One instruction pairing is expressly discouraged: telling an aircraft to maintain a high rate of descent and reduce speed at the same time. The two are normally incompatible: a jet pointed steeply downhill accelerates, and any significant speed reduction in the descent may force a temporary level-off. A controller may instead ask for "maximum speed", "minimum speed", a specified speed, or "minimum clean speed": the minimum at which the aircraft can fly without deploying lift-augmentation devices, speed brakes or landing gear.
Cancelling IFR
A flight that no longer needs its IFR clearance can give it up, but Doc 4444 controls the wording tightly. The change from IFR to VFR flight is acceptable only when initiated by the pilot-in-command, in a message containing the specific expression "CANCELLING MY IFR FLIGHT", together with any changes to be made to the current flight plan. The initiative may never come from the ground: no invitation to change from IFR to VFR is to be made, either directly or by inference: a controller sighing about how much easier things would be if the flight were VFR is exactly the inference the rule forbids.
The reaction from the ground is equally scripted. Normally, no reply is made other than the acknowledgement "IFR FLIGHT CANCELLED AT (time)": no approval, because none is needed; the rules change at the pilot's word. Two duties attach: when the ATS unit holds information that instrument meteorological conditions are likely along the route, the pilot should, if practicable, be so advised (a warning, not a veto), and the ATC unit receiving the cancellation shall as soon as practicable inform all other ATS units to whom the IFR flight plan was addressed, except those whose areas the flight has already passed. The pilot who cancels IFR into deteriorating weather has traded a protected clearance for VMC minima on the strength of their own judgement; the advisory exists because that trade has killed people.
Wake turbulence categories
Behind every wing generating lift trail two counter-rotating vortices, strongest behind heavy aircraft flying slowly in clean configuration: an invisible hazard that can roll a light aeroplane beyond its aileron authority. Doc 4444 grades aircraft into four wake turbulence categories by maximum certificated take-off mass:
SUPER (J) — aircraft types specified as such in Doc 8643, Aircraft Type Designators; the Airbus A380 is the type the category was created for.
HEAVY (H) — 136,000 kg or more, except the types listed as SUPER.
MEDIUM (M) — less than 136,000 kg but more than 7,000 kg.
LIGHT (L) — 7,000 kg or less.
The category travels with the callsign where it matters most: for aircraft in the SUPER or HEAVY categories, the word "super" or "heavy" shall be included immediately after the aircraft call sign in the initial radiotelephony contact with ATS units. One word, once per frequency, and every controller who works the flight knows to protect the traffic behind it. Doc 4444 also carries an alternative, finer-grained scheme of seven wake turbulence groups (A through G, graded by mass and wing span) that an ATS authority may approve instead of the four categories; the exam works in the categories, and so does the rest of this lesson.
Wake turbulence separation minima
The minima come in two families. The time-based (non-surveillance) minima protect departures and arrivals counted in minutes. For arriving aircraft, landing behind a preceding aircraft:
HEAVY landing behind SUPER — 2 minutes.
MEDIUM landing behind SUPER — 3 minutes; MEDIUM landing behind HEAVY, 2 minutes.
LIGHT landing behind SUPER — 4 minutes; LIGHT landing behind HEAVY or MEDIUM, 3 minutes: the light aeroplane always waits longest, because it is the one a vortex can flip.
For departing aircraft using the same runway, parallel runways separated by less than 760 m (2,500 ft), crossing runways where the second aircraft's projected flight path crosses the first's at the same altitude or less than 300 m (1,000 ft) below, or parallel runways 760 m or more apart under that same crossing-path condition:
HEAVY taking off behind SUPER — 2 minutes.
LIGHT or MEDIUM taking off behind SUPER — 3 minutes; behind HEAVY, 2 minutes.
LIGHT taking off behind MEDIUM — 2 minutes.
One minute more from an intermediate part of the runway — the same pairings become 3, 4, 3 and 3 minutes respectively when the following aircraft starts its take-off partway along the same runway or a parallel closer than 760 m, because it lifts off earlier along the path the leader's vortices occupy.
Two special runway situations complete the time-based family:
Displaced landing threshold — where the projected flight paths of a departure and an arrival on the same runway are expected to cross, the departure-style values apply in both orders, departing behind an arrival and arriving behind a departure: HEAVY after SUPER 2 minutes; LIGHT or MEDIUM after SUPER 3; LIGHT or MEDIUM after HEAVY 2; LIGHT after MEDIUM 2 minutes.
Opposite direction — a heavier aircraft making a low or missed approach while a lighter one takes off on the opposite-direction runway, or lands opposite-direction on the same runway or a parallel closer than 760 m: the values step up to HEAVY behind SUPER 3 minutes; LIGHT or MEDIUM behind SUPER 4; LIGHT or MEDIUM behind HEAVY 3; LIGHT behind MEDIUM 3 minutes.
The opposite-direction row is a known trap: older question banks answer it with 2 minutes, the value before the tables were amended; the current text says 3 and 4.
Where an ATS surveillance service is provided in the approach and departure phases, the distance-based minima replace the clock:
Behind a SUPER — HEAVY 9.3 km (5.0 NM), MEDIUM 13 km (7.0 NM), LIGHT 14.9 km (8.0 NM).
Behind a HEAVY — HEAVY 7.4 km (4.0 NM), MEDIUM 9.3 km (5.0 NM), LIGHT 11.1 km (6.0 NM).
LIGHT behind a MEDIUM — 9.3 km (5.0 NM).
They apply when an aircraft operates directly behind or crossing behind another at the same altitude or less than 300 m (1,000 ft) below, or when both use the same runway or parallel runways separated by less than 760 m: below and behind is where wake sinks to.