Transponder operating procedures and phraseology
Learning objectives
- State when you must operate the transponder, when you may press IDENT, and what Mode C must do in flight.
- Identify the correct code to select in each situation: emergency, radio failure, unlawful interference, an ATS instruction, no instructions, and no service.
- Explain how ATS verifies your Mode C level and what happens when the readout does not match.
- State the standard SSR phraseology, how codes are spoken, and which items you must read back.
The idea
A radar screen only knows about you if your aircraft answers it. The transponder is that answer: a small box that replies with a number, and often with your height. If it is off, you are invisible to the controller and to the collision-warning systems of other aircraft. So the habit is simple: keep it working, use the number you are given or the number that fits your situation, and talk about it in short, fixed phrases that nobody can mishear. This lesson covers the habit, the numbers and the phrases.
Why the transponder stays on
Secondary surveillance radar (SSR) is a radar system made of ground interrogators and airborne transponders. The "code" it displays is the number given to a particular reply signal that a transponder sends in Mode A or Mode C. How the replies work in theory is taught on the SSR principles page; here we stay with what you do.
Think about who needs your reply. A controller's radar picks you out only when your transponder answers. Another aircraft's airborne collision avoidance system (ACAS) works from transponder signals and advises its pilot about conflicting aircraft that carry transponders. If your box is off, you drop out of both pictures.
If your aircraft carries a serviceable SSR transponder, you operate it at all times in flight, whether or not the airspace uses SSR for ATS.
That means "outside radar coverage" and "uncontrolled airspace" are not reasons to switch it off.
There is one exemption. Aircraft without sufficient electrical power do not have to operate the transponder at all times, except in airspace the competent authority has designated for mandatory transponder operation. The guidance says this is meant for aircraft such as sailplanes whose power supply cannot run a transponder permanently; aircraft with electrically powered engines are not covered by it.
On the ground, local aerodrome regulations published in the AIP may require the transponder to be operated on the movement area, so that ATS and other aerodrome units receive surveillance data. In formation flights, the guidance says that after join-up all aircraft except the leader should be told to "squawk standby".
IDENT: only when asked
The IDENT feature makes your return stand out on the controller's display. It is a request tool for the controller, not a way to announce yourself.
You must not use the IDENT feature unless ATS asks for it.
When ATS says "SQUAWK [(code)] [AND] IDENT" or "TRANSMIT ADS-B IDENT", that is the request. "IDENT" means press the IDENT button.
Describe this figure
A generic panel shows four numbered parts, explained in a list below it. Part 1 is the mode selector with positions OFF, SBY, ON and ALT; a serviceable unit is operated at all times in flight, SBY is on but not replying, and ALT is altitude reporting, kept on unless ATC says otherwise. Part 2 is the code window, here showing 2000, the code you set when told to squawk. Part 3 is the IDENT button, pressed once and only when ATS asks. Part 4 is the reply light, whose style varies by unit. A dashed note says layout and labels are aircraft-specific: check your equipment and the POH/AFM.
The figure shows an invented, generic control head, not any manufacturer's design. Along the top is a mode selector with four positions: OFF, SBY (standby), ON and ALT. Below it sits a four-digit code window, and beside the window are an IDENT button and a small reply light. Labels point to the facts: ALT is where Mode C is reported, IDENT is pressed only on request, and the code window shows what ATC will see.
Control layout and mode labels (for example OFF, SBY, ON, ALT, GND, TST) and the way to select IDENT vary by transponder model. Check your aircraft's equipment and POH/AFM.
A few words you will hear, in plain language: "SQUAWK" means set this code; "STANDBY" means the transponder is on but not replying; "CHARLIE" means altitude reporting (Mode C).
Choosing the code
Every code you select answers one question: what does ATC or another pilot need to know about me right now? The special codes shout a problem. The normal codes say "here is my identity" or "I am here, but nobody is looking after me".
First the three special codes.
- 7700 for an emergency.
- 7600 for radio-communication failure.
- 7500 for unlawful interference.
To indicate an emergency, you select 7700, unless ATC has already told you to use a specific code. Even then, you may select 7700 whenever you have a specific reason to think it is the best course of action. To indicate radio-communication failure, you select 7600. To indicate unlawful interference, you attempt to select 7500; if circumstances warrant, you use 7700 instead.
Now the normal codes.
Otherwise you: select codes as instructed by the ATS unit; without code instructions, select 2000 or another code prescribed by the competent authority; and, when not receiving air traffic services, select 7000 to improve detection by suitably equipped aircraft, unless the competent authority prescribes otherwise.
2000 and 7000 are not twins. 2000 is the "no instructions yet" default, for example when ATS has not assigned you a code. 7000 is the conspicuity code for when you receive no ATS. Both can be replaced by a code the competent authority publishes nationally. The wording does not say which of the two prevails when both seem to apply, so learn the wording as written and do not guess.
In Portugal, aircraft flying VFR outside controlled airspace that carry a transponder and are within SSR coverage set code 7000. The AIP also gives code 2000 for aircraft that enter the FIR without ATC instructions after coming from an area without SSR coverage. Check the AIP in force.
| Code and meaning | When to select it |
|---|---|
| 7700 Emergency | An ATC-given code stands, but you may still select 7700 with a specific reason. |
| 7600 Radio failure | Your radio communication has failed. |
| 7500 Unlawful interference | Attempt 7500; if circumstances warrant, select 7700 instead. |
| 7000 Not receiving ATS | To improve detection, unless the authority prescribes otherwise. |
| 2000 No code instructions | No ATS instructions on the code: 2000, or the code the authority prescribes. |
| Otherwise, select the code the ATS unit instructs. 2000 and 7000 are different codes with different roles. | |
Describe this figure
The table has three columns: the code shown as a four-digit display, its meaning, and when to select it. 7700 is for an emergency; an ATC-assigned code stands, but the pilot may still select 7700 with a specific reason. 7600 is radio-communication failure, and 7500 is unlawful interference, where 7700 may be used instead if circumstances warrant. 7000 is for when no air traffic service is received, unless the competent authority prescribes otherwise, and 2000 is for when ATS has given no instructions about the code, or the code the authority prescribes. A footer says that otherwise the pilot selects the code the ATS unit instructs.
The code card shows each code as a four-digit display with its meaning and when you use it. The three special codes sit together: 7700 emergency, 7600 radio failure, 7500 unlawful interference. Beneath them are the normal codes: 2000 for no instructions and 7000 for no service. Every row carries a written meaning, so nothing depends on colour.
Describe this figure
Six questions run from top to bottom, each with a Yes exit to a result box on the right and a No exit down to the next question. Emergency leads to 7700, with a footnote: unless ATC has already given a code, though the pilot may still select 7700 with a specific reason. Radio failure leads to 7600, and unlawful interference leads to attempting 7500, or 7700 if circumstances warrant. A code assigned by ATS leads to selecting that code. No ATS instructions about the code leads to 2000, or the code the competent authority prescribes; not receiving ATS leads to 7000, unless the competent authority prescribes otherwise.
The decision flow reads from the top. If you have an emergency, select 7700. If your radio has failed, select 7600. If you are the subject of unlawful interference, select 7500. If none of those apply and ATS has assigned you a code, select that code. If ATS has given you no instructions about the code, select 2000 or the code the competent authority prescribes. If you are not receiving ATS, select 7000 or the authority's code.
When you change code, avoid passing through or briefly selecting 7500, 7600 or 7700 by accident, for example while dialling 7000.
When the code on the screen is wrong
If the code on the controller's display differs from the code assigned, you are asked to confirm the selected code and, if needed, reselect it. If the discrepancy persists, you may be asked to stop operating the transponder, and the next control position or units are told.
When asked to confirm, you check the Mode A setting, reselect the assigned code if necessary, and tell ATC the code shown on your transponder controls.
If you are squawking 7500 and ATC asks you to confirm it, you either confirm or do not reply. ATC treats no reply as confirmation that 7500 is not an accidental selection.
Other places the emergency codes appear
You meet the same codes in other lessons, so here is only a pointer. 7700 is one of the listed distress signals. An intercepted civil aircraft with a transponder selects 7700 unless the ATS unit instructs otherwise. An aircraft subjected to unlawful interference endeavours to set 7500 and notify ATS of significant circumstances and deviations. After radio failure you set 7600; a VFR flight continues in VMC, lands at the nearest suitable aerodrome and reports arrival by the quickest means. In an emergency descent, the guidance says to set 7700 among other steps. The full procedures are taught on the interception, radio failure and distress pages.
Mode C and the level check
Mode C adds your pressure-altitude to the reply. Pressure-altitude is an atmospheric pressure expressed as the corresponding Standard Atmosphere altitude. Because it is based on the standard datum, it does not depend on the subscale you have set. The detail is on the altimetry and SSR pages.
If your aircraft has serviceable Mode C equipment, you operate it continuously unless ATC says otherwise.
Why does ATC check it? Unless the competent authority prescribes otherwise, each suitably equipped ATS unit verifies the displayed level at least once, on initial contact or as soon as possible after. So expect it on your first call: the controller compares the readout with the level you report.
Describe this figure
The flow starts when the ATS unit checks your displayed level at least once, on first contact or soon after. The controller compares the readout with the level you report; if it is within tolerance, the level is verified. If not, the controller asks you to check the altimeter setting and confirm your level. If it is still wrong, the controller chooses one of three options: ask for the alternative transponder; ask you to stop Mode C, if position and identity are not lost; or keep Mode C running and override the label level with your reported level, where local instructions allow. A note says a wrong level can also trigger an ACAS advisory.
The sequence runs top to bottom. First contact: the controller compares the readout with your reported level. If it is within tolerance, the level is verified and nothing more happens. If it is outside, you hear "CHECK ALTIMETER SETTING AND CONFIRM (level)". If it is still wrong, ATC chooses from three options, described next. No tolerance number is given here.
The guidance describes it like this. If the displayed level is outside the approved tolerance, you are told and asked to check your pressure setting and confirm the level. A wrong subscale shows up as a mismatch between the level you report and the readout, which is why ATC asks about the altimeter setting.
If the discrepancy persists, ATC may:
- ask you to use an alternative transponder;
- ask you to stop Mode C or ADS-B altitude transmission, if this does not lose position and identity;
- keep the transponder running and override the level on the label with the level you reported, where local instructions allow.
The matching phrases are "CHECK ALTIMETER SETTING AND CONFIRM (level)" and "STOP SQUAWK CHARLIE WRONG INDICATION".
You are on a short cross-country with the transponder on ALT. You call a unit for the first time and report your level. The controller replies "CHECK ALTIMETER SETTING AND CONFIRM (level)". Before you argue, look at your subscale: a wrong setting is the mismatch the controller is looking for. Take it seriously, because ACAS accepts erroneous Mode C replies and can issue a resolution advisory based on them. A wrong altitude report is a collision-avoidance hazard, not only an ATC nuisance.
Aircraft identification and transponder failure
Aircraft identification is the group of letters and figures that equals (or is the coded form of) your radio call sign, used to identify you in ground-ground ATS communications. The aircraft address is something different: a unique 24-bit combination assigned to an aircraft.
If your Mode S or ADS-B transmitter has an aircraft-identification feature, it must transmit the identification in the flight plan or, with no flight plan, the aircraft registration, unless the operator holds an approval to use something else. If the displayed identification differs from what is expected, you are asked to confirm and, if necessary, re-enter it. If it still differs, ATS informs you, corrects the label where possible and notifies the next units. The guidance adds that your radiotelephony call sign should match the identification your transponder transmits, unless ATC instructs otherwise.
Some ADS-B equipment offers emergency and urgency modes. Some first-generation avionics send only a general emergency alert, whatever code is selected.
If the transponder fails where carriage is mandatory
Failure after departure: ATC tries to let the flight continue to its destination per the flight plan, but you may have to accept specific restrictions. In some traffic situations, especially just after take-off, you may have to return to the departure aerodrome or land at the nearest suitable aerodrome acceptable to the operator and to ATC.
Failure before departure that cannot be fixed:
You tell ATS as soon as possible, preferably before filing the flight plan; you insert "N" in the surveillance equipment field for total unserviceability, or the letter for the remaining capability if partly failed; and you follow any published procedure for requesting an exemption from carrying a working transponder.
The guidance adds that the aircraft should be allowed to fly as directly as possible to the nearest suitable aerodrome for repair, possibly with a changed time, level or route. In that field, "N" means no surveillance equipment for the route or unserviceable, "A" means Mode A with four digits and 4 096 codes, and "C" means Mode A and Mode C.
All flights in a transponder mandatory zone (TMZ) must carry and operate transponders able to work on Modes A and C or on Mode S, unless they comply with alternative provisions the air navigation service provider has prescribed for that airspace. A TMZ is airspace of defined dimensions where carrying and operating pressure-altitude-reporting transponders is mandatory. The airspace page owns the detail.
Phraseology
The rule is to use standardised phraseology wherever it is specified, and plain language only when it cannot serve. The guidance calls the phrases a core set, not an exhaustive list. Words in ( ) are filled in; words in [ ] are optional.
The core SSR phrases, with the pilot's answer where one is given:
| ATS says | You answer or do |
|---|---|
| "ADVISE TRANSPONDER CAPABILITY" | "TRANSPONDER (as shown in the flight plan)" or "NEGATIVE TRANSPONDER" |
| "FOR DEPARTURE SQUAWK (code)" or "SQUAWK (code)" | Set the code and read it back |
| "RESET SQUAWK [(mode)] (code)" | "RESETTING [(mode)] (code)" |
| "CONFIRM SQUAWK (code)" | "SQUAWKING (code)" |
| "SQUAWK [(code)] [AND] IDENT"; "TRANSMIT ADS-B IDENT" | Press IDENT |
| "SQUAWK LOW"; "SQUAWK NORMAL" | Do as asked |
| "SQUAWK STANDBY" | Select standby; replies are suspended |
| "SQUAWK MAYDAY [CODE SEVEN-SEVEN-ZERO-ZERO]" | Select the emergency code |
| "RE-ENTER [ADS-B or MODE S] AIRCRAFT IDENTIFICATION" | Re-enter the identification |
| "SQUAWK CHARLIE"; "TRANSMIT ADS-B ALTITUDE" | Switch on altitude reporting |
| "CHECK ALTIMETER SETTING AND CONFIRM (level)" | Check your subscale, then confirm your level |
| "STOP SQUAWK CHARLIE WRONG INDICATION"; "STOP ADS-B ALTITUDE TRANSMISSION [(WRONG INDICATION, or reason)]" | Stop altitude transmission |
| "STOP SQUAWK [TRANSMIT ADS-B ONLY]"; "STOP ADS-B TRANSMISSION [SQUAWK (code) ONLY]" | Terminate as instructed; some aircraft cannot run Mode S and ADS-B independently |
Two more. When ATS suspects a loss of communications, it may say "IF YOU READ [SQUAWK (code) or IDENT]" and then "(SQUAWK or IDENT) OBSERVED. POSITION (position) [(instructions)]". And if your selected level differs from your cleared level, a controller may say "CHECK SELECTED LEVEL. CLEARED LEVEL IS (level)" without saying the value they observed.
Saying and reading back codes
Codes are spoken digit by digit, except codes of whole thousands, which are a digit plus "THOUSAND". For example, 2400 is "squawk two four zero zero", 1000 is "squawk one thousand" and 2000 is "squawk two thousand".
SSR codes are mandatory read-back items. A bare "roger" is not enough: say the code back, for example "squawking" followed by the digits.
This content does not replace ATO instruction or your aircraft's POH/AFM. Practise the phrases aloud until the digit-by-digit habit is automatic.
Exam traps
"Outside radar coverage, or in Class G, I can switch the transponder off." Wrong. If it is serviceable, you operate it at all times in flight regardless of whether SSR is used. The power-limited exemption is for aircraft like sailplanes.
"Press IDENT when I call ATC so they find me." Wrong. Only when ATS requests it.
"7500 is radio failure and 7600 is unlawful interference." Wrong, they are the other way round: 7500 is unlawful interference, 7600 is radio failure, 7700 is emergency.
"2000 and 7000 are interchangeable." Wrong. 2000 applies when you have no code instructions; 7000 applies when you are not receiving ATS. Both are subject to the competent authority's prescriptions.
"A wrong altimeter subscale changes what ATC sees." Wrong. Mode C reports pressure-altitude on the standard datum; a wrong subscale shows as a mismatch with the level you report.
"'SQUAWK STANDBY' means wait for a code." Wrong. It means select standby, so replies are suspended.
Explain it back
You are flying VFR in uncontrolled airspace, receiving no ATS, and your radio then goes silent. Which code do you have on the transponder before and after the failure, why, and what should the transponder be doing in terms of Mode C the whole time?
Self-check
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You are flying in uncontrolled airspace with a serviceable transponder and an aircraft with ample electrical power. Nobody is providing you with a service. What must you do with the transponder?mcq · easy
Show answer
A serviceable transponder is operated at all times in flight, whether or not the airspace uses SSR for ATS, and 7000 is the code for when you receive no ATS (unless the authority prescribes otherwise). Switching it off is wrong because there is no such freedom; the exemption is only for aircraft without sufficient electrical power. Standby sends no replies, so you would be invisible. Limiting it to a TMZ confuses a mandatory-carriage zone with the general rule.
Reference: SERA.13001(a); SERA.13001(c); SERA.13005(b)(3)
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Match the situation to the code. Your radio has failed, and later you realise the aircraft is also subject to unlawful interference. Which is the correct pairing of code to the situation as the rule words it?mcq · medium
Show answer
7600 indicates radio-communication failure, and 7500 is attempted for unlawful interference, with 7700 used instead if circumstances warrant. Swapping 7500 and 7600 is the classic mix-up. 7700 is the emergency code, not the radio-failure code. 2000 is only the default when you have no code instructions and none of the special situations applies.
Reference: SERA.13005(a)(2); SERA.13005(a)(3)
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On first contact the controller says: "CHECK ALTIMETER SETTING AND CONFIRM (level)". What does this tell you, and what is the sensible first action?mcq · medium
Show answer
This phrase follows a mismatch between the Mode C readout and the level you reported. Because Mode C uses the standard datum, a wrong subscale shows up as that mismatch, so you check your setting and confirm your level. A verified level would produce no such phrase. IDENT is used only on request. 7600 is for your own radio failure, and you are clearly receiving the controller.
Reference: SERA.13010(b); GM1 SERA.13010(b); Appendix 1 to AMC1 SERA.14001, 2.3.12
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Scenario: you are cleared with "SQUAWK TWO FOUR ZERO ZERO". Later, ATC asks "SQUAWK STANDBY". Which answer describes the correct handling of the first call, and of the second?mcq · hard
Show answer
SSR codes are mandatory read-back items and 2400 is spoken digit by digit, since only whole thousands use the word thousand. "SQUAWK STANDBY" means select standby (replies suspended), not wait for a code. 'Roger' is not an acceptable read-back. 'Two thousand four hundred' is the wrong number format. IDENT and 7000 are unrelated to the standby instruction.
Reference: SERA.8015(e)(1)(iii); SERA.14035(a)(1)(iii); Appendix 1 to AMC1 SERA.14001, 2.3.8
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Name the only situation in which you press the IDENT button, and give the phrase that signals it.short · easy
Show answer
IDENT is a request tool for the controller. You must not use it unless ATS asks for it, and the requests are the phrases "SQUAWK [(code)] [AND] IDENT" and "TRANSMIT ADS-B IDENT". Pressing it on your own, for example when you call, is not permitted.
Reference: SERA.13001(b); Appendix 1 to AMC1 SERA.14001, 2.3.7
Key concepts
Written with ASD-STE100 writing principles: short sentences, one topic per sentence, active voice, one word for one meaning.
Transponder operation in flight
The pilot must operate a serviceable SSR transponder at all times in flight. This applies whether or not the airspace uses SSR for air traffic services (ATS). The pilot must also operate Mode C continuously, unless ATS says otherwise. Rule: SERA.13001(a); SERA.13010(a) Do not: stop operating a serviceable transponder in flight.
Special codes
Select 7700 to indicate an emergency, unless ATS has assigned a specific code. Select 7600 to indicate radio-communication failure. Select 7500 to indicate unlawful interference, or 7700 when circumstances warrant. Rule: SERA.13005(a)(1)–(3)
Normal codes
Select the code that ATS instructs. Without code instructions, select 2000, unless the competent authority prescribes another code. When the aircraft receives no air traffic services, select 7000 to improve detection by suitably equipped aircraft, unless the competent authority prescribes otherwise. Rule: SERA.13005(b)(1)–(3)
IDENT feature
The IDENT feature highlights the aircraft on the controller's display. The pilot must not use IDENT unless ATS asks for it, with the phrase "SQUAWK [(code)] [AND] IDENT". Rule: SERA.13001(b) Do: select IDENT only when ATS asks for it.
Mode C level verification
Unless the competent authority prescribes otherwise, ATS verifies the Mode C level at least once, on initial contact or as soon as possible after. If the level is outside the approved tolerance, ATS asks the pilot to check the altimeter setting and confirm the level. If the discrepancy persists, ATS can ask the pilot to stop altitude transmission, provided that position and identity are not lost. Rule: SERA.13010(b); GM1 SERA.13010(b) Do: check the altimeter setting and confirm the level when ATS asks.
Code phrases and readback
Readback is mandatory for SSR codes. Say each digit of a code separately, except whole thousands: say the digit and "THOUSAND". ATS says "SQUAWK (code)" to assign a code and "CONFIRM SQUAWK (code)" to check it. Rule: SERA.14035(a)(1)(iii); SERA.8015(e)(1)(iii) Do: give the readback of each SSR code, digit by digit.
Transponder failure
The pilot must tell ATS as soon as possible if a transponder fails before departure and cannot be fixed. For total unserviceability, insert "N" under SSR in Item 10; for partial failure, insert the letter for the capability left. After departure, ATS tries to let the flight continue, but the pilot can receive specific restrictions. Rule: SERA.13020(a); SERA.13020(b)(1)–(3) Do: tell ATS before filing the flight plan, if possible, and follow the published procedure for an exemption from carrying a working transponder.
Transponder mandatory zone (TMZ)
In a transponder mandatory zone (TMZ), all flights must carry and operate an SSR transponder for Mode A and Mode C, or for Mode S. Alternative provisions can apply when the provider of air navigation services (ANSP) prescribes them for that airspace. Rule: SERA.6005(b), (d)
Flashcards
Print them: A6 landscape cards (PDF, front and back) · 4 cards per A4 landscape sheet (PDF, cut lines) · Key figures deck (PDF, 8 figures)
Print double-sided, flip on the short edge.
010-06-02-01Which code indicates an emergency?
7700. Select 7700 to indicate an emergency, unless ATS has assigned a specific code. Even then, the pilot can select 7700 for a specific reason.
010-06-02-02Which code indicates radio-communication failure?
7600. Select 7600 to indicate radio-communication failure.
010-06-02-03Which code indicates unlawful interference?
7500. Select 7500 to indicate unlawful interference. When circumstances warrant, select 7700 instead.
010-06-02-04Which code is selected for interception?
7700, unless the ATS unit instructs otherwise.
010-06-02-05Which code is the default when ATS gives no code instruction?
2000. Select 2000 unless the competent authority prescribes another code.
010-06-02-06Which code is the conspicuity code when no ATS is received?
7000. Select 7000 to improve detection by suitably equipped aircraft, unless the competent authority prescribes otherwise.
010-06-02-07Which code does the pilot select when ATS instructs a code?
The code that ATS instructs.
010-06-02-08When must the pilot use IDENT?
Only when ATS asks for it. The pilot must not use IDENT unless ATS asks for it.
010-06-02-09What does the IDENT feature do?
It highlights the aircraft on the controller's display.
010-06-02-10When must the transponder be operated in flight?
At all times, whether or not the airspace uses SSR for air traffic services. The transponder must be serviceable.
010-06-02-11When must Mode C be operated?
Continuously, unless ATS says otherwise.
010-06-02-12How often does ATS verify the Mode C level?
At least once, on initial contact or as soon as possible after.
010-06-02-13What does ATS mean by CHECK ALTIMETER SETTING AND CONFIRM (level)?
The displayed level is outside the approved tolerance. Check the altimeter setting and confirm the level.
010-06-02-14How is the code 2000 spoken?
Digit and THOUSAND: squawk two thousand.
010-06-02-15How is the code 2400 spoken?
Each digit separately: squawk two four zero zero.
010-06-02-16Is readback of an SSR code required?
Yes. Readback is mandatory for SSR codes.
010-06-02-17What does ATS say to confirm a code, and what does the pilot answer?
ATS says CONFIRM SQUAWK (code). The pilot answers SQUAWKING (code).
010-06-02-18Which phrase does ATS use for the emergency code?
SQUAWK MAYDAY [CODE SEVEN-SEVEN-ZERO-ZERO]. The code is 7700.
010-06-02-19What does the pilot insert in the flight plan's surveillance equipment field for total SSR unserviceability?
N. For partial failure, insert the letter of the remaining capability.
010-06-02-20What must the pilot do first if a transponder fails before departure?
Tell ATS as soon as possible, preferably before filing the flight plan.
010-06-02-21How many Mode A codes are available?
4 096 codes, each with 4 digits.
010-06-02-22How long is the aircraft address?
24 bits. It is a unique combination for each aircraft.
010-06-02-23What does a transponder mandatory zone require?
All flights must carry and operate an SSR transponder that works on Mode A and Mode C, or on Mode S.
010-06-02-24What does ATS try to do after a transponder fails after departure?
Let the flight continue to its destination per the flight plan. The pilot can receive specific restrictions.
010-06-02-25What pressure reference does Mode C report, and what can cause a level mismatch?
Pressure-altitude, based on the Standard Atmosphere, independent of the pilot's subscale. A wrong subscale causes a mismatch with the reported level.
010-06-02-26In Portugal, which code does a VFR flight outside controlled airspace set on its transponder?
Code 7000, when the aircraft carries a transponder and is within SSR coverage. This is a Portuguese rule from the AIP. Check the AIP in force.
Sources
- Commission Implementing Regulation (EU) No 923/2012 (SERA): Article 2; SERA.6005; SERA.13001–SERA.13020; Appendix 1 — via EASA Easy Access Rules for Standardised European Rules of the Air (SERA), August 2025, with the related AMC/GM.
This page is educational. It does not replace instruction at your ATO or your aircraft's POH/AFM. Rules change: check the regulation in force.