Greg, you could make a small electronic circuit to turn the normally closed output from the oil pressure switch to you alarm to a normally open output to run the hobbs meter. But it is probably much easier to just unscrew the existing oil pressure switch, install a nipple with a "T" on the end, and then reinstall the existing oil pressure switch in one end of the "T" and an eleven dollar hobbs oil pressure switch in the other end of the "T."
gl
--- On Sun, 9/23/12, Gary LaPook <garylapook@pacbell.net> wrote:
From: Gary LaPook <garylapook@pacbell.net>
Subject: [NavList] Re: prices of N.A.
To: NavList@fer3.com
Date: Sunday, September 23, 2012, 10:40 AM
Here's the deal. Airplane engines are required by regulations to have a tachometer and the type of tachometers that are installed ar "recording tachometers," here is an example:
http://www.aircraftspruce.com/menus/in/tachometers_mitchell.html
These record one hour of real time for one hour of engine revolutions at full power so read less when the engine is operated at a lower rpm. Federal Aviation Regulations require periodic inspections (usually required every one hundred hours of "time in service" which is the actual time the plane is off the ground) and this time can be determined simply by logging time off and time on but it is more convenient to simply use engine operating hours which is a close enough approximation for the regulation. Using a
recording tach is a good measure of engine
wear since it is actually counting the revolutions of the crankshaft and displaying this information in comparable hours by dividing by the number of revolutions per hour. Since these record time even when the plane is on the ground, they record more than the required "time in service," so inspections may be indicated more frequently than actually required.
A Hobbs meter is a real clock that records accurate time so does not run slow when the engine is operated at less than full power and this is the standard instrument installed in rental aircraft so that the full rental cost can be billed even if the renter pilot operated at less than full power, which is normal. The Hobbs meter is an electric clock that is connected to an "oil pressure switch" installed on the engine so when the engine starts, and the oil pressure goes up, the clock starts running. When you shut the engine down at the end, the oil pressure goes down and the clock stops
running. You then read out the digital number of hours at the end and subtract the starting reading to determine the number of hours you pay for. This maximizes the rental income for the owner of the plane.
(Greg, you might want to install an oil pressure switch for your hour meter. If you have a low oil pressure horn on your boat, you might be able to utilize the out put of that oil pressure switch. See:)
http://www.aircraftspruce.com/catalog/inpages/hobbsswitch.php
gl
If you want to minimize maintenance costs for your own airplane then you can connect a hobbs meter to a "squat switch" connected to the landing gear that detects when the plane is off the ground so records only time in service as required by regulations, but this complication is rarely done.
--- On Sat, 9/22/12, bill
<billyrem42@earthlink.net> wrote:
From: bill <billyrem42@earthlink.net>
Subject: [NavList] Re: prices of N.A.
To: NavList@fer3.com
Date: Saturday, September 22, 2012, 11:12 PM
Apologies all around. Hit send instead of spell check.
Examples below, but to the point are engine hours based on a time clock or rpm?....
A private aircraft is for ....
Recreational power boats may operated over a wide range of speeds/rpm with widely-varying mpg/gpm and related engine wear.
Do "hours" account for this variable?
Bill B.
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