Compass card-why bother?

[BigEndBob]

Most training aircraft i have used have had compass cards fitted.
How many instructors bother to teach the student the use of.

How many find the compass is usually way off anyway.(say 5-10 degrees)
How often does this put the poor student/pilot off track.

Forgive my ignorance but how often should the compass be swung?
In my experience it should be every six months.

It seems to be a piece of kit that is often neglected.

[rmcdonal]

Are we talking about the compass, or compass card? I have always found the card showed the compass to be off by a max of 2deg. normally only 1.
Hell I can’t even fly S+L to 1-2deg.

[foxmoth]

How many instructors bother to teach the student the use of.

I certainly teach the use of it during the nav phase, I point out the fact that the error is normally of an order that will not make much difference, but should always be checked.:ok:

[AerocatS2A]

The compass shouldn't be 5-10 degrees out. If it is then it's probably because the pilot has stuck his big metal clipboard on the dash right underneath it, or there are electrics operating that shouldn't be operating if you intend using the compass for primary navigation. The compass card will sometimes have stipulations as to what radios and other equipment should be on or off.

I thought this would be a rant about "what's the point of a compass card which has 1 - 2 degree differences from the compass." If you've tried flying a compass heading you'll know that 1 or 2 degrees is completely irrelevant.

[stevef]

The swing interval is 36 months unless there have been engine changes, lightning strikes or certain electrical modifications. Like Aerocat says, badly-placed magnetic items or the field from electrical circuits will have an effect on the readings.

[IO540]

The compass correction card should be no more than 1-2 degrees out, and only a robot can hold a heading to that accuracy anyway :)

My autopilot can't, that's for sure.

[Sleeve Wing]

I certainly teach the use of it during the nav phase, I point out the fact that the error is normally of an order that will not make much difference, but should always be checked.:ok:
Yep, teach it so that the student realises it's part of the equation but then (and for once I agree with IO540 !) let's forget it in the bigger scheme of things.
If the compass has been swung and adjusted properly, the error is so small relative to all the other possible variations that very few people will fly that accurately - except maybe on a good autopilot.
Track-crawling and scud-running forever !
Rgds, Sleeve.
P.S. Isn't it called the DEVIATION CARD ?

[BigEndBob]

I find that at least 1 in 3 club aircraft have compasses that show say for example 360 on 360, 085 on 090, 185 on 180 and 260 on 270.

So that every time the student turns onto a new hdg. the compass needs re-setting.

I always have my students cross check the compass/DI with runway hdg. this is when the errors usually show up.

(Yes the runway mag. hdg is taken into account, pitot on/off, radios on/off as req. and who knows whats buried under the runway).

[Whopity]

Don't forget the Compass is full of alcohol and therefore unfit to fly!

[rjt194]

We do, of course, take the compass and deviation card from our erstwhile companions, the mariners.

They have, for the past century and a bit, had a method for doing away with that pesky bit of cardboard. Where navigational accuracy is paramount they fit a patented Kelvin Binnacle and compass. (http://cgi.ebay.co.uk/GIANT-1-5-ft-KELVIN-SPHERES-BINNACLE-BRIDGE-COMPASS-WOW_W0QQitemZ7405841257QQcategoryZ66638QQcmdZViewItem) Such an item would take pride of place in the light aircraft of any discerning gentleman (or lady).

[BigEndBob]

Was this item invented before or after Brunel built the first iron ship?
Or did iron clads come before.

[rjt194]

My understanding is that it was invented as a result of the ironclads. Before that ships were all wood (albeit with iron nails and guns), so the effect on the compass was minimal - or could be dealt with easily using a deviation card :}

[Whopity]

I wonder what Francis Chichester would have thought of the suggestion that the deviation card is redundant. Clearly nobody operates light aircraft for long distances over water any more. Knowing what the deviation is, is more to do with working out where you are, than being able to steer it. The average PPL could probably survive quite well without a compass, especially if they have a GPS on board.

At the end of the day though if we did not calculate the deviation we would not know how accurate the compass was! Having calculated it, it might as well be written down so that all can see how accurate the compass is.

[P.Pilcher]

On the back of the compass card is the signature of the person who adjusted the compass and the date. It's nice to know that it has been done. A number of years ago an aeroplane at the school where I worked suffered a catastrophic engine failure and the instructor just managed to get it back on the runway with oil spewing everywhere. Our maintainance engineer (who did not work on the field) turned up in his van a few hours later with a box of tools and a new engine in the back. This he proceeded to fit and once the work was completed I was asked to do an air test before the compass was re-swung. The engine behaved perfectly, but as I lined up I noted a 30 degree error in the compass reading. After the test, half an hour on the airfield's compass circle and the compass was once again reading in the region of 1 degree from the magnetic heading. This was in the days before reliable GPS units and I certainly would not have liked to take an aircraft on a cross country with such an error.

P.P.

[BigEndBob]

Why bother with compass card?....because half the aircraft i have flown have some compass headings 5-10 degrees off.

For instance fly west set di/compass, fly east and its off several degrees.
Or compass/di perfectly aligned just before turn point, then on checking after turn 5 degrees off.

So what do we do ground aircraft until it is reswung at £xxx cost..or do clubs just put up. I have known aircraft go through CofA's and still have major/same errors.

At least basic training aircraft compasses should be test swung every six months with all the bashing they get.
With radios on and pitot OFF.

[fireflybob]

As an aside it is also interesting to note that most modern airliners only have one magnetic compass - ie the one in the middle between the forward windows.

The displayed heading on the pilot's display is derived from true heading (from the aircraft inertial system - accurate to tenth of a degree or better) and the addition of variation from the nav system's computed position and a database of world magnetic variation. The experts tell us (and I think they are correct) that this gives a consistently more accurate magnetic heading than the old slaved compass system.

Whilst the likelihood of a modern airliner having to fly on the magnetic compass alone for heading information is fairly remote, a magnetic compass complete which current deviation card is still a mandatory fit!

Finally, I think you will find that residual deviation on a direct reading compass must be three degrees (not the pop group or glidepath angle!) or less after the compass has been swung.

[Whopity]

BEB

Try switching on the Pitot Heat, that may bring it back to the swung values.

[BigEndBob]

Switching on pitot heat, landing lights or any other high energy load doesn't usualy effect the compass much...couple of degrees.
Its just that compasses seem to be neglected.

And some are swung with the heat off and some with the heat on.

[Oktas8]

Does a light aircraft pilot need a deviation card?

Do we need to swing the compass regularly?

Do we need to know that the compass, although quite accurate, is not perfect?

Three very different questions! My vote is no, yes, yes respectively.

I agree that it's awkward trying to work out whether the pitot heat should be on! So why not scrap the conventional deviation card, and replace it with a similar card with two things written on it - the date of the last compass swing, and whether pitot heat should be on or off.

Cheers,
O8

[homeguard]

Compasses are swung with the engine running and all electrics switched on!

Further errors will be experienced when all of the electrics are not swtched on.

[foxmoth]

Switching on pitot heat, landing lights or any other high energy load doesn't usualy effect the compass much...couple of degrees.

This will actually depend upon the aircraft and the heading you are on - with some aircraft on certain headings this can be considerably more than "...couple of degrees"!!:sad:

[Oktas8]

Compasses are swung with the engine running and all electrics switched on!

Maybe in your part of the world...