IFR Clearance Readback and Lost-Communication Decision
You are captain of a twin-turboprop on an IFR schedule from Plaisance (FIMP) to Roland Garros (FMEE), cleared via the SID at FL240. Departure is at 0610 UTC; SSR code 4271. Five minutes after takeoff, Mauritius Approach issues: 'Turn right heading 070, climb FL260, cross 30 DME FIMP at or below FL180; traffic opposite direction at FL200.' You read back: 'Left heading 070, climbing FL260, cross 30 DME FIMP at or below FL180, code 4271.' ATC immediately corrects: 'Negative, right heading 070, readback correct.' You acknowledge correctly. At 30 DME, while climbing through FL175, radio communication fails; the transponder still squawks 4271 and the last assigned level was FL260. Weather is VMC, fuel normal, and the aircraft is radar-identified. You are now 35 nm from FIMP, in controlled airspace, and expect further clearance at FL260.
1. What was the significance of the incorrect readback, and what should the crew have done before accepting the corrected clearance?
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The readback error was on the most safety-critical element: turn direction. A clearance is not safely accepted until the readback matches the clearance, because the readback is the last barrier against a wrong turn or level. With opposite traffic at FL200, a left turn instead of right could have reduced separation and triggered a TCAS event.
On hearing ATC's correction, the crew should:
- Stop any turn or altitude change not yet commenced; if one has commenced, immediately verify with ATC and correct it.
- Re-state the full corrected clearance: 'Right heading 070, climb FL260, cross 30 DME FIMP at or below FL180, squawk 4271.'
- Cross-check the heading bug, flight director, autopilot, altitude preselect and FMS route against the corrected clearance.
- Ask for confirmation if any element remains ambiguous.
The crew should not treat the correction as a formality. The correct action is to verbalise the change, confirm it on the flight deck, and only then fly it.
2. State the lost-communication actions you would take, including transponder and route, and explain why.
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Under the ICAO IFR lost-communication procedure adopted for Mauritian controlled airspace, the crew should:
- Squawk 7600 and continue to listen on the last assigned frequency, 121.5 MHz and any relevant VOR/NDB frequency.
- Maintain the last assigned speed and level, or minimum flight altitude if higher, for 7 minutes after the failure. At the moment of failure the aircraft was at 30 DME passing FL175, cleared to FL260 with a crossing restriction at or below FL180 at 30 DME. The restriction was already satisfied, so the aircraft should continue the climb to FL260 and maintain it for the 7-minute period.
- After that period, adjust level and route to the flight plan. Because the weather is VMC, the crew should remain in VMC and consider landing at the nearest suitable aerodrome if that is safer than continuing IFR to FMEE.
- Attempt contact through other ATC units, a relay aircraft, or SATCOM/company frequency, and advise the destination of the situation.
The critical point is not to turn or descend immediately on losing radio. The aircraft was radar-identified, so ATC can see the 7600 squawk and provide separation; an unexpected manoeuvre could conflict with the opposite-direction traffic.
3. If ATC had not corrected the readback and the aircraft had turned left, who is responsible for separation, and what should the captain do?
Read the worked answer
ATC is responsible for separation between IFR aircraft in controlled airspace, but the pilot remains responsible for the safety of the aircraft and must not fly a clearance that is evidently wrong or ambiguous. Responsibility is therefore not a reason to accept a bad clearance silently.
If the aircraft had turned left and the crew recognised the error, the captain should:
- Stop the turn and request immediate clarification.
- If a collision hazard is perceived, take avoiding action under the emergency authority, using TCAS or visual avoidance, and advise ATC at once.
- State the actual heading, level and intention using standard phraseology.
- File a report if the event affected separation.
The captain's judgement should be based on the principle that ATC separation does not relieve the pilot from challenging an incorrect instruction. The readback and the challenge are both part of the same safety barrier.