Directional Gyro Failure Mode

[Bob Axsom]

I fly our RV-6A in Cross country air races that have some extremely tight turns as well as some mild ones. Before a race I do a lot of preparation work and have a race checklist with a page for each leg. One of the key items is the heading outbound after the turn at the end of the current leg. The objective is to stand it on it's ear and get moving in the new direction as precisely and as quickly as possible (high speed in the wrong direction is worse than inefficient). Anyway I had a RC Allen DG that was drifting badly so after living with it for a while I bought a new Sigma-Tek DG and the function was so good it was near perfect. OK get the picture?

I raced in the Rocket 100 at Taylor Texas last month with my nearly new DG. Turns 5 and 6 were less than a mile apart and each one was on the order of 90 degrees. Because of the wind I planned very high rate of descent between these two turns in order to come out under the headwind on the way to turn 7. I was really relying on the DG for the rollout because the Pictorial Pilot heading display goes blank even on standard rate turns and these are way beyond standard rate (very high bank and using elevator to bring it around). Here is the point of all this lead-in: As I am making turn 6 the DG looses its stability (tumbles?) and I just have to guess at the rollout point (which turned out to be an undershoot) and the DG was indicating some random heading after I was cruising level to turn 7. I reset it of course but I had this feeling that it was not performing as well as it had before. Last week I had to put the plane in for the 2 year static system and encoder check for IFR operations. After it came back I made a test flight of 0.7 hours and it seemed to me that the DG was drifting similar to the old RC Allen unit.

Is this a normal failure mode for this kind of operation? What is the actual internal failure? What can be done to prevent recurrence while retaining a vacuum driven DG?

Bob Axsom

[F1R]

Continual g loading is hard on any gyro. Perhaps consider a digital like the RC allen, or keep the vac system for your IFR cert, but add a quick panel like an Extra 300 to quickly remove the AI and DG. In their place(also on a matching quick panel you might carry in a gym bag for racing) consider an MGL exteme EFIS with a lightning fast 50 channel GPS or a Dynon with a digital heading and solid state inertial sensors., or perhaps an Aspen EFIS like they put in the Extra 300 touring model. Try your thread in the glass panel section for fresh ideas there.

[Bob Axsom]

Any others out there especially instrument mechanics?

Bob Axsom

[Bob Axsom]

I sent an e-mail to Sigma-Tek to get their opinion on this. I will post their response when received if possible.

It has been suggested that this would be a better fit in the "Steam Gauges" section. Management do as you wish with this.

Bob Axsom

[Bob Axsom]

I sent an e-mail to Sigma-Tek earlier and there has been no response yet. This afternoon I called the company and after explaining the problem/questions to the lady I was forwarded to her by the operator, I was told that Jim Hearn is the cognizant engineer and he is out today. He should be in tomorrow and he will be the one that answers the e-mail. More to follow...

Comment: I am surprised that there has been so little response here.

Bob Axsom

[Sam Buchanan]

Originally Posted by Bob Axsom

I sent an e-mail to Sigma-Tek earlier and there has been no response yet. This afternoon I called the company and after explaining the problem/questions to the lady I was forwarded to her by the operator, I was told that Jim Hearn is the cognizant engineer and he is out today. He should be in tomorrow and he will be the one that answers the e-mail. More to follow...

Comment: I am surprised that there has been so little response here.

Bob Axsom

Bob, I suspect the lack of response is due to mechanical gyros being pretty much off the radar of most RVers these days. They are rarely installed or even considered anymore since few have any interest in a vacuum system in their planes. The new digital avionics offer so many advantages over mechanical....so much so they are no longer considered experimental.

Hope you have success getting your instruments serviced, but at some point you may want to seriously consider going digital. You will never look back.

[Bob Axsom]

One exposure that I have is a digital readout on the Pictorial Pilot. Even in a standard rate turn the display blanks until I settle out on a heading. I need something that is analog, continuous and preferably not dependent on an electrical system. I've had total electrical failures in hard IFR - not good. But, I may have to move in that direction.

Bob Axsom

[Sam Buchanan]

Originally Posted by Bob Axsom

One exposure that I have is a digital readout on the Pictorial Pilot. Even in a standard rate turn the display blanks until I settle out on a heading. I need something that is analog, continuous and preferably not dependent on an electrical system. I've had total electrical failures in hard IFR - not good. But, I may have to move in that direction.

Bob Axsom

Bob,

The Dynon does not blank out even during steep turns. This little puppy (with built-in backup battery) would be a great addition to your panel:

http://dynonavionics.com/docs/D6_intro.html

Not only would you have a turn and heading indicator that works all the time with no error or precession, but a complete backup 6-pack!

Come on, join the 21st century.

[Bob Axsom]

I just spent a large amount of time, effort and money modifying my panel to accommodate an SL-30 and indicator while retaining my old Terra VOR/LOC/GS. It pains me to go through that again but your reference is impressive for price, size and function because of your personal recommendation. I will study it.



Bob Axsom

[DEWATSON]

Bob,

Thread highjack!!!!!!!! Maybe I should post somewhere else or PM you....

Have you ever thought about winglets for the 6A?