Flex Take-Off Speed Bugs
 

The load sheet is complete and my T/O weight is 68T. I select the correct speed card and bug the speeds..but wait I may be able to Flex..Out come the performance tables for the runway in use and yes indeed I can Flex to 48 degees.

I notice that the VI VR V2 are all higher than the rated speeds as I would expect since I have reduced my thrust.

I have heard it said that you should only bug the flex V1 and the other speeds should be set from the rated speed card. I dont understand this and was hoping someone could explain the theory..

Thanks

Take a look first at http://www.pprune.org/forums/showthread.php?t=292416

Don't know if one could explain that. Might put V1 greater than VR.

i disagree... you should not change anything.

if yr flex is 48,then the speeds are calculated precisely to take off with yr weight. be it v1 vr or v2. it has been calculated for the flex thrust.

now if u mix them up... like johnny says...there will be a chance the v1 will be higher than v2.

remember the speeds are calculated via thrust input. and if u just use yr v1 from the flex,and vr v2 from the std speed cards... when u reach vr,u might not have enough speed for lift because remember...you are still on flex thrust.

it may be very complicating to you,but just sit back,have a ciggy,and think about it alright??:ok:

Think of it this way: The flex thrust rules require that at any point, you must be able to advance to RATED TO thrust for you actual ambient temp. So, Vmcg and Vmca should be based on rated thrust.

And even though you're only using thrust equivalent to a +48C day, the wing is still flying in the cooler actual ambient temp. So, you'll use less roll than the perf. charts for +48C would predict.

Performance wise, I wonder if any serious problems arise. For the 737NG (700W) the V2 numbers (at 55T or 115K lbs) don't seem to vary:

(i) from max assumed temperature reduction (25%) D TO-2 at 60 deg AT, to RATED TO-2 thrust, the V2 remained virtually identical.

(ii) from FULL DERATE-2 (20K) thrust, up to MAX RATED (24K) thrust, the V2 remained vitually identical.

At limiting situations (Obstacle, Climb) there wouldn't be much of a difference between the flex thrust, and rated, I would think.

And in contaminated/slippery runways, there's no flexing.

I did note, for the NG, it would be possible for Vr to be 2 knots less than V1, using your 'advice'.

What are the differences in speeds we are talking here?

Trying to improve on the approved methods, seems unnecessary. (Like brake stands to improve short field takeoff).

So, what's the purpose?