LIMIT: FIELD would be the length of the runway itself, given the parameters used.

LIMIT: CLIMB (or OBS) would be the need to clear an obstacle, or meet a minimum climb gradient, given the parameters used.

 

A lot of people forget that takeoff performance includes initial climb figures, too.  The CLIMB limit would be one of those times where TOPCAT is using a lower assumed temp or higher trust rating in order to meet climb performance requirements.

 

Have been waiting for someone to mention climb performance. Take off performance calculations are based on a minimum climb gradient single engine (for twins)  A Higher flap setting will give you a shorter take off roll but decrease climb performance, lower flap setting gives you a longer take off roll but increased climb performance.

 

 

A lot of people forget that takeoff performance includes initial climb figures, too.  The CLIMB limit would be one of those times where TOPCAT is using a lower assumed temp or higher trust rating in order to meet climb performance requirements.

 

 

Thanks for the explanation. Also, if no obstacles are in the way, should FIELD technically be the standard LIMIT code? Does that make sense?

 

 

A Higher flap setting will give you a shorter take off roll but decrease climb performance, lower flap setting gives you a longer take off roll but increased climb performance.

 

Precisely this.... 

 

Most heavy takeoffs above 700,000LBS rarely are at Flaps 15. The only time I ever get a Flaps 15 calculation is when I am at moderate loads (550-680k LBS) paired with a 9-10k foot runway. 

 

However, 90% of the time, takeoffs are at Flaps 5 due to the takeoff thrust required being slightly lower which reduces costs for the airline and the chances of an engine failure on departure.

 

Last week out of LAX, I had my first Flaps 15 departure in a while due to my weight being 680,000LBS and using the shorter (10,100feet) runway 24L.

Although, the thrust required was Full-TO with bleeds off.-This was the case due to the increased drag from the Flaps 15 setting that needed to be overcome with an increased level of thrust.

 

Even though the takeoff roll was relatively short, the intial climb was only a shallow 11-13 degrees...

 

 

Thanks for the explanation. Also, if no obstacles are in the way, should FIELD technically be the standard LIMIT code? Does that make sense?

 

In theory, yes.  If one were to have a "perfect" field, in the middle of nowhere, with no obstacles, the limiting factor would be the amount of pavement at your disposal, and that's it.  Makes sense to me.

Lots of good stuff here already, so just to confirm....

 

Heavier weights usually just mean you can reduce thrust by less.....untill you are at full thrust ofcourse.....at that point, depending on what is the limiting factor; Take off run, accelerated stop distance, nearby obstacle, far out obstacle (relatively far), or minum legal climb gradient (2.4%) that must be achieved by regulations on one engine.......it may or may not help to use a higher take of flap setting (the programm will let us know these days).

 

Higher outside air temps and tailwind (even if only 5kt) usually ask for flaps 15.

 

That is all talking B7772 by the way.....I have no experience with the -300.

 

 

Heavier weights usually just mean you can reduce thrust by less.....untill you are at full thrust ofcourse.....at that point, depending on what is the limiting factor; Take off run, accelerated stop distance, nearby obstacle, far out obstacle (relatively far), or minum legal climb gradient (2.4%) that must be achieved by regulations on one engine.......it may or may not help to use a higher take of flap setting (the programm will let us know these days).

 

Exactly.