Definition
An altimeter setting adjustment of approximately 1 inch of mercury (1" Hg), which corresponds to a 1,000-foot change in the indicated altitude. The term refers to the full width of the pressure scale window on a typical Kollsman-type altimeter, where moving the setting by one full bar shifts the indicated altitude by 1,000 feet.
Plain English
Turning the altimeter knob enough to change the pressure setting by 1 inch of mercury moves the altitude reading by 1,000 feet. That full 1" Hg change is called a full bar width.
Context Anchor
Seen during instrument flying when using the altimeter and attitude indicator together to hold altitude in straight-and-level flight.
Derivation
The phrase comes from the appearance of the altimeter's pressure-setting (Kollsman) window. The numerals across the scale are spaced so that one full "bar" of the scale equals 1 inch of mercury. "Bar width" simply names the visible distance of one full unit on that scale.
Why Pilots Care
Prevents altitude errors that could lead to terrain conflicts or airspace deviations when flying on instruments in nonstandard conditions.
Intuition Check
Do not read “full” as a large or aggressive control movement. Here it means one full reference-bar width on the attitude indicator, which is still a small, controlled pitch change.
Example Sentence 1
If the local altimeter setting drops by 1" Hg and the pilot fails to update it, the altimeter will read 1,000 feet high -- a full bar width correction off.
Example Sentence 2
In cold weather operations the crew used full bar width correction to account for the altimeter under-reading true altitude.