What is an approach light layout?
The pattern the approach lights make on the ground — how far the system runs from the threshold, how far apart the lights sit, and where the crossbars and side rows go — set by the type of approach the runway supports.
An approach light layout is the arrangement of approach lights on and across the extended runway centreline, fixed by distances measured from the threshold. The type of approach the runway supports decides the pattern: a simple system runs at least 420 m, and a precision system runs 900 m wherever possible.
Every layout is built from the same few elements: a centreline of white lights, one or more crossbars, and in CAT II/III red side rows and barrettes in the inner part. The lowest-visibility layout is the most closely packed, because the pilot has the least time to see the lights and align. Sequenced flashing lights then supplement the centreline in the outer approach, while the green threshold row and wing bars mark the runway end.
Frequently Asked Questions
Q1What is an approach light layout?
An approach light layout is the arrangement of the approach lights on the extended runway centreline and across it: how far the system runs from the threshold, how far apart the lights are, and where the crossbars and side rows sit. The type of approach the runway supports decides which standard pattern is used.
Q2What are the three standard layouts?
A simple system runs not less than 420 m with centreline lights normally every 60 m and one crossbar at 300 m. A precision CAT I system runs 900 m wherever possible, with lights every 30 m and one crossbar at 300 m. A precision CAT II/III system also runs 900 m, but the inner 300 m is built of barrettes with red side rows, and it has two crossbars, at 150 m and 300 m.
Q3Why is the layout longer for precision approaches?
A precision approach is flown to a lower decision height in poorer visibility, so the pilot needs to pick up the lights earlier and have more time to align with the runway and judge the approach. The longer, denser system gives that extra guidance. Both lengths are set in ICAO Annex 14, Volume I, Chapter 5, and apply wherever physically practicable.
Q4Where are the distances measured from?
From the runway threshold, outward along the extended centreline. The innermost centreline light is 30 m from the threshold in a precision system, and the crossbars are placed 150 m and 300 m out. Because every position is fixed from the threshold, the same layout reads the same way at every runway end.
Q5What does the CAT II/III layout add?
Barrettes in place of single lights over the inner 300 m, red side-row barrettes either side of the centreline over the inner 270 m, and a second crossbar at 150 m. Together they give the pilot much stronger roll, distance and runway-end cues in the lowest visibilities. The crossbars stay white; the side rows are red.
Q6What else is added to a layout, and what if the full length does not fit?
Sequenced flashing lights can supplement the steady centreline, and the threshold and wing bars sit where the pattern meets the runway. The recommended lengths apply wherever practicable. If the ground does not allow the full length, the shortfall is a design decision to be checked against the standard in force and agreed with the authority, not a field change.
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