How do planes land in fog?
Instrument landing systems guide the airplane precisely to the runway, and strict rules say what the pilots must see before landing. If they can't, they go around.
In this article · 6 sections
Two pieces: guidance and a strict rule
Landing in fog relies on two things. First, ground equipment that tells the airplane exactly where the runway is. Second, a firm rule about what the pilots must see before they're allowed to land.
The instrument landing system (ILS)
Many airports have an instrument landing system on their main runways. The FAA describes it as providing "an approach path for exact alignment and descent" of an aircraft on final approach.1 Radio beams tell the airplane whether it's left or right of the runway centreline and above or below the correct glide path, all the way down. The autopilot or the pilots follow that guidance.
The "decision" point
Every instrument approach has a minimum height, the decision altitude or height. US rules say the crew may only continue below it if the airplane is in a position to land normally and they can clearly see specific runway references, such as the approach lights, the runway threshold or the touchdown zone lights.2
If those references aren't there, the rule is simple: the pilots must immediately go around (a "missed approach").2 No judgment calls, no squinting. Then they can try again, wait, or divert to an airport with better weather. A go-around is a normal, practised manoeuvre.4
The lowest-visibility landings
For very low visibility, airports and airplanes can be approved for Category II and III approaches, which allow lower decision heights, or in some Category III cases no decision height at all. These require the right equipment on the ground and in the airplane to be installed and working, a crew of two pilots with the right authorizations and training, and for airlines, approval in their FAA operations specifications.3 The same strict rule applies: if the required runway references aren't visible when they're needed, the crew goes around.3
What it means for you
If you land in fog, it's because the equipment, the airplane and the crew were all approved for those conditions, and the crew saw what the rules require. If they weren't, you'd have felt a go-around or heard about a delay or diversion instead. Either way, the rule did its job.
Keep reading
- Go-arounds: why your plane suddenly climbed again
- Why is my plane circling? Holding patterns, explained
Quick answers
Can planes land in thick fog?
Yes, at suitably equipped airports. An instrument landing system guides the airplane to the runway, and the lowest-visibility approaches require special equipment on the ground and in the aircraft, plus specially authorized crews.
What happens if the pilots can't see the runway?
They go around. US rules say that if the required runway references aren't visible at the decision point, the crew must immediately fly a missed approach.
What is an ILS?
An instrument landing system, which the FAA describes as providing an approach path for exact alignment and descent of an aircraft on final approach to a runway.
Why was my flight delayed or diverted for fog?
If visibility is below what the approach allows, flights may hold, wait or divert to another airport. That's the rule working as intended.