Area Navigation Systems (RNAV) — Page 282, Lesson 278

We're starting a new topic now: Area Navigation Systems, or RNAV. This is the chapter where we look at how the Flight Management Computer — the FMC — actually ties the whole navigation picture together. Let me set the scene for you. The aircraft has two FMCs, and they work as a pair. One of them is selected to feed its information to the other aircraft systems — the autopilot, the flight director, the EFIS displays. The other FMC is the backup. Now, here's a key point: the two FMCs will not show exactly the same position. There will be a difference in position between the two FMCs, and there will also be a difference between the EFIS and the non-selected FMC. That's normal — you're comparing two independent computers, and they each compute their own position. If one FMC becomes inoperative, the serviceable FMC simply carries out all the functions. But if both FMCs fail, then the system degrades. The IRS information — the Inertial Reference System — is used directly in the EFIS, so you still get attitude and heading and basic position on the displays. But the automatic performance functions are lost. No auto-thrust management, no automatic flight planning. So you're flying on raw inertial data. Now let's look at the principle of operation. This is the heart of it: Triple IRS, Twin FMC. Positional information and heading from the triple INS/IRS — that's three Inertial Navigation Systems — is fed into the FMC. The FMC compares the three positions to check for any system having gross errors. If one IRS is way off, the FMC flags it. Then it averages the remaining good ones. That averaged position can then be compared with an external reference. The external references available are DME/DME, VOR/DME, or GNSS. So the FMC blends the inertial data with radio or satellite data. The FMC uses something called Kalman filtering to produce position and velocity. Kalman filtering is a mathematical technique that optimally combines noisy measurements to estimate the true state — here, position and velocity. This filtering can be done purely using the IRS information, or using a combination of IRS and external reference. So you have two modes: pure inertial, or blended inertial-plus-radio. Now, there's a fascinating operational detail at high latitudes. When operating at latitudes in excess of 84 degrees, the FMC de-couples the IRS. Here's how it works: the left FMC uses the IRS in the order left, centre, right. The right FMC uses the IRS in the order right, centre, left. Over a short period of time, each FMC changes its FMC position to the appropriate IRS position. So the left FMC locks onto the left IRS, the right FMC locks onto the right IRS. Why do they do this? It's about the mathematics of longitude. The calculation of change of longitude from departure is a function of the secant of latitude. At latitudes approaching 90 degrees, the secant increases rapidly. Let me give you the numbers: sec 86°00' equals 14.3356, and sec 86°01' equals 14.3955. Look at that — just one minute of arc in latitude changes the secant by over 0.06. That means a small error in latitude results in a large error in the calculation of change of longitude. So the two IRSs, which are physically very close together, would appear to diverge hugely in terms of computed longitude, even though their actual positions are nearly identical. By de-coupling — by having each FMC track its own IRS — you avoid that apparent divergence. So that's the architecture: triple IRS feeding twin FMC, with Kalman filtering, external references for comparison, and a special high-latitude procedure to handle the secant problem. That's the foundation of RNAV.