Once indicated the health of the engine probably... the moment she came alive, the dials in the cockpit would flip all sides; the roaring 5-cylinder radial engine bursting and coughing shortly before running smoothly at idle thrust... waiting impatiently to hit the red line and take a trip in the air.
Now only this dial remains, glass broken and needle gone, in a lifeless carcass in one of the remote corners of a deserted museum.