Updated October 2026
To squawk is to set a four-digit code in your transponder so a radar controller can pick your airplane out of every other target on the screen. Four codes matter more than the rest: 1200 for VFR flight with no code assigned, 7500 for a hijack, 7600 for lost radio communications and 7700 for an emergency.
ADS-B Out does a related job without waiting to be asked. It broadcasts your GPS position, altitude, velocity and that same code about once a second, and most of the airspace around Los Angeles has required it since January 1, 2020. Here are the modes, the codes, the rules in 14 CFR 91.215 and 91.225, and how we use them under the Los Angeles Class B at Van Nuys.
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What A Transponder Does: Mode A, Mode C And Mode S
A transponder is a radio that answers questions. When a ground radar interrogates it, the box replies on 1090 MHz, and the controller's scope turns that reply into a data block beside your target.
Mode A is the identity code. Each of the four digits runs from 0 to 7, which gives 4,096 possible codes. Mode C adds pressure altitude in 100-foot increments, which is what 91.215 means by automatic pressure altitude reporting. Mode S is selective. Every airframe carries its own 24-bit address, so a Mode S radar can interrogate one airplane at a time, and the same signal carries the 1090 MHz version of ADS-B.
One detail catches students on the oral. Mode C reports pressure altitude referenced to 29.92 inches, whatever you have set in the Kollsman window, and ATC's equipment applies the local setting before the number is displayed. Under 14 CFR 91.217 the encoder must agree with the altimeter you fly by within 125 feet. When a controller sees a mismatch, check your own altimeter setting first.
| Mode | What It Sends | What It Is For |
|---|---|---|
| Mode A | The four-digit identity code | Tells the controller which target is you |
| Mode C | The code plus pressure altitude in 100-foot steps | The altitude readout 91.215(b) requires wherever a transponder is required |
| Mode S | A unique 24-bit address, the code, altitude and data | Selective interrogation, and the base for 1090 MHz ADS-B |
The Squawk Codes Every Pilot Memorizes: 1200, 7500, 7600, 7700
The special codes come from the Aeronautical Information Manual, and the private pilot written test asks about every one. Learn each with its situation attached, because four bare numbers are easy to mix up at the moment you need one.
| Code | Meaning | When You Set It | Source |
|---|---|---|---|
| 1200 | VFR, no code assigned | Any VFR flight, at any altitude, unless ATC gives you a code | AIM 4-1-20 |
| 1201 | LAX Special Flight Rules Area | Before entering and while flying the SFRA over LAX | 14 CFR 93.95 |
| 1202 | Gliders | Glider flights, in place of 1200 | AIM 4-1-20 |
| 7500 | Unlawful interference | Only if the airplane is being hijacked | AIM 6-3-4 |
| 7600 | Lost two-way radio communications | When you can no longer talk to ATC | AIM 6-4-2 |
| 7700 | Emergency | Distress or urgency, if you are not already on an assigned code | AIM 6-2-2 and 6-3-2 |
| A discrete code | Assigned to you alone | Flight following, an IFR clearance, a Class B clearance | AIM 4-1-20 |
1200 is the default. AIM 4-1-20 says that unless ATC instructs otherwise, a VFR airplane squawks 1200 regardless of altitude.
7700 is the emergency code, and the rest of the guidance matters as much as the number. AIM 6-3-2 says that if you are already in radio contact on an assigned code, keep it, because the controller already knows who you are. Squawk 7700 when you cannot reach anyone immediately, then get on the radio. AIM 6-2-2 adds that it normally sets off an alarm at every position in the facility.
7600 means the radio has gone quiet. AIM 6-4-2 tells you to set it and not to assume anyone can see you. On an IFR flight, 14 CFR 91.185 takes over: in VFR conditions you continue VFR and land as soon as practicable, and in IFR conditions you fly its route and altitude rules. On a VFR flight you stay VFR and land somewhere sensible. A towered field such as Van Nuys can still clear you in with light gun signals.
7500 is the code nobody wants to set by accident. Under AIM 6-3-4, ATC never assigns it unless you have reported unlawful interference. If it appears on your data block, the controller asks you to verify it, and a slip gets answered in the clear. If you confirm it or stay silent, the controller stops asking and notifies the authorities. That is also why AIM 4-1-20 says to avoid passing through 7500, 7600 or 7700 when changing codes. Going from 2700 to 7200, you turn the first digit through 2200, not 7700.
How To Use The Transponder: Standby, ALT, Ident And Changing Codes
AIM 4-1-20 wants altitude reporting and ADS-B Out on whenever the airplane is on the movement area, taxiways and runways included. Departures switch on for the taxi out, arrivals go to standby or off at the parking spot. On most transponders that position is labelled ALT, but the AIM sends you to the flight manual of the airplane you are flying.
Press IDENT only when asked. It makes your target stand out on the scope for a few seconds, which helps exactly when the controller wants it. On first contact with a radar facility, give your altitude to the nearest hundred feet so the controller can check your Mode C readout, and read back every code.
| ATC Says | What It Means | What You Do |
|---|---|---|
| "Squawk 4523" | A discrete code, assigned to you | Set 4523 and read it back |
| "Ident" | The controller wants to confirm which target is you | Press IDENT once |
| "Squawk VFR" | Radar service is ending | Set 1200 |
| "Squawk altitude" | Turn altitude reporting on | Select ALT |
| "Stop altitude squawk" | Your Mode C readout is wrong or unusable | Check the altimeter setting, then turn altitude reporting off as told |
| "Squawk standby" | Stop replying for now | Select STBY |
| "Squawk mayday" | Emergency code | Set 7700 |
ADS-B Out And ADS-B In: What They Are And Why January 2020 Mattered
ADS-B stands for automatic dependent surveillance, broadcast: nobody interrogates it, it depends on your own GPS, and it goes to anyone listening. AIM 4-5-7 describes two links. 1090ES is an extended squitter built into a Mode S transponder. UAT is a separate transceiver on 978 MHz.
The rule is 14 CFR 91.225, and its main paragraphs apply from January 1, 2020. In Class A, from 18,000 feet up, only 1090 MHz equipment qualifies. Below 18,000 feet, in the airspace the rule lists, either link will do. Paragraph (f) says equipped aircraft transmit at all times unless ATC directs otherwise, so ADS-B Out is not a switch you turn off to stay quiet.
14 CFR 91.227 sets the performance: position accuracy better than 0.05 nautical miles, an integrity radius under 0.2 nautical miles, and an update at least once a second in the air. Four items in the message come from you: the code ATC assigned, the call sign, any emergency status and IDENT. Dial 7700 and that code goes out over ADS-B as well.
ADS-B In is the receiving half, and the AIM is plain that the regulations do not require it. It brings traffic and weather. TIS-B rebroadcasts radar-derived traffic every 3 to 13 seconds and, in the AIM's words, is not intended as a collision avoidance system. FIS-B sends METARs, TAFs, NEXRAD, NOTAMs, TFRs and PIREPs on 978 MHz only, and it does not replace a briefing. Every traffic display is advisory. You still look outside.
| 1090ES | UAT | |
|---|---|---|
| Frequency | 1090 MHz | 978 MHz |
| Hardware | Built into a Mode S transponder | A separate transceiver beside the transponder |
| Class A, 18,000 ft and above | Allowed | Not allowed |
| ADS-B airspace below 18,000 ft | Allowed | Allowed |
| FIS-B weather | Not on this link | Broadcast on this link |
| Anonymous mode | Not available | Available on 1200, with no flight ID and no ATC services |
Where A Transponder And ADS-B Are Required: 91.215, 91.225 And The LA Mode C Veil
The two rules list almost the same airspace, which is why pilots say "Mode C and ADS-B" as one phrase.
| Airspace | Transponder With Mode C, 91.215(b) | ADS-B Out, 91.225 |
|---|---|---|
| Class A | Required | Required, 1090 MHz only |
| Class B and Class C | Required | Required |
| Within 30 NM of an Appendix D airport such as LAX, surface to 10,000 ft MSL | Required | Required |
| Above a Class B or C ceiling, inside its lateral limits, up to 10,000 ft MSL | Required | Required |
| At and above 10,000 ft MSL over the 48 states, except at and below 2,500 ft AGL | Required | Required in Class E |
| Class E at and above 3,000 ft MSL over the Gulf of Mexico, within 12 NM of the coast | Not listed | Required |
| Class D, and Class E or G outside the areas above | Not required; if installed, operate it in controlled airspace | Not required |
Under 91.215(c), a working transponder gets used, Mode C included, in all controlled airspace and not only where the table requires one. Aircraft built without an engine-driven electrical system, such as gliders and balloons, have a narrow exception inside the 30-mile ring.
Appendix D to Part 91 lists 37 airports in Section 1, and Los Angeles International is one. Burbank and Van Nuys are not listed. That changes nothing for us, because Van Nuys sits well inside 30 nautical miles of LAX. Every flight from our ramp starts inside the Mode C veil.
Equipment fails, so both rules have a way out. With Mode C or the whole transponder inoperative, you can ask the controlling ATC facility at any time to continue to your destination or a repair shop. With no transponder at all, ask at least one hour before the flight. 91.225(g) uses the same two time limits for ADS-B.
Transponder Inspections Under 91.413, And What Happens When One Fails
14 CFR 91.413 says no one may use a transponder unless it was tested and inspected to Part 43 Appendix F within the preceding 24 calendar months. Calendar months run to the end of the month, so a test on 10 March 2025 is good until 31 March 2027. The work belongs to a properly rated repair station, a Part 121 or 135 maintenance program, or the manufacturer that installed the unit.
For IFR, 91.411 adds a second 24-calendar-month test of the static system, altimeter and altitude reporting system. Instrument examiners ask to see both entries, and our private pilot oral exam guide lists them with the other inspections.
If the box quits in flight, tell the controller, who can authorize you home under 91.215(d). If you hear "stop altitude squawk," check the altimeter setting before blaming the encoder. Back on the ground, squawk it in the other sense of the word. In a hangar a squawk is a defect written up, and ours go to our own maintenance shop before the next pilot takes the airplane.
Flying Under The LA Class B: Practical Squawk Procedures From Van Nuys
Van Nuys is a towered Class D airport under the Los Angeles Class B shelf, with Burbank's Class C 11 miles east and Whiteman 6 miles northeast. Every one of our 20 airplanes carries ADS-B Out, because the field sits inside the LAX veil even though Class D alone does not demand it.
The transponder goes to altitude reporting before the airplane moves and stays there until the parking spot. A local lesson toward the Simi Valley practice area is normally flown on 1200, below the Class B floors and clear of Burbank. Ask SoCal Approach for flight following and you get a discrete code. Set it, read it back, and wait for "radar contact" before assuming anyone sees you. When you hear "squawk VFR," the code goes back to 1200 before anything else.
The airspace next door has its own equipment line. Burbank's Class C needs two-way radio contact plus Mode C and ADS-B Out, and the Class B needs a clearance as well. The LAX Special Flight Rules Area needs no clearance, but 14 CFR 93.95 fixes the details. You squawk 1201, fly VFR on the Santa Monica VOR 132-degree radial and level at 3,500 feet southeastbound or 4,500 feet northwestbound. Speed stays at or below 140 knots, with anti-collision and position lights on. Our Los Angeles airspace guide walks through the corridor, and Class B airspace covers the clearance.
Where Transponders Show Up On The Written, The Oral And The Instrument Rating
The private pilot written exam asks the codes, the veil and the 24-month inspection, usually as a scenario.
The Instrument Rating ACS goes further. It lists transponder and ADS-B operation as a knowledge element, and it grades you on setting the code a clearance gives you. Our instrument rating in Los Angeles is $11,700, with 20 of its hours in the Redbird MCX simulator, where you can copy clearances and fly lost-communication scenarios as often as a lesson needs. Starting from zero, the private pilot course comes first, from $10,400 with $2,500 down.
Related Reading
- Los Angeles airspace guide, the Class B shelves, the SFRA and the corridors around Van Nuys.
- Class B airspace, the clearance, the equipment and the student pilot endorsement.
- Airspace classes explained, every letter from A to G with its equipment rules.
- Radio calls for student pilots at Van Nuys, what to say to ground, tower and SoCal Approach.
- The private pilot written exam, where squawk codes turn up as test questions.
- Instrument rating in Los Angeles, where a squawk code arrives inside every clearance.
- Required equipment and documents — ATOMATOFLAMES, FLAPS, ARROW and AVIATES under 91.205.
Transponder And ADS-B FAQ
What Does Squawk Mean In Aviation?
To squawk is to set a four-digit code in the transponder so ATC radar can identify your airplane. The word runs through ATC phraseology, as in "squawk VFR" and "squawk standby," and in a hangar it means a defect written up for maintenance.
What Does Squawk 7700 Mean?
7700 is the emergency code, and it normally triggers an alarm at every position in the radar facility. If you are already talking to ATC on an assigned code, AIM 6-3-2 says keep that code instead.
What Happens If You Squawk 7500 By Mistake?
The controller asks you to verify the code. Say in the clear that it was a mistake and change it. If you confirm 7500 or do not reply, ATC stops questioning, keeps following you and notifies the authorities.
What Is Squawk 7600?
7600 means lost two-way radio communications. On an IFR flight, 14 CFR 91.185 then sets your route and altitude. On a VFR flight you stay VFR and land as soon as practicable.
What Is The Difference Between A Transponder And ADS-B?
A transponder replies only when a ground radar interrogates it. ADS-B Out broadcasts your GPS position, altitude, velocity and code about once a second without being asked, accurate to better than 0.05 nautical miles under 14 CFR 91.227.
Do I Need ADS-B To Fly Out Of Van Nuys?
Yes. Class D alone does not require it, but Van Nuys lies well inside 30 nautical miles of LAX, an Appendix D airport. That puts a Mode C transponder and ADS-B Out on the list from the surface to 10,000 feet MSL.
How Often Must A Transponder Be Inspected?
Within the preceding 24 calendar months, under 14 CFR 91.413, to Part 43 Appendix F. For IFR flight, 91.411 adds a separate 24-month test of the static system, altimeter and altitude reporting system.
Hear A Real Squawk Assignment From The Left Seat
The codes make more sense once you have heard a controller assign one and watched your instructor dial it in. A 60-minute discovery flight from Van Nuys is $229 in a Piper Warrior, you fly it yourself, and it is credited in full toward training if you enroll within 24 hours. Already flying and weighing the instrument rating? Book a free consultation with an instructor.
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