Plain English Meaning
On the VSI, '.5' is just a quick way of saying 'five hundred feet per minute.' It's halfway between zero and the '1' mark, which stands for 1,000 feet per minute.
Full Definition
A shorthand notation used when reading the vertical speed indicator (VSI), where '.5' represents one-half of a thousand feet per minute, or 500 fpm of climb or descent.
Context Anchor
Seen on the vertical speed indicator during an instrument scan, especially when checking how fast the aircraft is climbing or descending.
Derivation
.5 is decimal notation for one-half. On this instrument, the numbers are read as thousands of feet per minute, so one-half of one thousand equals 500 feet per minute.
Why Pilots Care
Precise rates like .5 allow pilots to maintain exact vertical profiles on approaches and assigned altitudes.