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The quiz.

Most of us take our transponders for granted. That little box offers eight values in each of four digits. Multiplying 8 x 8 x 8 x 8, you get 4096 possible codes. There are about 9,100 aircraft airborne reported by FlightAware as this is being written. Do you see a problem? Unfortunately discrete codes are not altogether discrete, meaning that national management is required to keep aircraft in the same area from being given the same squawk.

Assigning transponder codes comes down to the same circumstance as applying for Internet IP addresses ... now that they are hard to get, each applicant must justify their need. So it is with Centers, TRACONs, the military, Industry, Unique Purpose, and Experimental Activities. Answers on page 23.

1. What is the difference between a discrete and a nondiscrete code?

a. A discrete code is a one-off code given to only one aircraft in the National Airspace System.

b. A nondiscrete code is given to multiple aircraft as discussed above.

c. A discrete code identifies a particular aircraft; a nondiscrete code does not.

d. Indiscreet codes are not authorized.

2. These days you are given a single squawk code that will not change even if you pass through multiple Air Route Traffic Control Centers (ARTCCs). What is that kind of code called?

a. An external code c. A function code

b. An internal code d. A Center code

3. This code applies to you if you fly VFR in the immediate vicinity of KLAX and to VFR gliders not in contact with ATC (a fairly new assignment):

a. 1000

b. 1201

c. 1255

d. 1273-1275

4. You plan to penetrate the Air Defense Identification Zone or ADIZ under VFR. In other words you'll be on a Defense VFR or DVFR clearance. What code range might you be assigned, and who exclusively assigns them?

a. 4401-4443 and 4466-4477, assigned by ATC

b. 4434-4437, assigned by FSS

c. 1207-1272, assigned by FSS

d. 0100-0400, assigned by Service Area Operations

5. Code 1276 would come in handy if:

a. You have a bird strike.

b. You are going to penetrate the ADIZ when unable to communicate with ATC or another aeronautical facility.

c. You are going to fly through a Prohibited Area.

d. You plan on landing in Area 51.

6. You are on an authorized Search and Rescue Mission. Your code is:

a. 1277

b. 4453

c. 4400

d. 7601

7. Codes 5061, 5062, 5100 and 5200 are reserved for:

a. Use in Alaska

b. Transitioning Stadium TFRs

c. Potomac TRACON

d. Self-important privileged characters

8. If you are in Class A airspace, what is the general rule regarding transponder codes:

a. Expect a discrete squawk in the range 3100-3500.

b. You may be given a military code if no discrete civilian codes are available.

c. Expect a discrete code between 2100-2500.

d. You can expect a code that is the first two digits of your flight level followed by two random numbers that are unique to you.

e. Request any code that tickles you, perhaps the month and day of your anniversary.

9. Your aircraft will be operating on a flight plan that requires frequent or rapid changes in assigned altitude in more than one stratum (that is, in and out of Class A airspace) or other conditions of flight not compatible with a stratified code assignment such as flight testing. ATC should tell you to: (May have multiple answers.)

a. Squawk 4000.

b. Maintain VMC conditions.

c. Request a block altitude.

d. File a request at least 30 days in advance with the controlling Center, and they'll assign you a squawk that's the ID number sequentially assigned to your application.

e. Report after landing whether you used your Sic-Sac.

10. Which transponder code will never be assigned?

a. 1111

b. 7777

c. 0000

d. It's secret, ya know? Very hush-hush.

11. Bonus round: You sold that hi-tech startup and bought a bizjet. It's time to fly to Hawaii. What will you be told to squawk as you reach the edge of the mainland radar coverage?

a. 2000

b. 2222

c. 3000

d. 3333


1. c. The first two digits of a squawk code are "code blocks." Multiplying 8 x 8 yields 64 possible code blocks. The base number of the code block--any code that ends with 00 in the last two digits--is nondiscrete and multiple aircraft meeting designated criteria could have that same code, such as squawking 1200 for VFR. If the last two digits are not zeroes, you have a discrete code. There are 63 discrete codes in each block, yielding a total of 4032 discrete codes available. As above, these are re-used across the National Airspace System.

2. a. An external code is a beacon code assigned to a specific flight plan with one or more route segments not contained within a single domestic ARTCC's airspace. By contrast, an internal code is used when all route segments will be within one domestic ARTCC's airspace. Internal codes are repeated in different centers, while external codes are normally unique.

3. b. The exception that proves the rule, (reference #1, above) code 1201 should be used by all aircraft meeting the specific requirements. Code 1000 is used exclusively by ADS-B aircraft to inhibit Mode A (non-altitude reporting) transmissions and for aircraft outside National Beacon Code Allocation Plan airspace. You would use code 1255 if engaged in aerial firefighting. Codes 1273-1275 are used with fixed-location "Parrot" transponders to calibrate radars using Calibration Performance Monitoring Equipment.

4. c. These are discrete codes assigned to DVFR aircraft solely by FSS, except for 1255 which is for aerial firefighters. Answer a. is reserved for Federal law enforcement. Answer b. would be fine if performing weather reconnaissance. Answer d. is the block of codes allocated to Terminal, Combined Center and Radar Approach Control (CERAP), Industry, Unique Purpose and Experimental Activities.

5. b. This code could spare you an in-flight intercept, as changing codes once having entered the ADIZ is a red flag to your Continental NORAD Region.

6. a. 1277 is squawked by designated SAR aircraft. Code 4453 is great if you're a high-altitude weather balloon. Code 4400 is really your lucky day since you're a U-2, or some other aircraft operating above FL600. Code 7601 is reserved for special use by the FAA.

7. c. Given the sensitive airspace around the capital, Potomac TRACON gets its own dedicated codes. Alaska users have to abide by the same code allocations as everyone else. You cannot transition an active stadium TFR even if you are on an IFR flight plan and squawking an IFR transponder code. No comment on the veracity of d.

8. c. See the Controllers Handbook, JO 7110.65V section 5-2-6 b, note 2. There is nothing special about the codes from 3100 to 3500, and ATC will never give you a military code, even though you might like one.

9. a. & c. See the Controllers Handbook, 5-2-6 b. 3. Given your altitude changes, it would also be prudent to request a block altitude, although the handbook does not specify that. Choice d. is utter nonsense. Surely, you didn't fall for that one, did you?

10. c. Per JO 7110.66D, Code 0000 should never be assigned or used. This code is part of an internal subset of codes assigned by En Route Safety and Operations Support. Code 1111 is part of a block of codes reserved for Terminal, Combined Center and Radar Approach Control (CERAP), Industry, Unique Purpose(s) and Experimental Activities. Code 7777 is assigned to DOD interceptor aircraft on active air defense missions and operating without ATC clearance.

11. c. Flights making an ocean crossing out of range of U.S. ATC radar will be given a squawk of 2000. This applies to aircraft over the Atlantic, Pacific and the Gulf of Mexico. Remember, even if there's no ATC radar out there, other aircraft with TCAS are constantly interrogating transponders within a 40-mile range.
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Date:May 1, 2015
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