This lesson closes the general procedures with the machinery of certainty: the radio check that proves the equipment, the read-back rules that prove the message, the radar phraseology spoken under surveillance, the fixed vocabulary of vertical position, and the data link that replaces voice altogether, including what is said when it breaks.
Radio check and the readability scale
A test transmission has a fixed four-part form: the identification of the station being called, the aircraft identification, the words RADIO CHECK, and the frequency being used. The reply names the aircraft, names the replying station, and gives the readability; both transmission and reply are recorded at the aeronautical station. The scale:
Readability 1 — unreadable;
Readability 2 — readable now and then;
Readability 3 — readable but with difficulty;
Readability 4 — readable;
Readability 5 — perfectly readable.
What must be read back
Safety-related parts of voice clearances are read back, and Doc 4444 fixes a list that is always read back in full:
ATC route clearances — the en-route clearance, in full.
Clearances and instructions to enter, land on, take off from, hold short of, cross, taxi and backtrack on any runway: everything that touches a runway.
Runway-in-use, altimeter settings, SSR codes, level, heading and speed instructions, and transition levels, whether issued by the controller or heard on the ATIS.
Everything else, including conditional clearances, is read back or acknowledged in a way that clearly shows it has been understood and will be complied with. The obligation then flips sides: the controller listens to the read-back and takes immediate action to correct any discrepancy it reveals. A read-back nobody checks proves nothing.
Saying your level
Vertical position has three names, and the transmission format marks which is meant. Against the standard setting of 1013.2 hPa, the words FLIGHT LEVEL precede the figures. Against QNH or QFE, the figures are followed by FEET or METRES: an altitude on QNH of 2,500 ft is "two thousand five hundred feet", spoken with the number rules of the transmission-standards lesson, while the same slice of sky above the standard datum would open "flight level...". The Q-codes behind altitude and height were set in this course's first lesson, and the format keeps them audible.
Radar phraseology
Under ATS surveillance the vocabulary is short and exact. Identification is announced with IDENTIFIED, with position appended as required, and its opposites are just as fixed: NOT IDENTIFIED with the reason, and later IDENTIFICATION LOST with reasons and instructions, or RADAR SERVICE TERMINATED when the service ends. Vectors arrive as headings, and traffic arrives as a clock:
Traffic information — TRAFFIC (number) O'CLOCK (distance) (direction of flight), with whatever else helps: unknown, slow or fast moving, closing, same or opposite direction, overtaking, crossing left to right, the type and level, climbing or descending.
Avoiding action — TURN LEFT (or RIGHT) IMMEDIATELY, a heading of three digits, TO AVOID TRAFFIC with its clock bearing and distance; and afterwards, CLEAR OF TRAFFIC.
SSR — SQUAWK with the code, SQUAWK IDENT to flash the identification feature, RESET SQUAWK for a fresh selection, STOP SQUAWK to end it.
Suspected loss of communications — IF YOU READ, followed by a manoeuvre or SQUAWK instruction, answered from the ground with OBSERVED and the aircraft's position when the reply appears on the display.