The Hobbs meter in most Stearmans is actuated, in some, by the master switch (bad idea), and in the majority by an oil pressure switch. Both accumulate time which includes test running, warm-up, taxiing, holding short, etc. on both takeoff and landing. That is an appropriate number for logging “Pilot in Command” time.
However, the requirement for complying with the 100 hour AD on the McCauley prop is a different matter. That 100 hour requirement is for “Operational Time”. Operational time starts when the wheels leave the ground on takeoff and stops when they touch the ground on landing.
In a retractable gear plane, the Hobbs meter is sometimes actuated by the gear-up switch. This occurs a little late on takeoff, and a little early on landing.
But what can be done in a Stearman? There is a product available on the market that provides an ideal solution to this dilemma.
This kit provides an air pressure switch that is installed in the line from the pitot tube to the air speed indicator. This switch actuates at a pressure developed by the pitot tube at an airspeed of 27 and 34 miles per hour. So it switches the Hobbs meter “on” just before reaching takeoff speed, and turns it off shortly after touchdown. (A lot later if you do a wheel landing ).
This then saves you running up a lot of $4 (or more?) per hour operational expense on your McCauley Prop.
This device called an Aero Meter is manufactured and distributed by:
4342 Postmark Dr.
Anchorage, AK 99502
The Aero Meter STC does not currently include the Stearman, but Cliff told me that they will issue any buyer a letter authorizing the use of their data for preparing a 337 field approval, and will give a refund to anyone that is not able to get one approved. The price is $199.95.
Some people put in a second Hobbs meter, one on the oil pressure switch and one on the Aero Meter. That gives you both time readings.
I would guess that most Stearman flights are an hour or less. Run some time studies on your typical flights and just see how much reduction you would get in accumulating time for the 100 hour prop AD. You could find 20 to 30 hours. Then figure out how quickly you would pay back the cost of installing an Aero Meter.