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Magnetic compass dip

Old 14th Jun 2021, 07:39
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Magnetic compass dip

Hello!

I read from light aircraft pilot training manuals (Pooley's Air Pilot Manual on Aircraft Technical) that magnetic compass is counteracted for compass dip effect by offsetting it's CG, and there if the aircraft is operated in the Northern Hemisphere the CG is offset to the south, and vice versa.
However, from my experience in maintenance on a limited number of aircraft, I have only noticed that in the IPC, there no mention that if you operate this aircraft in the Northern hemisphere you need to use this PN for the magnetic compass & vice versa in the Southern hemisphere.

Anyone come across any aircraft IPC that has mention on different magnetic compass PN based on hemisphere of operation?

Cheers!
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Old 14th Jun 2021, 09:17
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I imagine it is a light aircraft thing, of which I have no experience. I know that some types of compasses used for sailing are biased for the area where they are sold and can need modifying if used in areas with significantly differing angles of dip.

On large aircraft, the standby compass is constructed so as the centre of gravity of the rotating assembly with the compass card attached is below the pivot point, so countering dip in either direction – hence, only one part number in the IPC.
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Old 17th Jun 2021, 15:29
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Originally Posted by shumway76 View Post
Anyone come across any aircraft IPC that has mention on different magnetic compass PN based on hemisphere of operation?
No. In general, the IPC lists those parts required for the aircraft type design/type certificate. Since the certification only requires the aircraft to have a magnetic compass regardless of operational areas most OEMs only provide a single compass P/N. On the operational side, if the owner/operator will be operating more toward the extremes in the north or south hemispheres where the dip is more pronounced, they have the option to get their installed compass modified or purchase a new compass from a 3rd party vendor which most offer mag compasses tweaked for the respective hemispheres. But I seem to recall seeing one OEM who offered a factory north/south compass but this was through a SB and not the IPC.

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Old 18th Jun 2021, 01:41
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Originally Posted by QA1 View Post
On large aircraft, the standby compass is constructed so as the centre of gravity of the rotating assembly with the compass card attached is below the pivot point, so countering dip in either direction hence, only one part number in the IPC.
You can see that the CG needs to be below the pivot. That's what gives Northerly turning error and other compass anomalies.
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