Putting the spin on your gyros
You don't have to be Mr. Wizard to work up enthusiasm for our rapidly spinning friends, the gyroscopic instruments. Especially since it's so hard to imagine flying without them. We'd be left in the tiedown spot until the overcast lifts, or stuck on top when the fog rolls in. Gyros are as common as headaches at tax time, and more welcome in times of need than a solid-gold deduction.
Thanks to mass production, the gyro instruments common to light aircraft have achieved an amazing level of reliability and affordability. Brand new attitude indicators, for example, cost about $450, which sounds like a lot until you understand what's inside the case.
As the name implies, the instruments use a gyroscopic wheel or disc that is spun by either pneumatic or electrical power. Most commonly, the attitude indicator and directional gyro derive power from the pneumatic system, while the turn coordinator or turn-and-bank instrument is electrically motivated. Cost has influenced this picture, because vacuum-driven gyros cost less than their electric brethren.
Gyroscopic instruments work on the principle of rigidity in space. That is, the rapidly spinning gyro wheel tends to remain in place, resisting displacement. Instruments utilize this tendency so that, in essence, the airplane is allowed to move around it. By connecting the gyro through its gimbaled mounts to a variety of pointers, we can display movement about all three axes.
Primary in the flight-instrument stack is the attitude indicator (AI), also called the artificial horizon. (Instrument-flying texts awhile back moved toward new names for some of the gyro instruments, creating some confusion for those of us who learned about them under the old titles.) The AI mounts its gyro in a gimbaled mount, while the gyro itself rotates on a vertical axis. The gimbal-mount pivot axes are parallel to the wingspan (laterally) and parallel to the aircraft's centerline (longitudinally).
The heading indicator (or directional gyro, if you prefer) mounts its gyro wheel on its side, compared to the AI. This allows the instrument to read only turning movements. In a common heading indicator, there is no slaving function; you must align it with the wet compass first. For a couple of reasons it's good practice to periodically reset the HI to the compass reading. One, the gyro will have some internal friction and precess from the correct heading; and, two, it will suffer from apparent drift caused by the earth's rotation. Apparent drift is more pronounced at the poles, and to compensate HIs are calibrated with some drift built in, based on the expected latitude of operation.
In an effort to reduce pilot work load, slaving systems have been devised to constantly correct the HI to magnetic north. These days, you'll most likely find such a slaving system connected to a horizontal situation indicator (HSI), a device that combines the HI with a course deviation indicator, an omni bearing selector, and the appropriate flags.
Pneumatic gyro instruments can be awfully fragile. When the gyro is not spinning, it is susceptible to damage from rough handling; when it is operating, the gyro wheel's weight is spread evenly around the bearings that hold it in place. You've probably noticed that gyro instruments come in heavily padded containers. That's for good reason. What's more, gyros are sensitive to contamination, whether it's from water or dust or cigarette smoke. One overhaul shop we spoke with said that smokers are considerably harder on gyros than those who don't light up in the airplane. Fortunately for the gyros in our lives, the number of people who smoke while flying is relatively small.
In flight, keep your eyes peeled for sluggish gyro performance, such as excessive precession of the HI or a failure of the AI to show wings-level when the airplane really is. Usually, an AI with a poor sense of "Which way is up?" comes from contamination or sticking of something called the pendulous vanes. These small metal tabs are located on the housing of the gyro wheel and help the indicator erect itself. The vanes move in response to gravity, modifying the air flow to the gyro wheel and causing it to seek level. Since this happens every time the airplane is turned, the effect is intentionally subtle. You will notice that a prolonged turn in one direction will result in AI errors when you roll out.
How do you keep your gyros happy and healthy? According to our talks with personnel in instrument shops, ensuring that the gyros receive clean air is the most important. Contamination is the leading cause of gyro failure, followed by excessive vibration and plain old age. The moral, then, is to keep the filters clean and to check them on a regular basis.
In addition, we were told that it's fairly common to see gyro failures shortly after a dry vacuum pump has failed and has been replaced. It's possible, even in a vacuum system, for some of the carbon particles that make up the pump's vanes to migrate into the pneumatic lines. Make sure you remove and clear the lines after a pump craters.
A vacuum system has many advantages, but it is not the most efficient for high-flying airplanes. For that reason, many turbocharged and pressurized airplanes have pressure systems instead. These work in reverse of the vacuum models, and have an extra set of filters to protect the instruments from any junk that might come out of the pump. Another filter protects the pump from ingesting airborne garbage.
While we're on the topic of instrument power, it might be useful to know that the dry vacuum pumps in widespread use have both good and bad points. On the positive side, the dry pumps — as differentiated from pumps that are lubricated by the engine oil supply — are light, clean, and inexpensive. Often costing less than $200 for an overhaul, these pumps usually do their jobs for hours and hours without a peep of trouble. They don't have the long life of wet pumps, but they also don't need an air-oil separator and aren't prone to leaving an greasy tattoo on the belly of the airplane.
Still, the dry pump often fails without warning. Moreover, there are few sure-fire ways to predict when a pump will fail. Many times, it works perfectly right up to the point of massive failure. Dry pumps are fitted with a frangible coupling, which disconnects the innards from the accessory drive shaft so that the engine doesn't suffer any damage in the event of a pump seizure. In the vast majority of cases, breakage of the frangible coupling is the first and only sign of pump trouble.
For instrument-power redundancy, certification rules require a separate power source for the third gyro instrument, which can be either a turn coordinator or a turn-and-bank indicator. The primary difference between them is that the turn coordinator mounts the gimbal axis at a 30-degree incline; the turn-and-bank's axis is parallel to the aircraft's longitudinal axis. This means that the turn coordinator will sense rolling, as well as yawing movements, while the turn-and-bank reads only rate of turn. Some pilots still prefer the turn-and-bank because it is less sensitive in turbulence, while others prefer the turn coordinator because it helps curb over-controlling during partial-panel flying.
While the electrically driven turn-and-bank or turn coordinator will generally outlive the pneumatic instruments in the stack, there are still some signs worth watching for. Turn on the master switch (and avionics master, if the instrument is wired through that circuit) and listen for a smooth power-up and reasonably quiet running. The instrument should get to operating speed within 30 seconds or so, and the warning flag should be stowed. Be warned, however, that the flag only depicts the presence of electrical power; it doesn't guarantee that the instrument will work satisfactorily.
A proposal was offered to the FAA to approve an electrically driven attitude indicator in lieu of a turn coordinator for IFR installations, on the basis that a second attitude indicator would be much easier to read and interpret. Alas, this good seed of a proposal has not yet borne fruit.
Quite a few pilots who log many instrument hours a year have made room for this separate electric attitude indicator by replacing the normal turn coordinator with a smaller model that can fit in a clock cutout, for example. This upgrade can be expensive, though, with the electric AI costing upwards of $1,500 new, and a miniature turn-and-bank running $600 or more.
A more cost effective solution for many has been a backup vacuum system. You can find setups that use a second vacuum pump (either electrically driven or mounted to a spare accessory pad on the engine) or utilize the natural suction in the engine's intake tract. Either one is a highly recommended form of redundancy for the frequent in-cloud flier. Then again, some pilots use the redundancy theorem to convince their spouses and bankers that it's time to move up to a twin.
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