Quick Cruise Altitude Question

[Nelson L.]

As I've been looking through some of the charts and routes the jets have been flying, I've noticed that a lot of people are going for extremely high cruise altitudes on hops from LAX-LAS (and trips of similar distance). The aircraft will get to cruise height after a 15 minute climb, cruise for 5 minutes, then immediately begin its descent. Wouldn't it be more efficient if the aircraft were to cruise at a lower altitude to minimize the climb time? I don't see any flight in the SOCAL area, even to LAS, that would feasibly go past FL260/280, and for a lot of the flights FL200 would, in my mind, work out just fine. Is there an operational advantage for climbing so long only to have to start descending right away? In the picture below, I would have leveled off at FL190 or maybe FL210 to maximize cruise time, but maybe that's just me.

[Keith Smith]

Here's a real world track log from a Southwest 737 LAX-LAS flight on APR-10: http://flightaware.com/live/flight/SWA2 ... S/tracklog

They're doing the same thing. Fuel burns drop at the higher altitudes.

Somewhere, at some point, someone did the math and worked out that it was more economical to climb to higher altitudes for short periods of time than it is to stay at lower altitudes for longer periods of time with these types of engines.

[Nelson L.]

Huh, learn something new everyday. Seeing as the 732 burns about 100gph more in climb compared to cruise, I'm just going to accept it and not contemplate the math that went into that. I wonder if the passengers mind being in a continual climb/descent for the majority of the flight...

[Keith Smith]

the deck angles are so slight that the customers haven't got a clue.

[par2005]

I've been trying to figure out this exact dilema and yet noone can give me a definitive answer how to calculate that, except for "read POH".

Although I am more interested in these calculations with GA aircraft. (Longer climb vs longer lower altitude cruise)

[Peter Grey]

For all aircraft it is generally best to constantly climb until 1 of 2 things happen. You reach the optimum cruise altitude (which depends on atmospheric conditions, weight, and specifics of the aircraft) or you reach a TOD. This matters more for jet aircraft then prop aircraft.

To demonstrate this point a C172 @ 2000' will burn 10 GPH to do 110 kts TAS. At 12000' it will burn 9.5 GPH at max power doing 119 kts. To go 110 kts you would burn 8 GPH.

So we agree higher is better. Climb wise climbing the extra 10000' costs you 4.2 gallons. However it takes 25 minutes to do so. In that 25 minutes at 2000' you would burn 4 gallons. So it's a wash until you factor in 1 last part the descent. You can either keep 110 kts TAS in the descent (and burn less fuel then you would at 2000') or you can go faster and farther for the same fuel. This means you save fuel with a climb to descent.

In a jet it's even more important. An A319 at FL350 will burn around 5000 lbs per hour. At 10000' it's 5600 lbs per hour. You have an TAS of 320kts at 10000' and 460 kts at FL350. So once again higher is faster for less fuel.

To climb from 10000' to FL350 costs 2000 lbs of fuel and takes 13 minutes. In 13 minutes at 10000' you burn 1200 lbs. So you are 800 Lbs behind at TOC. However if you were to descend immediately and take 13 minutes to do so you would burn idle thrust fuel vs 1200 lbs if you were level at 10000' that time. So it seems like a wash, until you remember that during that descent you have a higher average TAS so you are further down range.

So the parabola flight path wins (until you reach the optimum altitude). So when do I stop climbing.

In all aircraft the optimum altitude is the one where you get the best specific range (NM/Lbs of fuel). For jets the higher the better (until you run out of climb ability). For props this is also true but makes less of a difference (see the C172 example where you gain all of 9 knots for a given fuel burn). This is all because you have less drag on the airplane the higher you go (among other reasons, but that's the simple answer).

The final factor is wind, in prop aircraft almost without fail the optimum altitude is the altitude where you have the greatest tailwind as the speed/fuel difference isn't that big.

In Jet aircraft wind can affect the optimum altitude but greater amounts are needed to matter. Jet aircraft have charts to figure this out. For example in that A319 it takes a 43 knot tailwind increase to justify a descent from FL350 to FL310 (given a bunch of factors).

Hopefully that answers the questions in play.

[Ryan B]

Educational answers hehe!

I basically just use the my understanding of TAS as my guide. In a non turbocharged piston high is good, but higher won't akways work. 8000 ft is usually the most efficient altitude for small GA without turbo charging. With turbocharged engines the higher you go the faster you go at less cost to you. Of course if it's a short flight there's no point in going so high. I use two variants of the Lancair Legacy. One with stock non turbo engine and one with a modded turbocharged engine. I can do 240 ktas at 8000 at about 13GPH in the non-turbo, or I can climb (with pretend O2 of course!) to FL240 at 320 ktas with similar burn rates in the turbocharged LEG2.

I guess over time you get a feel for how high you can most efficiently go with a small non turbo'd GA aircraft over a short distance. Another concept I consider is descent rate and how many miles per minute I'll travel laterally for a 500-1000 fpm descent. 1000 fpm is easier on your brain (math i mean not your ears hehe) but 500 is more typical for lite GA.

And agreed on what Peter is saying... you can pull your power pretty well during descent and even speed up during the descent... less fuel and more speed! I think that combination overpowers the climb consideration.

Peter would tell me if I'm wrong but what my reading jets are most efficient at the highest altitude they can go...Maybe it's slightly below service ceiling not exactly sure... LAX to LAS is a long trip though (can you tell I rarely fly jets) and FL300 or whatever isn't very high at all, consider your SID from KLAX, probably LOOP6 or whatever it is now... you'll be crossing LAX eastbound climbing above 10k likely.... It's not like you depart Ry 7R at LAX and go directly NE on your way.

[Nelson L.]

Once again, math reigns supreme... Unfortunately, the optimum altitude displayed on the 732 is almost always too high for routes in SOCAL (and even to NORCAL). For a general rule of thumb, I like to use (meaning its almost certainly not the correct way) 1/4 of my flight time climbing, 1/2 in cruise, and 1/4 descending and landing while in a piston. For jets, I cheat and look at what other people are using - mainly because I don't know enough about my aircraft's performance to do the correct calculations. For the descent in a king air from 16000, I usually run the aircraft up to VNE, so I definitely cover more ground - in jets though, I usually use IAS mode to descend, thus eliminating the speed advantage and leaving the fuel advantage. Thanks for the detailed reply Peter, covered quite a lot.

Ryan - I've gone into the flight levels with a turbocharged Cessna fluttering along at 140 knots

[Peter Grey]

When I don't have performance charts available I use this formula to figure out short range cruise altitudes:

Total flight distance / 6 = cruise altitude (round down)

This assumes that you descend at 3 degrees and that it takes half the flight. Most aircraft climb above a 3 degree profile at low altitudes so you can assume a 3 degree climb profile as well.

It's not exact but it gets you in ball park, try it a couple times in your favorite airplane and you'll figure out how to tweak it.

[Nelson L.]

Hmmm, neat little formula, but I don't exactly see how it works. KSNA-KSFO is 300nm, but I definitely can't go up to FL500? Thats assuming I've understood the formula correctly:

Total distance=300nm

300/6=50

FL500?

Did you mean: Total flight distance * (multiply) .6? But then again, that gives FL180, which isn't right either?



Edit - Saw the "short distance" part, but that would make the formula viable only for trips up to around 50nm? Pretty sure I'm still missing something.