Gettting head around Compass turns?!!?

[hightower1986]

Hi ive just Compass turns Solo and Dual and can not seem to be able work out how much to overshoot or undershoot without my little cheat card, not sure of the Technical name for it but the small card that shows 360 and what heading you should roll out on, Im in the Southern hemisphere and understand the whole ONUS thing but its the actual headings im struggling with, any ideas or tips from people on how to maker it easier, without just doing the whole timed turn thing! Thanks

[ExSp33db1rd]

I sympathise.

If you are using one of those little Boy Scout compasses hanging from the windscreen pillar, you can't read it to the nearest 5 degrees, anyway !

Make a stab at the ONUS principle, roll out and wait to see what heading you're on, then adjust. Might waste 30 secs. How far are you going to travel off course in that time ? and anyway, you're maintaining VFR, aren't you, look out of the window, decide where you are, where you want to go - and go. The little wobbling compass can only be a guide at the best of times, don't strive for theoretical accuracy and lose the Big Picture as a result.

Unfortunately I fear this doesn't help you strive after a rocket science accuracy demanded by your over-zealous instructor. If he / she wants that unnecessary accuracy get him / her to demonstrate how they do it. That's what you're paying them for.

Course, if you're trying to do an IFR let-down in a snowstorm into an airfield surrounded by mountains, it's a different story - but then you'd have suitable equipment. Horses for Courses.

Don't try and do a maths exam in a rattling steel cabinet. Commonsense, big numbers, KISS ( Keep It Simple, Stupid )

Probably not the answer you wanted to hear - but my opinion anyway !

Good luck.

[jxk]

In N hemisphere it's N early S late (NESL). Is it the reverse in S hemisphere?

[A and C]

The answer to a DI failure is not to try to "compass turn" the anticipation required and factors that effect the compass make in a very unsatisfactory way to turn on to a new heading.

The only way to make an accurate turn without a DI is to time a rate one turn at 3 degrees a second the turn indicator is set up for an accurate turn and the VOR indicator makes a very good time/direction calculator so you dont have to use too much brain power when doing the sums.

The only reason for teaching compass turns was to enable pilots to fly aircraft without electrical/vac systems, most pilots who fly these aircraft now use a GPS and fly a GPS track.

[Whopity]

Would you fly without a watch?
Turn on the Clock, as A&C says the only accurate way is to time it.

[jxk]

However if you do use a watch to time your turn you must make it a rate one turn.
(Airspeed / 10) + 7 is a rough way to calculate angle of bank required.

I reckon using the compass if you turn 10 deg EARLY before N and 10 LATE after S it's not far out.

[Sciolistes]

Forget about putting all that ONUS/UNOS and ANDS, SAND stuff into practice.

If you are in VMC the easiest thing to do is simply estimate your angle of divergence, find a cloud or feature at that point and turn the aircraft to it and then check the compass. You will be awfully surprised how accurate you can get it.

If in IMC, then it has to be timed turns as mentioned above.

[CapCon]

The above posts are all good points but compass turns are part of the NZ PPL syllabus so you've got to know it. I didn't even bother to work it out in my head. Hopefully you have a VOR instrument in your aircraft? If so then the easiest thing to do is set that so the heading reads North, you then have a visual guide.

Remember ONUS (Overshoot North Undershoot South) This means that if you're turning onto a heading anywhere between 270 through 000 to 090 you will need to overshoot/go past that heading and the opposite if turning onto a heading between 090 through 180 to 270.

Next, remember that E/W headings will have no error and N/S have the max error of 30degrees. Breakdown the 30 degree error between E/W and N/S but only every 10 degrees of error. For example between North and East: 000 = 30 degrees, 030 = 20 degrees, 060 = 10 degrees and 090 = 0 degrees

Example: If I was on a heading of 090 and was asked to turn onto a heading of 030. First I would look at my VOR instrument and say I would need to make a left turn. I could then see that there would be a 20 degree error turning onto 030, which means I would have to overshoot by 20 degrees. Therefore conduct a rate 1 turn to the left rolling out when the compass reads 010.

It would be much easier to explain with a picture, hope this helps. If not speak to your instructor.

:ok:

CapCon

[ExSp33db1rd]

In N hemisphere it's N early S late (NESL). Is it the reverse in S hemisphere?


Yes.

Overturn on North and Under turn on South = ONUS.

( Low pressure systems go the other way, too. ANTI-cyclones go ANTI - clockwise in the Southern Hemisphere, same with the bath water, tho; I can never remember which way that it goes in the other half of the World when I remember to watch, is it low pressure or high pressure as it flows out ? Guess high pressure, as it will presumably flow from High to Low ?)

The suggestion to stop the Compass turn around 10 deg before a Southerly heading, or go 10 degrees past a Northerly heading, and stop the turn exactly on the heading if vaguely pointing East or West works near enough for all practical purposes. Forget the Maths as I said before, the compass doesn't read accurately enough to make it worth all the effort, and remember, with those bowl type compasses the trick is to pull the number you want into the lubber line, most students turn toward the number, e.g. if you are steering 330 and want to turn on to 360, the number 360 will be on the left side of the lubber line, so turn away from it, ie. to the right, pull the number into the middle. Most don't. Much more important to turn towards the way you want to go rather than worry about being a degree or two off heading when you stop.