Hi everyone,

 

recently I developed a question on the V-Speeds, especially V1, as it is calculated on the NGX. So far I was unable to find a quick answer. Perhaps some of you know it or can point the finger to where I can find it.

 

During flight preparation V1 is calculated and displayed in the FMC. To my surprise, however, they don't seem do change with the available runway length that is entered on the same page the speeds are displayed. Until now, I was under the impression that V1 is dependend on the runway length. To form a proper question: How does the available RWY length affect the calculated and displayed V1-Speed at the NGX?

 

And while we are at it: Why is it, that in some thrust levels (e.g. 22K or 24K) the V-Speeds will not be calcullated? So far I thought this would mean that a takeoff with this setting was not possible with the given RWY-length available. If that were true the RWY-length is considered, with closes the ring to my first question

 

I'd appreciate answers from the experts in this forum.

 

Many thanks, and all the best,

Ingo

How does the available RWY length affect the calculated and displayed V1-Speed at the NGX?

 

Available RWY length on TAKE OFF REF page 1/2 affects only position update with GPS update turned off. It does NOT affects performance calculations. NGX calculations are pretty accurate, but it's not recommended to use FMC calculated speeds. In RW pilots are using RTOW charts, or some software like EFRAS and Octopus or calculate them in EFB.

 

Why is it, that in some thrust levels (e.g. 22K or 24K) the V-Speeds will not be calcullated?

 

They will.

 

And while we are at it: Why is it, that in some thrust levels (e.g. 22K or 24K) the V-Speeds will not be calcullated?

 

I have never seen that.

I've seen it not populate V speeds on lower thrust levels like 22K 20K if there was simply not enough runway to achieve the V1 speed under the weight, atmospheric conditions, and/or whatever else goes into that calculation.

 

I can't recall but think it was at MDW with a heavier load headed west. When I selected a higher takeoff thrust the V speeds then populated.

 

Not sure if this is correct or was my mind playing tricks on me and also not sure if such the case on a real NG.

It depends a bit which V1 speed you use. V1 is defined with respect to both Stop and Go criteria.

With respect to the stop criterion, V1 is the highest speed at which you can reject the take-off and still come to a complete stop within the runway length available.

With respect to the go criterion, V1 is the slowest speed at which you can continue the take-off with one engine inoperative and still reach a height of 55 feet by the end of the runway.

 

If your runway is long enough those two speeds will not be the same (V1-go will be lower than V1-stop), so you have a speed band within which you could either stop or go and both will be safe. If the runway is shorter, these two numbers will be closer together, until you reach the 'balanced field length'. If the runway length is equal to the balanced field length there will only be one V1 speed, below which you must always stop, and above which you must always continue, to avoid an overrun. This V1 does not depend on the actual runway length available, purely on the calculation when the amount of runway needed to accelerate and go is equal to the amount needed to accelerate and stop. If the FMC uses the balanced field calculations, V1 will indeed not depend on runway length.

 

More information in this FAA booklet: http://www.faa.gov/other_visit/aviation_industry/airline_operators/training/media/takeoff_safety.pdf