Quick question for you Boeing experts out there.When the FMC calculates and displays the optimal altitude in the VNAV or Cruise page of the FMC, does it take the entire route length into mind?The reason I ask is that in some simulations for FS9/FSX, the optimal altitude shows something like FL410 on a route of a couple of hundred miles. With this in mind, getting up to FL410 would probably be possible if there were no restrictions by ATC, but then the distance between TOC and TOD would be so minimal it wouldn't seem worth it. I'm not sure on the economical side of it....?Would be good to hear an "official" verdict! :(

Quick question for you Boeing experts out there.When the FMC calculates and displays the optimal altitude in the VNAV or Cruise page of the FMC, does it take the entire route length into mind?The reason I ask is that in some simulations for FS9/FSX, the optimal altitude shows something like FL410 on a route of a couple of hundred miles. With this in mind, getting up to FL410 would probably be possible if there were no restrictions by ATC, but then the distance between TOC and TOD would be so minimal it wouldn't seem worth it. I'm not sure on the economical side of it....?Would be good to hear an "official" verdict! :(

Hi there,The FMC gives quite a lot of very useful information and saves us pilots digging out a perfomance manual numerous times in a flight by presenting us with certain values.You've mentioned 1 point here that is actually split into two channels on the FMC.The first one is the Optimum altitude, this is the most efficient altitude the FMC suggests the aircraft flies, however it does not take the minimum cruise time criteria into account, i.e it would suggest a value that may not necessarily be achievable and allow any duration of cruise prior to reaching top of descent. It simply says look, if you want to fly this thing efficiently, fly at this altitude. But i don't care how long you're up there for.Whereas the TRIP ALT you see displayed on the INIT REF page is takes into account your origin and destination, cost index, weight etc to give you a "minimum cost" altitude. It takes more into account and is a far better reference for selecting a cruise altitude for a short flight. It will generally be lower than the OPTIMUM ALT for a short flight because although you save fuel at higher altitude, you have the thrust levers wide open for the climb all the way up there which, unless you are going to cruise at that higher altitude for a reasonable amount of time, does not allow you the benefit in terms of fuel savings. All these Values are Advisory however and it is up to the crew to make the final decision based on prevailing conditions. e.g, if its turbulent at FL370 and your max is FL380 in the FMC, rather cruise at FL350 to give you a better manoeuvring margin. Hope this Helps someTawanda

Tawanda, perfectly explained! Thank you very much this is very useful.

Interesting note is that if the leg is long enough for FL410 they may go there even if for a few minutes. For example, at my location (KCRP) we have SWA service to Houston Hobby (KHOU) with a short duration flight of about 32 min but they climb to FL280 then start the descent after only about a 5 min cruise.

Interesting note is that if the leg is long enough for FL410 they may go there even if for a few minutes. For example, at my location (KCRP) we have SWA service to Houston Hobby (KHOU) with a short duration flight of about 32 min but they climb to FL280 then start the descent after only about a 5 min cruise.

I was on an SWA flight from PHX-LAS at the end of November that was basically the exact same thing - less than 10 minutes at cruise, we basically just flew a big arc up and down.

As a peculiarity on this particular subject you'll often find that short range flights fly higher than long range ones, as they dont need to carry all that weight in fuel, and could therefore reach higher altitudes quicker.

As a peculiarity on this particular subject you'll often find that short range flights fly higher than long range ones, as they dont need to carry all that weight in fuel, and could therefore reach higher altitudes quicker.

And thats why you only ever see Easyjet A319's up at FL410!

As a peculiarity on this particular subject you'll often find that short range flights fly higher than long range ones, as they dont need to carry all that weight in fuel, and could therefore reach higher altitudes quicker.

Yep, I've been at 410 LAX-PHX before, very short flight, yet when I went PHX-BFI it was like 350.

Hi guys,We try to fly as optimum as possible. This means we will climb upto highest possible altitude achivable. The altitude is depening on route length, weight, cost index, temperature and wind. For short stage flights plan for atleast 5minutes of cruise. A rule of thumb for the A320 at least is to take the route lenght in nm add 100 to it and that will be your flight level. And Sam: I don't think you'll find any easyjets at FL410 (I know they still have 737ngs, but they are being send back to their lessors) - let alone any of the A320 series ;) I'll let you guess whyKeep the blue side up,Martin Dahlerup

Maybe something to do with the service ceiling of 39000 feet... :(

I was on an SWA flight from PHX-LAS at the end of November that was basically the exact same thing - less than 10 minutes at cruise, we basically just flew a big arc up and down.

Similar thing for me flying from Coffs Harbour (YCFS) to Sydney (YSSY). I was fortunate enough to be in the jump seat and we spent less than 10mins in cruise before descending again!

To go high on short or longhaul trips is equally beneficial for constant descent procedures that are being adopted and accepted more and more by ATC (esp. Eurocontrol) i.e. when the traffic situation allows, ATC clears you direct from TOD cruise level to "ILS established" starting a constant minimum drag low power cruise-descent (depending on height sometimes from over 200 miles from the threshold) and only changing configuration when passing the final approach fix or outer marker. Saves fuel, catering, less noise and better for the environment.