Learning hub · Radio Navigation · Lesson 2

Transponders, ADS-B and collision avoidance

Air traffic control and other aircraft can see you if you carry a transponder, but the last line of defence against a collision is still your own eyes. This lesson covers secondary radar, the squawk codes you need, ADS-B, FLARM, ACAS and a good scanning technique.

About 12 minutes, plus the quiz. Part of Radio Navigation. It builds on VOR, NDB, DME and GNSS.

1. Secondary radar and the transponder

A primary radar sends out a pulse and listens for the echo from the aircraft’s skin. It needs no equipment on board but gives only position, and it is poor at small or slow targets. Secondary surveillance radar (SSR) works differently. The ground station sends a coded interrogation on 1030 MHz. Your transponder answers on 1090 MHz with a code, and sometimes with your altitude. The controller sees a labelled symbol instead of a plain echo.

• Mode A replies with a four-digit code, each digit 0 to 7, which gives 4,096 codes. This is your squawk.
• Mode C adds the pressure altitude from an encoder, in 100 ft steps. ATC converts it to height using the QNH. Modes A and C are often called Mode A/C.
• Mode S is a more advanced system. Each aeroplane has a unique 24-bit address, interrogation is selective (the ground station asks one aircraft at a time), and the reply can carry more data. Mode S transponders also support ADS-B.
• IDENT: when ATC asks you to “squawk ident”, press the button. Your symbol is highlighted on the controller’s screen for a few seconds.

Set the transponder to ALT (Mode C on) unless you are told otherwise, and to STBY on the ground until ready to taxi or depart, as your checklist says. A transponder on STBY does not reply. Some airspace is designated a transponder mandatory zone (TMZ), where SERA requires a transponder capable of Mode A and C, or Mode S, to be carried and operated. TMZs are published in the AIP.

2. Squawk codes to know

Code	Meaning
7700	General emergency
7600	Radio failure
7500	Unlawful interference (hijack)
7000	SERA code for when you are not receiving an air traffic service, unless your State prescribes otherwise (see below)

SERA section 13 (SERA.13005) sets these codes. The pilot selects 7700 for an emergency (unless ATC has directed another code, though 7700 may still be chosen if it seems best), 7600 for a radio-communication failure, and tries to select 7500 for unlawful interference (7700 may be used instead if circumstances warrant). Otherwise the pilot sets the code ATC gives. With no instruction, SERA says 2000 or another code set by the authority, and 7000 when not receiving an air traffic service, unless the competent authority prescribes otherwise. Because States can prescribe other codes (the UK, for example, uses additional conspicuity codes in some circumstances), check the AIP of the country you are flying in. SERA.13001 also says that a serviceable transponder should be operated at all times in flight, and SERA.13010 says Mode C should be operated continuously unless ATC says otherwise.

When ATC gives you a code, set it, read it back, and do not change it without being told. When you change codes on the knob, take care not to pass through 7500, 7600 or 7700 by accident. If your radio fails, follow your training: set the code, and follow the lost communications procedures, as covered in the VFR phraseology lesson.

3. ADS-B, FLARM and ACAS

ADS-B Out (automatic dependent surveillance, broadcast): your aeroplane takes its position from GNSS and broadcasts it, with altitude and identity, a couple of times a second. In Europe it is usually carried on the same 1090 MHz frequency as a Mode S transponder (“1090 ES”). Anyone with a receiver can see it. The advantage over radar is that it does not need a ground radar, so it also covers remote areas. ADS-B In is the other half: your aeroplane receives ADS-B from other traffic and shows it on a display or EFB. This helps awareness, but it only shows traffic that transmits, so it cannot replace a lookout. SERA also refers to ADS-B emergency functions: if you have them, select the emergency mode when you declare an emergency.

FLARM is a small device widely used by gliders and light aircraft in Europe. It predicts your track from GNSS, broadcasts it on a licence-free radio band, and warns you when another FLARM-equipped aircraft is likely to conflict. It works only with other FLARM units (and compatible devices), and does not talk to the transponder system.

ACAS (airborne collision avoidance system, known in the USA as TCAS) is carried mainly by larger aeroplanes. It interrogates the transponders of nearby traffic and calculates the risk. A traffic advisory (TA) says traffic is near. A resolution advisory (RA) tells the crew to climb or descend, with a vertical manoeuvre. SERA states that the pilot-in-command remains responsible for collision avoidance, including manoeuvres based on ACAS resolution advisories. A light aeroplane will seldom carry ACAS, but you may share the sky with aircraft that do, and a transponder set to ALT makes you visible to their systems. Also, none of these tools sees every aircraft: gliders and many balloons may have no transponder at all.

4. See and avoid: how to look

In visual conditions the rule is “see and avoid”, and it is not an optional extra. Your eyes see sharply only in a small central area. The rest of your view is good at noticing movement and poor at detail. A distant aircraft on a collision course has no relative movement: it stays in the same place in your windscreen and just grows. That is why it is easy to miss.

• Scan in sectors. The FAA handbook suggests short eye movements of no more than about 10°, with each sector observed for at least one second. Focus at a distance, not on the dashboard.
• Look outside most of the time, with short glances inside. Keep head-down time short, especially near an aerodrome.
• Clear before turning, climbing or descending. Raise the wing, or lower the nose, to see what is hidden underneath or behind.
• Know the blind spots of your aeroplane: the wing, the door pillars and the nose hide traffic, and a high-wing or low-wing type hides different parts of the sky.
• Keep the windscreen clean. Dirt and insects can hide a small target, and haze and low sun make aircraft hard to see.
• Use the radio and the traffic information. A traffic call is a prompt to look; it does not mean you see it.

Transponders, ADS-B and FLARM supplement your lookout. They do not replace it, and a screen display is not a licence to look inside. Real flights use approved equipment, official data and the flight manual. Rules on equipment and codes are national: check the AIP and your school’s manual.

5. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

1. Two invented aeroplanes approach head-on at 110 kt and 120 kt. You first see the other aeroplane when it is 3 NM away. How much time do you have, in seconds, before they meet? Why is this less than it feels?

   Show the answer

   1. Closing speed = 110 + 120 = 230 kt, which is 230 NM per hour.
   2. Time = 3 ÷ 230 = 0.0130 h, which is about 47 seconds.
   3. You need time to see, to decide and to act. The aeroplane also looks small and is not moving across your view, so you may notice it late. Once you have spotted it, you still need time to decide and to act.
   4. The lesson: scan often and early.

2. A Mode C transponder reports a pressure altitude of 3,400 ft. The QNH is 1003 hPa. Roughly what is the altitude on QNH? (Use the rule of thumb of about 30 ft per hPa, and remember that pressure altitude is referenced to 1013 hPa.)

   Show the answer

   1. The difference between standard pressure and QNH = 1013 − 1003 = 10 hPa.
   2. 10 hPa × 30 ft = about 300 ft.
   3. The pressure level is lower than standard, so the true altitude is lower than the pressure altitude: 3,400 − 300 = about 3,100 ft on QNH.
   4. This is why ATC applies the QNH to your Mode C. Your own altimeter, set to QNH, would show about 3,100 ft.

3. You use eye movements of 10°, each held for 1 second, to scan a 180° arc ahead. How long does one full scan take? Is that reasonable, and what do you do near an aerodrome?

   Show the answer

   1. Number of sectors = 180 ÷ 10 = 18.
   2. Time = 18 × 1 s = 18 seconds, with no time spent looking inside.
   3. That is a fair guide, but in practice you cannot scan this slowly all the time. Mix looking outside, a glance at the instruments, then outside again.
   4. Near an aerodrome, traffic converges and the risk is higher, so shorten the time inside the cockpit and keep up the outside scan. Use the radio to build a mental picture.

6. Watch

A video on the transponder modes and the ident button, which goes with section 1. Check what it says against your course material, and note that squawk code practice varies between countries.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

A video on how ADS-B works, which goes with section 3. It comes from Canada, so equipment rules and frequencies there may differ from Europe.

Open on YouTube if the player does not load. Not made by VectorEFB.

A short training tip on scanning for traffic, which goes with section 4. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB.

7. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

Next: Instrument approaches, ILS and GNSS. Back to the learning hub.