Six Minutes at Altitude?


#1

All,

I have a question regarding Southwest Airlines’ flights between Dallas Love (KDAL) and Houston (KHOU). I have noticed that these flights are typically flying at 30,000 to 33,000 feet. Based on the details of the flights it appears that they are at cruise altitude for only 5-6 minutes.

Question: Why would they climb to such an altitude with such a short cruise time? Is this just pure economics of fuel savings, or does the airspace between and Dallas and Houston require such an altitude?

Thanks,

Jeff


#2

Fuel efficiency. The higher you are, even if for just a few minutes, the better the fuel efficiency.

Even a one or two percent savings in fuel saves the airlines millions of dollars a year.


#3

On such a short trip the savings in fuel burn at that altitude is highly unlikely to be sufficient to offset the fuel burned getting there.

I would think it’s more due to local traffic patterns and density.


#4

As I said, even at higher altitudes for just a few minutes can save an airline millions of dollars, even taking into account the fuel needed to climb to altitude. Also, the airlines can take advantage of starting their TOD (top of descent) at a higher altitude and thus save a little bit more.


#5

It is true for a jet - it always pays to climb as high as you can, and then descend. Even if you are only at cruise for a very short time, it uses less fuel to climb, and then descend, than to level off and cruise at a lower altitude.
The only time this doesn’t work is when the winds increase rapidly as you climb (I am referring to a headwind, of course), then it may pay to level off at a lower altitude.


#6

Yep the most fuel efficent trip you can make will look like a bell. Up and then right back down.


#7

Message received and understood.

Spiker Bravo Six Actual out.


#8

My employer takes their Citation V down to KHUT for maintanence which is only about a 7 minute flight, they’ll take the plane up over 10,000 as long as it’s vfr. I went with them once and we flew it at 9,000 because there were clouds just above us, it only took us less than 2 minutes to get there. (we were pretty empty)


#9

I’m fully aware of the principle involved, I just didn’t think it would be of that great a benefit on such a short flight given the fuel burn to gain altitude.

We’ve got a local flight ex. BWI to PHL that never gets over 10K feet. Granted, it’s only half the distance of HOU to DFW, but they don’t really fit Leardvr’s bell shaped curve. Perhaps a flattened bell.


#10

I think that might be due to the heavy plane congestion in the NE corridor. A lot of flights descending into the NYC airports go right over BWI and PHL.


#11

The more fuel effecient engines make climbing higher more effecient. Using FLTPLAN.com and checking TOTAL fuel burn for various trips shows much higher than expected altitudes give the best fuel economy. For the airplane I fly, they seem to be pretty accurate also. I often get questioned whether I really want to go that high, and unless I get held down due to traffic, I do. Try it on different models of the most fuel effecient aircraft.

Older, less effecient engines (maybe 727), weren’t that way. It took a much longer leg to make the higher altitudes a benefit.

Skydawg


#12

I guess that, like the 727, I’m a product of the era. :wink:


#13

I imagine it would also depend on if the aircraft can climb straight to a given altitude or needs to do a step climb to reach it. If it’s the latter I don’t think there would be any fuel savings.


#14

THEIR??? Oops. Try to weasle out of that one!


#15

No weaseling, just a correction to my posting.


#16

No no, quite the opposite. In the old pure turbojet or low bypass engine it was always better to go as high as possible. It’s just that “as high as possible” on the older airframes was lower than todays airframes. The engine would go there but the airframe would not. In the newer high bypass engines it is still true but they are more forgiving of a low altitude dash.


#17

Most jets can climb straight to FL300, without stepping. On a short hop, they can quickly get to FL200-220.

I know of a Citation 560 that flies a 90nm route almost exclusively. They go 16,000 one way for ATC and 230 the other direction. 17-20 minutes average trip time with 2 minutes at 230.


#18

I don’t understand the need for step climbs. Is it just a way to build some speed up during the level-off or is it needed to burn off fuel? Is it just for turbojets?
I ask because even the PC12 can climb straight up to FL300 without step climbing. Granted, the rate of climb above FL250 is pretty poor but levling off doesnt seem to help, any speed buildup is quickly lost a few seconds after pointing the nose skyward.
G4? Skydawg? Leardvr?


#19

From what I’ve heard, it’s done for the fully fuel laden flights, where they can’t climb high enough under their current weight, so they have to wait a bit to lighten up. I could be wrong though.


#20

Step climbs are to burn fuel; many of the long range airliners can only make the low 30s (or high 20s) with full tanks. So they hang out at 310 for a while, then climb to 350, and probably on to 390 as they lighten up.