Right                                Wrong
-----                                -----
Beech[craft]                         Beach[craft]
Comanche                             Commanche
descend                              decend
definitely                           definately
gauge                                guage
hazard                               hazzard
Hobbs (an hour meter)                Hobb's, hobbs, Hobbes (as in Calvin and)
Monterey (California)                Monterrey (not CA, but Mexico)
propeller                            propellor
turbulence                           turbulance

And some words which are frequently confused:

advice (I'd like a bit of ...)       advise (please tell me)
descend (to lose altitude)           decent (proper; in good taste; moral)
flare (part of a good landing)       flair (with panache)
hangar (a place for airplanes)       hanger (a place for clothes)
it's (contraction for "it is")       its (possessive, belonging to it)
loose (not fully attached)           lose (to misplace or forget; to reduce)
roll (aerobatic maneuver)            role (part in a dramatic production)
yoke (aircraft control)              yolk (yellow part of an egg)
you're (you are)                     your (indicating possession)