CHT guage failed

[voorheesh]

I just completed annual on 1950 170-A and my CHT guage stopped working. It is an old guage on left panel with a red line of 460 deg f. I checked the wire to the rear most left cylinder and it appears intact. Shaking/exercising the wire did not change anything. The wire does not appear to be bent or damaged. The guage indicates about 80 deg and fluctuates about 10 deg but does not react when engine operates. During the annual, we cleaned the engine with solvent and rinsed with H2O. Everything seems dry now. Prior to the annual this guage worked well. It would indicate as high as 380 deg in climb on a hot day and would settle down to 300/310 in cruise. Any ideas on how to fix or where to get a replacement guage/wiring? I don't want to get involved with any fancy digital stuff. Thanks for any help/advice.

[bsdunek]

I replaced my failed CHT with a Westach from Aircraft Spruce: http://www.aircraftspruce.com/catalog/i ... stcht1.php
Works for me, and the price is right.

[pojawis]

As I recall, I was once informed that the Westach gauges with the red-tipped needles (pointers) are not TSO'd. The guages with all white needles are for certificated aircraft. Can anyone verify this?

[jrenwick]

Would it matter, given that a CHT gauge is not required equipment on a 170?

John

[pojawis]

If it's illegal to install a non-TSO'd gauge in a 170, then the answer is affirmative. Conversely, if it is legal to install a non-TSO'd gauge (and associated parts) in a 170, then it would be of no concern, legally speaking. I reckon the next question is if there's any real difference in the two, aside from the pigmentation; functional difference? Reliability?

[GAHorn]

A CHT gauge is only required equipment on aircraft which have moveable cowl-flaps. (See FAR 91, required equipment)

Therefore no CHT gauge is required in our 170's.

Even if it were required equipment, there is no requirement for a TSO'd gauge in our aircraft.

Voorheesh, I suggest you send your gauge to a competent instrument repair shop for testing and if it checks out OK... have them supply a new thermocouple, as it appears yours will have failed. (It's probably iron-constantine, but the shop can determine that when you send it along with your gauge.)

Also, be aware that the red-line for your engine is not 460-F. It's 525-F or 275-C. While the shop has your gauge they can re-mark it. (I suspect that someone subtracted 32-degrees rather than added it to the 9/5 X C equation.)

I have used Aircraft Quality Instruments 2649 S Custer Wichita, KS 67217. Phone: 316-942-9933 for my old AN gyros. Perhaps they can help you. I can also recommend Stahl Air Instruments 817-430-8589. Ralph Stahl did all the original restorations of my instruments and does superb work.

click to ENLARGE. *- Note: Oil Temp revised to 240-F w/SAE 50 oil per Type Cert.

[LBPilot82]

I recently installed Electronics International engine guages in my 170 and really couldn't be happier. They are a bit on the pricey side but I feel they are well worth the expense. While I don't have a CHT, I do have a single EGT, Volt/Amp, and Oil Press/Temp. They are easy to read and extremely accurate. When the day comes for a complete top overhaul, I will most likely go with a six cyl EGT and CHT setup. They are FAA/PMA'd and TSO'd. While not required for CHT or EGT, I feel better that they are. Also, all their instruments are approved as replacements for primary instruments.

BTW: I think they charged me about $15 per instrument to recalibrate and reface the instruments for the C-145.

[voorheesh]

Thanks for the advice!

[GAHorn]

Harlow, I remembered I had a pdf file from Alcor describing how to test a thermocouple. It's in the MX Library.

[Brad Brady]

Voorheesh,
If you are concerned about CHT ( and I think you should be) Before I spent $700 on refurbing an old single point system....Like Dad has for the Werner, (new every thing and refurb gauge) that is in the Fokker D-6 he is rebuilding, I would think hard about spending $1700 and get a JPI or such six point CHT and EGT system. Oh that $1700 is, if you have a son that will do the install and paper work for nothing ......Brad

[GAHorn]

Or spend only $99 for an entire new gauge including thermocouple:



http://store.wagaero.com/product_info.p ... ts_id=8104

[Robert Eilers]

I recommnd replacing the cylinder head sensor gasket and wire connection before buying a new gauge. I experienced something similar after an annual. Apparently I had disturbed the connection between the cylinder head sensor gasket and the gauge. I replaced the cylinder head sensor and rewired to the gauge - gauge worked fine afterward.

[GAHorn]

I agree completely, Robert. The only problem is gasket-thermocouples come in two different types.... (be certain the wire colors are the same on the replacement...) and either one typically costs $50 or more to replace. By the time one buys a thermocouple, (and discovers it doesn't solve the problem) and has the gauge tested (and finds it's no good)...one could have purchased an entirely new gauge and thermocouple and be finished with the problem. (Although that price seems suspect...a similar Alcor gauge alone costs more than that.)

Here's a complete gauge-kit with thermocouple and user-settable limits-needle that costs $92.


http://store.wagaero.com/product_info.p ... cts_id=162

[voorheesh]

I think Robert is probably right on the source of my problem but I am ordering the alcor guage since it is affordable and will give reliable temp indication. Here in the central valley, 100 deg plus temps are common May-Sept and CHT is, for me as important/more important than oil temp. If I can figure out a good place to put it in the panel, the next guage after CHT will be carb temp for the conditions here from Oct-April. Thanks again for everone's help.
Harlow

[Mike Smith]

Aircraft Quality Instruments 2649 S Custer Wichita, KS 67217. Phone: 316-942-9933
Stahl Air Instruments 817-430-8589. Ralph Stahl

George,
Would you recommend either of these for a vacuum powered ADI overhaul?
Mike