Fitting a compass

[John Bone]

has anyone got experience of fitting a compass in the flight panel? I have a Foxbat and am not happy with the compass as it is rather small and positioned in front of the right hand seat. I would like to put a new larger compass directly in front of the 'drivers' seat. I thought of installing a vertical card but have heard that they are very temperamental. I have heard good reports of the Silva range. I would be interested to hear from anyone who has installed a compass and how they tackled any adjusting necessary to compensate for metallic interference from the other instruments and surrounding metal.

 

 

[Guest Maj Millard]

There is of course a recognized proceedure used in the industry called 'swinging the compass'. You simply tie a piece of string on it and swing it around your head...no,no,no just joking. Have 'swung' many over the years, and incidentially they use a completely different proceedure in the US, to what we do here to achieve exactly the same result. Do not recall the use of any 'little magnets' to get a correct readout at all, and find that method extremly suss !!. You should however have all electrics switched on, as in flight, as they alone can cause interference by creating magnetic fields.

 

The actual proceedure is a little prolonged and I won't cover it fully here. It is of course carried out periodically, and after all new installations, including fitment of any new instruments or avionics in the panel, and also after fitting a different, or new engine to an aircraft. Generally you go to all four cardinals, noting the error at E/W and N/S. Then you adjust out only HALF the noted error and check again. The aim is to get all points reading within 2 deg of correct, in reference to a 'Master compass' and then this error is noted on the compass error card, for pilot use in flight.

 

Compasses here must be S biased, not N biased, usually indicated by a N or S on the back. Exercise caution when trying to swing compasses in European imported aircraft as they may be fitted with a N biased compass. From experience you'll be lucky to get them within about 15 deg of correct, which of course is not acceptable.

 

Also tried to swing a compass in an Ag plane once without success. It had had several recent encounters with powerlines, and the whole fuselage was magnatized from high voltage going through it. Couldn't get it any closer than 20 deg, so we just gave up. I have been looking at vertical card compasses also, anyone got any for sale. I do think they have an easier to read indication...................................

 

 

[Guest Maj Millard]

Yes interesting, can't say I have ever heard of them or encountered them in my 35 years of aircraft maintenance. I don't have a lot of experience with the vert card type compass, and I wonder if they are type specific to that type ??................

 

 

[Guest Maj Millard]

I'm thinking it may have been a N biased compass ?.....that is an unorthadox way of doing it, and not approved, but may have been the only way to get it somewhat accurate ?....And what of course, if it decided to drop off some where between the black stump and Whoop Whoop ?.............................................................

 

 

[Yenn]

Those balancing balls are a throwback to sailing. If you see photos of the helm of old sailing ships you will probably see the balls each side of the binnacle which houses the compass.

 

I don't know if Silva make an aviation compass, but from my experience their costs are excessive.

 

When swinging, if you have a taildragger, make sure it is in the flying configuration with the tail up.

 

Aviation compasses have the magnets mounted internally to ease the swinging process, asll you do is move a couple of screws.