Air Law

Collision Avoidance and the General Rules

Rules of the Air · Lesson 2 of 3•26 min read

The general rules are the working heart of the rules of the air: the rules that bind every flight, visual or instrument, from engine start to shutdown. This lesson works through them in the order a flight meets them — who gives way to whom in the air and on the ground, the lights and signals by which aircraft and aerodromes talk to each other, minimum heights and cruising levels, the flight plan and the clearance that let a flight into controlled airspace, and the procedures for the days when things go wrong: a dead radio, or worse.

Right of way in the air

The collision-avoidance rules start from two duties that never switch off. First, vigilance: it is the PIC's responsibility to see and avoid, and nothing in the rules (no clearance, no right of way, no instrument flight plan) relieves the PIC of the duty to take the action that best averts a collision. Second, proximity: an aircraft shall not be operated in such proximity to other aircraft as to create a collision hazard. Formation flight is the agreed exception: it is allowed only by pre-arrangement between the pilots-in-command, in controlled airspace also under conditions set by the competent authority, and each aircraft must stay within 1 km (0.5 NM) laterally and longitudinally and 30 m (100 ft) vertically of the flight leader.

When two aircraft do meet, the rules assign the roles unambiguously. The aircraft with the right of way maintains its heading and speed (predictability is its contribution to the encounter) while the other gives way and keeps well clear, avoiding passing over, under or in front of the other unless it passes well clear, wake turbulence taken into account; that is why the figure's give-way aircraft crosses behind. Between different categories of aircraft converging at approximately the same level, the more manoeuvrable gives way to the less:

Power-driven heavier-than-air aircraft — give way to airships, sailplanes and balloons.
Airships — give way to sailplanes and balloons.
Sailplanes — give way to balloons.
Power-driven aircraft — give way to aircraft that are seen to be towing other aircraft or objects.

Between aircraft of the same category the geometry decides, and the figure shows all three encounters:

Converging — at approximately the same level: the aircraft that has the other on its right gives way.
Approaching head-on — or approximately so, with danger of collision: each alters heading to the right.
Overtaking — the aircraft being overtaken has the right of way, and the overtaking aircraft, whether climbing, descending or level, keeps out of the way by altering heading to the right, and stays clear until entirely past, however the relative positions change in the meantime.

Check you understood

Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.

1. Warm-up
An air traffic control clearance must be obtained:
2. Applying it
Two aeroplanes approach an aerodrome at the same time to land. Which has the right of way?
3. Pushing it
A VFR flight operated as a controlled flight finds that continuing in VMC to its destination will not be practicable. The first action foreseen by the rules is to:

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What happened in the pastPSA Flight 182 — San Diego, 25 September 1978

On a clear morning in September 1978 a PSA Boeing 727 descended toward Lindbergh Field, San Diego, while a Cessna 172 on an instrument-training flight climbed out beneath it. Approach control called the Cessna twice; at 0900:21 the 727's captain reported traffic in sight, and two seconds later was cleared to maintain visual separation and sent to the tower. That clearance handed the whole of this section's first duty to the flight deck: the aircraft was now separated from the Cessna by nothing but the crew's eyes. The Cessna, assigned 070 degrees, turned to about 090, onto the 727's own track, ahead of it and below.

Twenty seconds later the first officer no longer had the Cessna. The tower called it again at twelve o'clock, a mile; the 727 answered, okay, we had it there a minute ago, and then, I think he's passed off to our right, which the controller took as a report of the Cessna clearing, not of the crew losing sight of it. In the cockpit the doubt was spoken and left unresolved: are we clear of that Cessna; supposed to be; I guess; I hope. In the approach room the new conflict-alert function sounded 19 seconds before impact; the controller, who had the crew's word that they saw the traffic and would separate visually, reasoned that no conflict existed and did nothing, and no procedure then required him to warn the pilots. The 727 overtook the Cessna from above and behind and struck it at about 2,600 ft. Both aircraft fell into a residential area. All 135 aboard the 727 died, the two in the Cessna, and seven people on the ground.

The probable cause, in the form the Board finally amended it to, was the crew's failure to comply with a maintain-visual-separation clearance, including the requirement to tell the controller when visual contact was lost, and the ATC procedures that authorised visual separation in a terminal area when radar separation was available; contributing were the controller's failure to advise the crew which way the Cessna was moving, the Cessna pilot's failure to hold his assigned heading, and the controller's improper resolution of the conflict alert. One Board member went further in dissent: see-and-avoid, he wrote, is outmoded in a high-volume terminal area and should not have been the last redundancy left. The recommendations answered him in part — a terminal radar service area for San Diego, a review of every airport shared by airliners and light aircraft for the same, and visual separation in such areas only at a pilot's request. The duty this section starts with never switches off, and the accident is the reason it is no longer left to work alone.

The sector angles are exam material and worth engraving: red on the port (left) wing tip covering 110 degrees from dead ahead leftwards, green on the starboard (right) wing tip covering 110 degrees from dead ahead rightwards, and white on the tail covering the remaining 140 degrees astern: 70 degrees either side of dead astern. The three sectors add to 360, and they turn the night sky into information: see red and green together and an aircraft is coming at you head-on; see a lone white light and you are behind it, inside its 140-degree tail sector — which is exactly why the overtaking rule's cone is 70 degrees each side of the plane of symmetry. An overtaking aircraft is, by definition, one that can see only the white light.

Marshalling signals

On the apron, hand signals replace radio. A marshaller (trained, identifiable, and using illuminated wands or fluorescent bats when needed) guides the aircraft with a standard vocabulary, and the exam expects a pilot to interpret it. The signals a pilot must recognise on sight, described as the marshaller makes them:

This bay / identify gate — arms fully extended straight above the head, wands pointing up.
Continue straight ahead — arms bent at the elbows, wands beckoning upward from chest height to head height.
Turn to your left — the marshaller's right arm and wand extended at 90 degrees to the body, the left hand beckoning the aircraft forward.
Turn to your right — the mirror image: left arm and wand extended, right hand beckoning.
Normal stop — arms and wands fully extended out to the sides, then brought slowly together above the head until the wands cross.
Emergency stop — arms and wands thrown abruptly above the head, wands crossed: stop immediately.
Slow down — arms extended downward, wands patting toward the ground.
Cut engines — arm and wand extended forward of the body at shoulder level, the hand and wand then drawn in a slicing motion across the throat.
Chocks inserted / chocks removed — arms above the head: wands turned inward toward each other for inserted, jerked outward for removed.

Light signals from the tower

When a radio is not available (a failure, or an aircraft not fitted) the aerodrome control tower falls back on a signalling lamp: a directed light, steady or flashing, in green, red or white. The same colour means different things to an aircraft in the air and one on the ground, so the exam loves this table. To an aircraft in flight:

Steady green — cleared to land.
Steady red — give way to other aircraft and continue circling.
Series of green flashes — return for landing; the clearances to land and taxi will follow in due course.
Series of red flashes — aerodrome unsafe, do not land.
Series of white flashes — land at this aerodrome and proceed to the apron; clearances follow in due course.
Red pyrotechnic — notwithstanding any previous instructions, do not land for the time being.

To an aircraft on the ground:

Steady green — cleared for take-off.
Steady red — stop.
Series of green flashes — cleared to taxi.
Series of red flashes — taxi clear of the landing area in use.
Series of white flashes — return to your starting point on the aerodrome.

The aircraft answers without a radio too. In flight by day it acknowledges by rocking its wings (though not on base leg or final approach, where a wing-rock at low level is a risk no acknowledgement is worth); in flight by night, by flashing its landing lights twice, or its navigation lights twice if landing lights are not fitted. On the ground by day it moves its ailerons or rudder; by night, the same double flash.

Minimum heights

Except when necessary for take-off or landing, or with permission from the competent authority, an aircraft shall not be flown over the congested areas of cities, towns or settlements, or over an open-air assembly of persons, unless at such a height that, in the event of an emergency, a landing can be made without undue hazard to persons or property on the surface. That is the basic requirement: the rule protects the people below, not the aircraft, and it is written as an outcome — whatever height an emergency landing clear of the crowd requires. The specific numeric minimum heights for VFR flight are part of the visual flight rules, taught in the last lesson of this module.

Cruising levels: flight levels or altitudes

Two neighbouring rules fix the language of vertical position in the cruise. The cruising levels at which a flight is conducted are expressed in two currencies:

Flight levels (FLs) — for flight at or above the lowest usable flight level or, where a transition altitude is established, above the transition altitude. A flight level is a surface of constant pressure referenced to the standard setting of 1,013.25 hPa.
Altitudes — for flight below the lowest usable flight level, or at or below the transition altitude; an altitude is measured against local pressure.

Why the changeover exists, where the transition altitude sits and how the two altimeter settings work is the business of the altimetry module later in this course; this lesson's rule is simply which term applies where.

Simulated instrument flight

Training under a view-limiting device (flying "under the hood" to practise instrument flight in a real aircraft) creates a deliberately blind pilot, so the rules allow it only under two cumulative conditions: the aircraft is fitted with fully functioning dual controls, and a qualified pilot occupies a control seat as safety pilot. The safety pilot must have an adequate field of vision forward and to each side, or else a competent observer in communication with the safety pilot must occupy a position from which their view adequately supplements the safety pilot's. Someone on board must always be able to see and avoid.

On and around the aerodrome

An aircraft operated on or in the vicinity of an aerodrome (in the circuit, joining it, or leaving it) shall observe other aerodrome traffic for the purpose of avoiding collision, and conform with or avoid the pattern of traffic formed by other aircraft in operation. Two defaults then shape the circuit: all turns are made to the left, on approach and after take-off, unless otherwise indicated or instructed by air traffic control (which is why the standard traffic circuit is left-hand), and the aircraft lands and takes off into the wind, unless safety, the runway configuration or air traffic considerations determine that a different direction is preferable. Into the wind means the lowest ground speed for a given airspeed: the shortest ground roll and the gentlest touchdown.

The ATS flight plan

A flight plan is the information an air traffic services (ATS) unit needs about an intended flight, filed in advance so the system can serve and watch over it. A flight plan shall be submitted before operating:

Any flight, or portion of one, to be provided with air traffic control service — every controlled flight files a plan.
Any IFR flight within advisory airspace — airspace where an advisory service, short of full control, is provided.
Flights within or into designated areas or along designated routes, when the competent authority requires it to facilitate flight information, alerting and search-and-rescue services, or coordination with military units or with ATS units in adjacent states to avoid the possible need for interception.
Any flight across international borders, unless otherwise prescribed by the states concerned.
Any flight planned to operate at night, if leaving the vicinity of an aerodrome.

The timing rules bracket the submission. A flight plan for a flight to be provided with air traffic control or advisory service, or crossing international borders, is submitted not more than 120 hours before the estimated off-block time (EOBT, the time the aircraft plans to start moving), at least 3 hours before EOBT for flights subject to air traffic flow management measures, and otherwise at least 60 minutes before departure. A plan may also be filed from the air: it must then reach the appropriate ATS unit at least 10 minutes before the aircraft is estimated to reach the intended point of entry into a control area or advisory area, or the point of crossing an airway or advisory route.

A filed plan must be kept honest. All changes to a flight plan for an IFR flight, or for a controlled VFR flight, are reported as soon as practicable to the appropriate ATS unit. A delay has hard thresholds: if departure slips more than 30 minutes beyond EOBT for a controlled flight, or more than 1 hour for an uncontrolled flight with a filed plan, the plan is amended or a new one submitted and the old one cancelled. And in flight, three inadvertent changes oblige a controlled flight to report to ATS:

Deviation from track — adjust heading to regain track as soon as practicable.
True airspeed (TAS) off plan — if the average TAS at cruising level, between reporting points, varies or is expected to vary by plus or minus 5 per cent from the TAS in the flight plan, inform the appropriate ATS unit.
Time estimate off plan — if the estimate for the next applicable reporting point, flight information region boundary or destination aerodrome, whichever comes first, is found to be in error by more than 2 minutes (or another period the competent authority prescribes), notify a revised estimate as soon as possible.

The plan is closed at the far end, because an unclosed plan launches a search. Unless otherwise prescribed, an arrival report is made (in person, by radiotelephony, via data link or by other prescribed means) as soon as practicable after landing, to the appropriate ATS unit at the arrival aerodrome, by any flight whose plan covered the whole flight or the remaining portion to destination. Where no ATS unit exists at the arrival aerodrome, the report goes by the quickest available means to the nearest ATS unit. The report contains five elements: aircraft identification, departure aerodrome, destination aerodrome (in the case of a diversionary landing), arrival aerodrome, and time of arrival.

ATC clearances

An air traffic control clearance is the system's authorisation to proceed, and it shall be obtained before operating a controlled flight, or a portion of a flight as a controlled flight. It is requested through the submission of a flight plan to an air traffic control unit (the plan is the request), though clearances for minor portions of a flight, a taxi or a runway crossing, are requested directly by radio. Two escape valves matter to the exam. If a clearance is not satisfactory to the pilot-in-command, the PIC may request an amended clearance, which will, if practicable, be issued: the PIC is never forced to accept a clearance the flight cannot safely follow. And a clearance is not permission to break any rule: it protects the flight from other traffic, nothing more.

The second escape valve serves the controlled VFR flight that runs out of weather. When it becomes evident that flight in visual meteorological conditions (VMC), weather at or better than the prescribed visibility and cloud-distance minima, will not remain practicable, the pilot works down a ladder of four options:

Request an amended clearance — one enabling the flight to continue in VMC to destination or to an alternative aerodrome, or to leave the airspace requiring a clearance.
Continue in VMC and notify ATC — if no such clearance can be obtained: notify ATC of the action being taken, leaving the airspace or landing at the nearest suitable aerodrome.
Request authorisation as a special VFR flight — if the flight is within a control zone: a clearance to fly below the visual minima in a control zone, the controlled airspace around an aerodrome, described in the next lesson.
Request an IFR clearance — if the pilot and aircraft can fly it.

The ladder's logic: stay visual if at all possible, tell the system what you are doing, and never blunder on into cloud uncleared.

Position reports

A controlled flight feeds the system its position. As soon as possible after passing each designated compulsory reporting point, the flight reports the time and level of passing, together with any other required information; additional reports over other points are made when requested, and where no reporting points are designated, reports go in at intervals the ATS unit prescribes. A full voice position report runs, in order: aircraft identification; position; time; flight level or altitude; next position and the time over it; and the ensuing significant point. ATS can lighten the load: a flight may be exempted from routine position reports, typically where surveillance coverage makes them redundant.

Communication failure

A radio failure does not leave a flight without rules; it hands it a script. First, try everything: attempt contact on the other frequencies appropriate to the route, try relaying through other aircraft, and set the transponder to code 7600, the radio-failure code that tells every control room what has happened. An aircraft that is part of the aerodrome traffic keeps a watch on the tower for instructions by the light signals taught above. Then the script forks by flight rules and weather:

A VFR flight — continues in visual meteorological conditions, lands at the nearest suitable aerodrome, and reports its arrival by the most expeditious means to the appropriate ATS unit.
An IFR flight that meets VMC — if the PIC decides to continue visually rather than press on under IFR, the pilot sets code 7601, lands at the nearest suitable aerodrome, and reports arrival. The dedicated code, introduced in 2025, tells ATC at a glance that the flight is diverting visually rather than flying the full lost-communications procedure.
An IFR flight in instrument conditions — flies the full procedure below, as does one that cannot complete the flight visually.

The full IFR procedure keeps the flight exactly where the controller's plan expects it. The aircraft maintains the last assigned speed and level, or the minimum flight altitude if that is higher, for 20 minutes following its failure to make a required report or, where surveillance service is provided, following the moment the transponder is set to 7600, whichever applies. After the 20 minutes it adjusts level and speed to the filed flight plan (as amended by any delay or modification messages) and proceeds along the last received and acknowledged route clearance to the designated navigation aid or fix serving the destination, holding there if needed. It commences descent at, or as close as possible to, the expected approach time (EAT) last received and acknowledged (or, if none was received, at or close to the estimated time of arrival), completes the instrument approach procedure specified for that aid or fix, and lands, if possible, within 30 minutes after the estimated time of arrival or the last acknowledged EAT, whichever is later. The expected approach time, met here for the first time, is ATC's promise under delay: the time at which the flight can expect to leave its holding and begin the approach.

One currency note: 20 minutes is the rule since 1 May 2025, when the EU realigned SERA with the ICAO value and created code 7601. Before that date the European rule was 7 minutes, and older training material and question banks may still carry it. For the exam, know the current pair: 20 minutes, and 7601 for the IFR flight diverting visually.

Unlawful interference

An aircraft that is being subjected to unlawful interference (a hijacking or other unlawful seizure) shall endeavour to notify the appropriate ATS unit of three things: the fact of the interference, any significant circumstances associated with it, and any deviation from the current flight plan that the circumstances make necessary. The purpose is written into the rule: the notification enables the ATS unit to give the flight priority and to minimise conflict with other traffic. The word is endeavour: the rules recognise that a crew under duress may be unable to transmit; the discreet transponder code for unlawful interference belongs to the transponder lesson later in this course, and what happens on the ground to such a flight belongs to the security module.

The general rules, memorised

Right of way: holder maintains heading and speed; power-driven gives way to airships, sailplanes, balloons and to towing aircraft; converging — the one with the other on its right gives way; head-on — both alter right; overtaking (approach within 70° of the plane of symmetry) — overtaken has right of way, overtaker alters right.
Landing aircraft and final approaches have priority; between landing aircraft the higher gives way to the lower (no cutting in); taxiing aircraft give way to aircraft taking off; formation flight only by pre-arrangement, within 1 km/1 km/30 m of the leader.
Ground: head-on — stop or alter right; converging — give way to the right; overtaken has right of way; stop at every runway-holding position without explicit clearance; stop at lighted stop bars, proceed only when the lights are off.
Lights at night (end of evening civil twilight to beginning of morning civil twilight): anti-collision + navigation lights; red port 110°, green starboard 110°, white astern 140°; anti-collision lights also by day if fitted.
Tower light signals — in flight: steady green cleared to land, steady red give way and circle, green flashes return for landing, red flashes aerodrome unsafe, white flashes land here then apron; on ground: steady green cleared for take-off, steady red stop, green flashes taxi, red flashes clear the landing area, white flashes return to start; acknowledge by day rocking wings (not on base/final), by night two flashes of the lights.
Minimum height over congested areas and open-air assemblies: high enough for an emergency landing without undue hazard to persons or property below.
Cruising levels: FLs at or above the lowest usable flight level / above the transition altitude; altitudes below / at or below it. Simulated instrument flight: dual controls + safety pilot (view supplemented by an observer if needed).
Aerodrome: observe traffic, conform with or avoid the pattern, all turns left unless told otherwise, land and take off into wind unless safety/runway/traffic dictate otherwise.
Flight plan required: controlled flights, IFR in advisory airspace, designated areas/routes, international borders, night flights leaving the vicinity. Timing: ≤120 h, ≥3 h (ATFM), ≥60 min before departure, ≥10 min before entry from the air. Delay 30 min (controlled)/1 h (uncontrolled) → amend or refile. Report TAS ±5%, estimates off by >2 min. Close with the arrival report: identification, departure, destination (if diverted), arrival, time.
Clearance before any controlled flight, requested via the flight plan; unsatisfactory clearance → request an amended one. Controlled VFR losing VMC: amended clearance → continue VMC and notify → special VFR (in a control zone) → IFR.
Position reports at compulsory points: time and level, ASAP; message order identification, position, time, level, next position/time, ensuing point.
COM failure: 7600, try all means, watch for light signals; VFR — land nearest suitable; IFR meeting VMC — 7601, land nearest suitable; IFR in IMC — last speed/level (or minimum altitude) 20 min, then filed plan to the destination aid, descend at EAT/ETA, land within 30 min (rule since 1 May 2025; formerly 7 min).
Unlawful interference: endeavour to notify ATS of the fact, the significant circumstances, and any deviation from the current flight plan, so the flight gets priority.