Cessna 172 compass drift 30 deg in 10 mins

[RRTrentSymphony]

After many years in jets, I've finally had time for a reunion with the 172, the bird most of us started in. I line up the compass with the glareshield whiskey compass on my runup on the ramp and on taxi out, I see it drifting but ignored it for now. Now before T/O I line up the compass with the whisky compass again, the runway HDG was 210, the whisky said 210, the compass was around 240 so it set it back to 210 and took off. Leaving controlled airspace, I see the compass drifting again and mismatching again. To makes things worse, the attitude gyro also seemed particulary drunk and drifted around (at one point i was making a constant 300 feet/min climb straight ahead with the gyro showing 6 deg pitch down and 10 deg bank... i was only at 90 knots!).

Any ideas to why the compass was drifting so much, it might be my bad memory but I'm sure it shouldn't drift that fast. There was no mentioning of it in the aircraft log, just a note saying don't trust attitude gyro too much its a bit drunk (yes the mech wrote that!), flight into IFR prohibited until gyro replaced. That explains the gyro but the compass is still a mystery. I have all electronic devices turned off (except the aircraft lights, avionics and radios obviously). There were lots of telecom attenas in the area though, might this affect the compass?

P.S. to make matters worse that flight, the vis went down to 4000 with haze, i was 1500 AGL, gonna request vectors but the big jets and military guys keep buzzing on the tower and approach frequencies that i don't get a chance to squawk a word. Saved 7700 for later (luckily did not have to use it), and used VOR (yes! luckily it worked flawlessly), to get back to visual range of airport. Got told to orbit as the big jets approach and takeoff... tons of birds in my orbiting area, most of em vulture sized... like dogfighting with em, looking out and maneuvering. Thought the tower forgot me, then they say for me to make a dash for the runway in the traffic gap. (amazing that i only lined up with the runway at the point I get the miniums callout in the jets i fly in) Luckily on the ground in one piece. Should I write up about the compass? Seems the pilots yesterday had no issue with it...

[No Fly Zone]

"Should I write it up?" Hell Yes! Does not matter what others have experienced; YOU had a problem with it and a qualified tech needs to have a serious look. Flying without a reliable compass is really dumb.:=

[TopBunk]

Q. Would you accept those tolerances in your jet?

Why should they be acceptable in a C172/PA28 etc?

[BOAC]

Check the engine vacuum pump?

[Desert185]

Aggravating at the least. Replace it if the vacuum pump and system is good. Might be loose plumbing.

On a related note. My whiskey compass was acting up, so did a bit of research and found the SIRS compass. EXCELLENT compass, and made in the U.K. Nice job, you blokes. :D

[phiggsbroadband]

Sounds like a problem with either the vacuum or the electrical gyro. I would think that both AI and DI are vacuum, but I could be wrong.
Most likely they are not getting up to the correct speed. Could be dirt on the gyro bearings or lack of lubrication.

[flyingchanges]

Ditto, vacuum pump or filter.

[A Squared]

Flying without a reliable compass is really dumb.:=

Well, actually, he did have a reliable compass. His problem was with his directional gyro. I've done plenty of flying in airplanes which didn't have (and never did have) a Directional gyro installed

[A Squared]

Should I write up about the compass?

Yeah, although I wouldn't call it a compass, because from your description it doesn't appear to be a compass you're having problems with.

Now, I'll preface this all with "probably" because I don't have any knowledge about your specific airplane. Generally, there's only one compass in a 172, and that's the whiskey compass. There is usually also directional gyro, but that is not a compass at all. It's a Gyroscope which indicates heading based on a heading you set in it, but has no ability to determine direction on it's own. (it's possible that a 172 has a slaved gyro which does sense magnetic heading, but that's not likely) The vast majority of directional gyros in light aircraft are vacuum driven. The Attitude indicators are also usually vacuum driven. The fact that both the attitude indicator and the Directional gyro are not functioning properly seems to suggest that there may be a problem with the vacuum system. Did you happen to note what the vacuum gauge was reading? It is also possible that you just have two gyro instrument (the AI and the DG) with worn out, dirty bearings and such which need to be replaced.

[mad_jock]

I hope this isn't teaching you to suck eggs but remember that all these systems aren't connected on your normal spam can.

I presume when you are saying compass you are meaning the direction Indicating gyro. This isn't like the ones we have on our work machines which are slaved to a flux valve to maintain direction.

There is no requirement for this to be working on a VFR machine. So the comparison with what you would expect in your jet is a false lead. Some single engines have never had one fitted or even an attitude gyro. Some older machines like a cub or super cub they are powered by a venturie nozzle outside in the air flow and to be honest at low speeds everything stops working.

The whisky compass or wet compass there is a requirement for it to be working.

The coming back to VFR flying after years of IFR work with an IFR flight deck can be a bit of a shock for some. Also the lack of help from ATC can also be a bit disconcerting.

If the horizon gyro is also not working it also tends to say that there is a problem with the whole system not just the horizon. As others have said its most likely the pump that's gone as both of them are showing issues with not being fast enough to remain ridged. If it was a filter it would be likely that it would only effect one instrument.

In the nicest possible way can I suggest you revisit your basic flying books on instrumentation. I say this as a current airline pilot who has maintained flying instructor ratings on light aircraft for 6 years after flying the line. I have checked out a few heavy tin drivers after long periods away from light aircraft and all of them were a bit too instrument focused initially. It s actually quite hard work to break away from IFR instrument scan and look out the window properly.

From memory all you need for a none complex single engine piston VFR daytime aircraft is.

1. clock
2. Wet Compass.
3. RPM gauge
4. Altimeter.

It may be more in your country.

[RRTrentSymphony]

Thanks everybody, written it up for the mechs to check the aircraft out. And yes I meant the directional gyro (yep i'm realising now how bad my terminology has gotten over the past few years).

This was a 80's 172N that's really showing its age... (flight already delayed from last week to have some other systems checked out)

[Mach E Avelli]

It's been a long time since I have had to fly with an old fashioned unslaved directional gyro, but I recall that they suffered two main errors.

1. Precession, due to earth's rotation: 15 degrees per hour
2. Drift, due to wear and friction: Obviously highly variable depending on the age and quality of the instrument. This could partially cancel out precession, or could go the other way and compound the error.

Somewhere in the recesses of the brain I recall being told that the combined worst case should not exceed 30 degrees per hour, but can't offer a reference. Resetting the DG every 10 to 15 minutes to the wet compass was a ritual.

In very high latitudes where the wet compass is useless, of course nowadays GPS has come to the rescue. But it is nice to have a good DG, knowledge of the individual instrument's errors and a method for resetting it against either the sun's true bearing or by calculating precession and adding or subtracting drift.

If your DG is out by 30 degrees in 10 minutes it is most certainly u/s regardless of your latitude.

[roulishollandais]

Did you or your passenger put your phone above the wet compass on the board?:p:p I am often mocking lazy astromers friends using compass to turn their telescops toward "North" , and show them the compass turning 60° and valsing with my phone near of it.They cannot think it is true and nobody warned them:cool:

Perhaps an electrical wires anomaly creates both issues on gyros and wet compass?
With your IFR flux vane compass ad supergyros you need a working wet compass too in an electrical emergency, take care where you place your phones! :oh:

[Lone_Ranger]

If they are using a compass to find north they don't know much about Astronomy
(hint: pole star)

[Oktas8]

From memory all you need for a none complex single engine piston VFR daytime aircraft is.

1. clock
2. Wet Compass.
3. RPM gauge
4. Altimeter.

Ts and Ps.

Don't care about RPM myself (I have ears for that :} )

[mad_jock]

Same here about the rpms.

Thought there was another one.

[roulishollandais]

If they are using a compass to find north they don't know much about Astronomy (hint: pole star).Sometimes the polar star is hidden by clouds or mountain, or light pollution, or in South hemispher, aso. And they have read that polar star is 0°44' from poles' axis and think compass is more accurate ...ignoring deviation.
Puting the telescope correctly is a boring work specially if you want to do photos with an engine on the telescope turning with the same axis than earth.
But I am sure that all the older pilots are still able to find their fix with height of stars almanach and exact time.;)

[roulishollandais]

From memory all you need for a none complex single engine piston VFR daytime aircraft is.
1. clock
2. Wet Compass.
3. RPM gauge
4. Altimeter

In France VFR daytime civilian aircraft flying GA over the ground in sight (arrêté du 24.jul1991) you need:
1. No clock
2. Compensable wet COMPASS
3. No rpm (ears!)
4. Adjustable Altimeter only if you fly inside controlled area
5. SPEED indicator
6. SLIP indicator

[Machinbird]

RRTrentSymphony
I have to compliment you on what was one of the funnier stories about aircraft systems knowledge- lack thereof. At least the aviator in you came through and saved the day.

As others have mentioned, possible causes are:
Insufficient vacuum from the vacuum pump (from memory this should be at 4" Hg),
Leaks in the vacuum system piping,
Blocked instrument air inlet filter (sometimes called a "vacuum filter")
Worn out instruments. (It is highly improbable that the instruments are electrically driven.)

Highest probability is a "vacuum filter" blocked with cockpit lint and the stuff smokers exhale. As you can imagine, this would not be an expensive fix.

Your experience with said 172 tells you a lot about how this particular aircraft operation handles their maintenance priorities and about their financial health.
Look at their fix for this problem, and if it is clearly a pencil whipped entry, then go elsewhere for your rental aircraft. The systems in a 172 are stone simple, but it would be worth spending some time on the aircraft manual learning the finer points of how it operates.

[roulishollandais]

Can I believe that "wiskey compass" is a directional gyro using vacuum pump and not a magnetic compass? :{
Happy if I am one of those who allowed you to laugh about ignorance to begin a happy new year !
Thank you for all your wise lessons and help us to enjoy sky safer.

[mad_jock]

roul its the wet compass, they call it a whisky compass because its filled with alcohol. Days gone passed when they leaked they only had whisky available to top them up with it. And when it leaked again you could smell it in the cockpit.

[roulishollandais]

Thank you mad_jock. WHISKY OR WHISKEY?:ok:

[cockney steve]

Ah Well, Roulis....that depends on the country of origin....Scotland,= Whisky
Irish = Whiskey....AFAIK, no other country is allowed to use either name to describe a locally-produced Spirit.
I was once gifted a bottle of "Manx Spirit"....this crystal-clear fluid smelt just like Whisky and was the most flavoursome and beautifully smooth I have ever sipped (though I do lead a sheltered life:} )
Originally, it was, indeed, called "Manx Whisky, distilled on the Isle of Man"
Scots had them take it off the label.

[Yancey Slide]

172's can be fitted with an electrical HSI or a vaccum-driven DG in addition to the normal magnetic compass on the glareshield (obviously we're not talking about the G1000 variant).

If both the DG and the AI were acting funny as described I'd be looking for a vacuum leak beyond the manifold where the sender for the annunciators are since I presume if one of the VAC L or VAC R lights was lit we wouldn't be having this thread (at least in the SP there are 2 pumps and the manifold is just a 4 port assembly with a marble for lack of a better description that rolls back and forth between the two sides depending on which pump is operating higher at the time. This marble seals off one side in case of a vacuum pump failure and floats to kinda balance the system in normal 2 pump ops.)

But that's just me.

[A Squared]

172's can be fitted with an electrical HSI or a vaccum-driven DG in addition to the normal magnetic compass on the glareshield (obviously we're not talking about the G1000 variant).

If both the DG and the AI were acting funny as described I'd be looking for a vacuum leak beyond the manifold where the sender for the annunciators are since I presume if one of the VAC L or VAC R lights was lit we wouldn't be having this thread (at least in the SP there are 2 pumps and the manifold is just a 4 port assembly with a marble for lack of a better description that rolls back and forth between the two sides depending on which pump is operating higher at the time. This marble seals off one side in case of a vacuum pump failure and floats to kinda balance the system in normal 2 pump ops.)

But that's just me.

Well, the original poster did say that it was a 1980's 172N, so the chances that it's an 172SP are pretty slim. I'd bet long odds that it had a single vacuum pump and a vacuum gauge with no annuciator lights.

But that's just me.

[Yancey Slide]

Been a while since I was in an N model, but don't they have some sort of light for the vac pump in addition to the dial gauge with the green pie slice? Memory's fuzzy but I thought it was just a single idiot light but honestly can't remember Most of my 172 time is in SP+ vintage.

[A Squared]

Been a while since I was in an N model, but don't they have some sort of light for the vac pump in addition to the dial gauge with the green pie slice? Memory's fuzzy but I thought it was just a single idiot light but honestly can't remember Most of my 172 time is in SP+ vintage.

Well, I remember a single red warning light sticking out of the panel on that vintage cessnas but I think it was an alternator out light.