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required visual reference
avia.	必需的目视基准 (与在进近到跑道上的飞机有关，指跑道的进近区域段或目视设备，能够使飞行员评估飞机相对于名义飞行路线的位置和位置变化率，以决定进近是否能够安全继续到完成着陆)
English thesaurus
required visual reference
avia., Canada	In respect of an aircraft on an approach to a runway, the section of the approach area of the runway or the visual aids that, when viewed by the pilot of the aircraft, enable the pilot to make an assessment of the aircraft position and the rate of change of position relative to the nominal flight path in order to continue the approach and complete the landing