Approach Procedures — Page 193, Lesson 270

I want to walk you through the approach procedures chapter, and we're going to start with the visual manoeuvring, or circling, concept. This is a critical part of instrument approach flying, so let's take it step by step. First, let's define what we're looking at. When you can't fly a straight-in approach, you might have to circle to land. The figure shows the Obstacle Clearance Altitude or Height for visual manoeuvre, which we abbreviate as OCA/H. This is the minimum altitude or height you must maintain while circling. Now, here's a key point: this OCA/H for circling is not 'aid' dependent. That means it doesn't change based on what navigation equipment you have on board. It is purely an aerodrome characteristic—it's a fixed value for that particular airport, regardless of whether you're using a VOR, an ILS, or anything else. Now, let's look at the numbers in the figure. You'll see a table with categories A, B, C, and D. These are aircraft categories, based on your approach speed. For each category, there's a corresponding OCA or OCH value. For example, for Category A, the OCA is 890 feet, with a height above aerodrome level of 375 feet. For Category B, it's 1210 feet, or 695 feet above the field. Category C is 1520 feet, or 1005 feet AAL. And Category D is 1840 feet, or 1325 feet AAL. Notice how the OCA increases as the aircraft category gets bigger—that's because faster aircraft need more room to manoeuvre, so they need a higher minimum altitude to ensure obstacle clearance. Now, let's talk about the recommended profile. The figure shows a glide path of 3 degrees, which translates to a descent gradient of 320 feet per nautical mile. That's a standard approach slope. Below that, you'll see a table with two columns: G/S KT, which is groundspeed in knots, and FT/MIN, which is the rate of descent in feet per minute. For a groundspeed of 80 knots, your rate of descent should be 420 feet per minute. At 100 knots, it's 530. At 120 knots, it's 630. At 140 knots, it's 740. And at 160 knots, it's 850 feet per minute. These are the rates you need to maintain to stay on that 3-degree glide path. Now, let's move on to the RNAV approach procedures based on VOR/DME. This is a different type of approach, and I want to be clear about the basic assumption. RNAV procedures based on VOR/DME are non-precision procedures. That means they don't provide vertical guidance like an ILS would. The reference facility is assumed to be composed of a VOR and co-located DME equipment. So you have a VOR for azimuth and a DME for distance, and they're at the same location. Before you start a flight using this RNAV system, you must ensure three things. First, the RNAV equipment is serviceable. Second, the pilot has current knowledge of how to operate the equipment to achieve the optimum level of accuracy. And third, the published VOR/DME facility upon which the procedure is based is serviceable. These are pre-flight checks—you can't just assume everything works. Now, here's a critical safety point: data insertion errors. These procedures require a computer to handle the data, and that computer has to be programmed with current promulgated data. This data is inserted by the operator or the crew, and here's the problem—the system has no method of checking for input errors. So if you type in a wrong waypoint or a wrong frequency, the system won't catch it. That means the computed positional information presented to the crew may well contain errors induced into the system. In other words, garbage in, garbage out. You have to be meticulous about what you enter. Finally, let's talk about accuracy. There are five factors that affect the accuracy of the VOR/DME RNAV system. First, ground station tolerance—how accurate the VOR and DME transmitters are. Second, airborne receiving system tolerance—how accurate your receivers are. Third, flight technical tolerance—how well you fly the procedure, your piloting skill. Fourth, system computational tolerance—how accurately the RNAV computer calculates positions. And fifth, distance from the reference facility—the farther you are from the VOR/DME, the more error can accumulate. Now, one more important point. This RNAV equipment may also be used when carrying out conventional, non-RNAV instrument procedures. But there's a condition: the procedure must be monitored using the basic display normally associated with that procedure. So if you're flying a conventional VOR approach using your RNAV system, you still need to monitor the raw VOR data on the basic display. And you must comply with the tolerances for using raw data on that basic display. You can't just rely on the RNAV computer alone—you have to cross-check with the raw data. Let me show you the figures so you can see these tables and profiles visually. So, to recap: circling OCA/H is aerodrome-specific, not aid-dependent. The 3-degree glide path requires specific descent rates based on groundspeed. RNAV VOR/DME approaches are non-precision, require pre-flight checks, are vulnerable to data entry errors, and their accuracy depends on five specific tolerances. And if you use RNAV for a conventional procedure, you must monitor the raw data. That's the core of what we've covered here.