I've flown a Piper that has a button labeled
CWS on the yoke, right next to the "Push-To-Talk" button. I never got a chance to push it though. What does it do?
Sounds like that plane has (or once had) an STEC autopilot installed -
CWS stands for Control Wheel Steering - basically it allows you to set a pitch attitude / turn rate using the yoke, which the autopilot will then maintain until instructed to do otherwise.
If the autopilot is still installed you also normally use the
CWS mode as part of the preflight routine to get the autopilot to activate so you can make sure you can overpower it (details on the preflight as well as using the
CWS feature can be found in the autopilot manual, which should be with the POH).
If the autopilot is no longer installed the switch technically should have been removed with the rest of the system and a blanking plate installed in its place.
CWS is Control Wheel Steering, an early autopilot feature that senses if the pilot presses the flying control, and adjusts the target pitch as a result. Forces over a certain threshold activated the CWS when selected, however the required forces are much below the autopilot override force, so the autopilot stays fully functional.
It is still present in the B737-NG autopilot select panel, the MCP. CWS-R activation played a roll in the crash of Flash Airlines flight 604.