Suddenly all planes but the Cessna loaded nosed up in the grass.
The Cessna 172 loaded in air and on final.
It could be moved to an airport using Location in the F10 menu; but on next launch was back in the air, as were the other planes nosed up in the grass.
Purged launcher by deleting Users\{name}\AppData\Roaming\flightgear.org.
After that all was fine.
Haven't the foggiest notion what corrupted the launcher, in the first place. Had just been switching and flying different planes; when suddenly it happened.