I think Fred is right, but it is useful to have engine time for at least oil
change intervals. I
have a Hobbs wired to a low pressure switch which is t-ed off the pitot
line, thus recording
actual flight time. Engine time is also recorded by most electronic
monitoring gadgets like
the EIS.
Cheers, Karl
>From: n3eu@comcast.net
>Reply-To: europa-list@matronics.com
>To: europa-list@matronics.com
>Subject: Re: Europa-List: Hobbs switch
>Date: Tue, 21 Oct 2003 02:44:49 +0000
>
>
> > This has probably been discussed a million times before. Typically when
>I
> > am involved in a certain section of the build I become focused and tend
>not
> > to assimilate some of the other stuff coming over the web. I need to
> > install a switch for the Hobbs meter. The first thought is to put a T
> > where the oil pressure transducer is and plug in the switch upstream of
>the
> > transducer. Are there any other pressure ports on the 914 that I'm not
> > aware of? What have others done to ensure accurate recording of engine
> > hours with the Hobbs?
> >
> > Thanks,
> >
> > Steve A143
> > Mesa, AZ
>
>Why do you really need to know accurate Hobbs time? As an amateur-built
>aircraft in the U.S., you're not bound in the slightest degree by any run
>time of any component anywhere. Admittedly, were you to sell the aircraft,
>airframe/engine time will be of interest to a buyer and thus how
>determined, but big deal.
>
>So, my "Hobbs" time is from that which a Taskem digital RPM display
>actually records in nonvolatile memory. Good 'enuf!
>
>The only place to tap into oil pressure is at the pressure transducer on
>all Rotax 91X, or the feed line to the 914 turbocharger. But the grizzled
>old aircraft mechanic in me says you do that you introduce a point of
>failure in a critical airworthiness system. It's not like there's a
>Service Bulletin on my other airplane specifically on this, which on
>failure causes oil to be spilled inside the aircraft cabin, until there
>ain't nuthin' in the sump!
>
>Thus preferable would be to simply read voltage at the Rotax transducer,
>and glomp a transistor to fire a stock Hobbs meter. It's a simple circuit,
>but if a builder can't do that, I think it best to at least shop for
>something that will record elapsed hours when fed a voltage, like when
>master is on -- even a stock Hobbs does that. It won't be exact engine
>hours, but so what.
>
>Regards,
>Fred F.
>
>
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