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Welcome to the IREX question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
Below question is from CASA IREX sample questions. There is a similar question given in Bob Tait's IREX book in Page # 139, Question 15.
But in this question they emphasize "By night". Would the answer be different when it comes to night ?
You are tasked on an IFR flight, by night, from point ALPHA to point BRAVO, 320 nm apart. You plan for A080 and an average groundspeed of 180 knots. The aircraft GPS is unserviceable.
Point ALPHA is an unlicensed aerodrome without any navigation aid. Point BRAVO has an NDB. There are no other navigation aids enroute, and IMC may be expected. With regards to the requirements of track keeping, what rated coverage (by night) for the NDB at point BRAVO would you assess as the minimum for the flight to proceed safely? Answer 6.39 X R = 320 R is = 50 nm
Concur with Stuart - in operations, you would be considering the AIP rated coverage at night. However, that is not material to the question's solution.
Aside - you might be able to fill in the following question of mine ?
Can't immediately see whence comes the 6.39 factor in your answer, although it is very close to 1/tan(9°) = 6.31 per ENR 188.8.131.52.1(b) at p ENR 1.1-27.
Generally, we don't get students to worry about trig stuff. However, most students seem to favour the Jepp whizz wheel which has sine and cosine scales incorporated so it is worthwhile knowing how to use them and they are there for a bunch of pilot calculations should you so desire.
You could figure the "correct" answer as
coverage = 320 x tan(9) = 320 x sin(9)/cos(9) = 50.7 which we would round off to 51.
I don't have a copy of the BT IREX book but I presume the 6.39 comes from an approximate work up in that text ?
Engineering specialist in aircraft performance and weight control.