Hi Guys,

I recently did my Aerodynamics exam during which I came across a real fine example of CASA questionnaire which I'm pretty sure I got wrong. From memory:

You are planning to take off from RWY 36 shortly after an large aircraft has departed. In what conditions would you expect the resultant wake turbulence generated from the preceding aircraft to be most prominent:

a) In nil wind conditions
b) A 10kt wind coming from the NW
c) A 5kt wind coming from the NW
d) A 10kt wind coming from the SW
e) A 5kt wind coming from the SW

At first glance I remembered that wake turbulence "drift(s) laterally outwards at about 3 - 4 knots" according to the textbook. For wake turbulence to linger on the runway after an aircraft has departed an opposite crosswind must be equal to or slightly greater than its outwards drift. Thinking this I chose to pick one of the 10kt answers as the crosswind component would be approximately 5kt. However I was unsure whether it would actually matter if there was a headwind or tailwind component to this as long as the crosswind component was "about 3 - 4 knots". So I ended up choosing the wind coming from the NW at 10kt for no particular reason. Was this the right answer?

Hi Rohan,

A bit late answering this one but better late than never. The answer I would go for would be (c) 5 kt from the NW. Light winds will tend to keep the wake turbulence in the area and 10 kt is more likely to blow the vortices away from the runway. Also, since the wind has a headwind component, the vortices will also tend to drift with the moving air mass and move along the runway towards the downwind end.

In all of the other answer options, wake turbulence is still present but not as prominent a problem as with answer (c):
(a) the vortices will drift sideways and away from the runway
(b) a 10kt wind will tend to blow the vortices off the runway. You need calmer conditions to keep the vortices in the area. However, they could still be a problem but not as much as in (c).
(d) and (e) will tend to move the vortices away from the following aircraft and so wake turbulence, though present, will be less of an issue.

Cheers,

RIch

Hi Rich

I'm a little confused by your answer to this. I may have misinterpreted it?!

Bob's CPL Aero book states that a light quartering tailwind would be the most prominent. So I would have answered a 5 Knots from the SW as this would not only allow the upwind vortex to linger along the RWY heading but, there may also be a likelihood of encountering the vortex even if the aiming point is ahead of the preceding aircraft? Am I completely off mark?

Rich

Hi Rohan,

There is different degrees that each factor will affect the vortex you will experience.

For it to be really bad you need the following things.
1. For the vortex to be uniterrupted
2. For the vortex to remain in the area
3. For the vortex to be strong enough to affect you.

1. The vortex generated naturally dissipates with stronger winds, this is why 10 kts is probably too much, this will cause it to dissipate the vortex quicker
2. It helps to have a crosswind to ensure the vortex remains stationary as they do tend to roll away.
3. Big aircraft taking off before smaller ones: This is why we have separation standards.

So Rich is right C would be the best answer to me.

Hi Kernowflyer,

The passage you refer to in the CPL Aero book is talking about arriving at the runway behind a heavy that is landing. In that case, the vortices will be blown along the runway by a light quartering tailwind, more into your likely flight path. So, in that situation, the tailwind is worse and option (e) would be the answer.

The question Rohan mentions above however talks about a jet taking off. In that case the light quartering headwind is going to keep the vortices on the runway and push them along the runway towards you. That will be the more dangerous situation.

Cheers,

Rich