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.

https://cimg1.ibsrv.net/gimg/pprune....74a3825dfc.jpg

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:

https://cimg4.ibsrv.net/gimg/pprune....eff2c0992c.jpg

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.

Oh dear. Some of the folk can get caught out from time to time.

Much simpler, but in a similar vein. Years ago, following a major rebuild of a bent bird, I was whistled up by the shop to do a reweigh. Quick walk around (as I do) before we started. "Where are the leveling lugs ?" sez I.

Embarrassing when you forget (or don't bother) to check the drawings and leave bits off during a rebuild .... I still wonder what else was left off and whether they talked someone else into doing a "turn-a-blind-eye" reweigh.

Even in big airlines unexpected things can happen on CofA test flights after major maintenance. In this case a small rubber seal was not put back in place on a wing fence with inyeresting results:-

Incidents and Accidents

Even in big airlines unexpected things can happen on CofA test flights after major maintenance.

And it doesn't have to involve test flights, just a plain old lack of knowledge and taking short cuts will achieve an unsatisfactory outcome. Many decades ago, I was in with one of the local Regulator's airworthiness PEs talking about whatever project. He, idly, mentioned a defect report which had just come across his desk regarding one of the two major airlines and the dispatch of a twin jet with one of the nacelle chine VGs missing. Apparently, the mx folk had deemed that missing component to be acceptable and had signed the aircraft to release pending spares. We had a rather urgent discussion on MELs, vortex flows, stall speeds, and asymmetric stall departures.

I never heard what the final outcome was but I suspect that there would have been some discussions over the road in the relevant mx hangar between tech services and mx ?

The best part about the reeds in a Cessna stall horn is you can use the yoke like a trumbone and play a tune . Very easy in smooth air , might miss a few notes in rough air . Demonstrated O Canada to an examiner for giggles once . He laughed when I told him I had not quite mastered the flight of the bumble bee yet or the Grenadiers March . Good times

Could you not have simply adjusted the angle of dangle of the wings? It sounds like the cams are both out.

If you do not know what I am on about see below,

Nutloose, we did that also. The shop had put the plane back together extremely well, so only a very minor cam change was required. The plane flew nearly hands off first flight, and perfectly the second flight. 185 amphibs are even less good in yaw than most Cessnas, as the water rudder system friction adds to the rudder friction, and reduces self centering and thus apparent rudder free directional stability. Interestingly, adjusting the cams on Cessna wings must be done with thought, as that adjustment puts a minute sweep into that wing, either forward or aft. Both wings must sweep the same way, or the plane will be imperfect (for a Cessna) in yaw if one wing is swept forward, and the other aft. It's a very tiny change, but I have found it can cumulatively affect things during previous post maintenance set up flying I've done in the past.

This exercise was needed because the hole in the [should have been installed] 185 wings for the stall warning pneumatic opening, is in a different location along the leading edge radius than the hole for the vane switch of the 182Q wings. We did not want to cut a slot in the leading edge of the wing to position the pneumatic stall warning part much farther up at the correct aerodynamic position for the 185. My 100 MPH tape trick achieved the objective without cutting a slot in the leading edge of the wing - which would have looked horrible!

yes, if i remember my figures correctly. the wing twist is only outboard of the strut, for my sins I have rebuilt lots of wings and indeed written off cessnas :)

Correct, the Cessna wing washout starts outboard of the strut, and the twist in the last bit of aileron span does the rest. The wing cam adjustment changes the angle of incidence of the whole wing though. Depending upon which way to turn the cam(s) (there is a dot to show), the rear spar is moved a tiny bit outboard or inboard, along with up or down as intended. It's a good system. I've set up other types which did not have wing angle of incidence adjustment per wing, and you have to fiddle a flight control instead.

I flight tested the finished product today, and it worked perfectly:

https://cimg7.ibsrv.net/gimg/pprune....5a72510b09.jpg
The shop riveted a small bar under the stall warning port as I had designed. All warning speeds within range today, released to service, and now looking for a buyer!


https://cimg9.ibsrv.net/gimg/pprune....594b878082.jpg

I found the video interesting….I always imagined the adjustment mechanism was much more involved than that. The “cams” work on the same principle as adjustable camber bolts on car suspension.

I’ve previously mentioned one Cessna I flew on my pre-GFT revision flight. It had just come out of long term maintenance and was found to sharply drop a wing when stalling. Myself and my instructor had to practice a full spin and recovery in it (it was in the PPL syllabus back then in 1973). I entered the spin in the normal way but it did something very strange and we ended up in an inverted spin with the prop stopped. My instructor asked me what I intended to do about it and I said “pull back instead of push?”. He nodded, I did and it came out again. It was grounded and the wing incidence was quite a way out. Now I know how.

An interesting thread. Ironically, the trainer I started on was a military derivative of a civilian type and the stall warning system was deactivated in service. There are several now on the civil register, so I wonder whether they had to reactivate the system

I flew a test program on a Siai Marchetti 1019, which is a derivative of a Cessna 305/L19/O-1 Bird Dog. The 305 does have a stall warning system, the 1019 does not. The 305 had a very benign, Cessna like stall, which was well warned aerodynamically.

Certification design requirements allow that the warning of the stall may be purely aerodynamic, without a supplementary system, of the plane is approved that way. For the 305, and indeed, the 185, the TCDS specifies a functioning stall warning system. It's all a matter of how it was certified, but if it was certified with it as specified equipment, it's gotta work!

Is adjusting the cams (sympathetically) on both wings going to change the stall-warning margin? Surely the relationship between stall warning vane/slot and the wing chord will be unchanged. Strikes me that all you will be doing is changing the alpha of the fuselage.

Happy to be shot down in flames on this, by the way.

It depends if it was dropping a wing or not when it hits the stall, with it running out of adjustment on the vane, it could have been possible the angle of attack of the wing was wrong, that is why I suggested checking them to ensure they were set correctly, various things can cause problems, Rudder trim being one, flap rigging being another, you do get people tweaking the flaps on Cessna's to level the wings, however that is not the recommended path, it is on some Pipers hence why bad habits may slip across.