PACKS off take-off question

[skelsey 765]

 

 

A lower V1 would be desirable at a short field, but how would the derate impact Vr and V2 if at all?

 

Depends on the most limiting factor for that particular takeoff.

 

Vr cannot be below V1, so if Vr is limited by V1 then Vr may become lower.

 

V2 must (amongst other things) be greater than Vmca, so with less thrust V2 may be lower.

 

Often it is the accelerate-stop distance that is most limiting on a short runway, and therefore the reduction in V1 is what leads to the potential for increased TOW; however, this will not always be the case on all runways in all situations.

[Copper. 78]

Thanks Simon. I've reached the limit of my knowledge. I'll disect what you've written and hopefully come out a little wiser. How did you become so conversant in aircraft performance if you don't mind me asking?

[skelsey 765]

No problem!

 

 

 

How did you become so conversant in aircraft performance if you don't mind me asking?

 

That's a very good question! I don't think there's any single source: just many years of reading articles, books and tutorials and quietly observing in forums. I also spent a number of years flying the Level-D 767, which came with all the tables (and a tutorial) necessary to manually calculate the takeoff performance, so applying all the corrections etc on a vital data card before each flight gave me an appreciation for the impact of some of the different factors (packs off vs packs on, for instance).

 

Having said all that, I think what really made it all click was when I started to understand the core physics behind it all, and the same applies for a lot of other aspects of this hobby: descent planning, for instance. The more I understand about the basic physics of lift, weight, thrust, drag, energy and the effect on all of the above of density/temperature etc, the more it all makes sense (and the easier it is to extrapolate to new situations).

[codechris 9]

I have to laugh because I was so in-over my head I was thinking it was packs, and not bleeds, hence the name of the thread!

 

But the important thing for me, and that's why I like diving in to the dispatch side of things, is I really get an idea of the plane, even though I'm far away from the actual aircraft. I'm diving in to payload and fuel numbers as well as the Topcat numbers. Yes, we can say that pilots do have to care about this, but maybe not to the extent that I have dived in, in trying to work out how to safely takeoff from this airport, all the while trying to keep in mind that an airline is there to make money. And that has been fantastic by hearing from others in what I need to go through in my head in order to get there.

 

In the sim world, we can practice all of this and find the balance ourself. An airline can't. I guess they bury their heads into the numbers of it all, and maybe (i don't know) test them out with a pilot in a sim to test a new airport, but I guess not, because they have all of the data and numbers to hand. The overriding factor that I've learnt, is to study the numbers and make adjustments where necessary and play close attention to them. Part of the issue with flying with a VA is that you know they CAN fly out of there and everything I've had so far has been a breeze (in terms of takeoff and landing anyway). This came out of a Vatsim event, I'd never even heard of that airport. And that has been the new challenge

[G550flyer 1,013]

 

 

A lower V1 would be desirable at a short field, but how would the derate impact Vr and V2 if at all?

 

As stated, there are a few things that the V1 is based on depending on the aircraft. In short field situations, the available runway distance will drive the V1. Short runways naturally reduce the amount of accel and stop distance, driving down the V1 and gross weight. The derate  makes the accel to rotate longer so the V1 on a short field must decrease in order to ensure stopping distance. If the same field becomes wet, V1 gets even lower which may result in a weight limit. When Vmcg limited, V1 will be bumped to Vmcg to ensure you don't commit before having ground control. When flying jets with powerful wing engines and light weight, you can reach a performance limit corner. Higher power aggravates Vmcg but it's required due to the short runway. I experienced this in the DC10 when light weight on ferry flights with crosswinds.

 

When distance is not a factor, the V1 could be higher as in a balanced field situation where accel distance is equal to stop distance. In general, derates won't change Vr or V2. When using full power or derated, it still takes the same amount of air flow over the wings to get airborne. It just takes longer time to reach rotate and a lower pitch for V2/V2 + 10 when derated.

 

In the balanced field concept, the higher V1 can result in a higher gross weight. In balanced fields on dry conditions, V1 becomes a ratio of rotate. In traditional performance, V1 is lower than Vr. If you lost the engine at V1, it takes longer to accel to rotate which increases your accel go distance. This in turn can reduce your gross weight to account for the extra distance. When the V1 is a ratio of rotate, it is higher and closer to rotate. This reduces the time/distance to accel to rotate engine out, allowing a higher weight. You will notice that the balanced field concept is mostly beneficial to light to medium type aircraft. Heavies require more stopping distance leading to lower V1s.