Transponder Code Sets

One of the most straightforward pieces of electronics has some oddities in its use.


Transponders have been with us since the 1950s. They have improved greatly over the years such as the addition of providing pressure altitude in 100-foot increments and other aircraft information but essentially perform the same basic function, aiding in the positive identification of a radar contact.

This is accomplished by two means. First, through the four-digit octal code assigned by ATC. Second, through secondary surveillance radar (SSR) that derives a target strictly from a transponder return presented on a radar display.

Octal Grouping

The transponder codes themselves are encoded in binary digits (1s and 0s), not the octal you enter or the more familiar base ten system we use daily. They become 12-bit codes, three bits (000) for each octal number entered. By representing the codes in octal format, only four knobs are needed vice twelve. Since the decimal numbers “8” and “9” are not used in octal we end up with 4096 possible codes (0000-7777). Remember, the next number after 17 in octal is 20 not 18.

It’s always fun to challenge a student to enter a code of let’s say 4069 and see how they react. Even many old-time pilots don’t realize the lack of those two digits in the transponder until pointed out to them.

Code Blocks

ATC deals with code blocks that produce discrete and non-discrete transponder codes. A code block is defined by the first two octal digits of the code (example 00##, 77##). This results in 64 different code blocks. When a transponder code ends in “00” this is considered a non-discrete code. The VFR code of 1200 is non-discrete since it can be used by many different aircraft simultaneously. Since there are only 64 code blocks this also means there are only 64 non-discrete codes ending in “00.” The one non-discrete code that should never be assigned or used is “0000.”

There are several non-discrete codes that actually apply to IFR flight though not the kind of flying your average general aviation pilot would be conducting. Non-discrete codes are typically reserved for radar facilities that are not equipped to handle discrete decode as well as for other purposes such as emergencies (7700, 7600, 7500). These three codes are universal emergency codes.

Discrete Codes

Whenever you receive a transponder code where the last two digits are anything but “00” you have a discrete code. There are 63 discrete codes in every block with 4,032 total. This is where it gets interesting since we know there are more than 4,032 aircraft flying at any one moment. Each ATC facility has a set of codes they can use broken down into primary, secondary and tertiary codes.

Through statistical analysis these are setup to minimize the chances that two aircraft with identical discrete codes will be in the same sector. Code blocks are also setup for use by ATC to assign to aircraft on internal flight plans, that is they will not go outside the boundaries of a particular ATC facility and thus there is no concern over any transponder code conflicts.

There is one discrete code that acts more like a non-discrete code and that is 1202. This is reserved for glider operations.

Due to the complexity of assigning a code to aircraft with external flight plans—those crossing various ATC boundaries—these codes are generated by the centralized ARTCC computer. A testament to how well the system works is how rarely you hear ATC request an IFR flight change their transponder code— other than to VFR.

An interesting observation is how newer transponders retain their code after you shutdown. This puzzled me as to why there would ever be a need to return to the same code after a shutdown. However, a momentary electrical blip, or if asked to recycle your transponder—if designed to reset to 1200 on shutdown—would have you squawking 1200. While electrical blips are rare, requests to recycle the transponder are more common.

Traversing a VIPTFR

Living now in the land of perpetual visits by Air Force One, I have found one clear case where retaining the code is important. Whenever you are in a VIPTFR you must have a discrete transponder code. Squawking 1200 at any time—even inadvertently—will result in a visit by the men in black on landing and possibly a fighter escort to boot.

Note that the penalties, even for an inadvertent VIPTFR violation, are ridiculously more severe than what should be meted out to some of the dumb and more dangerous things each of us has seen other pilots do.

Even if ATC tells you to squawk 1200, don’t when in a VIPTFR. This has happened to more than one pilot. When a pilot reports seeing the airport and is cleared for the visual approach they routinely cancel their IFR clearance and ATC instinctively directs them to squawk 1200. It is incumbent on the pilot to know they must retain their discrete code. Since VIPTFRs are now a way of life down here ATC is less likely to inadvertently advise a pilot to squawk VFR but this may not be the case in other areas.

Speaking of squawk, the original transponders were referred to as the “Parrot”—because they returned a signal when queried, as a parrot responds to the human voice. Thus when an ident was needed, pilots were asked to “squawk.”

The ADS-B Connection

ADS-B is the next step in the evolution of the transponder. By combining satellite information with the transponder output, ATC now has more precise positional information. ADS-B has given the transponder an added dimension in the cockpit where it no longer serves the needs of ATC but can now provide useful information to the pilot through ADS-B In.

While ADS-B In is not mandatory, you would be remiss in not adding this value-added feature. You not only get the free TIS-B (Traffic Information Service — Broadcast) data but also the free government-funded FIS-B (Flight Information Service—Broadcast) graphical weather information.

With FIS-B, you also get other useful information depending on your system such as AIRMETs, SIGMETs, PIREPs, TAFs, winds and temperatures aloft, NEXRAD imagery, SUA status, NOTAMs and more. One word of caution though, FIS-B weather is uplinked from ground stations, not downlinked from orbiting satellites such as the TIS-B information. So there is a good chance that you may encounter ground stations where coverage is less than ideal or not available at all, especially at lower altitudes.

For those who haven’t upgraded to ADS-B the deadline (January 1, 2020) is fast approaching. As more aircraft become ADS-B equipped you will begin to see marked improvements in air traffic flow management as ATC has more accurate information on the aircraft in its airspace.

Richard Lanning Ph.D. is a graduate of the U.S. Naval Academy and a pilot for more than 30 years. He is an ATP, CAP Check Pilot Examiner, and CFII.

This article originally appeared in the May 2018 issue ofIFR Refreshermagazine.

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