What's This Doohickey?

[Jeff_S]

Well, today the learning began in earnest as I did my first solo in the J, 10 days after getting signed off. It turns out flying it was the easiest part. I was met with a dead battery on start so we're definitely still in the shakedown phase for this plane.

On my first real thorough, unsupervised preflight I was going over all the systems, pushing all the drain buttons, etc., and found this other thingamajig on the belly of the plane. It's in the right-center of the attached photo, surrounded by the three screws. I couldn't find any reference to this in the POH, and I checked on a few neighboring J's and they didn't have the same thing.

Anybody have ideas on what this is? It may be related to any of the various avionics or other components that have been added to this plane, but if you can help me ID it I'd appreciate that.

Thanks!

[Buster1]

Since I have no clue, I'll offer it's the Flux Capacator overflow bleed port.

Of no help I know, but I gotta make a joke here once in a while.

[flight2000]

I was going to guess the pilot relief tube....

Brian

[Ron McBride]

On my F, there is a static system drain on the left side.  Yours looks loke you have one also, did someone add another drain on the right side?   Encoder?   Open the rear access panel and look, it may give your a hint.  Please let ys know what you find.

 

Ron

[Immelman]

Chaff dispenser

You know, for when ATC tells you to slow down..

[Jeff_S]

Ah, good to know there's a sense of humor! As I suspected, this is probably something so particular to my plane I'll have to dig into the avionics area and see what it could be. But if it really is a Flux capacitor then I got more than I bargained for with this plane!

[N57039]

Could be a temperature probe for an electric OAT.

[fantom]

Why not call the previous owner, and ask him?

[fitzmooney]

Could possibly be the remains of an air switch for your hour meter.

Usually they are a bit longer and wider so they come on at about 70 knots.

Cheers,

Mike

[Jeff_S]

Ah, interesting. The plane does have a "maintenance Hobbs meter" which is installed on the back bulkhead of the baggage compartment. Something to check into.

[Awful_Charlie]

Quote: Jeff_S

Ah, interesting. The plane does have a "maintenance Hobbs meter" which is installed on the back bulkhead of the baggage compartment. Something to check into.

[Jeff_S]

Mystery solved. This is definitely an airspeed-driven switch for that Hobbs meter. I had the access panel open today to look for any potential grounding problems that may be creating my pulsing whine, and I was able to trace the wires from this right up to that Hobbs meter.

So I've got that one figured out...one down, who knows how many to go!

[fitzmooney]

Hi Jeff,

Glad we sorted that out.

Mine is about 6 ft out, under the left wing in my 231 and triggers the Hobbs, which is on the right side of the centre console at 70 knots.

Not a good place on the belly, as it is prone to damaged by rubbish thown up by the prop.

I would put a meter or a light globe in series with it and see at what speed it comes on.

Cheers,

Mike

[Jeff_S]

I have a follow up question about this that may tie a bunch of things together.  Is this airspeed switch tied directly into the battery, i.e. would it activate without the Master switch being on, and cause a drain on the battery?  If so, then I wonder if in my naïveté I accidentally triggered it by putting on the canvas cover and having the strap pushing the lever back.  That could explain why an otherwise good battery may have been dead after 10 days of inactivity.

I guess if that were the case then this meter would show a sudden increase of 240 hours, but since it hasn't been in my normal routine to record this I have no idea what the numbers have been. All the logbook entries just use tach time. But if it is tied directly into the battery I may be further to solving another mystery. Just curious...

[Jeff_S]

Answer to my previous question: the airswitch doesn't seem to be activating much of anything at the moment. We tried it with both Master on and off and it didn't do a thing. Admittedly I don't know much about this type of Hobbs, so it could be wired into something else that requires the engine to also be operating, but at least I know that by itself this probably didn't cause a battery drain.

[carusoam]

It seems no final answer was given...

The M20R has an airspeed sensitive switch to keep the gear from retracting until flying speed is reached.  No squat switches are used.  If the switch fails to operate, the gear will not be raised.  The red button next to the gear switch is the overide that allows the raising of the landing gear.

The hobbs in the M20R is activated by the engine running.

Might you have one of these on your plane?

Check the POH for futher details...

[Jeff_S]

This is definitely not related to the landing gear. The wires from the airswitch go directly to a Hobbs that is on the back bulkhead. I saw some log entries from many years ago about the installation of the "maintenance Hobbs meter" so I'm sure that's what it is. The plane started life as a trainer with Flight Safety so I'm sure that had something to do with it. I haven't bothered to check to see if the Hobbs is still running after an actual flight.