OK. I'm no expert on this myself, but for someone who claims to have just figured out how an attitude indicator works, the author certainly doesn't explain it very well. I was left with the impression that the author is uncertain of the distinction between an accelerometer and a gyroscope. An accelerometer, like he says, can be likened to a hanging weight or pendulum. Accelerometers can measure the direction of gravity of a stationary object because the only force acting on the pendulum is acceleration due to gravity. However, as soon as the object is moved, other acceleration vectors are added to the already-present gravity vector.<p>(Imagine an accelerometer hanging from a rope. If you swing that rope in a circle around your head, the accelerometer will report that the force vector points to its bottom, away from your head, and not down to your feet as gravity alone would indicate.)<p>Which gets me to the OP. Because the article does not make a single mention of angular momentum (among other reasons), I would guess that the author doesn't understand how gyroscopes work. A gyroscope is essentially a spinning disc-shaped mass that resists change to its axis of rotation. If you built a housing that allowed it freedom of motion in three dimensions (called a gimbal (<a href="http://en.wikipedia.org/wiki/Gimbal)" rel="nofollow">http://en.wikipedia.org/wiki/Gimbal)</a>), it would remain on its original axis no matter how you tilted its enclosure (barring the inevitable energy loss due to friction, etc.)<p>So, in the case of an airplane, you would set the disc spinning on level ground. Then, while you're flying, any changes to your pitch or roll would be <i>around</i> the gyroscope (which, remember, is allowed to move freely relative to the cabin). The gyroscope wants to preserve its angular momentum, so it will still be spinning on the same axis relative to level ground. Any difference between the gyro and the cabin therefore indicates the orientation of the plane relative to the ground.<p>You'd probably get a more concise explanation from the Wikipedia page on Gyroscopes (<a href="http://en.wikipedia.org/wiki/Gyroscope" rel="nofollow">http://en.wikipedia.org/wiki/Gyroscope</a>). The Wikipedia page on attitude indicators (the visual display for a cockpit gyro) is also quite coherent and less prone to rambling (<a href="http://en.wikipedia.org/wiki/Attitude_indicator" rel="nofollow">http://en.wikipedia.org/wiki/Attitude_indicator</a>).<p>[EDIT: Toned down some grumpiness. I must be tired.]