Two Similar Callsigns and a Correct but Wrong Readback
You are the tower controller on frequency 118.7 during the evening peak. Two aircraft with similar callsigns are on the frequency: IGO 472 and IGO 427. Both are holding at the same holding point for runway 27. You transmit to IGO 472: 'IGO 472, line up and wait runway 27, behind landing traffic.' IGO 427 reads back: 'Line up and wait runway 27, IGO 427.' The content of the readback is correct but the callsign is wrong. At the same moment you are coordinating with approach control on the landline, and the landing aircraft is 1.5 NM final. The frequency is congested, one other aircraft is requesting start-up, and the ATIS is being updated. IGO 472 has not transmitted since checking in.
1. State your immediate actions and the exact transmissions you would make.
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The readback came from the wrong aircraft, so the clearance is not confirmed for anyone. Treat the runway as occupied until proven otherwise. First transmission: 'IGO 427, hold position, I say again, hold position.' That stops the aircraft that read back from moving. Second: 'IGO 472, confirm you are holding at the holding point, I say again, confirm holding.' Do not repeat the line-up clearance until IGO 472 has confirmed its position; if it has already moved onto the runway, the landing aircraft must go around. Third, if either aircraft is on the runway, transmit 'IGO 472, cancel take-off clearance, vacate runway 27 via the next taxiway' and instruct the arrival to go around. Tell approach control immediately that the arrival may need to go around. Only after both aircraft are positively accounted for do you reissue any clearance, and then one aircraft at a time with the callsign first and last.
2. Explain why a correct readback did not provide a safety assurance in this case.
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A readback is a check that the message was received and understood by the party it was addressed to. It is not a check that the right party received it. Here the words were correct, so a controller listening only for content would hear nothing wrong; the error was in the identity of the transmitter, which is exactly the element the readback is meant to confirm. Two conditions made this likely: the callsigns differ by only one digit in the same position, and both aircraft were at the same holding point performing the same phase of flight. Under those conditions the readback becomes a near-match rather than a mismatch, and the controller's attention is split by the landline coordination. The lesson is that the callsign is not a formality; it is the addressing mechanism. When two similar callsigns are active, the controller should slow down, use the full callsign in every transmission, and never accept a readback without matching the callsign first.
3. Rewrite the clearance and describe the communication practices you would adopt for the rest of the shift.
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Rewrite with the callsign at both ends and the condition stated after the clearance, not buried: 'IGO 472, I say again, IGO 472, line up and wait runway 27, behind the landing Airbus on 1.5 mile final, IGO 472.' The repetition of the callsign at the end gives the pilot a second opportunity to notice a mismatch. For the rest of the shift: separate the two similar callsigns in time and space where possible, for example by holding one at a different point; read back the callsign first when checking a transmission; and when a landline coordination is in progress, annotate or mark the strip so the frequency task is not resumed mid-thought. Ask IGO 427 to use its full callsign on every transmission. If the frequency becomes saturated, slow the speech rate rather than shortening the callsign, because shortening is what creates the ambiguity. Log the event and pass it to the supervisor as a communication occurrence.