PBN Errors RNAV
With PBNPBN —Performance Based Navigation systems, there are three things that could cause you to not be where you think you are in the sky. The first is that the route was improperly programmed long before you arrived in the cockpit. This is known as path definition error - you wanted to fly the route X, but an error was made in the input and so you are flying, say, half a mile to the right of route X. This error can be considered to be zero for all intents and purposes. The second error is caused by the inability of the autopilot to fly the input route correctly - in other words, the route is correct, but the plane cannot accurately stay on track. This is known as flight technical error. The final component is caused by a difference between where the nav system thinks you are and where you actually are - so you may actually be in position ZZ —Coordinated Universal Time but the navigation system thinks you are half a mile from position ZZ —Coordinated Universal Time. This is known, appropriately, as navigation system error. The total of all three of these errors, added together, is total system error. It is this total system error that must be within the required limit (e.g. 1 nm for RNP 1) for 95% of the time.