Two nav exam questions

[CharlieDeltaUK]

I'm after some help sorting out two questions arising from my nav exam prep:

1. The first question is about turning errors on the magnetic compass. Text book says they are greatest when turning through North or South and that "when heading towards the nearer magnetic pole, the magnetic compaiss is sluggish and will under-indicate the amount of turn". That all made sense to me. I was then doing one of the self-test questions at the back of the same book and reached the following question:

When turning through South in the UK, the pilot should
a) undershoot the desired heading
b) overshoot the desired heading
c) stop the turn immediately when the compass indicates the desired heading

So, I figured that South must be the nearest magnetic pole, and thus the compass would be sluggish and under-indicating, and so you have to undershootshoot the desired heading. Answer A. But, the actual answer is B. Where did I go wrong? The same question, but turning through North, does indeed say you should undershoot. But, the book suggests that turns to wards the nearest pole make the compass sluggish, whether that be north or south.

If I may ask another question on a different nav topic, this time about calculating density altitude. It's about the formula which says you add 120feet per 1degree variation from ISA temperature. There was an example question about an aircraft on an airfield elevation 825feet, QNH 999 and OAT of 28degrees. The density altitude works out as 3,105feet. The temp difference from ISA is 12.5 degrees, so you multiply 12.5 by 120 feet. The bit I don't understand is this: why is the density altitude higher? Surely, if the actual OAT is higher than ISA, the equivalent altitude for the same air density would be lower rather than higher.

Thanks in advance for some learned (even not so learned) comments

[24Carrot]

In the Northern hemisphere, the North pole is closest to you geographically. That's what they mean by 'the nearest pole'.

Remember UNOS:
Undershoot North, Overshoot South
(in the Northen Hemisphere).

[BackPacker]

why is the density altitude higher? Surely, if the actual OAT is higher than ISA, the equivalent altitude for the same air density would be lower rather than higher.

Actually, no. Remember that air density/pressure have an inverse relationship with altitude: The higher you go, the lower the air density/pressure.

If the OAT is higher than ISA, the air at ground level is thinner. Under ISA conditions, this thin air could normally be found a few hundred feet up, but is now found at ground level. In other words: the equivalent altitude for the same air density (the density altitude) is *higher*.

Density altitude, in essence, is the altitude where, under ISA conditions, the same air density can be found as under current conditions on the ground.

[CharlieDeltaUK]

Two clear replies in almost as many minutes. Thanks both.:ok:

[IO540]

When turning through South in the UK, the pilot should
a) undershoot the desired heading
b) overshoot the desired heading
c) stop the turn immediately when the compass indicates the desired heading

I have to confess that I found the above type of topic so hard to memorise that I simply ignored those questions and took a chance on it.

In the exam, you might get one question on this.

I successfully passed the CAA PPL, the FAA PPL, the FAA CPL, the FAA IR, and the JAA IR (aircraft tech) exams, ignoring this practically totally useless topic completely.

If you need to do turns on the compass, either do them gently, or do timed turns which are suprisingly accurate (3 degs per second).

[AndoniP]

ignoring this practically totally useless topic completely

when i filled in my mass & balance sheet for my skills test, the examiner accepted the aerodrome elevation i entered, however he did mention that i should have put in the density altitude of the aerodrome instead, as it was more relevant.

he then went on to tell me the story of an airline captain that took 3 friends up at Elstree, didn't get it off the ground properly and then ended up in the trees at the end of rwy08. if he'd considered the density altitude on that particular day he would have known he needed more runway.

[BackPacker]

Hmmm. Not sure I agree with that examiner.

It kind-of depends on where you filled in the aerodrome elevation. If it's filled in somewhere near the circuit height/threshold height parameters, then you use whatever is in the AIP, obviously. After all, you will want your altimeter to read 0 or airfield/threshold elevation when you land (having set QFE or QNH).

But when you enter the altitude as part of your take-off and landing run distance calculations, yes, you will need to work with density altitude somehow. However, the calculation charts I have seen so far all start working with pressure altitude, and the temperature compensation is worked into the calculation if you just follow the POH instructions. So with those calculation charts there's no need to *separately* calculate density altitude - the charts do that for you implicitly.