If waypoint 1 is 60degrees south, 30 degrees west, waypoint 2 is 60 degrees south 20

[gemma_600]

Can anybody PLEASE help me with this! If waypoint 1 is 60degrees south, 30 degrees west, waypoint 2 is 60 degrees south 20 degrees west, what will be the approx latitude on the display of the inertial navigation system at longitude 25 degrees west?

[BEagle]

Using http://williams.best.vwh.net/gccalc.htm the mid-point on the great circle track between those waypoints is at position S60:05.7 W025:00

[Stoney X]

Mind point between 20 and 30 west in 150nm. You started at 1800nm from the south pole. Using pythagorus gives you 1793nm from the south pole at the mid-way point. That 7nm more south than you started so S60:07 (roughly speaking of course)

(deleted to save HWD blushes ;))

Regards
Stoney

[High Wing Drifter]

Yup just realised boob and deleted, then noticed your post :O

Thank you Stoney :\

[Send Clowns]

This one needs a diagram for a sensible answer; ask your instructor or if that is too difficult send me a PM and I'll draw you up an explanation. BEagle's website relies on spherical trigonometry specifically excluded from the syllabus.

Failing that just learn the answer - 60°06'S. This is a really crap question, there are only two like it known in the question bank (unless Alex has come across any others recently?) and they are identical except the other is in the Northern Hemisphere.

Good luck!

Send Clowns
(former Gen Nav instructor, BCFT)

[DVR6K]

As Send Clowns says, just learn the answer. The other similar question in the question bank is the same but northern hemisphere. The only time in the whole of Gen Nav Oxford groundschool we were told to learn an answer to a question without bothering to explain it!! Makes sense really, far bigger gen nav fish to fry...

[High Wing Drifter]

If I recall the feedback correctly, you could answer at least one of those questions logically. It was either the only possible answer or there were two on the right side of the Rhumb line but one at an unreasonable latitude.

[BEagle]

It strikes me as being a very silly question indeed. What is its objective?

3 possible answer options - slightly more than 60S, slightly less than 60S or exactly 60S might be reasonable, but for a 4th answer to be offered implies that the candidate should be able to calculate the exact answer - or a reasonable approximation using some rule of thumb.

An exam I was asked to proof read wanted an answer to be given for the angle of bank required to achieve a Rate 1 turn at a given TAS. 2 of the answer options were obviously incorrect, but of the other 2 one was precisely correct using basic trigonometry and algebra and the other was correct using the 'first 2 figures of TAS plus 7' rule of thumb. The answer the examiner expected was the latter; however, I told him that both answers should be acceptable - and if he only wanted the 'plus 7' option to be the right answer then he'd obviously have to change the precisely correct option.....

[Strepsils]

BEagle - If I remember correctly the 4th answer was an RTFQ - RTFA in that it looked correct but had East instead of West. A sneaky gotcha, but still left the one correct answer by deduction instead of calculation.:ok:

[PB4]

Convergency = 10sin60 = 8.6°
Conversion angle = 4.2°
=> angle to join 60°N30°W to 60°x'N25°W is 1/2 conversion angle => 2.1°
Distance = (10x60)cos60=300nm => 1/2 way = 150nm
Then use the 1 in 60 rule on the CRP or I used Delta lat = 150sin2.1=5.49nm
So I would say answer is 60°5.49N

Please let me know if I'm wrong

What's the point exactly.. (apart from taking time during the exams) just read the damm INS :D

[High Wing Drifter]

Beagle,

With regard to the Rate 1 turn question, I think it is a perfectly reasonable question and the rule of thumb approach for an issue that is most likely to occure inflight is logically the most correct answer.

Obviously, if the CAA inconsistently apply this style of question then that's another matter.

[BEagle]

HWD - yes, the ruel of thumb is reasonable. But it would be wholly unreasonable to desribe the precise correct answer as being not the correct one because it wasn't the one the Examiner was expecting.

That's like saying Pi is:

1. 4.13
2. 22/7
3. 3.14159
4. 2.2

..and then saying that answer 2 is the correct one.

[Catster]

fwiw I think the point of the INS question is to check it is understood that a navigation system will follow the Great Circle between 2 waypoints and not the rhumbline!

[High Wing Drifter]

BEagle,

The guidance I received was to select the best answer from those available. So in the case of two answers that are good enough for the job, I would guess the one that was arrived at use the most appropriate technique is the best answer.

Anyway, I think Catster has nutshelled it good style.

[XL5]

PB4 seems to have it under control. A rather silly OTT question. Working it out on a napkin over lunch ( nothing better to do on some layovers) gives a track displacement of about 10 miles towards the pole. Requires a knowledge of: relative Gt Circle/rhumbline position, d.long into distance and d.long into convergency. Then you draw the triangle and attack with elementary trig to sort it out, the entire exercise being sadly worthless in a practical sense.

[Send Clowns]

PB4 is correct - it is the reason for using half conversion angle that takes a diagram. If you're just going to learn an otherwise pointless technique for one question, you might as well learn the answer!

[Dick Whittingham]

Beagle:

Sir.I bear a rhyme excelling in mystic force and magic spelling, celestial sprites elucidate all my own striving can't relate

3.14159265358979323846...

Dick W

[Ropey Pilot]

HWD

Afraid I go with BEagle on this one (the rate 1 turn question) - although it would depend on the exact wording. But as it stands you are in an exam and not an aircraft and more importantly it does not seem to be asking what you would try and aim for during your turn before correcting slightly as appropriate in order to achieve a rate 1 turn but what you actually need to achieve in order to be sucessful (which is the exact answer).

I agree it would probably be of more use to know and examine the rule of thumb but if that was the logic applied to the entire ATPL process where would we be! If that (ROT) is the answer the examiner is after as BEags said one shouldn't include one that is more accurate and count it as wrong!

With ref to the point of the original qu I think Catster hit the nail on the head with the reason behind it. But rather than learning the 2 answers for N & S hemispheres (which takes up 2 portions of brain) just look at the basic diagram with the rhumb line and the great circle track with the bit more than, bit less than or exactly 60 degrees approach. You can then answer both qu while only rembembering one thing and if they do change the numbers you can answer anything they throw at you (whilst also learning the point I believe they were making in the first place which is that you don't always fly along the lines on your map / a straight line isn't always the distance between 2 point!)

[High Wing Drifter]

Ropey Pilot,
it does not seem to be asking what you would try and aim for during your turn before correcting slightly as appropriate in order to achieve a rate 1 turn but what you actually need to achieve in order to be sucessful (which is the exact answer).
Good point and put like that I would probably have to agree with you. I can't recall if I had that question in the exams, but I'm certain I would have gone for the rule of thumb answer had I seen it.

On a more general point, I do recall several questions in GenNav where it seemed you were not expected to work out the answer but to deduce the correct answer simply through a process of estimation and elimination. I finished the exam with minutes to spare, I'm sure I would not have completed all the questions if I had been sidetracked.