Just been doing the Annual and having fitted the new Whelen 600 LED wingtip lights I checked the effect of various things on the compass (AHRS, Sandel SG102 + MT102 fluxgate) reading.
The wingtip lights (0.3A steady plus 3A during a flash) have no effect. The landing and taxi lights (1.5A each) do a 0.5-1.0 degree change, which is not surprising since Socata didn’t use a return wire but returned them via the airframe
so some sort of loop area is created. I wonder how bad this was before I used LEDs because the old bulbs drew 4A each…
The pitot heat does another 0.5-1.0 degree. That draws 5A but I don’t know (without checking) how the return wire is done.
This is OK; ATC doesn’t need to know your heading within better than 1-2 degrees, and I suspect most peoples’ compasses or slaved compass systems are off by a lot more… When did you last check yours?
It’s also an interesting Q whether one should check the compass with the landing/taxi lights running; are these supposed to be ON during flight?
I read some accident report (on here somewhere) where a PA28 had some electrical problem which caused the gps to fail over the sea. The pilot then navigated roughly by compass. The compass however was (as should be) calibrated for electrical consumers being online and so was showing quite an error, causing the pilot to fly way off course.
Makes sense to note the compass readings without electric master switch on for such a case. Or bring an ipad…
Peter wrote:
It’s also an interesting Q whether one should check the compass with the landing/taxi lights running; are these supposed to be ON during flight?
This is a subject change, but during the bright part of the day, no overcast or above the overcast, having a steady landing light on reduces the ability to see you as you have less contrast with the background. See this article on how this effect was considered during WWII to allow aircraft to sneak up on Submarines.
If you have ever sat at an airport and watched arriving aircraft land, it is very difficult to determine if the landing lights are on until the aircraft is very close during the bright of the day on a cloudless day. A flashing light that changes the luminance back and forth can be spotted at quite a greater distant because the eye detects the contrast change very effectively.
Peter wrote:
The pitot heat does another 0.5-1.0 degree. That draws 5A but I don’t know (without checking) how the return wire is done.
Ours on the TB10 makes a bigger difference than that, as I think we may have discussed.
Obviously if various electrics (which you may have on or off during flight) have a material effect on the compass then there is no totally satisfactory answer to the question of whether they should be on or off for calibration.
My solution to the problem is to accept that the compass is not much better than a guide, useful to within 5-10 degrees, and if I ever find myself relying completely on the compass for accurate navigation (with no GPS or radio navaids) then I am likely to ask a radar unit to confirm my track.
UK CAA publish useful data on compasses and compensation in CAP562.
So if you look on the rear of the compass deviations card it should state what conditions were used – e.g lights ON, Pitot OFF etc.
Are you talking about the wet standby compass? Haven’t had anything to do with them in years. If I remember rightly we would do a check swing with everything switched on. Lights, all avionics and electrical loads. Swing the aircraft with the engine running and take a reading using a calibrated landing compass at the cardinal points. And adjust out the errors. We would aim to get with +/- 5 deg. max. Any more would usually mean a new compass. We would then write out a deviation card to be placed by the compass to give correct heading to steer by.
The 737 classics I worked on had no requirement to check swing the wet standby compass at all.
I suspect very few people ever look at the wet compass… I was referring to whatever compass system one has, which in many cases (most IFR aircraft) would be a slaved compass system, with a fluxgate whose output is DG/AHRS stabilised.
I am not convinced one should switch on everything. I think one should switch on all in flight typical loads.
Peter wrote:
I suspect very few people ever look at the wet compass…
I check the slaved compass against the wet compass before each flight. And of course a fair amount of the aircraft I fly (also IFR) don’t have a slaved compass system.
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?
Some time back RHS with some old duffer (20K hours plus, most of it heavy piston) into the back end of beyond on some procedural NDB approach all manually flown. As we break cloud I point out that we are 3 degrees off track. He says that’s due to the errors in the compass I check and he was right.
I’ve got a lot of respect for guys like him.
