Turn Coordinator issues

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majKPWK

Filing Flight Plan
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Dec 18, 2014
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majKPWK
I went flying today on a MVFR day in Chicagoland and was planning to go through a thin layer and pop up over the clouds for an IFR flight. A bit of background:

The airplane is a C-172 with a standard 6 pack. I did all the preflight and during taxi I made sure the turn coordinator turned in the right directions, which it did. It was a little laggy but it turned the right way, so I figured it was OK and made a mental note to keep my eye on it on climb out. Bases were ~2000 ft AGL, so I had some time in VMC to make sure the turn coordinator was working right, but it never really took much mind share until...

After take off, I went under the hood and entered a climbing left turn just as I got close to the bases. I immediately noticed I was in a much steeper turn than normal (both from still seeing the ground AND the artificial horizon), but the turn coordinator was showing "Standard Rate" in the correct direction.

The turn coordinator did not have the red flag up showing that it wasn't working. Even after pulling the circuit breaker and resetting, the same problem persisted.

Questions:
1) Any ideas on what could have caused this?
2) Had I not noticed the issue on the ground and/or had I not had the VMC buffer to "test" the instrument on climb out, aside from noticing conflicting information with the heading indicator and artificial horizon, how is a pilot meant to know the turn coordinator is malfunctioning without the red flag? Given that the heading indicator and artificial horizon are vac pump driven, my initial inclination would be to trust the turn coordinator if the flag isn't there.

FYI--for those interested, I ended up immediately canceling IFR and proceeding VFR to do practice approaches in VMC.
 
Are you sure the bank angle was wrong for standard turn rate? What bank angle were you at and what true airspeed? Did you try testing the TC by establishing what it indicated as a standard rate turn and timing it through 360 degrees to see whether it was more or less than 120 seconds?

One way to check turn rate information is to compare bank angle/speed with turn rate. For the speeds that a light single engine plane flies, there are two good approximations of the bank angle to get a standard rate turn:

  • 15% of true airspeed (e.g., at 120 KTAS, 120 x 0.15 = 18 degrees)
  • TAS/10 plus 7 (e.g., at 120 KTAS, 120/10 = 12, plus 7 = 19 degrees).
 
And one more thing to check (though it's not your initial problem). Make sure your clock is running accurately. I found that the POS Ashtech clock in my plane (evidentally pretty darn common once I researched it) runs slowly when the batteries start to go. The instructor couldn't figure out why I was screwing up my timed turns until we switched to a different timer.
 
Good point about the speed affecting bank angle. I should have thought to time the turn, but did not...so I'm not 100% certain that the turn coordinator was incorrect. However, I was climbing at around 90Kts and in about a 40 degree turn when it showed "standard rate"...I didn't do the calculations because I just knew it didn't look, nor feel, right.

I did use the clock on a VOR and a LOC approach, and it worked perfectly.

I thought about it and the plane did just get a new alternator. Any chance the alternator could be affecting the turn coordinator to cause this?
 
It shouldn't. The rotor speed is governed and should work fine across a reasonable range of supply voltages.

First thing is to time some standard rate turns at various airspeeds to determine if there is really any problem.
 
majKPWK said:
Good point about the speed affecting bank angle. I should have thought to time the turn, but did not...so I'm not 100% certain that the turn coordinator was incorrect. However, I was climbing at around 90Kts and in about a 40 degree turn when it showed "standard rate"...I didn't do the calculations because I just knew it didn't look, nor feel, right.
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At 90 knots (indicated or true -- 172's don't go high enough for the IAS/TAS issue to be significant in this regard), it should not take 40 degrees of bank to get standard rate -- half that at most. So, your gut feel would appear to be accurate (assuming you really were in a 40 degree bank -- might be hard to tell which one was wrong if you were under the hood).

I thought about it and the plane did just get a new alternator. Any chance the alternator could be affecting the turn coordinator to cause this?
Click to expand...
No. Bad turn rates out of the TC usually mean excessively worn bearings. Most likely time for an overhaul/exchange of the instrument.
 
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