We have four Mode A codes to cover — Page 126, Lesson 204

Let's talk about transponder codes first, because these are the exact numbers you'll be setting in the cockpit, and each one tells air traffic control something specific. We have four Mode A codes to cover. Mode A is the transponder mode that replies to a ground interrogation with a four-digit code — that's the identity squawk. The first one is Mode A code 7500. This code indicates unlawful interference — that's the hijack code. Now, an important point: its use does not imply that the fact is being generally advertised. In other words, setting 7500 doesn't broadcast to the world that you're being hijacked. Discretion and confidentiality will be preserved by the ATC authority until the pilot mentions the fact by RTF — that's radiotelephony, your voice radio. So ATC will keep it quiet until you actually say it over the radio. A pilot may prefer to use the 7700 squawk to indicate the severity of the situation — 7700 is the general emergency code, and it can be used instead to signal how serious things are. Next, Mode A code 7000. This code indicates that the aircraft is operating in an area where a radar service is available from an ATCU — that's an Air Traffic Control Unit — but the aircraft is not in receipt of that service. It implies that the aircraft is operating under VFR — Visual Flight Rules. So 7000 is essentially the "conspicuity" code for a VFR aircraft that's not being actively controlled. Then Mode A code 2000. This code is used to indicate that an aircraft is entering an area where a radar service is available and will be requesting that service. It's usually used by aircraft entering a domestic FIR — that's a Flight Information Region — from an Oceanic control area. So it's the "I'm coming in and I want radar service" code. Finally, Mode A code 0000. This code is reserved to indicate that the aircraft transponder is in some manner unserviceable or inaccurate. So if your transponder is broken or giving wrong information, 0000 is the code you set. Now let's move to a different topic — Restricted, Prohibited or Danger Areas. Each state has the right to restrict or prohibit flight in territorial airspace for reasons of security or safety. Such areas are known as danger areas, indicated by the letter D; restricted areas, indicated by the letter R; or prohibited areas, indicated by the letter P. These are detailed in the AIP — the Aeronautical Information Publication. Each area is designated by a code identifying the area and showing the altitude — usually in thousands of feet — to which the area extends. So the designator tells you both the identity and the vertical limit. Areas may be either permanently active, abbreviated PERM, or activated by NOTAM — a Notice to Airmen. Here's a key rule about the designator itself: the area designator — for instance, D001 — cannot be re-used for a period of not less than 12 months after the closure of the previously designated area. Why? This allows for a full reprint of the half-million topographical charts so that no confusion can exist. If a chart still shows D001 as active, you won't accidentally think a new D001 is the same old area. So to tie it together: the transponder codes are your electronic identity and emergency signalling, and the danger/restricted/prohibited areas are the airspace you must know about before you fly into it. Both are part of the Rules of the Air you'll be tested on.