Other than the powerplant, the most critical system for safety of flight are the gyroscopic instruments. Why do you think they call it instrument flight?
Unfortunately, the pesky things break and since these are life-and-death devices, you don’t want to mess around with deferred maintenance. So what do you do when those all-important gyros go kaput?
When the motor acts up, you select the course of action appropriate to the problem. The tracks are less clear with gyros, particularly since theyre damaged as much by installation and shipping as by mechanical failure. In this article, we’ll examine failure modes and the solutions, while explaining the options.
As you recall from ground school, a spinning mass becomes stable in space, independent of the earths gravity. Once stable, the mass can be used as a reference. Its that spinning mass that causes gyro problems.
The smaller the mass, the faster the rotor speed required for stability. Smaller is better in airplanes, so modern gyros have small rotors spinning very fast. The gyro mass, or rotor, must turn at nearly 10,000 RPM to remain stable.
There are two ways to power a gyro, air or electricity. Air-driven instruments predate the age of electricity in airplanes. Theyre simpler and cheaper than electric instruments.
Air is both the lifeblood and the poison of the instrument. Contaminants flow in and around the rotor, settling into the bearings. Soon, the bearings cant handle the speed, friction heats them up and the gyro becomes unstable or even tumbles.
Electrically driven gyros have tiny, high speed motors as their rotors. Theyre more expensive because of the tolerances required for a 10,000 RPM motor about as big as your thumb. These units are sealed against the elements so they tend to be more reliable.
Reliable is good, right? Who wouldnt pay more for better reliability? Yet the difference in price is not trivial. You can pay triple or quadruple the price of a new air-driven attitude gyro for an electric one. That expensive gyro is also no better than the airplanes electrical system.
But TCs are Cheap
Yet electric gyros do have a cost-effective niche. Less complicated gyros are we’ll suited to electrical power. A turn coordinator is a strapped-down gyro, meaning it doesnt have fully free gimbals. The rotor doesnt have to be as large and the bearings don’t have to support higher speeds.
In fact, the TC turns slower than the other gyros. This is why the early venturi-powered systems have a smaller venturi for the turn gyros. While most are electric, you can still find air-driven turn gyros. In an airplane with a conventional, engine-driven air system, you’ll probably have an in-line to limit air flow.
The turn gyro, whether needle-type rate-of-turn or a turn coordinator, is a gyro oriented fore and aft and tilted 45 degrees. Its gimbal is connected through a calibrated dashpot or spring that provides a fixed displacement depending on the rate of turn. The gyro isn’t considered stable, but merely gives an idea of rate of displacement away from the longitudinal axis.
The TC-type gyro is used in yaw-damper autopilots, too. Speaking of autopilots, most derive some information from the aircraft instruments. S-TEC and the Century I use the TC for turn information; its the heart of such systems. Just because the face looks the same and it fits in the hole, that doesnt mean a replacement will be a direct replacement. Get a model number before ordering. As long as these gyros have power, they last a long time. Their speed isn’t critical, because stability isn’t as important as with an attitude gyro.
DGs
More fragile than the TC, but more robust than an AI, the directional gyro has a rotor mounted such that its free to turn in the horizontal axis. A series of gears couple the gyro capsule to a vertical card. This is a direction-keeping gyro. Unless slaved to magnetic north electrically, you must tell it which way youre going. The gyro itself doesnt care.
A gimbal allows it to remain upright in most banks, while transferring the heading information. The problems with DGs are usually related to precession. Without slaving, they must be reset to correct precession, which is the gyro drift resulting from friction, gravity and the earths revolution beneath the airplane.
A good DG should precess less than three degrees in 15 minutes. If it drifts more, the gyro could be on its way out. It could also be starved for air, so a check of the gyro air supply, including replacing the air filter, is in order. A damaged bearing or other internal problem will sometimes cause the gyro to go into gimbal lock.
The spinning rotor energy is transferred abruptly to the gimbal and the card will spin like a roulette wheel possessed. Stick a fork in it; that gyro is done. Even if it spins-up normally the next time, the bearings are shot and its only a matter of time before it will die. Probably in the clouds.
HSI
The horizontal situation indicator (HSI) is basically a DG with a VOR indicator attached. The only difference gyroscopically is the location of the gyro (remote or panel-mounted) and whether the HSI is slaved to magnetic north.
Slaving circuits in HSIs (or the very rare animal, a slaved DG) keep up with normal precession. Theyll slew the card about three degrees per minute. If the gyro is bad, it can drift at more than three degrees per minute, leading you astray.
Century Flight Systems makes an air-driven, panel-mounted HSI, Bendix/King and S-TEC make systems with remote, electric directional gyros. The Century NSD 360A, an air-driven, panel-mounted HSI, is available in slaved and non-slaved models. The electric NSD 1000 is slaved.
Directional gyros often have heading bugs built in. In a non-autopilot airplane, these are handy heading reminders. In an autopilot, they provide heading-select coupling. In addition to the bug, many autopilots use the DG as a source of directional information. Like the TC, you need to know what you have before calling for a replacement.
Some HSIs have a bootstrap, an electro-mechanical synchro that converts the position of the azimuth card to a signal that can be used by other systems, such as a moving map or autopilot. If your DG or HSI has a bootstrap, you need a replacement that has one or else some stuff in the airplane wont work anymore.
Attitude
The artificial horizon, a.k.a. attitude indicator vertical gyro, is the most important and the one thats most finicky. These gyros are suspended in gimbals and free to move in two axes. Theyre subjected to gravitational and acceleration forces that try to induce errors. They have to be intentionally precessed to show attitude correctly, because the stable gyroscope may not line up with the spherical earth horizon.
While failure modes range from excessive drift to tumbling, the most dangerous failure is complete loss of function. You fly into an unusual attitude because the gyro doesnt show your true orientation.
There are operational symptoms you might notice before the gyro packs it in. Watch the AI as you start the engine (for air-driven gyros). It should do a dance called nutation as it erects. This happens as the first blast of air passes through the gyro and activates the erection vanes. If your horizon doesnt wobble and oscillate to stability when the air hits it, the vanes could be sticky. If the vanes don’t operate properly, the gyro will not be able to compensate for drifting and acceleration errors that occur naturally in flight. Or perhaps the end gimbals are sticky or a bushing has been contaminated. Whatever, it isn’t a good sign.
Nutation only occurs when the gyro starts from a dead stop. If you restart the engine after a brief pause, the gyro will still be spinning and wont erect right away. If your gyro has been shut down for less than 10 or 15 minutes, a perfectly good gyro could take 10 minutes to re-center.
Watch the gyros during shut down. To pre-position the erection vanes on the air outlets, the gyro at rest will be tilted in pitch and roll at rest. When the gyro spins down, it will slowly park itself. Notice the parked position. If the gyro begins to park differently (absent any aircraft movement with the gyro air off), you could be looking at a sick gyro. Gyro erection is not an issue for manual erecting gyros, but they will usually park and they will have erection vanes. Electric gyros will act the same.
Not all gyro problems are in the instrument. In fact, most are at least precipitated by the air supply. When a vacuum pump fails, it often throws contaminants, chunks of vanes large and small, into the gyro air. Filters become clogged, reducing the supply. Hoses get kinked by mechanics working behind the panel. Occasionally, we find vacuum hoses made of non-air hose, which kinks and collapses under the systems small vacuum. Take care of your vacuum system and your gyros will last longer.
One frequent question relates to the gyroscopic instruments ability to withstand aerobatic maneuvers or some combination of roll and pitch adding up to 85 degrees. Positive G maneuvers and most roll axis maneuvers are fine.
Should you have to replace any gyro instrument, never, ever remove a serviceable unit from the original, manufacturer-approved shipping container until the gyro is ready to install. Under no circumstances should any gyro be set down on a hard surface. To swap pneumatic fittings, put the unit on thick, dense foam, or hold between your thighs. (Weve seen $2000 gyros chucked in a bench vice!)
Rules of thumb, if a gyro drops the diameter of a pencil, its damaged. If you put an instrument down hard enough to hear it, it has been damaged. The life has been decreased to about 100 hours or less. No kidding.
Decision Tree
Okay Bunky, youve decided that your gyro is shot. What are you going to do about it? Your choices are four: Get it repaired, get it overhauled, get an exchange unit or buy new. The most popular option, and the best one, is to procure an overhaul/exchange unit. But lets look at all the options.
Can you yank your DG and ship it to the instrument shop for repair, just as you would the transponder? Sure. Just be aware of a few considerations. The primary one is handling. Gyros don’t like to be man-handled. Their bearings are extremely smooth and round. When the rotor is spinning, the weight is distributed evenly. At rest, the weight is on a few points.
Any impact to those points will cause brunelling, small dents in an otherwise perfect surface. The dents cause friction, wear, and at the extreme, gimbal lock and tumble. For this reason, a new gyro should go from the shipping container to the panel, without stopping to rest on a bench. Wrestle the gyro, and you increase the opportunity for damage.
This is the reason loaner instruments are rare. A wise instrument technician once said that every gyro is good for one round trip shipment between overhauls. After you get the instrument to the shop, you have to open it up. Most repairs require getting to the guts and that level of disassembly drives up the labor cost.
Instrument repair is contraindicated unless the problem is simple (a burnt-out lamp, a sticky flag thats easily accessible) and the shop is close by. If the instrument is new, repair is okay. If the instrument is complex-an NSD 360A-overhaul it.
Overhaul Exchange
An overhauled gyro is new where it counts, the bearings. We arent aware of any reputable instrument shop that reuses the bearings during an overhaul (at least until the FAA decides to require a TSO on those bearings and internal parts). If the bearings are new and the rotors are precision balanced and the unit meets or exceeds all new specifications, why not go overhauled?
During overhaul, even the cases and dials are repainted, if necessary. From the drivers seat, you wont be able to tell and from a service-life standpoint, the overhauled unit will probably last just as long. The biggest caveat in overhaul exchange is being sure to order and exchange like instruments. All gyros arent created equal and instrument shops weve dealt with have had customers trying to swap units for different, and even lesser, types.
This hurts the instrument shop, which depends on a constant flow of good cores to overhaul. A directional gyro thats full of mud and fish eggs isn’t caviar and you can get a nasty bill from the instrument shop to set it straight.
Should you have your instrument overhauled or get an exchange? It could take weeks to have your unit overhauled. In most cases, you can have an exchange overnight, if not within hours. If the newly overhauled unit was built up correctly by a reputable shop with a proper warranty, youre every bit as protected. You can be confident of a long and happy life as you had with the original unit, perhaps longer.
Buying New
The artificial horizon quits, so you want to buy a new one. you’ll pay a premium price for new instruments. The difference can be as little as $100 for an R.C. Allen artificial horizon to $2500 or more for a KI 256 FD indicator/ attitude gyro.
The only reason you might want to buy a new unit is to upgrade your capability. Perhaps add an HSI instead of a plain old DG, or change to an attitude indicator with a warning flag. But generally, buying a new gyro to replace an older, plain-vanilla TC or AI with no autopilot functions is not cost effective. Better to go with an exchange from a good shop. It will be faster and cheaper.
Where to Buy/Repair
Finding an instrument shop can be nearly as bewildering as finding a trustworthy mechanic. Somewhere, there’s an attitude gyro waiting for yours to fail. When it happens, you’ll probably call your mechanic or avionics shop and cry for help. Acting on your behalf, theyll place an order for the exchange depending on availability, location and past service. Frequently, you wont know where the unit is coming from. Sometimes the unit will arrive with a yellow tag that isn’t from the instrument shop listed on the order, either.
The instrument business is strange. There are a finite number of gyro instruments, a slightly larger quantity than the number of holes in aircraft instrument panels. As instruments are swapped among the industry leaders, there’s usually one available for you at any time. Unless you live near Dallas, where the number of instrument shops is exceeded only by the number of Chevy Suburbans, you wont have a big choice of local shops. But dealing with a local shop has advantages. Eye contact and an easy conversation about the product or services is always worth the effort.
Lower volume usually means a little higher price, but you don’t have the exposure and added cost of shipping. Local shops are better able to troubleshoot your airplane right on the ramp, in conjunction with your avionics or maintenance technician. They may not have the precise unit you need, but they may be able to find a good alternative. Even if they don’t have the right instrument, theyll be able to call on one of the national shops for help.
All things considered, a reputable local instrument shop with good references is our first choice, even if the shop is 50 miles away. The service is usually worth the premium price.
National Shops
Airplanes get around. Thats their job. Therefore, it makes some sense to buy your instruments from a nationally recognized source. In the unlikely event that the unit fails, any maintenance professional anywhere can make an exchange and your warranty is still valid.
There are a handful of recognized instrument shops around the country. All are good, in our experience and competitive on price and service. Choice depends on availability, shop preference and the prevailing price and warranty policies. Among the big instrument companies weve had a good relationship with are:
Aviation Instrument Services, BF Goodrich Castleberry, J. D. Chapdelaine, Mid-Continent Instruments (and Mid-Continent West, Mid-Continent Northwest), Otto Instruments Service, Pacific Southwest Instruments and The Gyro House. This is a small list and shouldnt reflect negatively on others not listed.
Manufacturer
What about buying instruments from the source? AIM, Sigma-Tek or RC Allen? This is like buying a radio directly from AlliedSignal. They probably wont sell to you because they have a distributor/dealer network. If they did sell directly to an end user, it would be at full list. No benefit here, even if the instrument manufacturer is still in business.
Core Value, Contacts
Those tumbling and shrieking gyros are a valuable commodity. When you make an exchange, the instrument overhaul agency will quote your final price predicated on their getting a reusable, repairable core. The instrument shops raw material cost is that core, so it should be treated with respect. If they cant use it, or have to spend more than the usual amount of time and parts to make it serviceable, their cost, and ours, goes up.
As with any aircraft maintenance decision, if you can tend to the replacement before youre under the gun of an important trip, you’ll be far better off. We recommend replacing gyros pro-actively at the first sign of trouble, especially the all-important AI, a failure of which can be a nightmare in the clouds.
If you have questions about gyro problems, you can call the instrument specialists at the shops listed in the file linked below. In our experience, they would rather answer a question or two than have an instrument failure that could have been avoided or ship the wrong instrument to a customer.
Also With This Article
Click here to view the Gyro Replacement Checklist.
Click here to view “The Guts of a Gyro.”
Click here to view “Replace a TC With an AI? Forget it, Says FAA.”
Click here to view the Gyro Addresses & Contacts.
-by Gary Picou
Gary Picou is Aviation Consumers avionics editor.
