Proficient Pilot

Inertia

March 1, 2011

During a recent rainy-day gabfest at my home airport, one of the local pilots asked me to explain the principles of inertial navigation and why, in this age of GPS, inertial navigation systems (INSs) are still in use by the airlines. I told them that this is a subject more easily explained in writing than during the informality of hangar flying. “So write about it; I can wait,” came the retort, which is why this month’s subject deals with something that I ordinarily wouldn’t discuss in this space. I hope you find it interesting even though most general aviation pilots will never have a chance to use INS.

Principles of operation first: Assume that a blindfolded man is in the right seat of a stationary sports car. Before getting under way, the driver informs him of their precise location (using geographical coordinates) and the direction in which the automobile is headed. In other words, the man has been “programmed” or “loaded” with what he will need to maintain track of his position.

The driver starts the engine and accelerates the car to 60 mph. Because of his mass (inertia), his body resists this acceleration, a resistance he feels as the seat pushes against his back. The greater the acceleration, the greater the push. When the automobile has stopped accelerating, the passenger estimates that it has reached a speed of approximately 60 mph. In the meantime, he keeps track of how long the automobile maintains this speed. After 30 minutes, the driver slows to 30 mph, a speed change detected by the passenger as his body pitches slightly forward. “Aha,” he says to himself. “I have traveled 60 mph for 30 minutes and, therefore, have traveled 30 miles in a northerly direction.” Thirty minutes later the car comes to a screeching halt. “Now,” he says, “I have traveled an additional 15 miles [30 minutes at 30 mph] for a total 45 miles.” By carefully keeping track of acceleration, deceleration, and time, the passenger always knows his position with respect to the starting point. Had the driver turned the car in different directions, the blindfolded passenger would have been aware of these course changes by the amount in which the inertia of his body forces him to lean left or right during the turns.

Human beings, however, cannot accurately measure acceleration, keep track of time, and account for all of the velocity changes that occur during even a short ride. This, though, is exactly what an inertial navigation system does, and it does so in three dimensions. The heart of INS consists of three extremely sensitive accelerometers that measure acceleration in all three planes of an airplane’s motion: fore and aft, right and left, and up and down. No movement of an airplane goes undetected. For example, they can measure any acceleration between 0.0004 and 10 Gs. This means that an INS can measure the acceleration of an object requiring an hour to accelerate from 0 to 1 mph. At the other extreme, it measures the acceleration of a missile that goes from a standstill to 60 mph in only a quarter of a second. All aircraft speed, course, and altitude changes—no matter how slight—are fed into the INS computer, which continuously converts this information into track and distance data that is constantly displayed to the pilot on a display unit.

Because INS accelerometers must measure acceleration with respect to the Earth, they must be kept aligned with the Earth’s surface. This is done by mounting them on a small platform that is gyroscopically held parallel to the Earth’s surface in the manner of an attitude indicator. This inertial platform is known as a “stable table” and is far more precise than a conventional attitude indicator.

Before the advent of GPS, inertial navigation was the primary method used by the airlines and the military to navigate oceans and other remote areas of the world where radio aids were sparsely located or nonexistent. INS is totally self-contained. It is independent of radio signals, the Earth’s magnetic field, satellites, and the observation of celestial bodies. Nor does it transmit energy (like radar) and is unaffected by wind, sea, or atmospheric conditions. Because INS is oblivious to exterior factors and cannot be jammed by an enemy, it found early military acceptance. Germany was first to use inertial navigation when it installed guidance systems on its lethal V-2 missiles during World War II. When the preset distance had been travelled, an on-board computer sent a cutout signal to the rocket engine, and the missile then fell upon its prey. These inertial systems were thankfully crude—the London Blitz could have been far worse.

INS has been too expensive, complex, heavy, and reliant on AC power to have ever found popularity in general aviation. One exception was in 1971 when Flying Tigers Capt. Elgin Long used INS during a grueling, 36,000-mile flight around the world via both poles in a Piper Navajo.

Why is INS still found aboard modern airliners? Simple. It provides a reliable backup for GPS in case of satellite outages and so forth. The inertial platform also provides incredibly accurate attitude data to attitude indicators, radar stabilization systems, and autopilots. Because INS is seldom used for actual navigation any more, such systems are now referred to as inertial reference systems, or IRSs.

Barry Schiff won a prestigious world aviation speed record from the USSR during the Cold War. Visit the author’s website at www.barryschiff.com.