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Safe Flight Lift Detector: A Guide, A Rant


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A bit of light reading if you're bored over the weekend.

TL;DR: Stall switch failed on me, and the $2500 price tag for a new replacement is downright stupid.

A few months ago I noticed that the stall switch (lift detector) on the 201 I'm currently flying didn't work too well - in that the stallhorn might or might not activate even as the airplane entered imminent stall condition during the last moments of flare during landing. Interestingly, around that time I also noticed that during descent (in excess of 500fpm) in clouds/rain I would hear a distinct, but faint, high-pitched tone from the ceiling area.

A few days ago I returned home after a rather challenging X/C trip that mostly took place in the soup, even with a "fun" encounter with light icing too. Upon arrival late at night, I hangared the plane and plotted -D-> to MYBED. The next morning I returned to the hangar to take care of a few things, and at this time I turned the master on to check the voltage since I had it trickle charging overnight. As soon as I flipped the master on, the stall warning came on in full force and wouldn't quit. Playing with the lift detector vane didn't help (although I could hear a very faint pitch change in the stall warning tone) but pulling the STALL WARN circuit breaker stopped the madness. I searched this forum and found out that I was not the only one to encounter this issue:

I opted to leave the airplane alone at that time and to see if any supposed moisture buildup in the stall switch would clear up in a few days. It didn't. After letting it sit for 4 days the horn was still blaring as soon as I turned the master on, so I removed the switch from the airplane (not the worst, but not easy either) and confirmed with a multimeter that even when not activated, there was only 1.2kOhms between N.O and Common.

The Mallory Sonalert SC628 (the generic P/N of the 201 stallhorn) has a current draw of approx. 7mA at 13VDC using data interpolated from the datasheet. This roughly translates to a roughly 1.8kOhm impedance assuming no other resistance in the circuit. When a 1.2kOhm "resistor" (i.e. faulty switch) is added in series with this device, the voltage differential across the Sonalert drops to 4.6V, but the thing about these smaller Sonalerts is that despite their listed operational voltage of 6~28V, they actually activate around 1V and are quite loud even at 3V.

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I have disassembled the switch as much as I can. The Safe Flight lift detector system is attached to the leading edge curved piece via 2 MS20470AD4-3 (or -4?) rivets which can be drilled out with a 1/8" dia. drill and punch. The lift detector assembly itself is basically a custom-order variant of the Honeywell BA or BE series SPDT microswitch housed inside a custom formed sheet aluminum cover then secured via two long MS20470A4-?? soft rivets, although the rivets are bucked so that the bucktail looks identical to the head. One could try drilling these out too, but they are prone to rotating with the drill bit, so I found the best method of removal was to carefully use a file to shave off one of the heads, then to use a 1/8" dia. punch to press/pull the remains out.

The vane lever (constructed of stamped stainless steel) is secured at its fulcrum using a pin, which also can be pressed/pulled out of its place using a very small diameter punch or some other improvised device (I used an awl to expose enough pin on the other side, then carefully pulled the rest out with a needle-nose plier. It appears that the switch itself is not one of Honeywell's hermetically sealed types, which in my opinion is a huge no-no given the fact this switch is literally exposed to oncoming air/rain/debris. Thus, the plastic plunger slips right out of its place, so care should be taken not to lose this part in addition to the aforementioned lever pin.

The Honeywell (or rather, its subdivision "Micro Switch" in Freeport IL) switch can be opened up to reveal its conductive innards by separating the upper "roof" from the main body, albeit with much difficulty. Safe Flight uses some sort of yellow electrical tape in an apparent attempt to seal this joint but I have found the tape's material to age very poorly, peeling unevenly and leaving an unsightly pattern of sticky residue upon eventual removal. Alcohol and/or brakleen easily removes this though. The "roof" is secured to the switch body via a combination of press fit and 3 tiny pins, 2 located on the LH and RH sides, and one at the rear. I learned this after-the-fact when the roof separated from the body and 3 pieces of plastic debris fell out with it. In my case, the breakage was clean and CA glue (aka superglue) did the trick.

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The switch innards are very simple; in the image above, the switch's Common conductor (on the far right) is connected to a springy copper arm that extends from the right to the far left. The arched shoulders on this arm bias the arm upwards as such to normally make contact with the upper electrode (the gold colored rectangular tab on the left) which is connected to the N.C. port, but this port is not threaded/used in this switch. When the vane at the wing L.E. lifts during high AoA, the opposite end of the vane lever pushes down on the plastic plunger, and the bottom end of the plunger pushes this copper arm down, causing it to detach from the N.C. electrode and to make contact with a similarly-shaped N.O. electrode below. A distinct clicking sound can be hear d during this operation.

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I rinsed down the switch body with liberal amounts of contact cleaner and then tested the contacts again with a multimeter; the check was OK. Chances are some conductive debris got lodged between the Common and N.O. electrodes. Reassembly of the switch consists of reattaching the "roof", securing it with the 3 tiny pins, inserting the plastic plunger (flat end goes into the switch), then reattaching the vane lever with the securing pin. I have taken the liberty of securing the switch to the Safe Flight housing using 2 sets of #6-32 screws, AN960-6L thin washers and MS21044N06 nylock nuts, and likewise for securing the assembly to the leading edge curved piece as well.

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Especially disappointing (and frankly, anger inducing) is that this switch was removed, factory repaired, and reinstalled according to a logbook entry dated 15 January 2016, which means this switch only lasted just over 5 years and about 150 hours T.I.S. before failing again. (The previous owner apparently didn't fly much) According to a Cessna 170 forum thread, a factory overhaul for an unheated lift detector (which basically involves Safe Flight switching out the Honeywell switch and putting it back into the housing) cost $400 in 2007 and according to another MS thread, $1200 in 2017.

That is f***ing outrageous.

Mind you, the price above is for overhauls/repairs. Aircraft Spruce lists a new Safe Flight Model 164 switch at $2575.

A Honeywell BE-2R-A4 switch costs $12 on Sager Electronics. Even if, say, a custom-order variant of the BE series switch was batch-produced with an MOQ of 10,000 for $1,000,000 ($100 ea.), FAA PMA certification efforts cost $400,000, the aluminum housing cost $50 per unit, and the labor to assemble one was a generous 2 hours at $100 per shop-hour, this amounts to a valuation of $390 per switch, which translates to a 660% markup.

During my days as an avionics tech, I've seen some unremarkable products sold at absurd prices (such as the Astro-tech LC-2 clock, a $10 Walmart kitchen timer sold at $400) but the Safe Flight lift detector takes the prize for the dumbest piece of s*** sold at the most d e n s e markup considering it's tech straight out of the Industrial Revolution of the 18th century. I've seen TSO'd comm radios sold for less than this switch.

I understand that businesses need to generate profit to stay afloat; I may have given the lift detector a pass if it incorporated some ingenuity or special attribute that reflected some sort of significance in its R&D. My day job involves selling industrial electromechanical devices with a comfortable (but fair) markup because they're some of the best performers on the market and I know the engineers put in so much effort designing and refining them in years past and present. But the lift detector is nothing more than a custom-order microswitch from the 1940s with no meaningful R&D since, probably, the 1950s or 60s. To sit on decades-old tech and offer them for a markup so high that it might as well get RVSM is beyond my comprehension.

Well, maybe it's not that much beyond comprehension. I know Safe Flight has some pretty technically advanced products such as their powerline detectors and helicopter pedal shakers, and I can only assume that their lift detectors contribute only a tiny portion of their revenue portfolio at this point, so all things considered, they couldn't care less about these relics of the past. Normally, the nature of the market would dictate that someone else would pick up the slack to offer reasonably-priced competition, but given the current rate at which new airplanes are produced and the relatively high entry barrier of PMA certification, such a venture is unprofitable and unattractive.

If anything reflects painfully well the shattered remnants, the empty shadows of a once-hopeful general aviation industry, it is the flat-out unimpressive Safe Flight lift detector and its ludicrous pricing. At the end of the day, this is just one of my biggest pet peeves that I know is beyond my personal control, but one that I still can't help but be frustrated about.

If you've made it this far, kudos to you for actually sitting through this entire monologue. Thanks for coming to my Ted talk.

Edited by Minivation
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A bit of light reading if you're bored over the weekend. TL;DR: Stall switch failed on me, and the $2500 price tag for a new replacement is downright stupid. A few months ago I noticed that

Ah yes of course . Thanks for the heads-up! Yes, I am an A&P with an avionics tech background. I used to do quite a few bench tests and even some component level repairs when I used to work a

I realize I may have spoken a bit too soon about the lift detector "taking the prize" for the highest markup (plenty of horror stories about voltage regulators and prop heat control units I see) ... e

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I think I read far enough... maybe I didn’t...

But...

I can only say a cautionary note... that may be unrelated...

There are proper ways to get a safety device overhauled... and who can do the work...

If you followed those guidelines congrats...

If you followed your own guidelines... you may want to consider what you post publicly... :)

Of course, if you are an avionics tech or an A&P... now would be a good time to mention that kind of detail...

+1 on the ridiculous price of the stall vane and switch... wait until you have a heated one...

Nice pics...

For reference...

I receive more advice about things I write... nobody is perfect...

Friends trying to help each other out...

Best regards,

-a-

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35 minutes ago, carusoam said:

I think I read far enough... maybe I didn’t...

But...

I can only say a cautionary note... that may be unrelated...

There are proper ways to get a safety device overhauled... and who can do the work...

If you followed those guidelines congrats...

If you followed your own guidelines... you may want to consider what you post publicly... :)

Of course, if you are an avionics tech or an A&P... now would be a good time to mention that kind of detail...

+1 on the ridiculous price of the stall vane and switch... wait until you have a heated one...

Nice pics...

For reference...

I receive more advice about things I write... nobody is perfect...

Friends trying to help each other out...

Best regards,

-a-

Ah yes of course :). Thanks for the heads-up!

Yes, I am an A&P with an avionics tech background. I used to do quite a few bench tests and even some component level repairs when I used to work at a 145 shop.

As for the legalities, the work done on the switch, a "component part", is a minor repair, minor alteration done with reference to AC43.13-1B (accepted practices), much in the same way someone might repair a nav light bulb receptacle, for example.

 

Edited by Minivation
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I have a friend from college that was a corporate officer at Beechcraft back in the 80's. He told me then the company basically set aside 33% of the gross price as product liability reserve. I can imagine SafeFlight is in an even worse position as it is a "safety device" so that every time an airplane stalls and crashes, they go to court. I feel your pain, I just spent short of 2 AMUs to have my heated unit overhauled. It makes you wonder if Cessna was not brilliant with the little "wind whistle" stall warning. No contacts, just a little reed to vibrate.

 

 

 

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It truly amazes me how the strangest maintenance items seem to pop up out of nowhere where aircraft ownership is concerned.  I had a frustrating right wingtip strobe issue that only went away when I finally replaced both strobes with LEDs; we never could track that issue down.  I’m not an A&P, so I usually end up scratching my head, calling smart guys like you and picking their brain, and digging around the web until one of us figures it out or the answer magically appears.  Eventually.

My wife and I own older (paid for) domestically made vehicles, and I own and maintain a 1978 Kubota diesel tractor.  (Same year as my J by the way...)  I have learned one big life lesson many years ago that applies here- if ya buy something expensive that’s gonna to need maintenance, think about parts and getting it fixed or replaced when it breaks.  Sure applies to aviation, doesn’t it?  

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Ah—Fun topic: Outdated and poor aviation designs semi-supported at high prices.  

I had a propeller heat control module fail.  The replacement was $3500 as I recall.  

I took apart the old module (hey, I’m a retired engineer & can’t restrain myself at times). Inside I found a single-sided PCB with a 555 timer, a couple of diodes, one connector and three relays.  That’s it. 

The Bendix connector is admittedly expensive, as are good relays, which these were not.   A generous estimate for the component bill of materials is $250.  Or in China a contract manufacturer might pay $30.  

Like the OP I repaired the thing and now I have a spare. 

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Years ago, the OECO voltage regulator in my M20J failed. The overhaul/replacement cost seemed exorbitant at the time (can’t remember what it was). I replaced the power transistors with equivalents I bought at Radio Shack for a couple of bucks each, took it to work and adjusted it (it had never been quite the correct voltage) and it’s probably still working somewhere.

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When you recognize the manufacturer of the switch... (Honeywell)

you know it is the parent company for Bendix King...

It wouldn’t be all that difficult to make an industrial switch to last for decades in a harsh environment...

They probably make one already...

if it were BK... they would sub it out to somebody else...

Thanks for the update MiniV!

Best regards,

-a-

 

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11 hours ago, Minivation said:

If you've made it this far, kudos to you for actually sitting through this entire monologue. Thanks for coming to my Ted talk.

Awesome post and thanks for the detailed info!    My stall warning system is intermittent, but I think it's a connection at the sonalert.    If it turns out to be the switch, I am now confident it can be tackled without too much trouble, thanks to your post.

 

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7 minutes ago, EricJ said:

Awesome post and thanks for the detailed info!    My stall warning system is intermittent, but I think it's a connection at the sonalert.    If it turns out to be the switch, I am now confident it can be tackled without too much trouble, thanks to your post.

 

Data point: Mine had similar intermittency and I, too, first suspected poor connections at the Sonalert.  Turns out it was the Sonalert, itself!  I believe it is an SC628N available in the aviation aisle at DigiKey for around $25.

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Just now, MikeOH said:

Data point: Mine had similar intermittency and I, too, first suspected poor connections at the Sonalert.  Turns out it was the Sonalert, itself!  I believe it is an SC628N available in the aviation aisle at DigiKey for around $25.

Yeah, I already got spares, one for $7.59 off ebay, in case it's bad.

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5 minutes ago, MikeOH said:

Data point: Mine had similar intermittency and I, too, first suspected poor connections at the Sonalert.  Turns out it was the Sonalert, itself!  I believe it is an SC628N available in the aviation aisle at DigiKey for around $25.

I keep extras. The gear one is what usually fails. It’s more complex ( and an inch taller) because it’s intermittent beep. 

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I think the statute of limitations has run out by now, and I wasn't an A&P at the time.

I used to hang out at the crop duster place back in the 90s. The Air Tractors would eat voltage regulators on a regular basis. It was costing Joe $1500 to get them repaired. He asked me to take a look at one. I trouble shoot it and it was a small signal transistor which was available at Radio Shack. It cost $0.85. I put it back in the Air Tractor and fired up the PT6 and everything was working again! The strange thing was that every part in that voltage regulator was available at Radio Shack, which is strange because they had such a limited selection of these kind of parts. Even the regulator IC was available, it was the only one Radio Shack carried. It almost made me think the original engineers purposely built it with Radio Shack parts. There was only 4 small signal and one power transistor along with a 16 pin IC. I stopped troubleshooting them and would just replace all the parts, I could do it in about 15 minutes. The total parts bill was ~$7.00. Joe said my regulators lasted longer than the ones he paid $1500 for. I never charged him.

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I flew with a guy that had one of those installed. He slows down to where it shows almost stalled and I'm like we're not even close to being stalled. And just for good measure, we roll into a 60° bank and were able to turn at about 98 mph. The safe flight was completely buried off scale, so I'm like well if you're basing your approach speed on this thing you're flying 12kt too fast.

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34 minutes ago, jetdriven said:

I flew with a guy that had one of those installed. He slows down to where it shows almost stalled and I'm like we're not even close to being stalled. And just for good measure, we roll into a 60° bank and were able to turn at about 98 mph. The safe flight was completely buried off scale, so I'm like well if you're basing your approach speed on this thing you're flying 12kt too fast.

I think you are confusing a stall vane with an AOA detector.

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Mine is attached to a speaker... it gives a tone as a stall warning...

Its a simple on / off switch that works so well (sense the BK humor about to come out...) I flew around for about six months without it sounding once... I thought I Was really controlling airspeed well... :)

But, after watching so many MSer landing videos...  where the stall warning sounds, and tires chirp at nearly the same time...

My stall warning wasn’t working for those six months...

 

Pref-flight... I always tested the vane for it working... (mechanically)

Electronically... I had Gill batteries... and leaving them on to test the vane was not fully sensible...

 

So I had a dirty switch... it got cleaned, and Concorde batteries got installed, so a complete test of the stall warning system gets used all the time now...

The biggest challenge for maintenance on this switch... is how far away it is from the nearest access panel...

PP thoughts only, not a mechanic...

Best regards,

-a-

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I realize I may have spoken a bit too soon about the lift detector "taking the prize" for the highest markup (plenty of horror stories about voltage regulators and prop heat control units I see) ... even within my own experiences I think the one that takes the grand prize for the highest markup (although the end price isn't too expensive) is the Cessna solenoid surge protection diode (P/N 0770783-*). Behold, the true pinnacle of cutting-edge technology, backed by millions of dollars of R&D, countless weeks of testing, and most important of all, a shiny new Form 8130-3: a 1N4007 rectifying diode terminated on both ends by TE Connectivity PIDG series ring terminals. For just $55, you too could enjoy this marvelous piece of tech...

...or you could fabricate one yourself for literally $0.52, $0.02 (for the diode), $0.15 (for the #10 ring terminal) and $0.35 (for the 1/4" terminal).

Image result for cessna 0770783-1

That aside, I got the lift detector reinstalled in the airplane, ops test good. The weather here in the east coast has been absolutely dismal (going on 20 days of straight rain and sleet - I haven't seen the sun for more than 2 weeks) so I'll have to wait till next weekend (fingers crossed) to bring the plane up to make sure the switch is aligned so that it activates at the right AoA.

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On 4/7/2021 at 4:00 PM, AerostarDriver said:

@MinivationWere you ever able to flight check your repair. I have two of these which nether work as expected.

Sorry for getting back to you late! Just came back from an extended trip out to Texas.

Yes, I've since slapped the switch assembly back on, and I'm glad to report it works well. It did take 2-3 flights to get the position adjusted to exactly where I want it, but after that it chirps a few degrees above critical AoA, exactly where it was before the switch broke.

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I’m confused, I thought that an A&P couldn’t replace a leaking compass diaphragm, how can one overhaul a lift detector/ stall vane.  There are no manuals covering this repair.

And for those who don’t want to pay Safe Flight, Mooney had a replacement built in house.

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