One of the members of my club wrote just such an application for us,
and it's been working great for the last two years. There's a drop
down list for members, plane, and tow pilot, and when the plane takes
off you simply push a big green button. When it lands you push a big
red button, and the flight time is filled in. The pilot uses a drop
down list after he lands to inform the system of his tow altitude. The
data feeds into our billing system, so there's no more need to
decipher illegible scrawls on paper logs.

The only thing that doesn't meet your request is that this application
runs under Windows. I've alerted the author to this thread, and he may
chime in.

-John