What is it?

It warns you when you're low enough to hit ground. Useful in
instrument flying.

According to FAA's "Instrument Flying Handbook":

"Below 10,000 feet, a striped segment is visible. Above this altitude,
a mask begins to cover it, and above 15,000 feet, all of the stripes
are covered."

- Tom

On Sat, 22 Jul 2006 16:43:49 -0400, "Bob Chilcoat"
wrote:

The altimeter in this illustration http://en.wikipedia.org/wiki/Altimeter is
almost identical to the one in our Archer. I've been puzzling over what
that little black and white hatched area is on the dial ever since I bought
into the plane. It doesn't rotate with the Kollsman window knob, although
perhaps it rotates with the thousands hand, since it appears in this
illustration to be exactly opposite the hand. Anyone know what it's used
for? Inquiring minds want to know.