Similar Callsigns and an Ambiguous Descent Clearance
At 07:42, VT-ABC, a Cessna 172, is level at 5,000 ft on a training flight, monitoring Approach on 124.35. VT-ABD, an ATR 72, is on the same frequency, 8 NM ahead and descending. The controller transmits: 'ABC descend 3,000 feet, QNH 1006, report level.' Almost immediately VT-ABD reads back: 'Descending 3,000 feet, QNH 1006, VT-ABD.' The controller does not correct the readback and then says: 'ABC, confirm passing 4,000.' The VT-ABC pilot has not received a clearance since being level at 5,000 and now hears two transmissions using similar shortened callsigns. The frequency becomes blocked by another aircraft requesting weather. The pilot must decide whether to start descending and how to resolve the ambiguity without creating a second conflict.
1. What should the VT-ABC pilot do immediately, and why?
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The pilot should not descend on the basis of the first transmission. The clearance used only the shortened callsign 'ABC', and VT-ABD immediately read it back. That creates a reasonable doubt about which aircraft was addressed. A descent without a confirmed clearance could put VT-ABC into conflict with unknown traffic, especially since the controller has not corrected the readback.
The safe action is to maintain 5,000 ft, keep the assigned heading or route, and make a short, full-callsign transmission that states the current level and asks for confirmation. Do not wait silently for the controller to notice the error. Also monitor the frequency for a correction to VT-ABD. If the controller confirms that the clearance was for VT-ABC, then read back the full clearance before descending.
2. Write the transmission VT-ABC should make, and explain why each element is needed.
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A suitable transmission is:
'Approach, VT-ABC maintaining 5,000 feet. Confirm the descent clearance is for VT-ABC, QNH 1006.'
Why this works:
- Full callsign removes the ambiguity created by 'ABC' and distinguishes VT-ABC from VT-ABD.
- 'Maintaining 5,000 feet' tells the controller the aircraft has not started descending and gives immediate traffic situational awareness.
- 'Confirm the descent clearance is for VT-ABC' asks a direct question without accusing anyone of error.
- 'QNH 1006' lets the controller confirm the altimeter setting if the clearance is indeed for VT-ABC.
After the reply, read back the clearance in full, including altitude, QNH, and callsign. If the controller says the clearance was for VT-ABD, remain at 5,000 ft and continue the flight.
3. If there is no reply after two attempts, what should the pilot do next and what should be avoided?
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After two unanswered attempts, the pilot should continue to maintain the last confirmed level, 5,000 ft, and the last confirmed route or heading. Try the previous controlling frequency or another published frequency for the area if one is available. If the frequency is blocked, wait for a gap and make one concise call rather than repeated long transmissions.
The pilot should avoid starting the descent merely because the controller later says 'ABC, confirm passing 4,000'. That transmission still does not confirm the clearance was for VT-ABC. Avoid changing to 121.5 unless there is an emergency or a genuine communications failure; a busy approach frequency is not itself a failure. If the pilot becomes uncertain about traffic separation, request a traffic advisory or vectors once contact is established. Preserve the last clearance and do not improvise an altitude change.