Gen Navigation feedback question?

[Crosswind Limits]

Has anyone seen this question in their feedback literature and more importantly know how to do it?

An INS/IRS has an azimuth gyro drift of 0.01 deg/hour. After a 12 hour flight at 500 kts groundspeed, what is the accumulated position error due to the drift rate?

a)60nm
b)6nm
c)12nm
d)2nm

The answer in the feedback is given as b)6nm
but having discussed this with someone else we feel that to do the question you need to be given the lat which we don't have!

Any bright ideas much appreciated!

Thanks.

[Scratch One Bandit]

I think this is right, but I hope someone else will confirm this.

OK, you need the average drift, which is 0.06 degree/hour. And we travel 6000 nm in 12 at 500kts. Then it's the 1:60 rule. Dist=Angle x Range over 60 (sorry, it's hard to type out a formula).

So we have Dist off = 0.06 x 6000nm over 60.

Which gives 6nm.

I hope this helps (if it's right! :D )

[ 15 August 2001: Message edited by: Scratch One Bandit ]

[David Webb]

Crosswind Limits,

A good question, but I doubt a JAR one. Azimuth gyro drift rate will give rise to track error computation (unbounded) and groundspeed computation (dependant upon Schuler loop). If taken at simplistic face value I feel that the answer offered by Scratch One Bandit is incorrect. ).01 deg/hour times 12 hours = 0.12 degrees. The rest of SOB computation is correct, but the answer will be 12nm. I do not know where the question and feedback answer came from, but I do know that much which is purported and reported to be exam feedback is often untrue. I have spent the last ten years sifting fact from rumour.

Aye,

David Webb ex (Friday 10th) PPSC

[Cobbler]

DW,

The question is actually pretty much verbatim a genuine JAR question... I had it 6 months ago, and had previously seen it in the feedback.

SOB's answer is correct. The easiest way to understand what's going on is to draw it out in little triangles (which I can't do on this screen!!)

After the first hour, the gyro has drifted 0.01 degrees, giving a track error of (500 / 60 * 0.01)(using the 1 in 60 rule), or 5/60nm.

By the end of the second hour, the drift has increased to 0.02 degrees, i.e. a FURTHER track error of (2 * 5/60)nm, in addition to the 5/60 already incurred. So the cumulative track error is now (1+2) * 5/60.

Similarly over the remaining 10 hours. The final cumulative track error is (1+2+3+...+11+12) * 5/60, or about 6nm.

A truly ridiculous question. Learn the answer, rather than the method, and it'll be an easy mark!!

[Spartacus73]

Cobbler is right, it is a JAA question.

Both methods - SOB's & Cobbler's - will give you the right answer.

The mathematical basis for this is based on Integration or Calculus :eek: . The simple way to think of it, is as a mean error, therefore if you apply the error at the half way point, 0.06 degrees, you will always get the right answer even if they change the numbers!

Good luck!

[Scratch One Bandit]

Hurrah! I was right!

And stop calling me SOB, you gits! :p

[Crosswind Limits]

Cheers SOB and others! :p

[Paul Hickley]

I can confirm that this is indeed a genuine JAR Gen Nav exam question as we have had several students at Oxford report it and their recollections tallied consistently. It first came up about 6 months ago. Scratch One Bandit and Cobbler do have good rough sensible solutions and Spartacus 73 has correctly realised that it a calculus problem (which, in my view, is outside the Learning Objectives). I don't believe that David Webb's assessment is right, although I would like to know which answer the JAA mark as correct. Because the equations can't really be laid out on a ASCII-based thread such as this, the answer is attached on the URL below or type in www.v1rotate.freeuk.com. (http://www.v1rotate.freeuk.com.)

As I say in my answer, this is a question that has absolutely no relevance whatsoever to becoming a practical airline pilot. However, if you're thinking of a job as a systems engineer at Litton or Honeywell......

Paul Hickley
General Nav Subject Specialist
Oxford Aviation Training
www.virotate.freeuk.com (http://www.v1rotate.freeuk.com)

[ 24 August 2001: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]

[Send Clowns]

Agree, Paul, both with your answer to the question of course but more importantly your assesment that it is not only outside the learning objectives but also completely irrelevant to a pilot!

SC
Nav General, SFT

[Paul Hickley]

For those of you who tried to phone me at work about this question, it's probably easier to e-mail me at work on [email protected]

Or, of course, you could post a reply here.

Paul Hickley

[mad_jock]

Area under the graph will always work!!!

MJ