Attitude Gyros

Question:
How long should my gyro last?
Answer:
Well, that depends. At TGH we have seen gyroscopes that have lasted and worked properly in aircraft for a number of years and then we have seen gyros that have failed in just a few days. This holds true for gyros that are factory new as well as for gyros that are overhauled.

How well do you treat your gyro?
One of the primary factors in determining the life of a gyro is how carefully it is handled during transportation and during installation into the aircraft.

A gyroscope is an EXTREMELY delicate instrument. Dropping a gyroscope a mere 1/8th inch onto a bench top causes sufficient force to “Ball Dent” the units primary bearings. This means that the balls within the main race of the bearing cause an impression of themselves to be created on the inside of the bearing race due to the force of being dropped. The bearings within a gyroscope can be operating at speeds from 12,000 RPM up to 25,000 RPM, depending on the type of gyro. Dents on the inner race will cause friction as each ball within the bearing passes by the dent. Friction causes further wear on the bearing. Further wear causes additional friction and so on until an avalanching effect occurs. Friction is the main cause of gyro precession and ultimate failure of the unit.

Also you don’t have to drop the gyro to cause ball denting. Rough landings can have the very same effect. This is evidenced by the high occurrence of this type failure on training aircraft. Students just learning to fly often have bumpy touchdowns, this results in gyros that have less than expected life spans.

Did you replace those hoses and filters?
We often get vacuum operated gyros that have failed due to ingestion of carbon dust. The source of carbon dust is the vacuum pump. The vanes within a dry vacuum pump are manufactured from a material very much like pencil lead. As the vanes rotate around the pump housing they are moved outward by centrifugal force. The edges of the vanes rub against the pumps inner housing and wear down, just like a pencil. As a result of this wear the vanes generate carbon dust. When the engine is shut down, the carbon dust is sucked into the aircrafts gyro hoses and filters (remember the pump created a vacuum within the gyro, when the pump stops the vacuum must be filled). If your filter is no longer operating efficiently that dust will go into your gyro. Many times pilots tell us, “But I just put a new filter in”. Perhaps you did, but did you change the hoses? The carbon residue builds up and lines the hoses as well as the filter, particularly when there has been a catastrophic pump failure, like a shattered vane. If you don’t replace the hoses as well as the pump you are exposing your gyro to premature failure. The hose costs about $25.00 plus labor to install, the gyro costs hundreds or perhaps thousands, depending on the type, to repair or replace. So the next time you replace that filter think about doing the vacuum hose as well.

Complexity of the Gyroscope
While the cost of a gyroscope is directly proportional to the complexity of the unit, the lifespan is often inversely proportional to the complexity. Simply put, the more parts there are the more it costs and the more likely it is that one of those many parts will fail. Gyroscopes that have meter movements in them, such as Power Flags or NAV Flags are significantly more delicate than gyroscopes without those options.

Inactivity
Yes, not using your gyroscope can cause it to fail as well. The bearings within the unit are lubricated. If the gyro is not spun up every now and then, the lubricant will collect at the bottom of the bearing and can harden or dry out. It is recommended that you spin up the gyro every 30 days or so and allow it to run for at least 10 to 30 minutes in order to redistribute the lubricant.

In Summary
The lifespan of your gyroscope is very dependent upon the environment in which it is used and on how well you maintain your aircraft. When we overhaul a gyroscope at TGH we totally disassemble the unit and replace all of the bearings with factory new FAA approved bearings. We dynamically balance the rotor and gimbal assemblies. We perform a calibration utilizing test equipment that is traceable to the National Institute of Standards and Technology. We then allow the unit to run-in for hours before performing a final Acceptance Test Procedure. We are confident enough in our work to provide you with a full 1-year warranty on parts and labor. However even allowing for all of this we cannot accurately predict the lifespan of any one unit. You should ideally realize hundreds of hours of use, but that is totally dependent on how the unit is treated and the environment in which it is used.

Question:
How much vacuum is needed for my gyro to run properly?
Answer:
In most vacuum gyroscopes the rotor will actually start to spin up at around 2.5 InHg (Inches of Mercury). However with that low of a vacuum the gyro is not spinning fast enough to provide accurate and reliable information. Typically you should set your regulator to be at 5.5 InHg. Just as important is the amount of airflow through the instrument. It is possible to have sufficient vacuum without having sufficient airflow. Modern vacuum operated gyroscopes require at least 2 cubic feet per minute of airflow through the unit at approximately 5.5 inches of vacuum.

Question:
At what voltage level will my electrically operated gyroscope become unreliable?
Answer:
All electrically operated gyroscopes have an allowable tolerance for the input power. Within those tolerances the unit will operate reliably, outside of those tolerances the unit may appear to operate however the information it is providing may not be accurate. The accuracy of a gyroscope is directly proportional to the speed of the gyro rotor. When calibrations are made on the unit they are made at a specific rotor speed depending on the design of the unit. The actual speed at which the rotor is currently operating is dependant on a number of items, such as the condition of the bearings, proper rotor balance, and input power.
Raising or lowering the input power will cause the rotor to run either faster or slower. This results in affecting the accuracy of the gyroscope. Listed below are typical tolerances for the standard operating voltages, however you should always read the information provided by the manufacturer for your specific gyroscope.

14 VDC systems – Typical tolerance is from 11.5 VDC up to 16 VDC
28VDC systems – Typical tolerance is from 26.5 VDC up to 31 VDC
115 VAC systems – Typical tolerance is from 110 VAC up to 120 VAC