Separation Near Aerodromes

The closer to the runway, the tighter the choreography. This lesson gathers Doc 4444's aerodrome-vicinity procedures: how the runway-in-use is chosen and what a pilot may do about it, the visual approach and its conditions, the holding stack and who gets out of it first, the expected approach time that governs a delay, and the rule that lets a departure roll while an arrival is still inbound. It closes with three pieces of separation machinery the syllabus files under miscellaneous: own-separation clearances in VMC, essential traffic information, and the circumstances under which separation minima may be reduced.

The runway-in-use and the wind

The runway-in-use is the runway or runways that, at a particular time, the aerodrome control tower considers most suitable for the types of aircraft expected to land or take off. Separate runways may be designated for arrivals and departures. Normally an aircraft lands and takes off into wind, unless safety, the runway configuration, meteorological conditions, the available instrument approach procedures or air traffic conditions make a different direction preferable; beyond the surface wind, the tower also weighs the aerodrome traffic circuits, the length of the runways, and the approach and landing aids available. A runway may also be nominated for noise abatement, steering departures and finals away from noise-sensitive areas, with limits. Runways should not be selected for noise-abatement landings unless they carry suitable glide path guidance: an ILS, or a visual approach slope indicator for operations in visual meteorological conditions. And noise abatement shall not be a determining factor in runway nomination in five conditions:

When the surface is adversely affected (snow, slush, ice, water, mud, rubber, oil or other substances).
For landing with a ceiling below 150 m (500 ft) or visibility below 1,900 m.
For take-off with visibility below 1,900 m.
When wind shear is reported or forecast or thunderstorms are expected on the approach or departure.
When the crosswind component including gusts exceeds 28 km/h (15 kt), or the tailwind component including gusts exceeds 9 km/h (5 kt).

In all of those conditions the runway is chosen on operational grounds alone, and the noise-preferential option carries no weight.

Two pilot rights sit inside this machinery, and both are learning objectives. First: to expedite departures, ATC may suggest a take-off direction that is not into the wind, and it is the pilot-in-command's responsibility to decide between making such a take-off or waiting for take-off in a preferred direction. The offer saves time; accepting a tailwind or crosswind that the aircraft's performance cannot carry is entirely the pilot's call to refuse. Second: a pilot-in-command, prompted by safety concerns, can refuse a runway offered for noise-preferential reasons. The practical consequence of turning down the runway-in-use, either way, is patience: the traffic flow is built around that runway, and the aircraft that wants a different one waits for the gap that can accommodate it.

Essential local traffic

In the last mile the hazards are not all airborne. Essential local traffic is any aircraft, vehicle or personnel on or near the runway to be used, or traffic in the take-off and climb-out area or the final approach area, which may constitute a collision hazard to a departing or arriving aircraft. Information on it, when known to the controller, is transmitted without delay to the departing and arriving aircraft concerned, and described so as to be easily identified: "mower left of runway 24 threshold", not "traffic in the vicinity".

Visual approaches

An IFR flight can shed the instrument procedure when the weather makes it unnecessary. A visual approach may be requested by the flight crew or initiated by the controller, in which case the crew's concurrence is required, and controllers are told to be cautious about initiating one for a crew that may not know the aerodrome and its terrain. The conditions: the pilot can maintain visual reference to the terrain, and either the reported ceiling is at or above the level of the beginning of the initial approach segment, or the pilot reports (at that level or at any time during the procedure) that the meteorological conditions give reasonable assurance that a visual approach and landing can be completed.

The syllabus asks pointedly what happens to separation, because students guess wrong: separation SHALL be provided between an aircraft cleared to execute a visual approach and other arriving and departing aircraft. Visual approach does not mean see-and-avoid; the controller keeps separating. Only for successive visual approaches does the duty shift: the controller maintains separation until the succeeding pilot reports the preceding aircraft in sight, then instructs them to follow it maintaining own separation, with a wake turbulence caution when the leader is heavier (or both are heavy) and the spacing is below the wake minimum, the pilot then owning the spacing.

Instrument approaches and the unfamiliar crew

The approach control unit specifies the instrument approach procedure to be used; a crew may request an alternative and, circumstances permitting, should be cleared for it. When a pilot reports (or it becomes clearly apparent) that they are not familiar with the procedure, the controller spells it out: the initial approach level, the point at which the base or procedure turn will be started, the level for the procedure turn, and the final approach track. But when the aircraft is to be cleared for a straight-in approach, only the final approach track needs to be specified (the document's own exception), with the navigation aid frequencies and the missed approach procedure added when deemed necessary. And a crew that gains visual reference early has earned nothing: the entire procedure must still be executed unless the aircraft requests, and is cleared for, a visual approach.

The stack, the queue and the clock

When arrivals outpace the runway, the holding stack forms, and Doc 4444 orders it top to bottom: levels at a holding fix are assigned to facilitate clearing each aircraft to approach in its proper priority: normally, the first aircraft to arrive over the fix should be at the lowest level, with following aircraft at successively higher levels, so the next to approach is always at the bottom, and everyone steps down as the stack drains. One refinement: when extended holding is anticipated, turbojets should when practicable be permitted to hold at higher levels to conserve fuel, while retaining their order in the approach sequence: altitude is fuel to a jet, and the order in the queue is bookkeeping, not geometry.

The approach sequence itself is built to land the maximum number of aircraft with the least average delay, with a priority list cut into it:

An aircraft which anticipates being compelled to land — engine failure, fuel shortage, anything affecting safe operation.
Hospital aircraft, or aircraft carrying any sick or seriously injured person requiring urgent medical attention.
Aircraft engaged in search and rescue operations.
Other aircraft as determined by the appropriate authority.

A pilot may also opt out of the queue: one who indicates an intention to hold for weather improvement, or for other reasons, shall have that action approved. But when the other holding aircraft want to continue their approaches, the waiting flight cannot block the bottom of the stack: it will be cleared to an adjacent fix to hold awaiting the weather change or re-routing, or, alternatively, given a clearance placing it at the top of the approach sequence so the aircraft below it can land.

The clock on all this waiting is the expected approach time taught with the Doc 4444 definitions: the time ATC expects the delayed aircraft to leave the holding fix to complete its approach. Its procedures: an EAT shall be determined for an arriving aircraft that will be delayed 10 minutes or more (or another period set by the authority), and transmitted as soon as practicable, preferably not later than the start of the initial descent from cruising level: the crew's diversion arithmetic needs it before the descent, not in the hold. A revised EAT goes out without delay whenever it changes by 5 minutes or more, and whenever a hold of 30 minutes or more is anticipated, the EAT is transmitted by the most expeditious means available. A cousin covers the en-route hold: an aircraft held anywhere other than the initial approach fix gets an onward clearance time (the time it can expect to leave that fix), plus warning of any further holding expected ahead.

Releasing a departure against an arrival

One geometric rule lets the runway serve both directions of traffic at once. When take-off clearance is based on the position of an arriving aircraft making a complete instrument approach, a departing aircraft may take off in any direction until the arriving aircraft has started its procedure turn or base turn leading to final approach. After that turn has started, the departure may still go, but only in a direction differing by at least 45 degrees from the reciprocal of the approach direction, and only if the take-off is made at least 3 minutes before the arriving aircraft is estimated over the beginning of the instrument runway. The logic: before the turn, the arriver is still manoeuvring away from the field; once it turns inbound, the only safe departure is one that carries the departing aircraft out of the arrival's path with time in hand. A straight-in approach has no turn to hang the rule on, so time replaces geometry: against a straight-in arrival, a departure may take off in any direction until 5 minutes before the arriver is estimated over the instrument runway, and in a direction at least 45 degrees off the reciprocal until 3 minutes before the arriver is over the runway's beginning, or until the arriver crosses a designated fix on the approach track, where the ATS authority has established one with the operators.

Own separation, essential traffic, reduced minima

Three pieces of chapter-5 machinery close the lesson. First, the own-separation clearance that the airspace-class list already hinted at: in Classes D and E, in VMC, during the hours of daylight, an ATC unit may (when an aircraft requests it, the pilot of the other aircraft agrees, and the ATS authority has authorised the procedure) clear a flight to fly maintaining own separation to one other aircraft and remaining in VMC, for a specified portion of the flight at or below 3,050 m (10,000 ft), during climb or descent. The consequence is the whole point: for that portion, vertical and horizontal separation by ATC is simply not provided: the cleared flight must keep itself clear of the other. An IFR flight so cleared receives alternative instructions in case VMC cannot be maintained, and a pilot who sees the conditions deteriorating must inform ATC before entering IMC and proceed on those alternatives.

Second, the safety net under that gap: essential traffic is controlled traffic to which the provision of ATC separation is applicable but which, in relation to a particular controlled flight, is not (or will not be) separated by the appropriate minimum. Whenever two controlled flights constitute essential traffic to each other, essential traffic information shall be given to both: the direction of flight, the type (and wake turbulence category, if the other aircraft is heavier), the cruising level, and its position: as an estimated time over the reporting point nearest to where the level will be crossed, a clock-code bearing ("traffic at your two o'clock") and distance, or an actual or estimated position. Own-separation clearances generate it by design; an infringed minimum generates it by accident.

Third, the circumstances under which the horizontal separation minima may be reduced. En route, provided a safety risk assessment shows an acceptable level of safety and the users have been consulted:

When special aids let the pilot fix position accurately and fast communications pass it to ATC without delay.

When surveillance-derived position with rapid, reliable communications is available to the unit.

When special aids let the controller predict flight paths rapidly and verify actual against predicted positions frequently.

When RNAV aircraft operate within the coverage of aids maintaining their accuracy.

In the vicinity of aerodromes, three simpler cases are added:

When the aerodrome controller can provide adequate separation with each aircraft continuously visible to that controller.

When each aircraft is continuously visible to the flight crews of the others, who report they can maintain own separation.

When one aircraft follows another and the succeeding crew reports the other in sight and separation can be maintained.

The last miscellaneous learning objective is pure recall from the wake turbulence lesson earlier in this module: the non-radar wake-turbulence longitudinal minima. Arrivals: HEAVY behind SUPER 2 minutes, MEDIUM behind SUPER 3 and behind HEAVY 2, LIGHT behind SUPER 4 and behind HEAVY or MEDIUM 3; departures 2/3/2/2, with one minute more from an intermediate part of the runway. The values live, fully taught, in that lesson; here they need only be remembered as the time-based family the vicinity procedures lean on.