Hi there,

I got a debrief from a friend who recently did aerodynamics and advised me to study wake turbulence situations like the one below.

If you are in a 172 taking off on a runway which is intersected in the middle at 90degrees by another runway with a 747 on approach and the 747 does a go around, would you delay your rotation point or bring it forward?

I initially said delay rotation until a later time, but I cant exactly find any supporting material for this. Anybody have any suggestions?

hey 172, ive done aero also and didnt have anything like that. I did however get questions on similar things to page 5.10 in the aero book and also, purpose of design features, aerofoil terminology, the purspose/function of vortex generators etc. Definitions of things such as MAC etc. Hope this helps a little.

Hi 172,

It's hard to give you a clear answer to that one without seeing the actual question and the different answer options. What happens to the vortices is going to be very dependant on the wind strength and direction, at what point the go-around took place and things like that.

Having said that though it's hard to see how you could justify "rotating earlier" as an answer. Even if you could somehow rotate ealier, you'd be almost certain to climb up into the vortices. So, delaying rotation would make more sense if that was the only other option.

Does this question make any sense in real life flying though? Surely wasting available runway length by delaying your rotation point would also reduce your safety margin. It'd be great to avoid wake turbulence but you'd kick yourself if you ran out of runway trying to brake after an aborted take-off...

If that is the correct answer, it doesn't seem to make much sense. Opting to wait for an appropriate time and then departing as normal would seem the safer option in my opinion.

Cheers,

Rich

Hi Richard,

Courtesy of *Brook* I have found the question I have heard about.

Copy/Paste from "Brooks Exam Pre-brief" thread

Two rwys 27 and 36 intersect in middle (both questions)
Q9) B747 takes off on 27 1000ft before intersection of 36, does light ACFT on 36 wait 3, 6 or no minutes
Q10) B747 takes off on 27 1000ft after intersection of 36, does light ACFT on 36 wait 3, 6 or no minutes

Would just like to run my answers past you.

For 9) I think 3 mins because that is approximately the duration wake turbulence is significant at low altitude, also if you were to takeoff without delay you would likely punch right through it.

For 10) I reckon no delay is required (of course in an operational situation things would be rather different) due to the fact that wake turbulence will not be significant for the Heavy until it commmences its rotation.

Would my reasoning for the above questions also be correct if the B747 actually conducted a missed approach?

Hi 172,

According to the Manual of Standards Part 172 Section 10.12, specifically 10.12.1 and 10.12.3.2 (c),

* if the 747 rotates before the intersection, it would appear you should wait 2 minutes (and not 3 minutes as suggested by the answer options) Check the reference MOS Part 172 10.12.3.2 (c). Two minutes is required since the "Full Length Operations" Minima apply as specified in 10.12.3.6 (e) and this is 2 minutes for a LIGHT departing behind a HEAVY.

* if the 747 rotates 1000ft after the intersection, I agree, you would not need to wait at all.

If it was a missed approach, I would consider this to be the equivalent of a take-off before the intersection and again the 2 minute separation would have to apply.

Cheers,

Rich