Well not quite grounded...
What do you do when your airplane is broken? Take someone else's, of course.
I used pretty much the last summer-y day of the year to ferry the Fox Moth to Vintage Wings, at Gatineau. But first, I was surprised and amazed at what Stan had done to the clock -- which had never worked, to my knowledge.
Stan recognized that it was a 1931 version of a Hobbs Meter. He took it out and cleaned and serviced it.
How it works: there is a plunger that stick out the back of the unit about 1/8". In this photo it is depressed by weight of the clock as it sits on the shelf. The plunger is an interrupter, which stops the clock ticking. When the plunger is pushed in, the clock ticks. When it's extended, the clock stops.
But in use, the clock is in a receptacle in the panel which has an aneroid capsule in the back. This is hooked up to the pitot air source. The aneroid is lined up with the plunger. So, when the airspeed gets to about 50 mph, the aneroid expands, forces the plunger in, and the clock starts ticking. It records elapsed-time. And then of course after landing, when you slow down, the aneroid shrinks, the plunger extends, and the clock stops, recording the hours and minutes of your flight.
When you turn the knob counter-clockwise to zero the reading, you have rewound the clock.
Ingenious mechanical device!