The Latitude Nut

[Jim Logajan]

Went up to do some pattern work and to check the drift or precession of the club plane's Heading Indicator (HI). After engine start I set the HI to the magnetic compass value, It was about ten minutes later that I was at the end of the runway ready to take off. The HI differed from the compass by 30°. Oh oh, I thought, that's not good. But I went ahead with my test plan and once aligned with the center line of the runway I rotated the HI to 0° so that it would be easier to read off the progression of the deviation on each circuit of the pattern. Each circuit of the pattern averaged about 6.5 minutes. Every time I landed and had the plane aligned with runway I checked the HI.

After 45 minutes of flying I saw essentially no drift. If there was any it was under a couple degrees. Normally that would indicate that once up to speed the gyro was maintaining excellent rigidity in space relative to the distant stars. But the earth should have rotated such that the HI should have drifted 0.75 hrs * (15°/hr) * sin(44° latitude), or approximately 8°. I couldn't understand why I didn't see the drift so went researching. I discovered a heading gyro typically has something called a latitude nut (or latitude rider nut) that causes a precession of the gyro that matches the rotation of the earth at a given latitude but reversed sign. My research found these references:

https://www.pilot18.com/wp-content/uploads/2017/06/2-INS-Gyro-Instruments.pdf
https://en.wikipedia.org/wiki/Heading_indicator

It is possible I was told this at some point, but if so it didn't register. I have learned something about gyroscopic heading indicators today that was new to me and just thought I'd pass it on. Probably known to commercial pilots, but nice to know for this private pilot.

 

[PeterNSteinmetz]

I don’t think I have ever heard of this before. And the nut should be set for a specific latitude. I wonder if the one in my Cardinal was changed when I last had it worked on.

 

[Jim Logajan]

I don’t think I have ever heard of this before. And the nut should be set for a specific latitude. I wonder if the one in my Cardinal was changed when I last had it worked on.

My guess is that it would be set to some intermediate latitude like 45° or perhaps a bit south of that to minimize the drift for most areas of the U.S.

At about 14 minutes into this video the latitude nut is discussed. The whole video is informative about other interesting details on the operation of a directional gyro.

 

[rotorwrench]

My guess is that it would be set to some intermediate latitude like 45° or perhaps a bit south of that to minimize the drift for most areas of the U.S.

FYI: in general, the latitude wander is mainly set for northern or southern hemisphere operations. Each OEM has their own process for each hemisphere. That said, some OEMs do offer specific settings if you plan to operate in one latitude region like a polar region. So unless you plan to fly south of the equator you wont see much difference unless you decide to fly to Alaska and points north.