Fog can shut an airport down. The Instrument Landing System exists to prevent that, guiding aircraft down a precise electronic path to the runway even when pilots cannot see the ground until the last few hundred feet.

Two radio beams do the work. The localizer provides lateral guidance, keeping the aircraft lined up with the runway centreline. The glide slope provides vertical guidance, defining a descent path typically around three degrees. Marker beacons or distance measuring equipment tell the pilot how far they are from the threshold.

Approach categories

• Category I, decision height of 200 feet
• Category II, decision height of 100 feet
• Category III, landings with no decision height

Higher categories demand more equipment on the ground and in the aircraft, plus crew certification. Category III systems allow landings in visibility so poor that the runway lights are barely visible from the cockpit.