Encoded with a transponder's return signal is the transponder's four-digit "squawk" code and, if equipped with operating Mode C, the aircraft's pressure altitude. The ATC computer uses the local altimeter setting to convert the pressure altitude to indicated altitude. Then, the aircraft's altitude, along with the four-digit squawk code and call sign (tail number) is displayed on the radar screen beside the appropriate target. The computer can also display the type of aircraft and destination if the controller desires.
Several types of transponders are used in the general aviation world. The standard "Mode 3/A 4096" designation refers to the capability of the transponder to send 4,096 different squawk codes (four digits, each set at 0 through 7). "Mode C" refers to altitude-encoding capability. A transponder obtains altitude information in two fundamentally different ways. The first way is a pressure reading that an "encoding" altimeter transmits to the transponder. More common is the "blind encoder" method in which a sensor reads the pressure of the aircraft's static system and transmits this data to the transponder. In either case, the pilot has no way to "correct" erroneous Mode C information transmitted to ATC.
Many commercial airliners and other large aircraft have Mode S transponders with a number of enhanced features including the capability to use traffic alert and collision avoidance systems (TCAS) (see sidebar).
Transponder Requirements
All aircraft operating in Class A, B, and C airspace, or above 10,000 feet MSL, must have an operating Mode C transponder. Operating transponders with Mode C are also required within 30 miles of a Class B airport. Referred to as a "Mode C veil," this airspace is identified on sectional, WAC, sectional, and VFR terminal area charts.
It's not a requirement, but it is a good idea for you to use your transponder and Mode C whenever you fly. This gives ATC your aircraft's position and altitude, and allows controllers to keep aircraft they are talking to away from you. Better yet, by requesting flight following from ATC, you can get a heads-up on other traffic in the area as well.
Sometimes you'll hear a controller call out other traffic at a certain altitude, but the controller qualifies the altitude by saying "unverified." (That means the airplane is squawking Mode C, but the controller is not handling the aircraft and therefore does not have verbal confirmation of the altitude from the pilot.)
Not only does a transponder with Mode C let ATC keep track of our position and altitude, it allows aircraft with TCAS and TCAD (traffic and collision alert device) to get a heads-up on our position without ATC intervention. This means fast-flying commercial jets and other collision-avoidance-equipped aircraft will have a better chance of seeing and avoiding us.
If we fly in airspace where a transponder is required, FARs 91.411 and 91.413 require the transponder and altitude reporting equipment be tested and inspected every 24 months. Think of it as a third-class medical for the transponder and Mode C. Maintenance personnel conducting the test must make an entry in the aircraft maintenance logs noting the date of the inspection. This certification is valid through the end of the 24th calendar month following the inspection.
If our transponder or Mode C isn't working, it doesn't necessarily mean we're banned from flying above 10,000 feet, or in other airspace that requires the equipment. ATC facilities can authorize a deviation from the regulations to allow aircraft with inoperative equipment to operate in their airspace. All we have to do is ask ATC for the deviation. If our aircraft isn't equipped with a transponder, the request should be made at least an hour before arrival in the airspace.
Using the Transponder
Using a transponder is relatively simple, but a few rules apply. First, during the preflight inspection make sure that the transponder antenna is secure, and remove any oil or grease with a paper towel or rag. Then check the transponder as part of the cockpit preflight check to make sure it's turned off and not inadvertently set to the wrong code. Unless ATC assigns a specific transponder code for your flight, you use the standard VFR 1200 code. Set the transponder to standby mode while you're operating on the ground. This energizes the circuitry, but does not allow the transponder to transmit.
When you're cleared for takeoff, turn the switch to the "ON," or "ALT" (altitude reporting or Mode C) position, if so equipped, just before you taxi onto the runway. When you see the amber light on the face of the transponder blink, you know it is receiving and replying to ATC radar signals.
When providing flight following, the ATC controller gives us a four-digit transponder squawk code and often requests that we "ident," or press the transponder's IDENT button. Pressing the IDENT button makes the target flash (become brighter or more intense) on the controller's display, making it easier for the controller to identify our location. The controller will tell us when he has "radar contact" and where ("Trainer 00000, radar contact three miles east of Downtown Airport."). He may ask us to verify the altitude reported beside the target on his radar scope. All this ensures positive identification and checks the accuracy of our Mode C indication. If another pilot mistakenly enters our code and hits the IDENT, the discrepancy in location can be quickly noted and remedied.
If a controller doesn't see the altitude readout, he may ask us to "squawk altitude," or set our transponder to the "ALT" or Mode C setting. If ATC notes a discrepancy between our assigned altitude and the Mode C readout, he'll repeat the current altimeter setting, and ask us to verify our altitude. If the Mode C altitude discrepancy is 300 feet or greater, ATC will ask us to recycle our transponder (go to standby, then back to Mode C) or to "stop altitude squawk" (deactivate the Mode C by turning the switch to the "ON" position).
Squawk Codes
Squawk codes may seem to be random numbers, but there's a method to assigning codes. The numbers, assigned by the ATC computer, tell a lot about the aircraft. "The last two digits of the squawk code identify a specific aircraft and the first two digits provide information about that aircraft," says Pat Fahey of the Providence Terminal Radar Approach Control (TRACON) facility in Rhode Island.
"Each ATC facility is assigned banks of two-digit codes that have specific meaning within the system," Fahey says. "Each ATC facility has specific codes for VFR arrivals landing at the primary airport, for aircraft passing through the airspace and receiving radar advisories, and for high- and low-level departures headed in various directions. For example at Providence, 34, 35, and 36 indicate high-altitude westbound departures that will be handed off to Cleveland."
When ATC receives a handoff from another sector or ATC facility, the first two digits of the squawk code give the receiving controller an idea where the aircraft is going and what it's doing.
Several squawk codes are reserved for special operations and emergencies, and all pilots should know these codes. According to the Aeronautical Information Manual (AIM), pilots can use the general emergency squawk code - 7700 - in a distress or urgency condition. This code triggers an alarm or indicator at ATC radar facilities. If possible, pilots should also establish voice communication with ATC should such a situation arise.
Pilots can inform ATC of a loss of communications capability by squawking 7600. Instrument pilots are familiar with this code, but VFR pilots, even if they aren't communicating with ATC, can use it too. "This code also triggers an alarm," notes Fahey. "If we see an aircraft on our screen squawking 7600 and headed for an airport, we can call that airport, even if it's uncontrolled, and alert personnel to the fact that an aircraft is inbound with no radio. If we have IFR aircraft inbound to the airport, we can take the necessary steps to avoid potential conflicts."
Squawking 7500 tells ATC an aircraft has been hijacked, or unlawfully forced to a new destination. As with the 7700 and 7600, this code triggers an alarm at ATC. A controller will acknowledge the 7500 code and ask the pilot to verify it. In such a situation, ATC will provide flight following, comply with pilot requests, and notify the proper authorities.
The transponder may appear to be an innocuous black box, but the information it provides to pilots and controllers is critical to operating in today's airspace and high density traffic.
Electronic See and Avoid
Since the dawn of aviation, pilots have been taught to see and avoid other aircraft. The advent of ATC radar has allowed controllers to provide another set of "eyes" to help pilots spot traffic. Recent years have seen the advent of electronic devices that add one more means of alerting pilots to other air traffic.
The first is TCAS, the Traffic Alert and Collision Avoidance System. TCAS gives pilots a radar-like screen on their instrument panel that depicts the bearing, distance, and altitude of other aircraft with operating Mode C transponders. If an aircraft is too close, TCAS alerts the pilot and provides a command to either climb or descend.
TCAS is an expensive system mandated on airliners, but a more affordable version of the system called TCAD (Traffic and Collision Alert Device) is available. Like TCAS, it determines the proximity of other aircraft using their Mode C transmissions, but TCAD gives the pilot range (distance) and relative altitude of conflicting traffic only. No azimuth, or bearing to the traffic, is provided. The pilot must decide which way to go to avoid the traffic.
Considering that a pilot really only needs to know whether he should climb or descend to avoid hitting another aircraft, the information provided by TCAD is usually sufficient. The relatively low cost (around $9,000 uninstalled for TCAD versus about $20,000 for the least expensive TCAS) puts TCAD in financial range of general aviation aircraft owners. The only TCAD on the market is manufactured by Ryan International, 4800 Evanswood Dr., Columbus, OH 43229.
Robert N. Rossier
Related Articles
