In article om,
Jay Honeck wrote:

I used to assume that Class D existed because air traffic was once
heavier than it is today, and (as with all things government) newly-
useless facilities are slow to be closed. But now I'm not sure --
maybe they were *never* needed?

Castle Airport (formerly Castle AFB) was an uncontrolled field for
years after going civilian, and has just re-opened the control
tower. According to the traffic numbers[1] on Airnav, it has 579
operations per day.

Checking the traffic numbers of local airports on Airnav, it looks
like all the airports with over ~300 operations a day have control
towers. My experience with the ones in the 300-400 range (Sac Exec
(KSAC), Napa (KAPC), Santa Rosa (KSTS)) is that they really don't
need a control tower except when everyone decides to show up at
once. All the ones above that range (Palo Alto (KPAO), Livermore
(KLVK), San Carlos (KSQL)) have enough traffic that the control
tower is useful. Palo Alto and San Carlos have radar, and will
give vectors as needed. Livermore doesn't have radar, but does a
good job sequencing traffic as long as the position reports are
good. Bad position reports are a problem at uncontrolled airports
too, so I don't hold it against ATC when the position reports are
wrong.

None of the above airports have airline traffic. KSAC, KAPC, KSTS
and KLVK have jet traffic and multiple runways. KPAO (single 2400ft
runway) and KSQL (single 2600ft runway) are just piston and turboprop.

BTW, Iowa City lists 53 operations per day, and Oshkosh lists 283
through the wonder of averaging.

John
[1] These numbers are probably similar in accuracy to the flight
hours numbers, but I expect the numbers between airports to be in
the same margin for error.
--
John Clear - http://www.clear-prop.org/