So during flight planning I'm inserting all the info that can effect takeoff run such as wind, weight, engine de-rate and flaps yet I always pass V1 at a point where I just won't be able to stop on the runway anyone else have this problem?

 

...yet I always pass V1 at a point where I just won't be able to stop on the runway anyone else have this problem?

 

I'm guessing you're only using the FMC for this information, correct?

Have you tried or do you just feel like that? Those brakes can produce MASSIVE stopping force on RTO. Also remember that you are supposed to use spoilers.

 

And, as Kyle noted, FMC numbers do not guarantee that you can stop from V1 at that specific runway. These are tables for "balanced field" at specific weight and flaps (and some more conditions might be factored in, but not all). You can use them, even in real flight if you need, but you absolutely have to check, that corrected runway length available is in fact longer than required balanced field length.

 

So during flight planning I'm inserting all the info that can effect takeoff run such as wind, weight, engine de-rate and flaps

 

Are you using TOPCAT to determine your engine de-rate and flap settings based upon TOW and RWY length, wind, etc?

 

Michael Cubine

Yes just the FMC

 

Yes just the FMC

 

Thought so.  You'll need to use an airport-specific tool to get more precise numbers, like TOPCAT.

 

As Peter explained:

 

These are tables [the performance numbers in the FMC] for "balanced field" at specific weight and flaps (and some more conditions might be factored in, but not all). You can use them, even in real flight if you need, but you absolutely have to check, that corrected runway length available is in fact longer than required balanced field length.

(clarifications mine)

The FMC will return the calculated V1 speed for the conditions that you enter.  However, there is no guarantee that this V1 speed will result in the airplane stopping within the required runway length unless the takeoff weight has been used to compute the takeoff field length required and this field length required is at or less than the runway length.

 

The airline’s engineering department (or the contracted performance engineering service provider) will perform this check as part of the airport analysis process.  This analysis assesses all the requirements for takeoff, which include field length required, brake energy limits, tire ground speed limits, and one-engine-inoperative takeoff obstacle clearance, and provides a field length limit weight for takeoff.  This field length limit weight and an airport-specific climb limit weight are then used to determine the maximum allowable takeoff weight to comply with performance requirements, the lower of these two weights being limiting.

 

The TOPCAT program replicates this process and in fact uses much of the same AFM performance data that the performance engineer is using, albeit with computer programs.  If you want to determine a true takeoff weight limit for runway, you need to use a program like TOPCAT or real airport analysis.  I am told that TOPCAT does not include an option for NGX’s version of the B737, but there are some versions that are close.

 

If you use TOPCAT or a real analysis to determine the maximum takeoff weight, and then use the FMC to compute a V1 speed for that weight, you will find that the NGX’s runway performance is quite close. 

Also, do not be fooled by the amount of runway remaining at V1 speed.  At field length limiting weight about 60% of the runway will be used to accelerate to V1 speed, leaving only about 40% to stop.  If field length limit weight, takeoff field length required is 5000 ft., then 3000 ft. will be used to accelerate to V1 speed.  That leaves 2000’ to stop.  That can be a little disconcerting as the runway numbers on the far end get real big in the window.

 

I highly recommend TOPCAT if you plan on doing any real field length limited takeoffs.

 

Regards,

 

Rich Boll

Wichita KS

 

So during flight planning I'm inserting all the info that can effect takeoff run such as wind, weight, engine de-rate and flaps yet I always pass V1 at a point where I just won't be able to stop on the runway anyone else have this problem?

 

I am told that TOPCAT does not include an option for NGX’s version of the B737, but there are some versions that are close.

 

It has the 800 with same engines, the 700 it has with 22Ks instead of PMDGs 24Ks, but using one more derate degree in PMDG is quite enough to make them perform pretty much the same.