What is CAT II and III approach lighting?

It is the approach lighting provided for Category II and III precision approaches, the low-visibility ILS categories. ICAO defines Category II as a decision height lower than 60 m (200 ft) but not lower than 30 m (100 ft) with an RVR of not less than 300 m. Category III goes lower still: IIIA below 30 m (100 ft) or no decision height with RVR not less than 175 m, IIIB below 15 m (50 ft) or no decision height with RVR from 50 m to under 175 m, and IIIC with no decision height and no RVR limit. The lights must give the pilot roll, alignment and distance-to-go cues from very little visible length of pattern.

A row of white lights on the extended runway centreline, 30 m apart, running 900 m back from the threshold wherever possible, with two side rows of red barrettes extending 270 m from the threshold and two crossbars, one at 150 m and one at 300 m. In the inner 300 m the centreline is made of barrettes. The 150 m crossbar fills the gap between the centreline and the side rows; the 300 m crossbar is 30 m long, as in the Category I pattern.

ALSF-2, the high intensity approach lighting system with configuration 2. It is 2,400 ft long on the FAA standard (3,000 ft where the ILS glide slope is below 2.75 degrees) and has white centreline barrettes every 100 ft, a 1,000-ft bar, a 500-ft bar, red side-row bars over the inner 900 ft, sequenced flashers from 1,000 ft outward and green threshold lights. Some installations are dual-mode ALSF-2/SSALR, running as the simpler SSALR when Category I conditions exist.

The Category I pattern is a subset. Both share the 900 m white centreline and the 30 m crossbar at 300 m. Category II and III add the red side rows to 270 m, a second crossbar at 150 m and barrettes in the inner 300 m of the centreline. In FAA terms, ALSF-2 adds the red side rows and the 500-ft bar to the legacy ALSF-1 pattern and drops ALSF-1's separate red pre-threshold and terminating bars.

At low decision heights the pilot sees only the inner part of the pattern. The red side rows show the edges of the runway-aligned corridor, so the pilot can sense roll and lateral position and tell the inner pattern from the white lights beyond it. The 150 m crossbar gives a distance-to-go reference. Under the FAA rule for operating below DA/DH, a pilot may not descend below 100 ft above the touchdown zone using only the approach lights unless the red terminating bars or red side row bars are also distinctly visible, which is the job ALSF-2's red side rows do.

Yes. The approach lights are one part of the picture. ICAO also expects touchdown zone lights and runway centreline lights on Category II and III runways, along with the full runway edge, threshold and end lighting and the stop bars, taxiway centreline lights and signs that low-visibility operations rely on. Use the aerodrome's approved Category II and III documentation to see the full set for a given runway.

Reliability carries more weight. Under ICAO secondary power rules the inner 300 m of the approach lighting on a Category II or III runway is expected to switch over to the standby supply within 1 second, while the rest of the pattern has a 15-second figure. Check the aerodrome's own requirement. For work, treat the circuit as live until it is isolated, locked out and proved dead under the permit-to-work; the structures are elevated and in the runway protection area, so access and aircraft-movement controls apply, and capacitor-discharge flashers must be discharged as the manufacturer instructs before a head or cabinet is opened.