This article is from the Tolkien Newsgroups FAQ, by Steuard Jensen sbjensen@midway.uchicago.edu with numerous contributions by others.
[Conrad Dunkerson has written a much more detailed essay on this
question as part of his "The Truth About Balrogs" series, on the web
at: http://tolkien.slimy.com/essays/TAB6.html. That page also includes
links to other discussions of the issue.]
There is considerable disagreement on this point. Most agree that
the Moria passage does not provide convincing evidence for or against
Balrogs' ability to fly. (For example, while the Balrog does not fly
out of the chasm, it may not have had enough room to use its wings, or
its highest priority may have been the destruction of a rival Maia in
its domain.) Most also agree that as Maiar, Balrogs could conceivably
be able to fly even if they had no wings, or that they could have had
wings but remained flightless. Still, the two issues are certainly
related to some degree.
A number of facts have been taken as indirect evidence that Balrogs
could not fly (e.g. they never flew over the mountains to discover
Gondolin; at least two died falling from cliffs), but counterarguments
have always been found (the eagles defended Gondolin; the Balrogs fell
only after great injuries). Clearer evidence comes from "Of Tuor and
his Coming to Gondolin" in _Unfinished Tales_, when Voronwe says, "as
yet no servant of the Enemy has dared to fly into the high airs". Even
if Voronwe's information was complete, however, this still leaves room
for Balrogs to be able to fly at low altitudes.
The only known place where Tolkien may have made a direct statement
on Balrogs' ability to fly can be found in "The Later Quenta
Silmarillion (II)" in _Morgoth's Ring_:
Far beneath the halls of Angband... the Balrogs lurked still....
Swiftly they arose, and they passed with winged speed over Hithlum,
and they came to Lammoth as a tempest of fire.
Some believe that this passage clearly describes Balrogs flying, others
believe it just uses imagery of flight to indicate speed, and still
others believe it to be ambiguous.
 
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