The Gyro with an Attitude

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The accident database and university research seem to agree: if your attitudegyro quits while you're in thick soup, you're probably gonna die. So how come you don't have a backup?

The attitude indicator is the only instrument on the panel that provides a clear picture of the flight attitude of the aircraft. Without it, the pilot must try to piece together a mental picture of flight attitude by integrating information from several other instruments (turn-and-bank or turn coordinator, airspeed indicator,vertical speed indicator, altimeter) that provide only partial, indirect, and often delayed information about flight attitude.

Are you up to this task? Are you confident enough to play "you bet your life"?

Air-Driven Attitude Gyro

Most of our airplanes are equipped by an air-driven attitude indicator powered by the aircraft's vacuum or pressure system. The heart of the instrument is a gyroscope: a massive metal rotor disk that spins at about 10,000 RPM in the horizontal plane about a vertical axle. The gyro is gimballed with pivots in both the roll and pitch axes.

The rotor of the air-driven gyro is mounted in a sealed housing. Filtered air is brought into the gyro housing through passages in the rear pivot, the gimbal ring, and the side pivots. The air then blows against the rotor through two angled nozzles on opposite sides of the housing. The circumference of the rotor is machined with dozens of little bucket-like cutouts which allow the airflow to spin the rotor like a waterwheel (but a whole lot faster). Having done its work, the air then exits through exhaust ports on the lower part of the sealed gyro housing, whereupon it is sucked out of the instrument case by the vacuum pump.

The attitude indicator relies on the principle of gyroscopic rigidity in space—the rapidly-spinning rotor disk resists any attempt to disturb its orientation. Thus, the gyro provides a stable horizontal reference against which the pitch and roll attitudes of the aircraft can be measured.

The "artificial horizon bar" on the face of the attitude indicator is mechanically coupled to the gyro inside, so it remains parallel to the natural horizon. The instrument's little "symbolic airplane" is fixed tothe instrument case, however, so it follows the roll and pitch movements of the real airplane. Consequently, the relationship of the horizon bar to the symbolic airplane is the same as the relationship between the natural horizon and the real airplane.

Older AN-style instruments use gyro gimbals that are limited to about 100 degrees of bank and 60 degrees of pitch before hitting physical stops. The gyro will "tumble" if you exceed these limits by doing an aerobatic maneuver like a loop, roll, Immelman, or split-S (or by tailgating a 747 or C-5A). Once this happens, the instrument will not provide useful attitude indications unless it is manually "caged" with the aircraft visually established in straight-and-level flight. Many newer-design instruments, however, permit full 360-degree rotation about the roll and pitch axes without tumbling. Such "non-tumbling" instruments typically do not have manual caging knobs.

Tricky Erection

The only really tricky part of the attitude indicator is its erecting mechanism. When power (vacuum) is first applied to the instrument, the mechanism quickly erects the gyro spindle to plumb-vertical in order to align the artificial horizon with the natural horizon. In flight, the erecting mechanism continually compensates the gyro for precession errors in order to keep it aligned throughout the course of a long flight.

The design of the erecting mechanism is quite clever. In an air-driven attitude gyro, there are "pendulous vanes" suspended from pivots attached to the lower portion of the sealed gyro housing, one vane for each exhaust port. The vanes are positioned so that, when the gyro is erect, each vane half-covers its associated exhaust port.

If the gyro tilts so that its axle is not plumb, the pendulous vanes shift so that one exhaust port is more-than-half-covered and the opposite exhaust port is less-than-half-covered. The resulting imbalance of discharge air exerts a force on the rotor housing at right angles to the direction of tilt, causing the gyro to precess to the erect position. As soon as the gyro is erect, the pendulous vanes return to a balanced condition to remove the precessing force. Pure magic!

The erecting mechanism can can erect the gyro quite quickly at initial power-up because the gyro doesn't exhibit much gyroscopic rigidity at low rotor RPMs. Typically, the horizon bar becomes level within the first 10 or 15 seconds, and then its oscillations (caused by precessional nutation of the gyro) are damped out within a minute or so as the rotor comes up-to-speed.

Keep an eye on your attitude indicator's initial erection sequence; it's your best clue to the gyro's internal health. If the instrument doesn't erect quickly, the pendulous vane pivots may be contaminated and sticking. If the oscillations don't damp out shortly thereafter, the rotor may not be coming up to full rated speed (due to worn or contaminated spindle bearings, low vacuum, a pinched air hose, or a dirty air filter). On the other hand, if the horizon bar hardly oscillates at all during gyro spin-up, the gimbal bearings may be worn or contaminated.

Any of these early warning signs of impending instrument failure should be grounds for pulling the attitude indicator and bench-testing, overhauling, or exchanging it before further flight into IMC. Heed yourgyro's cry for help...don't wait for it to sieze in the soup!

Autopilot Pickoffs

Many autopilots—including almost all two- and three-axis autopilots—are coupled to the attitude and heading indicators. (Most simple wing-levelers, on the other hand, are coupled to a turn coordinator instead.) In autopilot-equipped aircraft, the gyro instruments are equipped with a special "autopilot pickoff" and an electrical connector towhich the autopilot's computer is attached.

The most common type of autopilot pickoff is electromagnetic (although a few autopilots use optical or pneumatic pickoffs). An electromagnetic pickoff consists of three small electromagnetic coils fixed to the instrument case, together with a moving metal vane attached to the gyro gimbal. The autopilot energizes the center coil with an alternating current, producing an oscillating magnetic field. The magnetic field is conducted by the metal vane to the two outer coils,thereby inducing an alternating voltage in those coils.

If the vane is perfectly centered between the two outer pickup coils, then the voltage induced in both coils is equal. If the vane moves off-center in one direction or the other, one pickup coil produces a higher voltage and the other produces a lower voltage. The autopilot compares the voltages coming from the two pickup coils and deduces the position of the metal vane (and therefore the gyro).

An attitude indicator in an autopilot-equipped airplane has two of these three-coil pickoff gizmos: one to sense the bank angle and the otherto sense the pitch angle. There is usually a third pickoff in the heading indicator or HSI—attached to the heading bug—that drives the autopilot's heading-hold function.

When flying on autopilot, a failure of the attitude gyro or its vacuum power source will cause the autopilot to try to follow the slowly-precessing attitude gyro as it spins down. In most installations, the autopilot has no way of detecting the failure, so it continues to flythe failing gyro until the aircraft assumes such a severe unusual attitude that it exceeds the autopilot's servo torque limits. Many in-flight structural failure and loss-of-control accidents (most of them fatal) have been attributed to gyro failure while flying on autopilot. In most cases, by the time the pilot became aware of the problem and tried to recover manually, the aircraft attitude was so severely upset (inverted and/or above Vne) that recovery was not successful prior to structural failure or terrain impact.

That's why it is so important for pilots of autopilot-equipped aircraft to maintain a vigilant instrument cross-check while the aircraft is on autopilot...and also to provide systems redundancy so that the attitude gyro won't fail in the first place.

Electric AI Backup

Of course, most singles that fly serious IFR are nowadays equipped with some sort of backup vacuum system. And twins always have two vacuum pumps. But this won't help a bit if the gyro instrument itself fails, or if a vacuum hose comes loose or chafes through. If you want the safety that only true systems redundancy can provide, you need both a backup power source and a backup instrument.

The ideal setup is to equip your airplane with two attitude indicators, one air-driven and the other electrically-powered. A 3-1/8"electric attitude indicator (RC Allen) sells for about $1,200 brand new, while yellow-tagged overhauls are available for a good deal less. This compares very favorably with the cost of a standby vacuum system, and provides a great deal of additional redundancy.

An electric attitude indicator is internally similar to an air-driven one, except that the gyro rotor is driven by an electric motor rather than by an air turbine. The rotor in the electric gyro is lighterbut spins faster (20,000+ RPM). The erection mechanism is also different (it works on the principle of magnetic induction rather than unbalanced airflow), but has the same effect of applying a corrective force at right angles to any gyro tilt, causing the gyro to precess back to plumb. Electric gyros have a considerably longer TBO than air-driven ones because they are sealed and not nearly as vulnerable to contamination.

If your aircraft is already loaded with equipment, you'll probably have a hard time finding a spare 3-1/8" panel hole in which to mount the backup electric AI. BF Goodrich/AIM makes a miniature electric attitude indicator that fits in a 2-1/4" clock hole, but it's not cheap (about $2,000). A less-expensive solution is to replace your standard 3-1/8" T&B or turn coordinator with a miniature 2-1/4" unit (about $350), or to replace your 3-1/8" VSI with a 2-1/4" VSI (about $250).

There is currently a petition for rulemaking before the FAA (submitted by Hal Shevers, CEO of Sporty's Pilot Shop) calling for the elimination of the FAR 91 requirement for a gyroscopic rate-of-turn instrument during IFR operations, provided that the aircraft is equipped with both air-driven and electrically-driven attitude indicators. Although this proposal seems to be eminently sensible, it is too early to tell whether or not the FAA will act favorably on it. But keep an eye out for an NPRM.