Definition
The traditional grouping of six primary flight instruments arranged in a standard pattern on the instrument panel: airspeed indicator, attitude indicator, altimeter, turn coordinator, heading indicator, and vertical speed indicator. The arrangement places the attitude indicator in the top center, with airspeed to its left and altimeter to its right; below sit the turn coordinator, heading indicator, and vertical speed indicator in the same left-to-right order.
Plain English
The classic six round dials a pilot uses to fly the airplane, laid out in a standard pattern so any pilot can find them in any cockpit.
Context Anchor
Seen in instrument flying and aircraft malfunction discussions, especially when an electrical failure may affect some cockpit instruments or displays.
Derivation
Called a 'six-pack' by analogy to a six-pack of cans or bottles -- six items in a neat rectangular arrangement (two rows of three). 'Primary' indicates these are the main instruments used to fly and navigate, as opposed to secondary or supporting instruments.
Why Pilots Care
In alternator or generator failure, pilots often rely on these instruments (especially vacuum-driven ones) to maintain controlled flight until power is restored or an emergency landing is made.
Intuition Check
Do not read “six-pack” as a drink or as a guarantee that every aircraft has six separate round gauges. Here it means the standard set of six basic flight instruments or their same functions on a modern display.
Example Sentence 1
During his instrument scan, the pilot's eyes moved smoothly across the primary six-pack, checking attitude, airspeed, and altitude in turn.
Example Sentence 2
The primary six-pack remained usable because the attitude and heading indicators were powered by the vacuum system.