Exam question(s) - can't be right, can it?
 

Been down the 'club today mowing over a few exams (all passed - yay!), but a couple of questions/answers seemed to be a bit wrong, or confusing at least. At risk of looking like a plonker, here they are (slightly paraphrased - I didn't pinch the exam papers!):
the one that confused me...

Q - you are flying with a strong tailwind, how does your best range speed change compared to flying in calm conditions?

A's - slower, faster, no effect, depends on weight.

I chose 'no effect', but the CAA reckon it's 'slower' - so can anyone describe why this might be the case? I think best range speed will give you best range irrespective of the wind speed/direction. Where's my thinking flawed?

...and the one that annoyed me:

Q - you are converging with a fast jet on reciprocal headings at approximately 600kts. Vis is 5 nautical miles, but you don't spot him until three. How long have you got to take avoiding action?

A's - 12s, 20s, 55s, all day.

I chose 12s as 600kts = 6s/mile => max time is 3*6 = 18s, ie at 20s you are dead. CAA apparently want you dead though as 20s is the 'correct' answer.

Oh, and there were only 49 answers to a 50 question General and Principles of Flight exam too. Does that make question 50 a trick answer?

Anyway, any people care to pontificate on the answer to the first question for me?

Cheers,

A

I fully agree with both your answers!

They're correct on the first one. Its easier to explain the converse of it.

Imagine you are flying into a headwind equal to your normal cruise speed - ground speed is zero - its only by going faster that you achieve any range at all.
In a tailwind, by going slower, you are getting more of a 'free lift' from the wind vector.
More technically, the best speed is the tangent of the drag curve that crosses the origin displaced left/right by the amount of tail/head-wind.

As for the second question - If the options you give are correct, I agree that you'll collide in 18 seconds, so (a) is your only option.

Most (all?) aircraft have a best endurance speed lower than best range speed. Accordingly, fly at best endurance speed, you'll be in the air for longer, and benefit more from the tailwind, giving a greater range.

Therefore slower will be the correct answer, provided that the tailwind component exceeds the difference between best endurance and best range speeds. For my aircraft (Pioneer 300), and as far as I can recall all of the aircraft I've flown the best endurance is around 5 knots slower than best range, therefore in the circumstances described, slower would be the correct answer. :8

Agree with both the 2 Ms on the first ? and would have gone with Marks explanation (though both are correct).
Second is obviously wrong!

Foxmoth - glad I pressed refresh before blowing you out of the sky! :}

Mark 1 - thanks for the explanation - I think I understand it easiest by drawing the graph from your technical description.

A

Foxmoth

"600kts gives 10 miles/minute not 6 seconds/mile"

Surely 10 miles/minute & 6 seconds/mile amount to the same. The point you missed was "you don't spot him until three". Hence 18 secs to oblivion.

RTFQ;)

Mariner,

Best endurance is the bottom of the drag curve, so best range in a tailwind will be between best endurance speed and the 'still air' best range speed.

And at 600kts how long to edit a post?:uhoh:

<<edit: Is that all "heads down" looking at computer, or "heads up" looking for fast jets?>>

I think you are all wrong with question 2. Einstein showed that as velocity increases time slows down and as such you will have all day.

Clearly the caa expects everyone to have a basic grasp of quantum theory:E

Does the examiner not debrief you on the answers as he should do?

Oh, and there were only 49 answers to a 50 question General and Principles of Flight exam too. Does that make question 50 a trick answer?

Shouldn't there have been 200 answers? (4 per question) Looks like your examiner doesn't look after his papers either!

The RTFQ prinicpal applies to the collision time question.

You have misquoted it......

There were in fact 200 possibilities, but the CAA appeared only to give 49 correct answers, despite asking 50 questions - does this make it clearer?

And yes, I was de-briefed. Maybe I am thick, but it wasn't until Mark 1's explanation that I fully understood why 'slower' was the correct answer. The justification for the second question was that the calculated 18s was closest to the 20s answer.

A