Definition
A single rotating shaft that mechanically links the turbine and compressor of a turbocharger, so that exhaust gas spinning the turbine wheel directly drives the compressor wheel at the same speed.
Plain English
One shaft with two wheels on it — exhaust gas spins one end, and that same spin drives the air pump on the other end.
Context Anchor
Seen in turbocharging descriptions that explain how exhaust flow is used to help the engine maintain power at higher altitudes.
Derivation
"Common" here means "shared" — the same shaft is used by both the turbine and the compressor, rather than each having its own.
Why Pilots Care
A failure of the common shaft stops both turbine and compressor at once, instantly removing boost and potentially forcing an emergency descent or diversion.
Intuition Check
Common does not mean ordinary here. It means shared by the turbine side and compressor side of the turbocharger.
Example Sentence 1
In a turbocharger, the turbine and compressor are mounted on a common shaft, so exhaust gas energy is converted directly into intake air pressure.
Example Sentence 2
Excessive heat can warp the common shaft and cause both the turbine and compressor to seize.