The instrument flight rules
IFR flight substitutes instruments and procedures for the view outside, and its rules open with the machine: an aircraft flown under IFR must be equipped with suitable instruments and with navigation equipment appropriate to the route to be flown. Then come the IFR minimum levels, protecting the flight from the terrain it cannot see: except when necessary for take-off or landing, or when specifically authorised, an IFR flight is flown at a level not below the minimum flight altitude established by the state overflown or, where none is established, over high terrain or mountainous areas at least 600 m (2,000 ft) above the highest obstacle within 8 km of the aircraft's estimated position, and elsewhere at least 300 m (1,000 ft) above the highest obstacle within 8 km. Compare the VFR heights: the instrument pilot gets double the clearance over mountains and a vastly wider protected radius, because estimated position is all there is.
IFR flight in level cruise uses the same semicircular logic as VFR, without the extra 500 ft: outside controlled airspace, a cruising level appropriate to the magnetic track: odd thousands eastbound (000–179), even thousands westbound (180–359), except where the competent authority specifies otherwise for flight at or below 900 m (3,000 ft) above mean sea level; inside controlled airspace the assigned level from ATC takes over. An IFR flight outside controlled airspace additionally maintains a watch on the appropriate radio frequency and reports position as the previous lesson described, so that the flight information and alerting services know where it is.
Leaving IFR is a deliberate act with a script. A flight electing to change from IFR to VFR notifies the appropriate ATS unit specifically that the IFR flight is cancelled, using the phrase cancelling my IFR flight, and communicates the changes to its current flight plan. No one else can make the change happen: an ATS unit never invites a flight to cancel IFR, directly or by implication. And the rules warn against optimism: the change should not be initiated when instrument conditions are likely along the rest of the route; a pilot who cancels IFR into deteriorating weather has traded a protected flight for an illegal one.
Interception: why it happens
Interception of civil aircraft is governed by the principle that it is a last resort, undertaken only when identification or assistance cannot be achieved another way. The circumstances in which a civil aircraft may find a fighter alongside form a short list:
Identification — the aircraft cannot be identified by any other means, and a state needs to know what is flying through its airspace.
Straying — the aircraft has entered airspace without authorisation, or deviates from its assigned track: it penetrates a prohibited or restricted area, or crosses a border off its cleared route.
Communication failure — a flight out of radio contact may be intercepted to inspect it and to shepherd it, since no one can ask it what it intends.
Suspected unlawful acts — a flight suspected of unlawful interference, smuggling or another unlawful purpose may be intercepted and instructed to land.
Interception: what the crew does
The intercepted aircraft's duties run in a fixed order, and the first is the one the exam asks most: follow the instructions of the intercepting aircraft, interpreting and responding to its visual signals. In parallel the crew notifies the appropriate ATS unit, attempts radio contact with the interceptor or its control unit on 121.5 MHz (the emergency frequency, the primary channel for interception), giving the flight's identity and the nature of the flight, repeating the call on the UHF emergency frequency 243 MHz, if practicable, when no contact is made, and, unless otherwise instructed by the ATS unit, sets the transponder to Mode A code 7700. If instructions received by radio from any source conflict with the interceptor's visual signals, the crew requests immediate clarification while continuing to comply with the interceptor's visual instructions: the aircraft with the weapons alongside outranks every voice on the radio until the confusion is resolved.
The visual signals themselves come in numbered series, and the exam quotes them. From the interceptor:
Series 1 — you have been intercepted, follow me: the interceptor rocks its wings and flashes its navigational lights at irregular intervals, from a position slightly above and ahead of, and normally to the left of, the intercepted aircraft and, after acknowledgement, makes a slow level turn onto the desired heading. The intercepted aircraft answers by rocking its wings, flashing its navigational lights at irregular intervals, and following.
Series 2 — you may proceed: an abrupt breakaway manoeuvre, a climbing turn of 90 degrees or more without crossing the intercepted aircraft's line of flight. The answer is a rock of the wings.
Series 3 — land at this aerodrome: the interceptor lowers its landing gear, shows steady landing lights, and overflies the runway in use. The intercepted aircraft answers by lowering its own gear, showing steady landing lights, following, and landing if the runway is usable.
And from the intercepted aircraft, three signals of its own:
Series 4 — the aerodrome you have designated is inadequate: raising the landing gear while flashing the landing lights, passing over the runway at a height between 300 m (1,000 ft) and 600 m (2,000 ft) while continuing to circle. The interceptor then either leads the aircraft to another aerodrome (gear up, series 1) or releases it (series 2).
Series 5 — cannot comply: the regular switching on and off of all available lights, at a rate distinct from flashing.
Series 6 — in distress: the irregular flashing of all available lights.
When radio contact is achieved but no common language exists, a small vocabulary of standard phrases carries the exchange, each pronounced as the rules spell it. From the interceptor: CALL SIGN (what is your call sign?), FOLLOW (follow me), DESCEND (descend for landing), YOU LAND (land at this aerodrome), PROCEED (you may proceed). From the intercepted aircraft: CALL SIGN followed by the call sign itself, WILCO (understood, will comply), CAN NOT (unable to comply), REPEAT (repeat your instruction), AM LOST (position unknown), MAYDAY (I am in distress), HIJACK (I have been hijacked), LAND followed by a place name (I request to land at that place), and DESCEND (I require descent).