Altimeter subscale setting to obtain altitude above mean sea level — Page 66, Lesson 50

I want to walk you through the altimeter subscale setting and the associated radio navigation terms you'll use in professional flying. Let's start with the Q-codes — these are three-letter codes that originated in Morse code telegraphy but are still standard in aviation today. First, QDM is the magnetic direction towards a facility — in plain language, if you're flying a QDM, you're on a magnetic heading that will take you directly to the VHF direction finding station, usually an aerodrome. Next, QDR is the magnetic bearing from a facility — that's the radial you're on, measured magnetically from the station outward. Then QTE is the true bearing from a facility, so it's the same idea as QDR but measured relative to true north instead of magnetic north. Finally, QUJ is the true bearing to a facility — the true-north equivalent of QDM. Now, the equipment that provides these bearings is called VHF DF, abbreviated VDF — VHF Direction Finding. Most air traffic control units, usually on the approach frequency, can give you bearing or direction information based on your transmission. The direction finding equipment can give you a "steer" — that's a QDM towards the aerodrome — or it can give you your true or magnetic bearing from the airfield, which would be QTE or QDR. Let me give you an example of how a pilot requests this. You'd say: "QDM QDM QDM Oxford Approach G-BODA request QDM G-BODA." Notice the transmission ends with the aircraft call sign repeated. That's a standard format. Here's a critical point: the heading you get from a VDF steer takes no account of wind effects — no drift correction. So if you fly that heading exactly, you'll be pointing towards the station, but crosswind will push you off track. You'll need to correct for drift yourself. Now let's talk about the class of bearing. This tells you how accurate the bearing information is. There are four classes: - Class A is within plus or minus 2 degrees — the most accurate. - Class B is within plus or minus 5 degrees. - Class C is within plus or minus 10 degrees. - Class D is less accurate than Class C — so worse than plus or minus 10 degrees. When ATC gives you a bearing, they'll also state the class so you know how much you can trust it. Next, let's cover radio test procedures. Before flight, it's wise to ensure your radios will transmit and receive properly. There's a standard procedure for each radio requiring a test, and it uses the Readability Scale. This is a 1-to-5 scale that describes how well you can hear the other station: - 1 — Unreadable - 2 — Readable now and then - 3 — Readable but with difficulty - 4 — Readable - 5 — Perfectly readable So if you call for a radio check, the reply might be "Readability 5" meaning your transmission is perfectly readable. The format of a test transmission should follow this order: 1. The identification of the station being called — for example, "Oxford Approach" 2. Your aircraft identification — "G-BODA" 3. The words "Radio Check" So a complete test call would sound like: "Oxford Approach G-BODA Radio Check." That covers the altimeter subscale setting context, the Q-codes for VDF bearings, the classes of bearing accuracy, and the standard radio test procedure with the readability scale. These are all fundamental procedures you'll use every time you operate in controlled airspace.