El Al -- Every Landing Always Late.

Iberia -- I Bet Everyone Retches In (the) Air

LOT -- Late On Takeoff (or, if they ever joined SkyTeam, Lands On Taxiway)

TACA -- Take A Chance Airlines

COPA -- COntinental's Panamanian Airline

and for some now-defunct carriers:

TWA -- Try Walking Across
BOAC -- Better On A Camel
BWIA -- Britain's Worst Investment Abroad
SABENA -- Such A Bad Experience...Never Again!