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## Wake Turbulence question in exam.

• Posts: 12

### lcanning created the topic: Wake Turbulence question in exam.

G,day Team another question ive got from the exam.

I was told that I was taking off behind a major airliner on RWY 36 heading straight North.

They wanted me to choose between a few options of winds on what kind of wind would cause the most risk during take off, In referring to wake turbulence
a) 10knots North
b) 5knots SW
c) 10knots SW
d) 10knots South

So I was sitting there thinking for a bit to choose between B & C as they are the closest winds to a cross wind. But i really didn't know which speed to choose from, as said in the text book that a 4-3 knot crosswind is most dangerous, So I choose C and Im pretty sure I got it wrong

Should have it been B ??

• John.Heddles
• Offline
• ATPL/consulting aero engineer
• Posts: 853

### John.Heddles replied the topic: Wake Turbulence question in exam.

These sorts of questions are always a problem as the subject is not cut and dried but flavoured with many subtle variations. So, for instance, we need to have some idea of the particular Type which is the generating aircraft, the class of aircraft in trail, relative speeds and weights, local terrain and so it goes on.

However, if you want to apply the oft-seen 3-4 knot crosswind criterion in this case, with a 45 degree angle (N/S and SW) then the crosswind fraction is about 70 percent so 5 knot from the SW would give you around 3.5kt so it's the obvious "exam" answer.

(Keep in mind that we get the answer using one of rules of thumb (if you have a good memory), the navigation computer grid (graphical solution), or simple trigonometry (the easy solution).

Engineering specialist in aircraft performance and weight control.