The International Cessna^® 170 Association

My compass swings about 10 degrees when I turn on the landing lights. Is this expected? The strobe light also "adjusts" the compass, but that's on all the time so it's not so much of a problem.

My compass is mounted on top of the glare shield. Landing lights and strobe are grounded to the wing structure, and I couldn't guess by what routes their current flows back to the battery/alternator, and how those currents might affect the compass. Is this normal, and you just live with it?

John

All compass swings should be with all exterior lights on and aircraft in flight attitude. Radios on. Engine running. I put my tailwheel on a rolling work bench (tied on) and pull it around the compass rose with a trusted person(my wife) holding the brakes. Takes 3 people this way but works. 3rd person is for chocking the tires.

phantomphixer wrote:All compass swings should be with all exterior lights on and aircraft in flight attitude. Radios on. Engine running. I put my tailwheel on a rolling work bench (tied on) and pull it around the compass rose with a trusted person(my wife) holding the brakes. Takes 3 people this way but works. 3rd person is for chocking the tires.

This gives corrections for flying with all exterior lights on, and I wouldn't have the right corrections for the way I usually fly, which is with the landing lights off.
I could swing the compass twice and put two cards on the panel, one for lights off, and the other for lights on. I'd prefer first to look for ways to remove the extra lights-on deviation, if that can be done.

John

I don't want to change your question jrenwick (waiting to hear suggestions) but this reminds me of a question that a cub owner wanted me to ask the forum. He has problems with metal affecting his compass and is wanting to know if there is some kind of remote compass that's mounted out on the wing. Seems to me that this would be ideal at least for rag wing planes? Is there such a thing?

As Phantomphixer said, compass swings should be done with everything on and the engine running. I do mine in a 3-pt attitude and it's worked fine.

I'd be curious about your landing light/strobe switches and/or wiring routing. I have not experienced any differences with the lights on or off. I'd get up under the panel and have a good look at how things are routed relative to the compass location. Also, check the condition of the wiring (old, cracked insulation, automotive grade, etc.). Sometimes a real small thing can make a big headache.

My stupid little el-cheapo timer, which uses a simple watch battery, would generate a 10 degree swing in my compass when it got within 12 inches of it - and it wasn't even on!! You might be surprised at how sensitive a compass can be.

My $.02 - you get what you pay for

doug8082a wrote:I'd be curious about your landing light/strobe switches and/or wiring routing. I have not experienced any differences with the lights on or off. I'd get up under the panel and have a good look at how things are routed relative to the compass location. Also, check the condition of the wiring (old, cracked insulation, automotive grade, etc.). Sometimes a real small thing can make a big headache.

Thanks -- good to know this is not an expected phenomenon.
The wiring to the landing lights looks original, although this 170 has an alternator, so the wiring between the firewall and the bus is not original. I'll recheck the routing of the fat wires, and make sure they're all down low behind the panel. I'm still suspicious of the grounding, because that's a lot of current taking an unknown path through the cabin framwork.

Thanks,

John

John,
I would not change the card if the main problem exist when turning on the landing light. At that time you should have the runway in sight so the compass is just along for the ride, right. Randal

I don't use the landing/taxi lights exclusively for landing. I first noticed this on departure from an airport -- I had the lights on to help incoming traffic see me, and while trying to establish my course I noticed my carefully-set DG wasn't agreeing with the compass. Turned off the lights, and voila! Back on the correct heading. I can live with this if it's a normal thing, but if it indicates that something isn't right I'd like to fix the problem.

John

It might be a good idea to check your wiring and make sure there is nothing else powered by that circuit that could cause this such as a gps powered from your cigarette lighter. Cigarette lighters are often powered from your landing light circuit if it isn't disconnected. You may want to check your compass backlighting also. the wires should be twisted to prevent affecting the compass. your problem is most likely your landing light wiring as previously mentioned. Check the routing and make sure the wires are not wrapped in a coil anywhere. If all else fails you could sheild the landing light wiring to prevent it from causing a problem with the compass.

AR Dave, My first C-170B had a electric remote compass, very accurate, and I liked it a lot. Seen some advertised but not lately.

I agree with FredM. I once installed a radio set in a 172 and amazingly every thing worked w/o any smoke escaping from the insulation that contains it. The only problem we discovered was when transmitting on the #1 com the alternator dropped off line every time. After talking to a radio tech.. I tywrapped a 1/2" single wire loop flat to the wire bundle that was located by the mike jack. End of problem. So any loops in the wire bundle can generate electrical fields that can do funny things other places.Brian

Large amperage circuits are common causes of compass swings. Airplanes that have such circuits usually include a placard about the matter, but not a correction card regarding it. (Examples include the King Air series with a warning placard about Air Conditioning and Cessna 425 series that have warnings about windshield heat causing compass errors of more than 10-degrees. I've seen some compasses have 180-degree errors in flight when some systems are operated.)

The standard procedure for swinging a compass does not include the landing lights being on. Usually compass cards have a Radios ON and Radios OFF corrections, but otherwise nothing other than warning placards about systems which may cause deviations.
The C-170 is also famous for flight control/yoke position errors. If you swing your compass, place the control wheel in the median elevator position (where it most commonly resides in flight.)
As for your landing light circuit, ...you may find that re-routing your light circuit wiring may help reduce the compass errors. Generally, D.C. current is the most onerous progenerator of magnetic lines of flux. A straight line will throw the flux off at right angles to the conductor. (That's why twisting the compass illumination wiring like safety wire will cancel out the flux of that circuit.) There are several "tricks" that can help the situation. One can actually "coil" or "loop" the wiring back upon itself to produce an anomalous flux, and One can wrap the line in lead sheeting. I once had to wrap a tachometer on an Aeronca in lead sheet to keep it from influencing the compass. Iron or ferrite "donuts" can be used to decrease magnetic flux emissions also. (Electronic supply houses will be familiar with that technique, but typically it's used on low-amperage circuits. I have no idea what size would be necessary in a landing light circuit, but would guess it would likely require one of about 2" diameter and about 2 feet of additional wire.)
Fabric and Steel Tube aircraft (like Cubs, Aeroncas, Stinsons, etc.) can have their entire fuselages become magnetized to the point of inducing some pretty wild compass readings. The fix sometimes involves de-gaussing the entire airframe. http://www.sacskyranch.com/ has a degaussing coil (for rent) and instructions for it's use for such aircraft. They also have an article about it at their website.
Hope this helped.

Thanks, George. I appreciate this, although it's mostly what I already knew. I'm still concerned about the field induced by the ground path, which isn't wires -- it's the metal structure of the airplane.

Mostly I wanted to know if it's common in 170s to find that turning on the landing light (or turning off the strobe light) affects the compass. I heard one person say he saw no effect, so that's an important data point for me.

Thanks!

John

I'm pretty certain the "return" path (ground) is not the cause of the deviation. It's most likely the wiring beneath/behind your panel and/or along the front face of your firewall.
My landing lights have no noticeable effect on my compass. The wiring runs primarily along the bottom of the panel, along/above the pilot's knees, so it's fairly removed from the compass.

John, you could also try moving your compass away from the panel, on the windshield. They have mounts for both the center strip and the "single piece" windshield.