Unbalancing the field

[doublepsych]

I am posting this here to get some useful real world input (as opposed to posting in other forums where the responses would likely be more academic), so thanks in advance for any info you wise old owls can share.

I think I have a fair understanding of what BFL is and does. What I don't know is when or how one would select a V1 to unbalance the lengths.

Is this essentially what a correction for wet, contam, or sloped runway does? Or are there other reasons. The only V1 adjustments I see in my aircrafts' (Beechjet) books is for the above factors...

Like most, I use the tab data 99% of the time because the assumptions for that data are met, but I'm studying for a interview coming up and this issue came up.

Thanks

 

[Stby One]

Most takeoffs in my airlines operation are in fact "unbalanced", meaning that we are not often right on the runway limit. If you have a long runway relative to your takeoff weight then a balanced field take off will see you reach your screen height (JAA 35ft dry, 15ft wet) well before the end of the runway, or in case of a rejected takeoff you will stop at the same point as you would have reached your screen height.

So this means that we are not using all of the runway (or stopway and clearway if applicable). We can now "unbalance" the field length by for example decrasing our V1. If the V1 is decreased you will need less runway to stop in case of a rejected takeoff. So your field length required from a stop point of view has decreased.
However, the effect of a decreased V1 will be an increase in field length required from a GO point of view. If you would continue your take off with an engine fail at Vef (1-2 sec before V1) then you will effectively be rolling a longer distance down the runway on one engine before you reach your screen height.

The field length required in the above case would be the equivalent of the "GO" case. The opposite would be true if we would increase V1.

An unbalanced V1 should not be confused with a wet, contaminated or sloping runway. As in the case I described it could as well have been a wet runway.

 

[MauleSkinner]

I am posting this here to get some useful real world input (as opposed to posting in other forums where the responses would likely be more academic), so thanks in advance for any info you wise old owls can share.

I think I have a fair understanding of what BFL is and does. What I don't know is when or how one would select a V1 to unbalance the lengths.

Is this essentially what a correction for wet, contam, or sloped runway does? Or are there other reasons. The only V1 adjustments I see in my aircrafts' (Beechjet) books is for the above factors...

Like most, I use the tab data 99% of the time because the assumptions for that data are met, but I'm studying for a interview coming up and this issue came up.

Thanks

Actually, the tables in the Beechjet don't give you the option of unbalancing the takeoff without having a wet or contaminated runway. The way they set up the tables doesn't allow for it, since it only gives a takeoff distance available table.

Many other airplanes have place to enter the charts for both a-stop and a-go distances, which would allow you to unbalance the takeoff. You may never see a clearway, allowing you an extended a-go, with a-stop on the runway, but you'll often see an overrun (stopway), that allows you to extend a-stop. Depending upon the airplane, the conditions, and the amount of runway you have, it may make the difference.

We used to get to an airport where using the stopway to unbalance the takeoff allowed us to make it home nonstop, where a balanced field takeoff wouldn't (obviously, this wasn't a Beechjet trip...as I indicated earlier, we couldn't have unbalanced the takeoff).

As Stby One said, most takeoffs aren't balanced field...you've got excess runway/takeoff distance available. However, in this case, adjusting V1 for an unbalanced takeoff doesn't really do you any good with respect to the amount of weight you can takeoff with for the amount of runway (unless you are operating off a contaminated runway). You're simply changing the parameters to favor a-go or a-stop, as appropriate.
Fly safe!

David