The aircraft transponder is part of a sophisticated communication system between your aircraft and air traffic controllers and large aircraft. The beauty of the system is how little you as a pilot have to do to make the system work.
Crude transponders were invented during World War II to extend the usefulness of radar. Radar allowed the Allies to track aircraft though clouds or at night; however, a radar operator could not tell whether a moving dot on the screen was friendly or an enemy. To solve this dilemma "friend or foe" transponders were installed in Allied aircraft. Pilots set a secret code in the transponder, and the radar operator could identify them as friendly. Other dots on the screen without codes were likely enemy aircraft.
Civilian air traffic controllers have a similar need to identify aircraft on their radar screens. They do this by assigning a squawk code to each aircraft typically as soon as communication is established. "Cessna Two-Three-Four-Five-Lima, squawk zero-four-one-three." Your reply is simply "Cessna Two-Three-Four-Five-Lima, roger," and you enter the transponder code 0413. The controller sees your coded blip on the screen within seconds, so there is usually no need to read back the code. From then on the controller can positively identify your aircraft on radar. Now you have the advantage of an extra set of eyes to point out traffic, help you to avoid restricted airspace, and even help you if you become lost.
As air traffic increased, there were times when aircraft paths appeared to be crossing on radar, but there was no collision hazard because one airplane was well above another. However, radar could not detect whether they would safely diverge. Therefore, an altitude squawk or Mode C was added to the system. When Mode C is activated on your transponder, altitude information is obtained from an encoding altimeter, or a blind encoder. From a pilot's point of view it does not matter if the signal originates from the instrument you are viewing or a blind box. However obtained, the transponder now has the digitized altitude it can transmit when interrogated.
You should turn on your transponder - and the Mode C altitude reporting, if installed - any time you fly. Not only is this good common sense, but it's required by the federal aviation regulations. To operate in or above Class C airspace (up to 10,000 feet msl), all aircraft are required to have a Mode C transponder. You're also required to use a Mode C transponder on flights within 30 nm of a primary Class B airport, up to 10,000 feet msl.
Despite their electronic complexity, the transponder is perhaps the simplest electronic device in the aircraft to operate. Typically there is one rotary switch with positions for Off, Standby (warm up), On (basic position, or Mode A, only), and Altitude (Mode C). (If you are wondering, there is no "Mode B" in the United States.) Some transponders also have a Test position that partially checks transponder operation, but the best test is confirmation from an air traffic controller that you are "radar identified," and he or she knows your correct altitude.
Prior to takeoff, the transponder should be turned to ALT, activating transmissions of both squawk code and aircraft altitude. Of course, if your airplane does not have encoding altimeter capability, you can only transmit the Mode A signal (the squawk code). The transponder should remain on for your entire flight unless you are instructed for some reason to "squawk standby." If you are doing touch and goes, there is no need to touch the transponder, but after your final landing it should be turned off once you exit the runway. Your trainer's checklist of pretakeoff and post-landing procedures should refer to transponder operations.
Transponder push buttons or knobs allow entry of four-digit squawk codes using 0 through 7. As there are only eight possibilities for each digit position, there are 4,096 possible codes (8 3 8 3 8 3 8 = 4,096). When no code is assigned, as on a VFR flight when you are not talking to a radar controller, the transponder code 1200 should be used. Some transponders have a VFR button that automatically enters the squawk code 1200. However, when talking to an air traffic controller you can expect to be assigned a discrete four-digit code. That code should remain in the transponder until the end of the flight. You normally change the code only if the controller provides a new code or instructs you to "squawk VFR." Then you enter 1200.
When setting a new code, avoid passing through the digit "7" on the far left. This is because all codes beginning with "7" signify an emergency and will set off alarms at air traffic control centers. You should memorize three emergency codes. 7700 means "Help! Mayday!" the same as if you declare an emergency using the radio. 7600 means you have lost communication with air traffic control, and 7500 means "I've been hijacked." If you squawk 7500 the controller will covertly respond, "Confirm you are squawking 7500." If you confirm, your flight will be carefully monitored, and you can expect law enforcement personnel to surround your airplane when you land.
Occasionally an air traffic controller will ask you to "squawk Ident." This is your cue to push the Ident button. It results in a brighter and larger blip on the radar screen for positive identification. Never push the Ident button unless requested! Some pilots have been taught to press Ident after setting their initially assigned code, but this is not the correct procedure.
The Ident feature can re-establish communication if you lose the ability to transmit but can still hear the controller's voice. When you realize your transmissions are not heard, enter code 7600. The controller will reply with "November Four-Four-Six-Seven-Golf, if you hear this transmission, Ident." You respond by pushing the Ident button, and communications have been restored. As in the game of Twenty Questions, the controller will ask a question and expect your response by pushing or not pushing the Ident button. "November Four-Four-Six-Seven-Golf, I understand you are unable to transmit. If you need help with navigation, push Ident." You respond as appropriate.
Air traffic radar sites consist of a double rotating antenna. One is for the primary radar that simply bounces radio waves off aircraft, with the refection displayed as a blip on the radar screen. This is called a skin paint. The second antenna is for secondary surveillance radar, the coded signal transmitted to interrogate transponders. Transponders then respond or squawk. There is an allusion that transponders are parrots, and they respond to interrogations by "squawking" answers.
Most aircraft taken on cross-country flights have Mode C capability and automatically transmit aircraft altitude in 100-foot increments, as well as the Mode A squawk (no altitude information) of discrete four-digit codes. Depending upon the specific electronic query received, your transponder will respond with either the discrete code or altitude. As the air traffic radar computer knows what question it has asked, it knows whether the digital answer is a digital code or an altitude report.
Other than to turn on and off Mode C, a pilot has no control over altitude information sent to the ground. It is unaffected by changing the altimeter's barometric-pressure setting. The air traffic control computer provides the barometer correction electronically, displaying your correct altitude to the controller.
A controller expects you to announce your altitude to confirm the altitude on his display before depending upon it. If you don't, the controller may ask, "November Six-Nine-Seven-One-Delta, confirm level at 2,000 feet." In the event your altimeter disagrees, check your altimeter setting. It is best to obtain a local setting from the controller. An incorrect barometric-pressure setting may be the reason for a lack of correspondence. However, if the setting is correct and your altimeter and what the controller sees on his screen still differ by more than 200 feet, the controller will probably make the following request: "Cessna One-Two-Three-Five-Lima, stop altitude squawk. Your altitude differs by 500 feet." Acknowledge with "Cessna One-Two-Three-Five-Lima, roger," and turn the transponder switch counterclockwise one click from Alt to simply On.
A reply light illuminates each time your transponder is interrogated. However, the blinking light does not indicate who is interrogating you, or what they want. Large aircraft with traffic collision avoidance systems (TCAS) and some military aircraft also interrogate nearby aircraft transponders, and these will cause the reply light to flash. In busy metropolitan areas your transponder may be interrogated so frequently by commercial aircraft and different radar sites that the light remains on almost continuously.
While the federal aviation regulations permit aircraft to operate without Mode A or Mode C transponders under special circumstances, all regulations are satisfied if your aircraft has a transponder and altitude encoder. The most important regulation requires a transponder and encoder to be on at all times while in the air. This gives air traffic control the benefit of your location. So even if you are not speaking to a controller, your transponder is proudly telling everyone, "Here I am, and I am at 2,500 feet."
Dr. Ian Blair Fries is a CFI, senior aviation medical examiner, and ATP, and holds a Lear 35 type rating. He serves on the AOPA Air Safety Foundation Board of Visitors and is co-chairman of the AOPA Board of Medical Advisors.
Just the facts
The transponder should always be turned On and set to Alt (if an altitude encoder is in the aircraft).
The transponder sends the four-digit squawk code and aircraft altitude to air traffic control. The code should always be 1200, unless another code is assigned by ATC. However, if there is an emergency squawk 7500 for hijack, 7600 for communication failure, or 7700 for emergency.
Only push Ident when requested.
Ian Blair Fries
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