Realigning the DI with the magnetic compass.

[ACARS]

Total hrs of 13 in a C172. For the 1st time the other day I was asked to realign the DI with the magnetic compass during the flight. Which made me think. I don't remember reading it anywhere. How often should one realign with the magnetic compass?

[The Rookie]

We were taught to do it every time you do a FREDA check (at every waypoint on your P/LOG) and I would do it every time you turn through more than 45 degrees just to be sure.

Not as critical in VFR flight in an area you know but absolutely essential when flying IFR.

The professionals all drive aircraft that have the DI slaved to the compass so don't have to worry as much as us PPL's.

The Rookie.

[wes]

my instructor makes me check it every fifteen minutes while VFR, it's probably good to glance at it often to check it against the mag compass...

[Luftwaffle]

Recheck it before using it to navigate, as The Rookie said. That includes before using an ADF and before starting your approach, even VFR. It helps you to find the correct runway at an unfamiliar airport, or to find the runway at all if you descend through an area of poor visibility.

Don't forget that the compass is inaccurate during turns and during and airspeed and pitch changes, so be sure it's stablized before resetting the DG.

I too was taught "15 minutes," but the time the DG takes to drift noticeably depends on which way you're going, what exercises you are doing, and on the condition of the gyros and suction pump in your aircraft.

[Grandad Flyer]

I read something recently, might have been in GASIL which said that there was a strong link between PPLs/ students not checking their DI against the compass and them getting lost and calling D+D.
So its definitely worth bringing this subject up.
It is extremely important to check it as often as you can. I was once told that if you were flying IFR and you weren't doing anything then something was up!
There is always something you can do.
But VFR, if you are using that same DI as your main reference then if its 20 degrees off and you fly on the heading for half an hour without checking it against the compass, just think how far off course you are going to get. It has been the cause of many light aircraft flying into places like Stansted and Heathrow zones.
So yes, check it regularly. Its worth asking your instructor to explain more about it. Don't ever think you have a question that is stupid, the only question that is stupid is the one you didn't ask.

[mik]

The Air Pilot's Manual vol. 4 "The Aeroplane - technical" by Trevor Thom, pub. Airlife, has a chapter on gyroscopic instruments, in which the whys and hows of aligning the DI with the magnetic compass is described.

Mik

[twistedenginestarter]

For most purposes you'd be better off using the GPS which is steadier and more accurate. You can't of course use it whilst you are doing your PPL. So get that over and done with as quickly as possible.

[twistedenginestarter]

I perhaps need to expand on that last posting.

You learn to fly (right up to ATPL) using Dead Reckoning a method of navigation using a compass, timepiece and wind information, and pilotage a method of navigation based on topographical maps. These are selected (perhaps) because they are the last resort methods that you may have to revert to one day. In every day life you really have no need to do either in the early 20th century.

You never need to know in which direction you are pointing rather than in which direction you are moving. The only slightly relevant occasion is when you are instructed by ATC "Heading 240" but even then if you don't guess the correct drift they'll soon notice and say "Heading 250". Pilotage only comes in to play if you ever have to do a non vectored approach so you have to work out the layout of the airfield.

The DI in a little plane is doing two things. The obvious thing is to tell where you are pointing but I've just explained you don't really need that. The other thing is to give you a reference to fly to.

You could just have a DI with one line on it. Set your heading with the GPS then align the line with a reference mark and hold the DI against the reference. It doesn't matter whether it is 065 degrees or 337 degrees.

That's just a bit of philosophy. The DIs in small planes can precess quite a bit so you should keep an eye on them every 10 mins. Don't keep adjusting the DI though as the errors in the compass are quite difficult to eliminate so you may be introducing as much error as the DI actually suffers from.

Of course watch for the obvious. Anything affecting the compass (eg golf clubs) and you are lost.

[Blue Hauler]

Most gyroscopes possess two properties – rigidity in space and precession.
Unlike the compass, which seeks out the north magnetic pole, the gyro simply is aligned by the pilot to a point in space. If we align the gyro against the compass, at that precise moment the gyro will indicate the direction of the north magnetic pole. But in fact we have aligned the horizontal axis of the gyro with a point in space, a ‘star’ perhaps.
Imagine the aircraft is located at the North Pole. As the earth revolves about its axis the gyro will drift (apparent precession) at the same rate as the rotation of the earth (360 degrees in 24 hours). Our ‘star’ which is located perpendicular to the axis of the earth is stationary but the earth is revolving. To us it appears that the gyro is drifting.
If on the other hand the aircraft is located at the equator, then our alignment to the ‘star’ is parallel to the rotational axis of the earth and drift or apparent precession will not be detected.
Apparent precession at the poles of 360 degrees per 24 hours equates to 15 degrees per hour at the poles and zero at the equator. At intermediate latitudes, apparent precession will vary, the rule being:
Apparent precession = 15 deg Sine Latitude per hour, and
Decreases in the Northern Hemisphere
Increases in the Southern Hemisphere.
Unfortunately most gyros also experience ‘real precession’ caused by imperfections in the mechanical components of the instrument. Real precession will of course modify its alignment capability.
Fifteen-minute checks and appropriate adjustment by the pilot will keep simple directional gyros aligned.


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Blue Hauler

[gaunty]

All good stuff, but if you understand how the instrument actually works you will also understand that when you "set" the card (realign the gyro) simply screwing the knob around to the "correct" position and letting go is not in itself enough.

You must hold (or push) the knob in the new position for sufficient time to allow the gyro to settle properly on its gimbals, resolve any acceleration errors and find its new position in relative space. You will find that it will be more accurate and the "precession" rate will be less. ;)

[NIMBUS]

twistedenginestarter,
SHAME....! :)
There is only one way to navigate-DEAD RECKONING! Map+Compass+watch!
Everything else is just slighty refined D.R!

PS. 'Course, I never fly without my GPS, but I always feel guilty afterwards!