July 2012
Returning to Waltanna (SN65) after a long trip, I noticed that my airplane, Bill, didn’t seem to be his normal bubbly self. I asked him why he was so quiet, and he said, “Well, I seem to be having some trouble with some of my gyros. Did you notice how fast the heading indicator precessed? Then on our last takeoff, the heading just danced around over a 90-degree arc.”
I said, “Bill, you know your gyros have been in the panel for 10 years. How about removing your gyros and taking them to the gyro doctor for a checkup?”
GYRO REPAIR FACILITY
Wichita has numerous specialty shops to repair almost every aircraft component. Terry Alderdice at Pressure Technologies suggested that I take my gyros to Nu-Tek Aircraft Instruments, Inc. in Augusta, Kan.
I called and talked to Steve Cannaby, owner and president, about my aircraft’s gyro symptoms. Cannaby said that with 10 years and 1,400 hours of operation, it probably was time to overhaul the heading indicator. That was all I needed to confirm that the gyros were coming out of the panel; then it was just a matter of taking Bill’s heading indicator and turn coordinator for the drive across town to Nu-Tek.
Gyro instrument repairs and overhauls are Cannaby’s specialties. He founded Nu-Tek Aircraft Instruments 25 years ago after almost a decade of experience in other Wichita-area instrument repair facilities. Today, Nu-Tek’s reputation brings repair and overhaul business from all over the world, including U.S. government locations.
GYRO CHARACTERISTICS
Typically, there are three gyroscopic instruments in our aircraft: an attitude indicator, a heading indicator, and a turn coordinator (turn & bank). These are powered by an electric motor, a vacuum, or pressure that energizes a spinning mass gyroscope that attempts to remain fixed in space as a reference.
When certified and released for use in an aircraft, a gyro instrument is balanced and mounted on bearings that are as frictionless as possible.
HOW GYROS WEAR
As time goes on, the gyro’s accuracy is compromised by its environment, contamination, time in service, and use (hours of operation). Tying down an aircraft on the ramp and allowing the instruments to bake and freeze adversely affects gyro life; the heat dries out bearing lubricants and the cooling creates condensation that corrodes internal components.
Vacuum pump carbon vane failures permit carbon dust and debris to backflow into instruments and contaminate the gyro components. Smoking in the cockpit or failure to change the vacuum filter regularly are other ways to contaminate and shorten gyro life.
Finally, either time in service or hours of operation will impact a gyro’s reliability and call for an overhaul of the component. Flight schools that fly aircraft 2,000 hours per year can attain up to 4,000 hours’ operation before requiring an overhaul, but most of us do not fly close to this type of utilization.
WHAT TO EXPECT
The typical owner flies approximately 75 to 100 hours per year, and time in service is the limiting factor. A gyro overhaul is usually needed every five to 10 years.
Why is there such a spread? Cannaby states that “it depends on the instrument and how you fly.” Gyros do not like to be banged against their stops. This causes the spinning gyro to place severe stress on bearings.
Also, a turn coordinator has fixed degree-per-second limitations. In the air, a standard-rate turn is three degrees per second. Turn coordinator stress comes when taxiing. When we make a 90-degree turn on the ground, how long does it take? Significantly less than 30 seconds.
Look at your turn coordinator in a turn from the taxiway to the runway. The little airplane is hard over in the direction of the turn—the gyro is against the stops and attempting to destroy itself.
Flying an attitude or heading indicator until it fails is clearly not a recommended practice. If you fly VFR only and experience a failure, you will likely be able to get down safely by looking outside the aircraft. But if you use your aircraft for flying in the clouds, losing an attitude indicator means reverting to the old needle ball-and-airspeed-type flying. Don’t assume you can rely on your autopilot in this failure scenario; some use both the attitude and heading indicators; others use the turn coordinator for reference. It might be a good idea to understand which instruments your autopilot requires—and practice flying while simulating failures of these key instruments.
RECOGNIZING IMPENDING FAILURES
What type of actions can you observe that point to impending failures?
Attitude Indicator
A healthy attitude indicator will jiggle some on startup. Nu-Tek’s website (see Resources at the end of the article) contains a video illustrating such a healthy startup. If the jiggle is very sluggish or not present, the instrument may be failing, and it’s time to look further into the situation.
Other symptoms of a failure are when the attitude indicator has the “leans” left or right, or if it pitches when flying straight and level.
Pitching was the first indication of trouble when Bill’s attitude indicator was failing the year after his other two gyros were overhauled—and so, off to Nu-Tek it went before I got into a situation when I needed to count on its reliable operation. This indicator is now the backup unit in my aircraft. Yes, I have two AIs: the primary is electric with a one-hour backup battery, and the backup unit is vacuum powered and was the original (and only) AI installed in 1999 when I bought the aircraft.
Another important lesson: I didn’t know this AI’s history until it needed overhaul. The tag on the removed indicator showed the last overhaul had occurred in February 1995—17 years ago. Aggh!
Heading Indicators
Heading indicators will start to precess, and setting the heading against the compass becomes a constant adjustment. After overhaul the standard is approximately one degree precession per 10-minute period. Ten years after installation of a new heading indicator with the S-TEC single-axis autopilot in my aircraft, the precession was up to one degree per minute—10 times the normal rate. In addition, the heading would occasionally change rapidly over a 90-degree arc.
These symptoms indicated that it was time to remove and overhaul this instrument. After overhaul, the heading indicator returned to service with precession significantly less than specifications.
Turn Coordinators
Turn coordinators today are typically electrically powered. The S-TEC Autopilot is a rate-based system and uses the turn coordinator for stability. Running this type of indicator to failure has the potential to damage the electric motor, and that will be a very expensive repair.
If you’re concerned that something isn’t right, look for standard rate turn indications not agreeing with the 10 to 20-degree bank angle displayed in the attitude indicator. Compare the indications with what you see out the cockpit windshield.
CONCLUSIONS
Gyros are as delicate as a carton of eggs. Treat them with care. If you have to tie down on the ramp, use aluminum window covers to reduce interior temperatures. Not only will your gyros appreciate the protection, but your avionics will like it also.
I installed a wet vacuum pump as a reliable way to address the carbon dust contamination problem. Accessories, Inc. here in Wichita is a wet vacuum pump headquarters. (See Resources for contact information. —Ed.)
“Take care of your gyro instruments and they will take care of you”—these are words worth taking seriously.
Charles Lloyd has logged 10,000 hours since his first flying lesson in 1954. He worked for Cessna Aircraft for 16 years, and retired as captain for a major fractional aircraft ownership company. His personal aircraft is a great business tool for his real estate investment company. Send questions or comments to editor@www.piperflyer.com.
RESOURCES
Nu-Tek Aircraft Instruments, Inc.
7169 SW Santa Fe Lake Rd.
Augusta, KS 67010
(316) 775-1144
(800) 338-7146
nu-tekinc.com
Accessories, Inc. of Wichita
4123 May St.
Wichita, KS 67209
(316) 946-0701
(800) 926-0701
accessories-inc.com
