Definition
A manufacturer-specified airspeed at which an airplane should be flown when encountering significant turbulence. It is chosen to balance two risks: flying fast enough to stay well above the low-speed stall and Mach buffet boundaries, while flying slowly enough that a sudden gust cannot impose structural loads beyond the airplane's design limits. The specific value is published in the Airplane Flight Manual or Pilot's Operating Handbook for that aircraft.
Plain English
The speed the manufacturer tells you to fly when the air gets rough. It is set so the airplane is neither close to stalling nor at risk of being damaged by a strong gust.
Context Anchor
Seen in the airplane’s flight manual or operating handbook, and in high-altitude discussions where turbulence can reduce the safe margin between stall and high-speed buffet.
Derivation
Penetration' here comes from the Latin penetrare, meaning to pass into or through something. The idea is the speed at which the airplane passes through a region of turbulent air. It is not about forcing through the turbulence, but about the speed used while flying in it.
Why Pilots Care
Flying at this speed reduces the risk of structural damage or loss of control when sudden gusts are encountered.
Grounding Statement
In rough air, the goal is not to force the airplane to hold a perfect altitude; the goal is to use the published rough-air speed and avoid harsh control movements.
Intuition Check
Turbulence penetration speed does not mean the fastest speed for getting through turbulence. It means the recommended speed for safely riding through turbulence.
Example Sentence 1
When the ride got rough at FL370, the captain slowed to the turbulence penetration speed listed in the POH.
Example Sentence 2
Maintaining turbulence penetration speed let the airplane ride through the mountain wave without exceeding load limits.