Flex T/O & Speed Corrections

[Macgyver]

Hey folks,

Im on the A330, and when calculating our speeds for a Flex T/O and using the corrections tables, we only have to adjust the flex temp and not the speeds unless there is a WET factor, where we then reduce both speeds and flex temp. ie) for QNH corrections, we just subract from the Flex Temp and V1/v2/vr corrections arent necessary. This is also given the Airbus FCOM 2 manual.

Cant someone please explain why the speed corrections arent required for QNH, Packs on/off, etc, but they are for WET?

Thanks for the help,

Mac.

[Macgyver]

If we're using a Flex T/O with a low QNH on a wet runway, according to our charts we adjust Tflex for the weight and the low QNH, but we only adjust the Vspeeds downwards for the wet runway, and NOT the low QNH. Whenever we do a Flex t/o, the only speed adjustments are to the wet factor.

[MD80rookie]

With QNH you adjust your weight (weight, altitude, temp - remember?).

Wet rwy, you only adjust speed for stopping capabilities / enough rwy to accelerate on one engine and reach 15 ft above rwy end.

Have fun!

[Escape Path]

The increase in allowable weight with QNH correction is usually not that big to generate a significant change in V-speeds on a mid-size (or bigger) jet, IIRC

Runway condition (wet or dry) does generate a change in V-speeds for the reason given my MD80rookie

[Aikon]

The reason is for FLEX TO, the less dense air (lower QNH) factor is compensated by a lower FLEX setting. Therefore there's no requirement to change the speed.

However, if you do a TOGA TO, then speed correction is required on QNH.

[rudderrudderrat]

Hi Macgyver,

It sounds like you have calculated the optimum V2 speed for your actual weight (mass), and adjusted the flex temperature for the performance changing effects of QNH, packs etc.

If you then use wet figures and reduce your V1 (to help with the stopping case) but then have an engine failure just afterwards, the adverse effect is that you'll be continuing from a lower speed and take longer to accelerate. In order to clear the 15ft fence at the end of TODA, the V2 is reduced slightly from the optimum previously calculated (but you will still be above V2 min)