On "glass" aircraft this issue is handled by the FMS/FMGC and the cost index. I have seen short segments where the box will have an optimum altitude of 230 while the max is much higher.

Another issue is getting clearance to the upper levels. While you will burn less fuel at the higher altitudes, that has to be balanced against the fuel used in either a ground delay or radar vectors trying to get up to the higher altitudes. On shorter segments, it's just not worth it.

Most dispatchers have the tools to analyize the flights and balance the altitude/fuel/block time issues.
Fly fast, live slow
 
tdscanuck
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RE: Questions About Cruising Altitude

Tue Nov 20, 2007 3:14 am



Quoting Flexo (Thread starter):
- Is the optimal cruising altitude generally lower for short (or in my case medium) range flights? And if yes, why is that?

Yes, because for short flights it may not be worth the extra fuel to climb higher, only to have to descend faster to make it to your landing field.

Quoting Flexo (Thread starter):
- On long range flights is higher usually better (= more efficient)?

All other things being equal, yes. Fuel burn per mile goes down with altitude.

Quoting Flexo (Thread starter):
- How come some business jets have a certified cruising altitude much higher than that of airliners (i.e. Gulfstream V: 51,000 ft)? Would that be desirable for airliners as well and why can't they do it?

Airliners would usually run out of practical lift before they got that high and the increased altitude increases the pressure differential on the fuselage, which decreases the fatigue life of the aircraft. Business jets have more wing for their size and very low cycle lives, so these issues aren't such a problem for them.

Tom.
 
CosmicCruiser
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RE: Questions About Cruising Altitude

Tue Nov 20, 2007 3:38 am

In our FMS the OPT. alt is based on Econ ($/NM) CI or max endurance
MAX alt is based on the lower of 43,200', 1.3g buffet or thrust limited climb. There is a penalty in climbing higher than OPT. This is out of our MD-11 man.

2000' abpve opt.......2.2% increase in fuel burn
2000' below opt.......1.3% increase " " "
4000' below opt.......4.2% increase " " "
 
pilotboi
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Joined: Thu Sep 06, 2007 8:16 am

RE: Questions About Cruising Altitude

Tue Nov 20, 2007 3:50 am



Quoting PhilSquares (Reply 10):
higher is normally better. That is in an ideal world

Ah, yes - I forgot to mention that I meant this is only true in an ideal world, and there are other factors. I was only comparing altitude to fuel.

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