Understanding Gyroscopic Instruments

These spinning devices enhance your attitude awareness

By Marc E. Cook

hank Jimmy Doolittle. In September 1928 he first tested some revolutionary new instruments, items that would allow improved situational awareness for the pilot and safe passage through clouds. Those two advances were the precursors to the modern gyroscopic attitude indicator and directional gyro, the two main devices in the instrument pilot's arsenal.

Today we take a full gyro panel for granted. Even the most mundane trainers come with a standardized package of attitude indicator (AI), heading indicator (HI), turn coordinator (TC), or, in its place, a turn and bank (TB). Together with the pitot instruments — airspeed indicator, altimeter, and vertical-speed indicator — the gyro system allows precise and safe trespass through the clouds.

Gyro instruments work on the principle of gyroscopic inertia. Inside each of the gyro devices is a spinning wheel or disc. Its inertia, once the wheel has been accelerated, tends to keep the disc stable about its axis of rotation. You may have picked up a bicycle wheel by the axles and tried to deflect it side-to-side while it was spinning; you would have noticed that it initially resisted the movement. That's the same principle used by the gyro instruments. Once the instrument is stabilized, say in level flight, any deviation in flight path will try to deflect the gyroscopic wheel in its gimbal mount. This movement — which is, in truth, the instrument case changing position relative to the gyro wheel — is translated to movement of a needle or card on the instrument's face. The gyro wheel is said to have stability in space.

While the three main gyro instruments use the same principles, there are significant differences inside the cases. The artificial horizon, for example, contains the gyro wheel spinning on the vertical axis. One basic gyroscopic concept is precession — any force applied to the gyro will result in movement of the gyro wheel 90 degrees out of phase. This dictates how the gyro disc is connected to the indicating mechanism.

In the AI, the gyro wheel is free to move about two axes, thanks to the construction of its gimbal mount. This means simply that the gyro is being held by an apparatus with pivots in two axes, which provides for both pitch and roll information on one instrument.

The heading indicator, on the other hand, places its gyro wheel spinning on the horizontal axis, the pivot aligned with the aircraft centerline. Its gimbal allows only one axis of freedom (vertical) and connects the mount to the card on the instrument's face through bevel gears. When the airplane begins to turn, the compass card on the front will begin to turn only when the gyro reacts to the yawing of the airplane during the turn.

Gyro instruments react to short-term movements of the airplane. In fact, the attitude indicator contains a set of weights intended to drive the instrument toward level flight by sensing gravity. These weights move the instrument face about 3 degrees per minute. So if you were to maintain a 30-degree coordinated banked turn for 10 minutes, you would look down to see the AI indicating level flight. Likewise, the heading indicator will succumb to precession, moving from the set magnetic heading over time. That's why you should periodically check it against the wet compass to make sure you're still on track.

Commonly, the AI and HI are powered by vacuum pneumatic systems. Along the periphery of the gyro disc are small, cup-like cutouts. A tube aligns incoming air pressure to act on these cutouts to spin the gyro disc. By evacuating the instrument chamber with help of the vacuum pump, ambient-pressure air comes rushing in, propelling the gyro wheel. This wheel spins quite rapidly, typically 10,000 to 15,000 rpm.

Most modern airplanes use engine-mounted vacuum pumps. They are simply ellipsoidal chambers into which is fitted a circular drive hub. Protruding from this hub are a number of carbon or aluminum plates, or vanes. They are free to move inside the hub's slots, drawing in air from one port and forcing it out another. Typically, the pneumatic system is connected to the port that is drawing in the air, hence the name vacuum system. Some airplanes do the reverse, providing pressure to the gyros on the panel; this is called a pneumatic-pressure system.

Why use suction at all? In the event of a vacuum pump failure, debris will not be scattered through the system and deposited into expensive, sensitive instruments. There is a filter inside the cabin that removes particulates from the air going into the gyros. It's generally true that the gyros will lead shorter lives in a smoker's airplane.

Since the AI and HI are usually powered by the same pneumatic system, it's prudent to have another gyro instrument independent from them. That's the turn coordinator or turn and bank, either of which is usually powered electrically.

The turn coordinator is the more recent development. Its gyro wheel spins on the horizontal axis, but the pivot is oriented transversely, parallel to the wing span. The turn coordinator's gimbal mount therefore runs along the airplane's longitudinal axis. In the turn and bank, this gimbal axis is perpendicular to the instrument face, meaning that the needle will show only movement in the yaw axis, or the pure turning of the airplane. The turn coordinator has its gimbal mounted 30 degrees off the longitudinal axis, meaning that it senses some portion of the rolling required to initiate a turn. This makes the TC a bit more sensitive. Both instruments are marked in what's known as a standard-rate turn, or a 2-minute turn. Hold the indicated markings for 2 minutes and you'll have made a 360-degree turn.

General aviation has been using gyro instruments long enough to have the bugs pretty well worked out. Notice on the ground how the instruments respond — those indicating movement about the yaw axis should move freely during taxi, and the AI should show any changes in pitch, such as you might have traversing the potholes in front of the FBO. If you hear one of the gyros whining over the sound of the engine, it's a good bet the instrument will not be long for this world. Also be alert for proper suction indications for the pneumatic instruments; the gauge should be in the green arc not far above idle speed and most definitely by the run-up rpm. Watch for too high an indication as well as one below the green arc; too much suction can spin the gyros beyond their design limits.

Don't forget, too, to include the suction gauge in your scan while practicing on the instruments — vacuum pumps give little warning before they fail suddenly and completely.