You can keep frequency congestion at busy aerodromes to a minimum if you stick to correct readbacks of ATC instructions.
AIP Gen 3.4 5.4 lays out mandatory readback requirements. You need to know these items:
- An ATC route clearance in its entirety and any amendments.
- Enroute holding instructions.
- Any route and holding point specified in a taxi clearance.
- Any clearances, conditional clearances, or instructions to:
- hold short of
- land on
- line up on
- take off from
- taxi or backtrack on
- any runway or HLS.
- If you receive an instruction involving a holding point, you must include the words 'holding point' in your readback.
- Any approach clearance.
- Assigned runway, altimeter settings directed to specific aircraft, radio and radio navigation aid frequency instructions. An 'expectation' of the runway to be used is not to be read back.
- SSR codes, data link logon codes.
- Level instructions, direction of turn, heading and speed instructions.
You also need to know which other items you do not need to read back.
For example, 'Report at ...'. These unnecessary readbacks clutter the radio frequency at busy aerodromes. Air Traffic Controllers won't appreciate it as they monitor multiple aircraft.
Remember: AIP ENR 1.1 2 and AIP Gen 3.4 5.4.