Stall vane not getting hot on pre-flight? (FIKI/TKS plane)

[dkkim73]

Hi All, 

I just went to green up some landings today and, during the pre-flight, the stall vane did *not* get hot as I was used to (the pitot tube was searing after a modest wait). 
It is reasonably warm (e.g. 70+F at least, plus sun) today after a cold morning. Is there a thermostat that prevents heating the vane? This behavior is new. Usually it has been getting very hot quickly. No popped breakers I saw. 

I recall there is some differential behavior between being in-flight and not. Unsure if things are different during the summer (picked up plane in Feb).

Hoping to travel this weekend so any  IFR-relevant advice appreciated! 

Thanks,
David
 

[exM20K]

It should get hot on the ground, and if it’s burned out, prepare yourself for a shocking bill.  But you can start by pulling it out and seeing if it’s getting power. Just make careful note of where it sat vertically.

-dan

[Fritz1]

in the FIKI Bravo the stall vane has two stage heat, low heat on ground, you really have to put your lips on it to fell it heating up, full heat when air pressure safety switch is actuated, typically around 60Kt, so you never feel full heat on the ground unless you cautiously pressurize the pitot tube to actuate the air pressure safety switch

[exM20K]

I agree that it’s not as hot as pitot tube on the ground, but you should feel heat. I don’t have the POH FIKI supplement handy, but I believe the amber Pitot Heat annunciation will illuminate if either fails. Some are set up differently. Blue when pitot heat on, for example.  Mine isn’t that way, just amber if it fails.

-dan

[dkkim73]

I do not recall the amber annunciator coming on, which I think in the FIKI plane it should if the pitot heat has failed (I don't recall the POH saying it lights with either). Come to think of it, I did *not* however check pitot heat in the air at speed. 
Looked for a schematic covering this part and did not find it. Please LMK if any one has it. 

Also heard back via Savvy, apparently Maxwell can repair these if need be. 

How straightforward is it to remove the unit? Can it just be pulled out forward by removing the visible screws? (any backing pieces that fall in, LOL, requiring rear access somehow?) I've always heard the positioning is crucial, can it just be put back in by empiric visual reference to the position within the screwholes, distances from edges etc. all marked down of course...

Thank you guys for the quick thoughts!
D

 

[Fritz1]

Call or email Frank Crawford at Mooney for service manual with wiring diagram, culprit may be air pressure safety switch

[dkkim73]

1 hour ago, Fritz1 said:

Call or email Frank Crawford at Mooney for service manual with wiring diagram, culprit may be air pressure safety switch

I found this this afternoon in the PDF manuals: 

Also I realize I had an IFR cert and altimeter calibration done last month. However, the vane had been heating up in the interval.

It sounds like a good next step is to check voltage, which I was just told is via an access panel on the wing. Anyone done this recently and have any tips?  

[EricJ]

If you can find the "Airspeed Switch Connector" you can test the pins there for voltage.   That'd at least narrow it down to before/after that connector.   I'm assuming the connector is probably more accessible than the vane itself.    On a J model (and others) the vane is a bitch to get at.

[dkkim73]

Good idea. It is supposed to be "near the pilot's left knee". Getting access to the harness might be the tricky part unless there are connectors to back-probe.

[Fly Boomer]

1 hour ago, dkkim73 said:

Looked for a schematic covering this part and did not find it. Please LMK if any one has it. 

Not sure if this helps, but on PDF page 24 of the following AFM Supplement is a schematic showing the stall vane heat connected to the airspeed switch.  Someone above suggested that the heat may not be on (or not full on) until the airspeed is beyond some threshold, and this schematic seems to support that.

https://www.mooney.com/wp-content/uploads/2020/12/SUPP0017H.pdf

[Fritz1]

Air pressure safety switch is high up on left side of pilot foot well hidden behind wire bundles, good idea to take seats out when digging for it, the connector is maybe 10" away from switch 

[dkkim73]

Thank you all. Update:

After reading and researching a bit, I went back to search for the pressure switch and also to re-verify everything. The plane was cool inside the hangar. When I turned on the pitot heat switch, the vane heated up  very quickly, within seconds, too hot to touch as I am used to...  So I guess on the positive side the heating element is probably not burned out (?would expect all or none). 

Perhaps the airspeed switch is sticking as Fritz suggested. Mooney manual warns against pressure-related damage (static only), so  I talked to the avionics shop that did the static cert. recently and they said it is standard to keep up pressure on the pitot side. Also it had worked since then. 

Another thought I had is that, given it's so hot in my experience, I wonder if the switch is in "flight mode" on the ground. Ie. without the dropping resistor. There are probably some voltage references I could check if I got to the wires. Looking on my back, there are a bunch of tightly-zipped bundles under the panel on the pilot's side, near where I would expect it. It might also be under the side fascia panel(?). @Fritz1 is it behind a side panel, or just buried in wire behind the main panel?  The manual says "near the pilot's left knee". I lay on the floor but didn't pull seats or cut zip-ties or look more than a layer deep for a few minutes, as I was just coming by for 30min before close. 

I'm trying to figure out what I might do to have caused this spuriously yesterday but can't think of anything. The pitot tube was definitely hot yesterday when the vane wasn't. Usually I can barely touch the vane for a moment and yesterday I could hold it. 

David
 

PS. Also wonder if something related to outside temp. Not that it should act that way, just thinking of thermal effects on the parts (airspeed switch). 

[dkkim73]

4 hours ago, Fritz1 said:

Air pressure safety switch is high up on left side of pilot foot well hidden behind wire bundles, good idea to take seats out when digging for it, the connector is maybe 10" away from switch 

Will look again when I get a chance. It occurred to me that this is the same switch that provides the "safety" against retraction of landing gear on the ground. So if it has actually been in "flight mode" (failing the other way, explaining the super-hot vane most of the times I've checked) that could be an issue, too. 

 

[Fritz1]

I replaced air pressure safety switch because stall vane was stuck in hot mode, my A&P IA cautioned me that the retraction safety part will typically fail soon thereafter, you have to cut zip ties to get to it, think it is above side panel but best to take side panel out to get to it, this is a real pita, I bought new air pressure safety switch, you might be able to have it overhauled if new not available

[dkkim73]

4 hours ago, Fritz1 said:

I replaced air pressure safety switch because stall vane was stuck in hot mode, my A&P IA cautioned me that the retraction safety part will typically fail soon thereafter, you have to cut zip ties to get to it, think it is above side panel but best to take side panel out to get to it, this is a real pita, I bought new air pressure safety switch, you might be able to have it overhauled if new not available

Very helpful. Not confirmed but sounds like this is a possibility. Out of curiosity did you buy a new one or get it repaired? Unclear from the web what the availability is. 

[Fritz1]

bought a new one, but that was 3 or 4 years ago, still have the old one, just in case it might be repairable, looks like mechanical switch with membrane inside