igorland
Topic Author
Posts: 27
Joined: Thu Mar 14, 2013 12:02 am

Question About Dispatch And FL

Thu Apr 25, 2013 4:42 pm

Hello,
Can someone help me understand how FL planning works? Optimum FL is determined by performance charts, correct? However, FL depends on the total plane weight, the total weight depends on the fuel weight, the fuel weight depends on adjusted distance, adjusted distance depends on the wind component at FL, which depends on FL! Catch 22. Does the dispatch give the pilot FL based on the considerations other than the optimum FL, and then the pilot, if he or she decides, asks to change it during the flight? Thanks a lot!!!
 
MSJYOP28Apilot
Posts: 293
Joined: Tue Jul 21, 2009 2:09 am

RE: Question About Dispatch And FL

Thu Apr 25, 2013 11:18 pm

The flight planning systems will optimize an altitude in lieu of the dispatcher inputing one. Reasons to file different would be turbulence requiring lower or higher altitude, thunderstorms requiring higher to get above some of the weather. Flight plannign systems dont always plan for temperature and ozone limitations so sometimes you need to file lower than optimal due to those restrictions. MELs that restrict the aircraft altitude such as an AC pack out or RVSM deferral effect the altitude filed. FL280 may be the higest you can go even though the optimal altitude is FL360.

Sometimes the flight planning system optimizes an altitude way too high or way too low of certain city pairs sometimes. It can be funny to sometimes get those calls when you miss the FL370 on a short leg or FL100 on a longer one. Most of the time, we know the routes and input a more realistic altitude but when you get busy this can sometimes be a more comical error to make.

Another thing is some flight planning systems optimize step climbs on short to medium range routes. On long hauls these step climbs are necessary but normally they are not for shorter flights and many crews ignore them anyways so I will just cruise them at the lower altitude planned by the system.

If weight and balance becomes an issue, you can file a plan lower so it can burn off more gas to get within landing weight limits.

Pilots do whatever they want with altitude. If they fly lower than planned, many airlines and FAA inspectors require they get new fuel burn numbers from dispatch. Sometimes, crews will request altitudes for better rides or to get above weather. Most of the time, pilots just ask ATC for higher or lower based on whatever altitude they want to fly. The vast majority of pilots just fly whatever altitude dispatch files.
 
igorland
Topic Author
Posts: 27
Joined: Thu Mar 14, 2013 12:02 am

RE: Question About Dispatch And FL

Fri Apr 26, 2013 3:41 am

MSJYOP28Apilot.
Thanks a lot! That made it clearer for me.
Cheers!
 
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Starlionblue
Posts: 17212
Joined: Fri Feb 27, 2004 9:54 pm

RE: Question About Dispatch And FL

Fri Apr 26, 2013 4:25 am

Quoting igorland (Thread starter):
Optimum FL is determined by performance charts, correct? However, FL depends on the total plane weight, the total weight depends on the fuel weight, the fuel weight depends on adjusted distance, adjusted distance depends on the wind component at FL, which depends on FL! Catch 22

If we just look at these basic inputs and disregard MEL items, airway restrictions, and weather other than winds, the computer system presumably performs the calculation for all applicable flight levels, then just chooses the one with the lowest burn.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
MSJYOP28Apilot
Posts: 293
Joined: Tue Jul 21, 2009 2:09 am

RE: Question About Dispatch And FL

Fri Apr 26, 2013 5:04 am

Most flight planning systems default to lowest burn in determining the optimal altitude since the company wants the fuel savings. But there is normally an option to optimize by speed and time. There are times when lower altitude saves time when headwinds are strong at higher altitudes. There are also aircraft that cruise more efficiently at a lower altitude than a higher even if the flight planning system wants it higher due to the fuel savings.

Airways restrictions are usually built into the route by the company flight/nav data people and the most a dispatcher would do is read a note given to him by the nav data people to file at a specific altitude. That's the same as well for ATC route restrictions where ATC has a cap on the altitude for volume.
 
igorland
Topic Author
Posts: 27
Joined: Thu Mar 14, 2013 12:02 am

RE: Question About Dispatch And FL

Sun Apr 28, 2013 3:01 am

Thank you, all, for your help! Best regards!

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