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Old 14th May 2022, 02:35
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Pilot DAR
Join Date: Aug 2006
Location: Ontario, Canada
Age: 61
Posts: 5,274
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Modifying a Cessna stall warning system

My most recent flight test project involves validating the installation of Cessna 182Q wings on a Cessna A185F amphibian. The wings were installed a number of years ago during a major repair, and a few details were "glossed over". After extensive evaluation, I was able to validate that the wings are aerodynamically, and structurally the same, so approval of this design change was worthy. There were a number of required systems changes (aileron cables, flap system, some fuel system details - and the stall warning system). I documented these, and described them in the approval. However, the 182Q has a vane/switch electrical stall warning, the A185F has the more simple pneumatic system the same as a C 150/152/172. The holes in the leading edge skin are in slightly different locations around the leading edge radius. It seemed easy to just screw in the pneumatic stall warning system parts in the hole in the changed wing intended for the electric switch. This was done. The only way to validate it is to fly it, so that's what I did - all flap and power combinations, no, it did not work. Indeed, the warning never sounded in this arrangement. So the plate with the slot was moved higher, and I reflew a number of times. At the very most extreme possible position of the plate, I could just get the stall warning to sound at the break of the stall. But in most cases, silence.

For reference, it is required that the stall warning horn (if equipped) sound at least 5 knots before the stall break, but not be sounding more than 10 knots before the break - in all configurations.

I offered to the shop that I would test and possibly approve the installation of the whole electric stall warning system from the 182Q, but this was uncertain, very expensive, and would require a whole new electric circuit in the A185F. Not a desired option. So, I got inventive. I used the world renown aluminum "100 MPH tape". and folded up a tab below the stall warning slot.

To my delight, it worked perfectly the first time! All stall warning speeds were perfectly bracketed within the 5 - 10 knot range. This was delightful, as it solved the problem, and did it right first time, so I did not have to go up and down a ladder cutting and folding more 100 MPH tape between numerous flights.

So the shop fabricated a more proper part:

Which I flight tested today. It worked pretty well. It'll need a little refinement, but that will be easily done, and I will fly the final validation, and calibration flying next week, then approve this changed part as a part of the wing change approval I have issued. So for those who cringe at seeing a roll of 100 MPH tape come out around a plane, sometimes there are good reasons! My lucky guess has saved me untold hours of fiddling and flight testing. Soon, the plane will be back on the market (asking price $450,000) entirely safe and legal.

The alarming part is that the "other" repair shop, who casually installed the 182Q wings on the A185F, (and documented it really vaguely) apparently did nothing to validate the stall warning system. My review of the logbook today showed me that that plane had flown nearly 100 hours over four years in regular private service - with no functioning stall warning system! How had this been missed in the initial release to service, over three annual inspections, and apparently no pilot ever noticing! The whole debacle was only noticed during a thorough prepurchase inspection, where the prospective buyer's inspector noticed that the wrong wings were installed, and started asking questions. The plane was then grounded, and I was called to issue the required approval to properly document what the configuration needs to be! It heartens me that every time I have flown a maintenance check flight, I have stalled the plane to assure handling, and correct warning.

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