Wake Turbulence

[callum91]

Hello

I recently experienced wake turbulence for the first time when I was cleared for take off shortly after a large helicopter was cleared take off, crossing the upwind end of the runway and was wondering what the procedure is for light aircraft pilots - was it my responsibility to decline take off clearance and should I have told someone?

It was only minor (like turbulence which can occur naturally) - a sudden bank of about 20 degrees and a stall warning just as I was climbing out where the helicopter had been.

Thanks

[Fly-by-Wife]

Callum,

A good question - ultimately you, as PIC, have responsibility for the safety of your aircraft. You would hope that ATCOs / FISOs would be advising you of hazards they should be aware of, but you should not rely on it.

In your situation, you could have replied to your clearance with "G-ABCD holding position, due wake turbulence from rotary traffic departure" - if you were expecting / anticipating a problem (which you probably will be next time!)

If you were to reply "holding", you would then HAVE to still wait for a new or confirm your existing clearance again before commencing your take-off, once you assessed the wake turbulence had dissipated, because you would obviously need to be aware of the larger picture - other aircraft movements, etc.

But if an ATCO was trying to get the rotary off, launch you with someone else on final - well they were cutting it too fine! If the other traffic has to go around because you are stationary on the runway - tough.

I'm not sure I would describe it as minor turbulence, either - you don't say how high you were, but I would not want that amount of roll with a stall warning anywhere near the ground - wingdrop plus stall = spin.

Well done for recovering, and you have probably learnt a lot today!

FBW

[reportyourlevel]

callum91

I'm an ATCO, not a pilot. I would never give take-off clearance until I'm sure I'll get the required wake turblence spacing, for example if I need three minutes I'll probably start giving the take-off clearance at 2min30s as by the time you've read it back and rolled, the three minutes will have elapsed. This is the way I was taught at college and it seems to work. I know some pilots will use a stopwatch to time the gap from the previous departure and not start their roll until the required time has elapsed. This is fine from my point of view.

It's useful to know that ATCOs consider the wake turbulence to be significant from the time the nosewheel lifts, or for a heli from the time the weight of the aircraft if transferred from the gear to the rotors. We also try to appreciate that a light heli will produce more turbulence than a light aeroplane and might try to give you a bigger gap if we can.

If you haven't done so already, you can send one of these http://www.caa.co.uk/docs/33/SRG1423.pdf to the CAA. I think it's mostly for research, but it helps towards a better understanding and therefore safer operation for everybody.

[Spitoon]

...for example if I need three minutes I'll probably start giving the take-off clearance at 2min30s as by the time you've read it back and rolled, the three minutes will have elapsedThe problem is that in callum's situation there is probably no vortex wake spacing requirement (certainly not a clearly applicable one from the info we have). Perhaps the best message that we can gain from callum's experience is that you can meet vortex turbulence anywhere (even if the minimum spacing requirements have been applied by ATC). Hence the need to be aware of the conditions in which vortices are generated and also what cases them to dissipate - or not - more quickly.

[callum91]

Thanks for your replies

I'm not sure I would describe it as minor turbulence, either - you don't say how high you were, but I would not want that amount of roll with a stall warning anywhere near the ground - wingdrop plus stall = spin.


I was not more than 150 feet above ground level at the time (my plane climbs slowly) and with the cross wind as well that was of concern to me - needless to say I did push the nose forward as well!

The problem is that in callum's situation there is probably no vortex wake spacing requirement (certainly not a clearly applicable one from the info we have).

I'm almost certain it was an EC 120 helicopter (just looked it up on the company's website) and as you say, if it is Light category, no spacing was required. I must admit, when I saw the helicopter I did wonder whether the controller might wait for wake turbulence.

If you haven't done so already, you can send one of these http://www.caa.co.uk/docs/33/SRG1423.pdf to the CAA. I think it's mostly for research, but it helps towards a better understanding and therefore safer operation for everybody.

I'm not sure if an SRG1423 applies to me since I'm only a PPL holder.

I have learnt for next time that you can get wake turbulence from any helicopter and I think I'll ask to wait in future.

Thanks

Callum