Definition
An airport at which a flight can be safely landed given the current condition of the aircraft, the capability of the crew, weather, runway length and surface, available approaches, fuel state, and any abnormal or emergency situation in progress. It is not necessarily the geographically closest airport, but the closest one that is genuinely usable under the existing circumstances.
Plain English
The closest airport you can actually land at safely right now, given how the airplane is performing, what the weather is doing, and what kind of runway and approach you need.
Context Anchor
Seen in emergency and abnormal-condition guidance, especially when a pilot needs to leave icing conditions and land as soon as practical.
Derivation
Suitable comes from an older sense of “to fit.” In this phrase, it means the airport must fit the needs of the aircraft, pilot, weather, and situation.
Why Pilots Care
In icing conditions performance can degrade rapidly, so reaching a suitable landing site quickly prevents loss of control or forced off-airport landing.
Grounding Statement
If ice is building and continuing the flight is unsafe, the pilot looks for the closest airport that can realistically be landed at safely.
Intuition Check
Do not read “nearest” as automatically meaning the closest airport by distance. Do not read “suitable” as meaning any airport with a runway; it means usable and safe for the conditions right now.
Example Sentence 1
After picking up airframe ice, the crew declared an emergency and diverted to the nearest suitable airport with a long runway and an ILS.
Example Sentence 2
With continued icing and no de-icing equipment available, ATC cleared the flight direct to the nearest suitable airport for landing.