Altitude question
 

Trying to fill lockdown I am using x-plane 11 FliSIm and trying to fly a 737-800 and very interesting it is too. I am a retired helicopter pilot so altitude/economy was not the problem it is for jet aircraft. (Although it was to some extent on the Commercial Chinook.)

Jet aircraft fly as high as possible for the best fuel economy. As trip distances decrease there must come a point when the shorter distance makes it uneconomical to climb to cruise at FL350 plus. Is there a formula/accepted practice that determines the best altitude to fly over any distance less than the minimum required to make flight at FL350 economical? I'd appreciate any details.

To be honest, pilots have limited formula for this today. Usually it is calculated either automatically by the flight planning system (and is on the OFP) not only for optimum fuel economy, but also other operational things like airspace structure, ATC overload etc. And then there is the FMC which gives us an optimum level as well, however sometimes questionable ones.

That said, apparently the best fuel economy is by flying a parabola, get up as high as possible and then idle the rest of the way down, I've seen cruised durations measured nearly in seconds, not minutes on very short sectors, but single digit minutes are quite common on those short ones.

If you're planning from first principles, then you look up the Optimum Altitude planning graph in the Aircraft's Flight Manual.

Not "as high as possible" - there is an optimum FL for fuel burn that is weight dependent. Most jets are capable of flying higher than the optimum, so you need to refer to the performance data in the ODM. Usual practice is to try to be within 2000 ft of optimum. Another consideration, especially for crews who regularly fly at high latitudes, is exposure to cosmic radiation - one transpolar sector gives about the same exposure as a chest x-ray.

For short sectors, practicalities as discussed in previous really determine what you can do. Typically, a shorthaul twinjet will take around 25-30 min to climb to FL350, and about the same for descent and approach. For a short sector, it's a good idea from the crew workload aspect to have at least 10 min of cruise; so, for say a 40 min sector, somewhere around the FL250 region is about as high as you will get.

What Denti said is right, but a parabolic flight path gives just about max workload for the crew and for ATC.

As another data point, which conforms to kenparry's estimate, on a BA A319 flight CDG-LHR (~185nm) we flew at FL240, according to the captain (I was pax that trip). Of course, once we got over London, we then had to fly holding patterns for 20 minutes. ;)

From another angle, commercial jet climbs and descents to/from FL300 require approximately 100nm each (3.3 miles per 1000 feet) - but that is definitely a rough rule-of-thumb that will vary with aircraft, weight, ATC, winds, most efficient cruise speed, etc. etc.

But those are practicalities separate from absolute best fuel economy calculations.

I'd suggest that on an otherwise unrestricted route,that you plan for a cruise segment of at least 10 minutes, you don't really want a parabolic up-down lob (you were talking B737 sized aircraft)..

Of course, the route may be too short for a 10 minute cruise segment, but that is prettty rare, .

The 10 minutes in the cruise in practice allows for all the necassary admin stuff (crew discussions, company chat, 'paperwork', and importanty a cup of tea and time to catch your breath).

That would mean on anything over about 50 minutes you have time to get to FL350, cruise and get back down (subject to any other constraints).

I've been retired 11 years, but I'm sure the basics haven't changed that much in the intervening years!

Indeed, they absolutely have not.

The more modern versions of Airbus 320 family FMS have embedded minimum-duration 15 minutes level segment for calculating the cruise altitude.

Although only introduced about 14 years ago.

The young keep discovering what the wise start to forget. 👍😁

To OP: Otherwise, for more guidance, Google hard for Optimum Altitude on Short Segment ​​​​​, there was a graph to determine. Cannot remember if AB or BG.

Climb to your descent point.
If it’s about economy and saving fuel, stuff the workload.
If the sector is that short, you do as much of your arrival briefing as possible before you depart.
Even more important if you are diverting to an alternate with min fuel.

As Capt Fathom mentioned I've seen flight plans that climb, cruise for a couple of minutes (<10), and then descend. I've had some that literally touch the climb altitude and immediately start a descend. Looking at a flight today - 18 minutes of climb, 11 minutes cruise @ FL300, then descent. A flight on a longer leg (2.5 hrs) also had 18 minutes of climb. A longer flight (8+50) had 24 minutes of climb. A 13+26 hr flight had a climb of 19 minutes. So the typical climb time is 20-25 minutes.

On shorter flights, in high density areas like NW Europe, NE U.S., you'll be assigned altitudes based on ATC airspace requirements and not aircraft performance reasons.


As Checkerboard mentioned airliners usual don't go to their maximum altitude. OPT ALT is 2000-4000' under MAX ALT depending upon aircraft type. Ignoring winds the flight plan brackets OPT ALT, slightly higher until slightly lower, than climb to a new cruise altitude slightly higher than OPT ALT and as fuel burns off waiting until OPT ALT is below your cruise altitude and you climb again. Typical plan is to climb to 1000' above OPT ALT and repeat once you're 1000-2000' BELOW OPT ALT after fuel burns off. Strong winds change the calculation as the impact on GS is greater than the inefficiency of not flying near OPT ALT. With very strong headwinds you might be 10,000' below OPT ALT. That's unusual but being 4-6000' below OPT ALT against headwinds isn't that unusual. With strong tailwinds you do see cruise altitudes right at MAX ALT but the performance margins are smaller so it there's some consideration that goes into accepting that altitude, especially if you're going to be in a non-radar environment and changing altitudes might not be an option.

Quick and dirty rule of thumb is to divide distance by 10 to get cruise attitude in thousands of feet: 200 nm/ 20.000 ft or, even simpler, 200 nm/ FL 200

Hi Capt Fathom,
M2dude agrees with you.
https://www.pprune.org/5884837-post56.html

At least according to a college course I took ~45 years ago, the most efficient cruise is what's known as a "climbing cruise". You set the power and speed for the best fuel burn (per mile) at that altitude, then leave it alone - as fuel burns off, the aircraft will slowly climb at that thrust lever setting and speed (not really constant thrust since the thrust will slowly decrease as the aircraft climbs). The trick being to pick the optimum starting altitude.
Unfortunately not very practical with ATC constraints, but the closer you can operate to a 'climbing cruise', the better.

Gentlemen

Many thank for your responses and all the info. There's a wealth of useful information there and many points that hadn't occurred to me, not the least a level cruise long enough for a cup of tea! The general concensus seems to be that its worth getting up there, ATC and airspace restrictions permitting, even if the level cruise is of a short duration. 185nm/FL240 as mentioned is a good guideline for shorter flights

I also appreciate the point that in crowded airspace cruise levels may be down to ATC requirements and not greatest economy.

My thanks again. X-plane is as close as I'm likely to get to the real thing - I'm 77 - so its as well to get it as authentic as possible.

Boslandew

Don’t worry mate - For many of us it is the same situation thanks to covid crisis...

The technique used by the U-2 and SR-71, there was a time when airlines were able to be assigned a block and cruise climb, traffic precludes that these days?

Didn't the old JP 3/4 have a graph or chart to give one the optimum level to climb to in event of unplanned diversion? It was in the flight check list.
Foggy memories!

Just stick to the crossover altitude.... and get there faster :cool: