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Compass accuracy

146fixer wrote:

None of the aircraft I work on now except the permit/experimental stuff have a wet compass. Most have a flux valve somewhere for magnetic heading inputs. Do more modern GA aircraft have a wet compass? Like the Cirrus?

According to the ops regs you must have a heading indicator which is a “magnetic compass or equivalent”. This is distinct from the stabilised heading indicator (gyro or EHSI).

The cheapest and simplest way of meeting this requirement is using a compass. Although I suppose a flexgate with remote display would also work.

ESKC (Uppsala/Sundbro), Sweden

My HSI (King something-or-other) IS magnetic – it has a fluxgate out in the wingtip. So presumably that would count, although I do have a wet compass as well. (A bit TOO wet actually – recently it developed a leak which meant that as you accelerated for takeoff it would dump some kind of yucky oil all over the glareshield. Fixed now).

LFMD, France

That is a slaved compass system, with the fluxgate sensor feeding a directional gyro which stabilises the jitter in the fluxgate’s output. It has multiple points of failure, as well as a loss of electrical power, which is why a liquid compass is required.

But I didn’t realise a separate slaved compass system would meet IFR certification requirements. It would need a separate power source e.g. a second alternator. Or, if some AI products are anything to go by, a battery with a specified endurance would meet this requirement.

In practice a $300 compass is the easy option and the vertical card compasses are actually very usable.

Administrator
Shoreham EGKA, United Kingdom

146fixer wrote:

Like the Cirrus?

It does. All the planes I flew have one. From C150 to 777.

always learning
LO__, Austria

I know a guy who used to be in the RAF and they calibrated their slaved compass systems to a fraction of a degree. This was in transport category aircraft. The whole hull was also de-magnetised. So amazing accuracy is possible.

My original KCS55 system – the classic GA slaved compass system – came with the plane in 2002 and never failed. It was always within a degree of the published runway bearing when lining up. It’s actually a very good system which can be picked up on US Ebay very cheaply – probably under $1k for the whole lot. The drawback is the ~10kg of wiring needed

In 2011 I installed the LH SN3500 EHSI while retaining the KMT112 fluxgate. That worked well also. Then in 2013 I installed the SG102 with the MT102 fluxgate and that system has to be calibrated. That was also accurate to within 1 degree on any published runway bearing. There is a story out there that if you change the SG102 to later firmware, it shifts the compass calibration but I have not seen that; if it does it is within 1 degree or so.

The above systems were never touched. They just stayed accurate. In the meantime the liquid compass (the vertical card type)

is “sort of there” on a good day. In flight it is probably within 5 degrees, is substantially more affected by electrical currents than the fluxgate out near the wingtip, and on the ground – for a non-obvious reason – it is much more affected by metal buildings than the fluxgate. One “could” fly with it if one had to, of course… and yes it has been calibrated, but on the ground, and really it should be done airborne.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

In the meantime the liquid compass (the vertical card type) is “sort of there” on a good day.

How much is your vertical card compass affected by turbulence? A standard wet compass becomes almost unusable.

ESKC (Uppsala/Sundbro), Sweden

It seems a lot better.

It is also readable without bending your brain

Vertical card compass thread.

It is no more accurate though, and does require vibration to make it work – usually there is plenty of that in a SEP.

Administrator
Shoreham EGKA, United Kingdom
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