AOPA Pilot Magazine - March 2011 - 26

PROFICIENTPILOT

BY BARRY SCHIFF

Inertia

uring a recent rainy-day gabfest at my home airport, one of the local pilots asked me to explain the principles of inertial navigation

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 and why, in this age of GPS, inertial navigation sysmatter how slight—are fed into the INS computer, tems (INSs) are still in use by the airlines. I told them which continuously converts this information into Barry Schiff won a prestigious world that this is a subject more easily explained in writtrack and distance data that is constantly displayed aviation speed ing than during the informality of hangar flying. “So to the pilot on a display unit. record from the write about it; I can wait,” came the retort, which is Because INS accelerometers must measure USSR during the why this month’s subject deals with something that I acceleration with respect to the Earth, they must Cold War. ordinarily wouldn’t discuss in this space. I hope you be kept aligned with the Earth’s surface. This is find it interesting even though most general aviation done by mounting them on a small platform that pilots will never have a chance to use INS. is gyroscopically held parallel to the Earth’s surface in the manPrinciples of operation first: Assume that a blindfolded ner of an attitude indicator. This inertial platform is known as man is in the right seat of a stationary sports car. Before get- a “stable table” and is far more precise than a conventional ting under way, the driver informs him of their precise location attitude indicator. (using geographical coordinates) and the direction in which Before the advent of GPS, inertial navigation was the prithe automobile is headed. In other words, the man has been mary method used by the airlines and the military to navigate “programmed” or “loaded” with what he will need to maintain oceans and other remote areas of the world where radio aids track of his position. were sparsely located or nonexistent. INS is totally self-conThe driver starts the engine and accelerates the car to 60 tained. It is independent of radio signals, the Earth’s magnetic mph. Because of his mass (inertia), his body resists this accel- field, satellites, and the observation of celestial bodies. Nor eration, a resistance he feels as the seat pushes against his back. does it transmit energy (like radar) and is unaffected by wind, The greater the acceleration, the greater the push. When the sea, or atmospheric conditions. Because INS is oblivious to automobile has stopped accelerating, the passenger estimates exterior factors and cannot be jammed by an enemy, it found that it has reached a speed of approximately 60 mph. In the early military acceptance. Germany was first to use inertial meantime, he keeps track of how long the automobile main- navigation when it installed guidance systems on its lethal V-2 tains this speed. After 30 minutes, the driver slows to 30 mph, missiles during World War II. When the preset distance had a speed change detected by the passenger as his body pitches been travelled, an on-board computer sent a cutout signal to slightly forward. “Aha,” he says to himself. “I have traveled 60 the rocket engine, and the missile then fell upon its prey. These mph for 30 minutes and, therefore, have traveled 30 miles in inertial systems were thankfully crude—the London Blitz could a northerly direction.” Thirty minutes later the car comes to a have been far worse. INS has been too expensive, complex, heavy, and reliant on screeching halt. “Now,” he says, “I have traveled an additional 15 miles [30 minutes at 30 mph] for a total 45 miles.” By care- AC power to have ever found popularity in general aviation. One fully keeping track of acceleration, deceleration, and time, the exception was in 1971 when Flying Tigers Capt. Elgin Long used passenger always knows his position with respect to the starting INS during a grueling, 36,000-mile flight around the world via point. Had the driver turned the car in different directions, the both poles in a Piper Navajo. blindfolded passenger would have been aware of these course Why is INS still found aboard modern airliners? Simple. It changes by the amount in which the inertia of his body forces provides a reliable backup for GPS in case of satellite outages him to lean left or right during the turns. and so forth. The inertial platform also provides incredibly Human beings, however, cannot accurately measure accel- accurate attitude data to attitude indicators, radar stabilizaeration, keep track of time, and account for all of the velocity tion systems, and autopilots. Because INS is seldom used for changes that occur during even a short ride. This, though, is actual navigation any more, such systems are now referred to exactly what an inertial navigation system does, and it does so in as inertial reference systems, or IRSs. three dimensions. The heart of INS consists of three extremely sensitive accelerometers that measure acceleration in all three Visit the author’s website (www.barryschiff.com). 26 • MARCH 2011

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AOPA Pilot Magazine - March 2011

Table of Contents for the Digital Edition of AOPA Pilot Magazine - March 2011