On Fri, 04 Aug 2006 04:55:32 GMT, Jose
wrote in :

http://www.alaska.faa.gov/ata/MACA.htm

PROPER CLEARING/SCANNING TECHNIQUES

An efficient scan pattern is paramount to visual collision
avoidance procedures. In developing a proper scan technique,
remember that when your head is in motion, vision is blurred and
the brain will not be able to identify conflicting traffic.
Therefore a constant motion scan across the windscreen is
practically useless.

A proper scan technique is to divide your field of vision into
blocks approximately 10 to 15 degrees wide. Examine each block
individually using a system that you find comfortable (e g. from
left to right or starting from the left and moving to the right,
then back to the left again). This method enables you to detect
any movement in a single block. It takes only a few seconds to
focus on a single block and detect conflicting traffic.

A moving target attracts attention and is relatively easy to see.
A stationary target or one that is not moving in your windscreen
is very difficult to detect and is the one that can result in a
MIDAIR COLLISION.

The time to perceive and recognize an aircraft, become aware of a
collision potential and decide on appropriate action, may vary
from as little as 2 seconds to as much as 10 seconds or more
depending on the pilot, type of aircraft and geometry of the
closing situation. Aircraft reaction time must also be added. By
the way, any evasive maneuver contemplated should include
maintaining visual contact with the other aircraft if practical.

There you have it. The FAA telling pilots, that for a 90 degree wide
scan field (45 degrees each side of center), it takes more than two
seconds per 15 degree scan block (at 3 seconds per block, that's 18
seconds per full scan), for a pilot to see a stationary traffic
conflict aircraft growing larger, and it can take up to ten seconds
see-and-avoid (for a total of 28 seconds per full scan).