Approach Procedures — Page 175, Lesson 253

Let’s start with the core idea: in instrument approach procedures, a fix is a defined point in space that you use to navigate, and its accuracy is absolutely critical. Why? Because the width of the MOC area — the Minimum Obstacle Clearance area — is built around that fix. If the fix is sloppy, the whole protected airspace is sloppy, and that’s a safety problem. Now, fixes are generally based on data from radio navigation aids, like a VOR. A fix can be an ‘on top’ to a beacon — meaning you’re directly over it — or it can be a point defined by a DME range on a specified radial, which is called a waypoint. Either way, the accuracy of the fix is paramount. Here’s the trade-off you need to remember. For ‘on top’ fixes, the type of aid is a limiting factor. A VOR is designed to give accurate track guidance — it tells you which radial you’re on very precisely — but it’s quite poor at giving an ‘on top’ indication. That’s because of the ‘cone of confusion’ directly over the beacon, where the signals become unreliable. An NDB, on the other hand, gives a better ‘on top’ indication, but its track guidance is worse. So you can’t have both from one aid. When you need an accurate ‘on top’ — for example, the outer marker of a CAT 1 ILS — a 75 MHz marker beacon is used. You’ll also hear it called a ‘Z’ marker or a ‘fan’ marker. It’s designed specifically for that purpose: giving a precise overhead indication. Now, what if a fix is specified using information from two separate systems, like a VOR radial and a DME range? Then the inaccuracies of each system must be aggregated — added together — to define a fix tolerance area. That’s the zone within which the fix could actually be, given the errors of both aids. Let me give you the assumed bearing errors for fixes. For a VOR, it’s ±4.5 degrees. For an ILS localizer, ±1.4 degrees. For an NDB, ±10.3 degrees. Notice how much larger the NDB error is — that reflects its poorer accuracy. Next, track guidance accuracy. The width of the MOC area also depends on how accurately the navigation aid guides you along the track. The track accuracy values are: VOR ±5.2 degrees, ILS localizer ±2.4 degrees, and NDB ±6.9 degrees. So again, the NDB is the least accurate for track guidance, and the ILS localizer is the best. Finally, fix tolerances for other navigation aids. Terminal Area Surveillance Radar, or TAR, within 20 nautical miles, has a tolerance of ±1.5 kilometres, which is 0.8 nautical miles. En route Surveillance Radar, RSR, within 40 nautical miles, is ±3.1 kilometres, or 1.7 nautical miles. And DME has a tolerance of ±0.46 kilometres, or 0.25 nautical miles, plus 1.25 per cent of the distance to the antenna. That last part is important — the DME error grows with distance, so the further you are from the antenna, the larger the tolerance. So the takeaway: every fix and every track has a defined tolerance, and those tolerances directly shape the protected airspace. The better the aid, the tighter the tolerance, and the more precise your navigation can be.