| .. | .. |
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| 1724 | 1724 | |
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| 1725 | 1725 | #### Answer |
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| 1726 | 1726 | |
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| 1727 | | -B) |
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| 1727 | +A) |
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| 1728 | 1728 | |
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| 1729 | 1729 | #### Explanation |
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| 1730 | 1730 | |
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| 1731 | | -An aircraft may use its abbreviated callsign once radio communication is well established with the ground station, and only after the ground station has itself first used the abbreviated form. |
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| 1731 | +Per ICAO Annex 10 Vol II / SERA.14050: an aircraft shall not use an abbreviated callsign until the aeronautical station has addressed the aircraft using the abbreviated form. The ground station initiates the abbreviation — only then may the pilot follow suit. |
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| 1732 | 1732 | |
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| 1733 | | -- **Option A** is partly correct but incomplete — it is the ground station's use that triggers permission. |
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| 1734 | | -- **Option C** (heavy traffic) and Option D (no confusion risk) do not independently grant abbreviation rights; the ground station must initiate it. |
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| 1733 | +- **Option B** ("once communication is well established") is vague and not the regulatory criterion. Communication being "well established" is a precondition for the station to decide to abbreviate, but the trigger for the pilot is the station's actual use of the short form. |
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| 1734 | +- **Option C** (heavy traffic) and **Option D** (no confusion) are factors the station considers before abbreviating, but neither independently grants the pilot permission to abbreviate. |
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| 1735 | 1735 | |
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| 1736 | 1736 | ### Q73: An aircraft fails to establish radio contact with a ground station on the designated frequency or any other appropriate frequency. What action must the pilot take? ^t90q73 |
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| 1737 | 1737 | |
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