Definitions — Page 15, Lesson 21

I want to walk you through the definitions that open this chapter on communications. We’ll start with something very practical: the range you can expect from your VHF radio. The range of VHF radio in nautical miles is about 1.25 times the square root of the aircraft height in feet. So if you’re at 10,000 feet, the square root is 100, and 1.25 times that gives you roughly 125 nautical miles. That’s a quick mental rule. A slightly rougher version works in flight levels: about 12 times the square root of the flight level. Flight level 100, square root is 10, times 12 gives you 120 nautical miles — close to the same answer. Now, the full formula also takes into account the height of the transmitting aerial when it’s not at mean sea level. That formula is: Range equals 1.23 times the sum of the square root of h1 plus the square root of h2. Here, h1 is the height of the ground aerial in feet above mean sea level, and h2 is the aircraft altitude in feet above mean sea level. So if your ground station is on a hill at 500 feet AMSL, you include that height in the calculation. The 1.23 constant is slightly different from the 1.25 in the simple rule — that’s because the full formula accounts for both ends of the path. VHF propagation is covered in greater detail in Chapter 7, so we’ll come back to the physics later. Now, the rest of this excerpt is an appendix — Appendix 1A — which lists abbreviations used in AIS publications, that’s Aeronautical Information Service publications. There are two important footnotes at the top. The first says that when radiotelephony is used, the abbreviations and terms are transmitted as spoken words. The second says that when radiotelephony is used, the abbreviations and terms are transmitted using the individual letters in non-phonetic form — so you say the letters themselves, not the phonetic alphabet words. Let me walk you through the abbreviations listed here. A stands for Amber. AAA, or AAB, AAC and so on in sequence, means Amended meteorological message — that’s a message type designator. A/A is Air-to-Air. AAL is Above Aerodrome Level. ABM is Abeam. ABN is Aerodrome Beacon. ABT is About. ABV is Above. AC is Altocumulus, a cloud type. ACARS is the Aircraft Communications Addressing and Reporting System. ACAS is Airborne Collision Avoidance Systems. ACC with a double-dagger symbol means Area Control Centre or Area Control. ACCID is Notification of an Aircraft Accident. ACFT is Aircraft. AFS is the Aeronautical Fixed Service. AFT means After, either time or place. AFTN with a double-dagger is the Aeronautical Fixed Telecommunication Network. A/G is Air-to-Ground. AGA stands for Aerodromes, Air Routes and Ground Aids. AGL is Above Ground Level. AGN is Again. AGNIS is Azimuth Guidance for Nose-In Stand. AIAA is Area of Intense Air Activity. And AIC begins the next entry, which continues beyond this excerpt. That double-dagger symbol you see next to some entries — like ACC and AFTN — refers back to the footnote: when used in radiotelephony, those abbreviations are transmitted as individual letters, not as spoken words. So you’d say “A-C-C” letter by letter, not “Area Control Centre” as a phrase.