“G-CD, squawk Alpha two one six one” — Page 249, Lesson 323

Right, let's pick this up with the transponder and how we actually use it in the air. We've just heard the controller say, "G-CD, squawk Alpha two one six one," and the pilot's correct response is, "Squawking Alpha two one six one G-CD." Let me break that down for you. "Squawk" is just the aviation term for setting a specific code into the transponder. "Alpha" is the phonetic for the letter A, and "two one six one" is the four-digit code. So the pilot is confirming they have set code A2161. That's the standard readback. Now, there's a very important facility on the transponder called Squawk Ident. This is a feature that lets the radar controller automatically identify your specific radar contact using SSR — that's Secondary Surveillance Radar — without having to rely on the specific code transmission. In other words, instead of matching your code to your blip, the controller can just ask you to press the Ident button, and your blip will flash or highlight on their screen. This facility is activated by selecting the 'Ident' feature on the transponder controller, and you do it only in response to a request to "squawk Ident." The key rule here is that pilots are not to squawk Ident except on instruction from the radar controller. You never press it on your own initiative. Next, let's look at Special Codes. The airborne equipment can transmit any of 4096 individual coded responses, and certain specific ones have been allocated specific meanings. These are the ones you must know cold: - Emergency is A/7700. - Radio failure is A/7600. - Unlawful interference is A/7500. There's also an important note here: if an aircraft has been radar identified by a transponder code and subsequently experiences an emergency, the pilot should stick to the identified squawk. Don't change it — stay on the code you've been identified with. In addition to those special codes, there are reserved codes with specific meanings: - A/7000 is for Conspicuity. This is set when the aircraft is in an area where radar is used to provide ATS — that's Air Traffic Services — but the pilot is not in receipt of a service. That's typically VFR outside CAS, which is Controlled Airspace. - A/2000 is used in the absence of any ATC direction or regional air navigation agreement, or when outside of a radar control or service area. This code is set when eastbound in the NAT region — that's the North Atlantic. - A/0000 indicates an unserviceable transponder. It's set as directed by ATC. Now let's move to the Altitude Reporting Function, which is Mode Charlie. Mode Charlie encodes and transmits the aircraft altitude with reference to SPS, which is 1013 hPa — that's the standard pressure setting. Whenever the transponder is operating, Mode Charlie should also be selected. At some point during initial communications with ATC, the pilot will be requested to "say aircraft level for Charlie." The pilot should report the aircraft level information accurate to the nearest full 100 feet. From this, the ATCO — the Air Traffic Control Officer — will determine the accuracy of the Mode Charlie data. If the Mode Charlie data is within plus or minus 300 feet of the stated level, Mode Charlie is deemed to be accurate. Finally, let's consider Transponder Failure. The failure of a transponder in the air will adversely affect the quality of the ATC service provided. While every effort will be made to permit the flight to continue to the destination in accordance with the filed FP — that's the Flight Plan — it may be that ATC clearances may be restricted. And if a transponder fails before departure and it cannot be rectified, the pilot is to... well, that's where we'll pick up next. Let me just make sure you've got the key numbers locked in: 7700 for emergency, 7600 for radio failure, 7500 for unlawful interference, 7000 for conspicuity, 2000 for the NAT eastbound, and 0000 for an unserviceable transponder. And remember, Mode Charlie accuracy is within plus or minus 300 feet of the stated level.