Definitions — Page 11, Lesson 10

I want to walk you through the definitions that govern how we speak numbers on the radio. This is foundational for every single transmission you will make as a professional pilot, so let's get it exactly right. First, there is a specific rule for numbers that describe Altitude, Height, Visibility, and Runway Visual Range (RVR). If those numbers contain whole HUNDREDS or THOUSANDS, we use the words "Hundred" and "Thousand". If they don't contain whole hundreds or thousands, we use single digits. Let me give you the examples from the book. The number 100 is spoken as "WUN HUNDRED" — not "one zero zero". The number 2500 is spoken as "TOO TOUSAND FIFE HUNDRED" — not "two five zero zero". The number 11,000 is spoken as "WUN WUN TOUSAND" — not "one one zero zero zero". And 25,000 is spoken as "TWO FIFE TOUSAND" — not "two five zero zero zero". Now, there is an important exception. For a Squawk code, like Squawk 6500, we do not use "hundred" or "thousand". Instead, we say "Squawk six fife zero zero" — using single digits. So the rule about "hundred" and "thousand" applies only to Altitude, Height, Visibility, and RVR. Next, let's move to VHF frequencies and channel spacing, which is shown in Figure 1.1. The bandwidth allocated to VHF frequencies is, for the most part, 25 kHz. That means the spacing between one channel and the next is 25 kilohertz. However, in busy types of airspace designated by the authorities, the spacing has been reduced to 8.33 kHz — which is exactly one third of 25 kHz. This creates many more channels in the same frequency band. Now, how we say and set these frequencies depends on the channel spacing. Channels spaced by 25 kHz require 5 digits, with no more than 2 digits after the decimal point. For example, a frequency like 118.10 has five digits total — 1-1-8-1-0 — and two digits after the decimal. Channels spaced by 8.33 kHz require 6 digits, with 3 digits after the decimal point. For example, 118.105 has six digits — 1-1-8-1-0-5 — and three digits after the decimal. However, ICAO has now set a standard that requires all 6 digits of communication frequencies be passed in RTF instructions — that's radiotelephony, the spoken transmissions. This means that VHF channels will include 6 digits in the transmission, irrespective of whether the channel is 25 kHz or 8.33 kHz spaced. So even a 25 kHz channel like 118.10 would be transmitted as "one one eight decimal one zero zero" — six digits. There is one exception to this rule: where the final two digits are both zero. In that case, only the first four digits need to be transmitted. So if the frequency is 118.00, you would say "one one eight decimal zero zero" — four digits — not "one one eight decimal zero zero zero". Finally, a practical note about the radio equipment. When using a 5 digit radio — which is a radio designed for 25 kHz spacing — only the first 5 digits of the given frequency need be set on the radio. So if you are given a 6-digit frequency like 118.105, you would only set 118.10 on a 5-digit radio. The radio cannot tune to the 8.33 kHz spacing, so you simply use the nearest 25 kHz channel.