Plain answers
Why do flights divert?
A diversion means the crew chose a different airport because landing there was the better option. It sounds dramatic on a tracking map. In the cockpit it is a briefed, fuel-planned decision from a list written before the engines started.
Every flight carries a plan B, by law
Airline flights are dispatched with alternate airports and the fuel to reach them plus reserves; that is a certification requirement, not a courtesy (US rule: 14 CFR 121.639, with equivalents worldwide under ICAO Annex 6). Using the alternate is the system working. The aircraft never departs with just enough fuel to make it if everything goes right.
The usual reasons, ranked by how boring they are
Weather below minimums. Every instrument approach publishes visibility limits below which crews do not attempt it (FAA Instrument Procedures Handbook, ch. 4). If the destination sits below its minimums and the forecast disagrees with reality, the flight lands where the numbers work. Airports built for low visibility publish their capability; see the precision-approach notes on our flight pages, sourced from each country's AIP.
Closed or avoided airspace. Volcanic ash, conflict zones, and state closures reroute traffic hours in advance. Our route pages say when a corridor is shaped by exactly this, sourced to EASA bulletins and state closures.
A passenger medical event. The most common headline-maker: someone on board needs a hospital sooner than the destination allows. Nothing about the aircraft is wrong.
Precaution after an indication. A sensor reading, a smell, a system message. Crews are trained to land and let engineers look rather than continue and wonder: an outcome we record in our government records data as precisely what it is.
What a diversion is not
It is not a crash averted by luck. The NTSB investigates accidents and serious incidents; routine diversions are so far below that bar that they are tracked as operational statistics, not safety events. A flight that diverted yesterday flies today.
Sources: 14 CFR 121.639 (alternate and reserve fuel); ICAO Annex 6 (flight planning); FAA Instrument Procedures Handbook FAA-H-8083-16B ch. 4 (approach minimums). Checked 2026-08. Our own numbers and records: methodology.