Hi guys, first of all well done on such an informative site with amazing support.
I have been studying this week and done 2 cyber exams 1st = 68% and today I managed 75% on the second.

One question was this one on exam 1.

You are planning a flight from Alpha to Bravo, in VH-OZY. The distance from Alpha to Bravo is 380 nm. Your flight planned ground speed is 193 knots. There is one NDB at Alpha and one NDB at Bravo. The GPS has been removed for repair. If there are no other navigation aids within range along the route, the least rated coverage for the NDB at Bravo to allow the trip to be planned as an IFR flight is -

AIP ENR 1.1 para 19.1.1(c) says that the distance between positive position fixes shall:

1. Allow for a 9° track error and still bring the aircraft within the rated coverage of the aid, and
2. Allow the time interval between positive position fixes not to exceed 2 hours.

If the only aid at Alpha and Bravo is an NDB, there will be no position fixes available between leaving Alpha and flying over the aid at Bravo.
If a track error of 9° is to bring you within the rated coverage of the aid at Bravo, the distance from Alpha to Bravo must not exceed 6.66 times the rated coverage of the Bravo NDB. Therefore, the minimum range required for Bravo aid is 380 ÷ 6.66 = 57nm.
IREX_010

The distance is 380nm and the ground speed is 193kt, so you are OK for the 2 hour requirement.

The answer to pick is 60nm.

I worked my answer out exactly the same and got 57nm. The answers available were
A. 55 B. 60 C.65 D.20
Given those answers I chose A. 55 because it was the closest to 57 and got it wrong. May I ask why the correct answer is 60? Should we always be rounding up?

Thanks for any help.
Drew

I also had this exact question in my Irex practise exam. After I submitted it the answer said 60. For some reason 6.39 was used instead of 6.66.
I dunno where that has come from, I asked my instructor about it and he had no idea what the 6.39 was for. The Bob Tait books all used 6.66 so I think this question just needs to be revised. Your working was all good.

Hello,
I get answer D. 20mn. Only because at 193kts GS you will cover the 380nm in just under 2 hours (118 minutes actually). Therefore you meet the minimum requirements of the fix not exceeding 2 hours and question asks for the MINIMUM range required from Bravo.

Remember! The figure of 6.66 is the equivalent to 9nm off after 60nm travelled to meet the minimum requirements of ENR 1.1 19.1.1c. That's why 6.66 is always used for these type of questions.

If you get these type questions in an exam or in real situations like I have flying Kalgoolie WA to Forest WA then onto Ceduna SA with a dead GPS in a PA28. Start with the minimum GS required to be within range of the aid then work from there.

From the answers supplied, 55nm would be correct for 162kts GS,160kts GS for 60nm, 158kts GS for 65nm. All these answers are, as stated in the question, for using an NDB. If you using a VOR, then you have to consider altitude as well.

Hope this helps,

Mister W.

The 6.66 is based on the one-in-sixty rule which is not really mathematically valid. The 6.39 is the correct trigonometrical solution. Now that CASA are presenting questions that require a typed numerical answer to be entered into a computer field, it might be safer to use the 6.39 rather than the slightly less accurate 6.66. Just in case you're interested, here is the derivation of the trigonometrical solution.

I have amended the text in the latest version of the IREX book to suggest you use 6.39 in future.