DGs and Precession- How Much/ How Often?

[Owad1971]

As I posted earler I had an experience last night during my dual XC- I noticed that I had to constantly check & reset my DG to my compass. It felt like I was doing it every 40-50 seconds...Is this normal? I already had a bad compass and bad VOR (s) in one of the rentals during a daytime XC....

Is this a sign of mantenance issues? I havn't found one Youtube video where the subject checks and recalibrates the DG even once. Are you people just editing that part out of the video

I don't say it enough and I will take the opportunity and to do so: Thank you All for sharing your insights and feedback!!!! This forum has helped me tremendously throughout my journey!! As I gain expierence and certifications I look forward to my chance to give back!

 

[BiffJ]

The amount and time it takes to change will depend on the instrument and probably on your maneuvers as well. Sticky or worn bearings will cause things to move quicker. Cold or heat can affect lubricant in the system which will also change things. So basically the answer to your question is as "much as it does and as often as it does it". In my opinion your DG should be looked at. A few minutes is one thing but less than that says the bearings are bad or something else is wrong.

Frank

 

[MAKG1]

That's excessive. Squawk it and mark the DG inop.

If you discover that in the air, fly the mag compass. You have to be aware of errors while turning or accelerating; they are quite substantial at mid-latitudes.

Check your POH to see if the DG is required for certification; I doubt it is, and it's not required by 14 CFR 91.205. The aircraft is still airworthy for day VFR as long as it is properly placarded. No IFR, but that's not what you're doing, right?

 

[eman1200]

for my first dual xc we learned that the DG wasn't working. flew the compass, which was a good learning experience. but in general, I check the DG vs compass after leveling off, then 'every now and then'.

 

[dmspilot]

Mere rotation of the earth will cause an apparent drift in the heading indicator of 15 degrees per hour before even factoring in precession. Standard practice is to check it about every 15 minutes.

40-50 seconds isn't normal. Are you sure you weren't just seeing magnetic compass errors? Try lining up on the runway for take off, set the DG to runway heading, fly a trip around the pattern, and check how far off it is after you land.

 

[hindsight2020]

Welcome to the non-AHRS club. Get used to it. Mech Gyros, the bane of my existence.

To your question. Yes, that DG is shot. Probably mechanical friction wear has rendered it excessively inaccurate (they never are all that accurate in the first place). It's overhaul/exchange time...again.

 

[Owad1971]

Welcome to the non-AHRS club. Get used to it. Mech Gyros, the bane of my existence.

To your question. Yes, that DG is shot. Probably mechanical friction wear has rendered it excessively inaccurate (they never are all that accurate in the first place). It's overhaul/exchange time...again.

Thinking back my 40-50 seconds was a little exaggeration it was like every 2 minutes.....

 

[Clark1961]

A few minutes is one thing but less than that says the bearings are bad or something else is wrong.

I had a bad compass in the 'kota. Thought it was the DG that had a problem...finally checked the compass against a painted rose and tried to adjust...ended up with a new compass. More than just a little frustrating.

 

[MAKG1]

Mere rotation of the earth will cause an apparent drift in the heading indicator of 15 degrees per hour before even factoring in precession. Standard practice is to check it about every 15 minutes.

40-50 seconds isn't normal. Are you sure you weren't just seeing magnetic compass errors? Try lining up on the runway for take off, set the DG to runway heading, fly a trip around the pattern, and check how far off it is after you land.

15 deg/hour from earth rotation is worst case. You'll almost always see less than that. It's a function of where the gyro axis is pointed. But several deg/hour is to be expected.

You'll also get some precession just from moving along the surface of the earth. On an airliner, this is only slightly less than earth rotation. It's marginal on a spam can. It adds when flying eastbound and subtracts westbound.

 

[poadeleted20]

The book answer for instrument flying is that if it drifts more than 3 degrees in 15 minutes, it's past its sell-by date and needs overhaul or replacement.

 

[Owad1971]

The book answer for instrument flying is that if it drifts more than 3 degrees in 15 minutes, it's past its sell-by date and needs overhaul or replacement.

Thanks...I am going to squawk this. I pray that when I get to my checkride that I don't get a broken plane.