Aircraft typically use radio telephony for communicating with air traffic control (ATC). The information and instructions transmitted are of vital importance for the safe and expeditious operation of aircraft. Incidents and accidents have occurred in which the use of non-standard phraseology was a contributing factor.
Radio transmissions can at times be distorted or garbled, due to sun spot activity or radio interference, causing communication to be misunderstood.
ICAO developed and promoted a suite of standard phraseologies for efficient, clear, concise and unambiguous communication. This ensures that correct and precise standardised terms are used at all times for ease of understanding. The phraseology suite is based on the English language, the official language of international aviation. All licensed pilots and air traffic controllers must meet minimum standards for English language proficiency.
As aircraft fly around the globe, ATC will assign altitudes known as flight levels (FL) for the safe vertical separation of aircraft. Flight levels are quoted in hundreds of feet. FL100 refers to an altitude of 10,000 feet. Separation between levels are in increments of 1,000 feet below FL290 and 2,000 feet above. Separation of 1,000 feet is permitted above FL290 when certain conditions are satisfied. These conditions are collectively known as reduced vertical separation minima (RVSM).
The higher a jet aircraft flies the less fuel it burns due to the decreased density of the air. Pilots seek to operate aircraft at the optimum altitude during the cruise phase of flight for maximum fuel efficiency. An aircraft cannot change its assigned altitude without prior approval of ATC except due to onboard emergencies or when taking evasive action to avoid danger.
[caption id="attachment_995400" align="alignnone" width="878"] -[/caption]
When a pilot flying at FL370 wants to fly at FL390, clearance will be sought from ATC using the ATC station name and the aircraft call sign in the phraseology, "Request clearance to climb to flight level tree niner zero.” The word "climb” is used rather than “ascend” as it is phonetically similar to the word “descend.” Likewise, if the pilot desires to fly at a lower altitude ATC clearance will be sought to “…descend to flight level tree seven zero.” ICAO phraseology aims at avoiding similar sounding words, as far as possible.
Read-back requirements were introduced to promote safety. The stringency of the read-back requirement is directly related to the seriousness of misunderstanding an instruction or acceptance of ATC clearances by a specific flight. Strict adherence to read-back procedures ensures that the clearance was received correctly by the particular flight.
An ATC’s clearance for an altitude change as above must read back by the pilot to ATC using the exact words. If an aircraft read-back is incorrect, the controller transmits the words “negative I say again” followed by the correct version. The pilot once more has to read
