Let’s start at the top — Page 175, Lesson 174

I want to walk you through a detailed Instrument Approach Chart—the kind you’ll use in real IFR flying. Let’s start at the top. First, we have the transition altitude. On this chart, it’s set at 6000 feet based on QNH—that’s the altimeter setting that gives you altitude above mean sea level—and 5923 feet based on QFE, which gives you height above the airport’s touchdown zone. The difference between those two numbers is 77 feet, and that 77 feet exactly equals the elevation of the runway touchdown point. So if you set QFE, your altimeter reads zero when you touch down; if you set QNH, it reads the field elevation. Now, moving to the Minimum Safe Altitude, or MSA. This is supplied by the controlling authority—the air traffic control or air navigation service provider. The MSA is the lowest altitude that gives you up to 1000 feet of obstacle clearance within a 25 nautical mile radius from the navigational facility on which the MSA is based. If the radius is something other than 25 NM, that will be stated explicitly. This altitude is for emergency use only—it’s not for normal navigation—and it does not guarantee that you’ll receive the navaid’s signal. When the MSA is divided into sectors, and each sector has a different altitude, those altitudes are called minimum sector altitudes. In this particular chart, one of those sector altitudes is 2300 feet. Next, a note about DME—Distance Measuring Equipment. The chart says: advise ATC if you are unable to receive DME. If you can’t receive it, an equivalent radar fix will be provided at 7.5 NM and 4 NM from the ILL ILS localizer. The LOC—that’s the localizer, the lateral guidance part of the ILS—is not available without the ILS DME. So you need that DME to use the localizer. The initial and intermediate approach are valid up to 220 knots indicated airspeed. And the ILS DME reads zero at the threshold of runway 27. Now, the ILS localizer frequency is 109.5 MHz, and its call sign is ILL. There’s an asterisk—*—which means part-time operation. So you need to check the times of operation. The IAF—Initial Approach Fix—is based on the BOVINGDON VOR/DME. Its frequency is 113.75 MHz, call sign BNN. The D in VOR/DME means Distance Measuring Equipment is available. If the BNN VOR/DME is unserviceable—out of service—you use the position at BOVVA, which is defined by the 321 radial at 32 NM from the Biggin VOR/DME. You’d see that on the JM STAR 10-2A chart. The Missed Approach Holding is based on the CHILTERN Non-directional Beacon, or NDB/Locator. Its call sign is CHT, frequency 277 kHz. The holding pattern is a race-track pattern, inbound on a magnetic heading of 293 degrees, to the overhead, then a left turn outbound onto 113 degrees magnetic. The route from the BNN VOR/DME is an approach transition. The 096 radial and the small arrowhead on the chart represent a cross radial—that’s a radial from another navaid that helps you pinpoint your position. There’s also a heading: 274 degrees magnetic, which is the QDM—that’s the magnetic bearing from the aircraft to the station, or the heading to fly directly to the station in zero wind. Next, you’re told to refer to the en route chart E(LO)2. That chart extends from the ground up to 1400 feet, and it operates from 0800 to 2359 local time, and from 1400 to 2200 local time when NOTAMed—meaning if a NOTAM changes the hours, those are the times. We also see the number 77 feet again—that’s the touchdown zone elevation difference we already covered. Then we have 1410 feet QNH and 1333 feet QFE, with that same 77-foot difference. Finally, the aircraft approach category is D. That category is based on a speed of 141/165 knots, which is 1.3 times VSO. VSO is defined as the stall speed or the minimum steady flight speed in the landing configuration. For this approach, the Obstacle Clearance Altitude/Height, abbreviated OCA/H, is 264 feet on QNH and 187 feet on QFE. That’s the lowest altitude or height at which you can descend on the approach while still clearing obstacles by the required margin.