Electric stall warners

[FullyFlapped]

My current aeroplane has one of these. it is the "vane" type, which you can test on the ground (with the power on) by moving it backwards, and the buzzer duly sounds. No power, no buzzer.

My question is this. What happens if you have a complete power failure ? I know all about stall buffet, but .... Is this not a serious design flaw ? My last plane had a simple but highly effective "pressure activated" system which required no power ...

FF :ok:

[Cusco]

I test mine by moving it forwards...........
How did your 'pressure activated' system get its important message across to you?

Does a little red flag with 'stall' written on it unfurl in your line of vision.?

Safe (but fast enough) flying.

Cusco;);)

[stevef]

Well, one advantage to having a vane-type stall warner is that it's easy to heat in icing conditions. Not so with suction units. I see your point though.

[Final 3 Greens]

My question is this. What happens if you have a complete power failure ?

Airmanship Airmanship Airmanship

[172driver]

My question is this. What happens if you have a complete power failure ?

Fly above stall speed ? :}

[NutLoose]

Dependant on the aircraft, would you not still feel the buffeting over the tail as the wing starts to stall at the root? so you still have an indication...

The vane type you are talking about is indeed off battery power and if that is totally dead it will not work, BUT the amount of current it draws, the battery would have to be seriously flat to not operate it.

[the dean]

hi fully flapped,

as nut loosed says...what do you mean by complete power failure..??

if you have alternator failure ( and of course doing regular cruise checks you will have noticed this before you run out of battery power ), then you will still have probably at least ( depending on the condition of your battery ) 45 minutes and possibly a lot more ( especially if you pull circuit brakers ...or switch off all non essential equipment )...so no problem as you will still have power to run your stall warning..and make arrangements to land at the nearest suitable airfield..

in any event..i am sure you would pay close attention to your airspeed indicator on approach anyway..so why should it be such a problem even if you lost the electric stall warning...? in the average small plane getting close to the stall is so noticeable in attitude, wind noise, buffeting..( some may vary a little and may not have all three ) that it is difficult not to recognise the onset of the stall. this is why on flight test some of the stalls given ( not the fully developed ) you are asked to recover ' from the first indication of stall '....that is for the examiner to see that you can recognise the early symptoms ...and initiate the recovery from that point..

good you should wonder ' what if '...but i do'nt see it should be a problem for you.

fly safe..:ok:

the dean.

[FullyFlapped]

Ah, good to see the usual cross-section of Proon replies (i.e. some helpful, some wildly assumptive, some trotting out good-old standard cliches, and just a little sprinkle of "smart-arse" thrown in for good measure - although Mr. "little red flag" has obviously never seen a vacuum-activated system, and should restrain his smart-arsery until he's done a little more reading!).

OK, here's the background : a few days ago, I lost all electrical power within no more than 10 minutes of take off, despite taking down everything as quickly as possible once I'd noticed a continuous discharge. We lost everything, including radio, flaps, and - and here's the reason for the question - undercarriage. No 45 minute grace : could be the battery is duff, I don't know yet.

I deployed the undercarriage manually, but there is no way of knowing whether it's actually down and locked without power. I took us off to the longest hard runway I could find, made sure my wife was briefed and secure as possible, and then attempted the gentlest landing I've ever made : which means as slow as safely possible.

Down and safe, all's well that ends well etc. But it only occurred to me afterwards that once I'd got the speed nailed, and then cut the power for the hold off (as low as I dared), that I never heard the stall warner go off at all ... (didn't notice at the time, but I was kind of busy !:ooh:)

Never mind, just wondered because you'd kind of expect them to have picked this up at design time ...

FF :ok:

[NutLoose]

Did you recycle the Alternator? I ask, as some of the pilots that fly our Aircraft are unaware of this, I realise you probably had your hands full, but selecting the Alternator off for a brief period of time, ( about 20 seconds) then turning it back on resets the Voltage regulator and brings the alternator back online. This may or may not have worked in your circumstances, or indeed if there was a fault elsewhere, but its worth knowing if you are unaware....... The Battery does sound a bit suspect......

Just as well it wasn't a DA 42, as prior to the Airworthiness Directives that are currenty whistling through, you would have ended up flying a twin engined glider.

[FullyFlapped]

Nutloose,

Good advice, but unfortunately didn't help (turns out the belt had come completely off!)

FF :ok:

[BackPacker]

I'm learning aerobatics and one thing they teach you is that the stall, more than anything, is associated with stick location. At 0g or +4g, Vs or Vs0 (bottom of the green or white arc) is completely meaningless. So as long as the aircraft is trimmed for a reasonable approach speed and you're not pulling the stick hard into your crotch (in case of a yoke, into your armpits), you can't (or rather, should not) stall.

In the circumstances, what you did sounds perfect. Find a long runway, nail the approach speed to something safely above the stall, flare and cut the power just inches above the runway. Keep pulling until you get strong signs of stalling behaviour, or where you feel that the aircraft is done flying. Then put her down gently.

If you're worried about the undercarriage not being locked, you might actually want to shut the engine(s) down completely while holding off, so that the props are not turning. If the gear fails, at least you're only shearing off one or two prop blades, not all of them, and are not shock-loading the engine. But I admit you have to have the spare mental capacity to do that.

Don't worry about keeping the runway occupied after landing. It's your emergency, not the ones in the circuit.

[david viewing]

I'm genuinely puzzled by this and indeed similar stories recounted at other times.

I cannot see how a battery could fail within ten minutes of takeoff simply because the alternator fails. There must have been enough charge in the battery to start the engine and even a small amount of remaining capacity would be enough supply radios etc for more than 10mins.

For instance even as little as 10% capacity (unlikely for an aircraft in regular use) would still be about 7AH (Ampere hours) which would require a current flow of 42A to completely discharge in 10mins. For the undercarriage retraction to explain that would require very high currents to flow.

Even if some load had discharged the battery, switching off all the avionics, lights and so would allow some recovery in battery voltage, surely enough to work the stall warner which is just a buzzer - 1A or so and probably works down to 6v anyway.

So I'm convinced there must be some other factor in play - a battery fault or an undisclosed load that can handle 50A or so without making a burning smell. Fan belt failure just doesn't meet the facts as described.

[Dave Gittins]

I've had enough experience of rotten old cars (and most light aeroplanes are electrically simpler than them) to know that if there was enough urge in the battery to start the engine, there wasn't that much wrong with it and if you started to conserve electricity as soon as the "low volt" light went on you should be OK fo some time.

I can only assume that the departing belt took an important cable with it .. or dead shorted one.

:confused:

[Rod1]

It is quite common for PFA aircraft to not have stall warners fitted. In my gliding days, I spent large amounts of time tacking up and down and going in circles just above the stall, with no stall warning. Unless your aircraft is unusual in its stall characteristics, or you are not listening to your airframe, you have very little to worry about.

Rod1

[Dave Gittins]

All those years ago when I was doing my training, it was common practise for my instructor to wait until I was downwind and then slap the black circular rubber sucker over the ASI and I was expected to complete the landing OK without it. (To calls of "Low and slow - lookout below" if I wasn't getting it right). Perhaps for that reason I tend to be heads up looking at the picture as I land, rather than heads down looking at the instruments.

I guess that after all these years and knowing what an aeroplane feels like on the point of stalling (slack controls, the attitude, the right picture and all that) means that the stall warner is one of the "instruments" I rely on least.
I suppose that is what the seat of your pants is for ... how long does it take get the "feel" ??

To digress a bit, one of our instructors in a chat on Saturday morning was saying how bad most airline pilots who want to fly a PA-28 are, as they are no longer instinctive aviators but systems operators, it having been perhaps decades since they flew a light aircraft.

(Hard hat to ON)

[gasax]

My tuppence?

10 mins to a flat battery - got to be something very wrong. In my Terrier we toured for a week without any power generation - although after the first couple of days I did handswing the engine. Got home about 20 hours later and the battery still started us! (but of course fixed gear).

If losing the stall warner is a serious design flaw then most aircraft have a whole raft of issues.

Stall warners are only added for certification purposes when there is not considered to be 'sufficient' natural buffet. What an acceptable level of buffet is has often baffled me. (Especially as differing variants of the same airframe do and don't have them and very frequently the G-reg airframes must have them but the least of the world doesn't need them). That does of course rather ignore the somewhat unnatural attitude that you need to get into to stall the thing in the first place and all those other clues like airspeed, g-loading etc. The same Terrier also had a vane stall warner - I found looking at the sky tended to give me a bit of a clue not the feeble buzzing.

[Contacttower]

Worth remembering that some of the most numerous GA types on the planet, eg Cub and associated types have been flown for years without stall warners. The lack of a warner would be a problem in a plane, for example some heavy jets, that doesn't give you fair warning of an oncoming stall!

[Final 3 Greens]

The Pup stall warner operates when flap is extended but not clean.

So flapless landings are without stall warner.

As I said earlier, its all about airmanship and dealing with the context.

[FullyFlapped]

I'm really grateful to those who seem to think that I need a lesson on recognising the symptoms of an incipient stall (in my own aeroplane) ...

Where exactly did I say that I needed a stall warner to land the b*****y plane ? :ugh::ugh::ugh:

There is no doubt that as I held off, the airspeed reduced sufficiently to make the stall warner go off under normal circumstances. It didn't, because obviously there's no fallback when the power's gone, and that's the only point I was making !

FF :ok:

[Cusco]

Take a chill pill FF:
Perhaps if you'd posted the circumstances as background information in your original posting you might have avoided some of the irony of subsequent replies.

I was genuinely interested in a pressure active stall warner, having never heard of it.

It does however sound a bit like the 'wire and cork float' type of fuel gauge on some a/c types.

However I'm not about to get the books out every time there is noise on PPRuNe.

Safe (and fully powered) flying
Cusco ;);)(aka Mr Smart *rse)

[172driver]

I was genuinely interested in a pressure active stall warner, having never heard of it.

Cusco, some Cessna models have it. To test it, you have to give your a/c a nice French kiss - really ! ;)

[englishal]

I don't have a stall warner......I imagine electric ones stop working in the absence of electrickery.

I would say though that this is a secondary piece of equipment, and not really nescessary in my view, other than for the JAA PPL ;) When I bought my plane, I didn't even notice that it didn't have one to tell you the truth, and I find them slightly annoying, especially if they're constantly chirping away at 5kts about stall. I suppose it can be quite satisfying when you touch down with the thing chirping away, especially on your YouTube vide :}

Anyway, not having a dig at anyone here, but I don't even listen for stall warning on landing (partly because I don't have one :8) but feel the aeroplane and it'll soon let you know when it wants to stop flying....Just don't be too high at that point!

[bookworm]

Is this not a serious design flaw?

I don't think so. The stall warner is not intended to be used routinely during flight -- it's a warning that something has gone seriously wrong. The failure that you're concerned about (stall warner fails to alert pilot to imminent disaster after electrical failure) would require two highly unlikely events: both the failure of the pilot to maintain a proper AoA and the failure of the electrical system. On that basis, it seems perfectly reasonable to design an aircraft with an electric stall warner.

[FullyFlapped]

Bookworm,

Fair point ...

FF :ok:

[Cusco]

!72:
I still don't know what signal the pressure active stall warner gives the pilot to tell him he's about to drop out of the sky, presumably it's not an electrically generated warning..........
I can understand 'on the ground testing' with a French Kiss: presumably it lets out a languid sigh, wiggles its pelvis a tad then drops off to sleep..............

Cusco:=

[BackPacker]

Cusco, you came across as a wind-up on your first post. Perhaps that's why nobody took you serious. Call me stupid but I now believe you really have never seen a pressure-activated stall warner. They're common on Cessnas, and the DA-40 has 'em too.

It's nothing more than a hole in the leading edge of the wing, just below the line where the air normally separates itself into a low pressure area above the wing, and a high pressure area below the wing. In other words, in normal flight, there is a high pressure (higher than static, I mean) pushing into the hole. Nothing happens in this case.

As you approach the stall, the line between low and high pressure shifts downward due to the higher angle of attack. The hole is now in the low pressure area (lower than static), and air is sucked out of the hole.

The hole is connected, via a tube, to a sort of whistle in the cockpit, which makes a loud buzz as soon as air is sucked out through it. That's why you french-kiss it as part of your preflight checks: by sucking on it, you should hear the buzz. And yes, that is yuck, so there are little harmonica-like things for sale that do the sucking for you.

The advantage over a vane-type like in the PA-28 or Robin is that this system requires no electrics with all the problems associated with that (like mentioned in this thread). The disadvantage is that sometimes insects fly (or even nest) inside the hole, and it's harder to prevent them from icing over compared to electric vanes.

[stevef]

Cusco:
Small port on the left wing leading edge connected to what looks like a kid's plastic 'parp' trumpet, internally just forward and above the doorpost. Works by pressure decrease as the AoA changes approaching the stall. Best tested on the ground by holding a handkerchief over it and sucking (just in case of wasps, etc!).
Edit: Cross-posting - Backpacker beat me to it!

[Cusco]

Thanks Guys:

Piper man all me life except 6 months in a C182: That had an electric vane type stall warner similar to Pipers.

Cusco.:ok:

[stevef]

Slightly off the immediate subject, some aircraft, such as the Seneca, have vane-type stall warners that can't be tested on the ground unless the landing gear squat switch is manually operated to give a 'weight off' signal. Annoying as it has two vanes, one dependant on the flap position. Most crew are probably not aware of this, so there is always the possibility of a malfunctioning stall warning system not being detected. Bad design fault in my opinion.

[Mike Cross]

Stall warner? Looked everywhere and can't find it. Similar problem with the flap lever, pitot heat, fuel pump, low voltage warning light, vacuum pump etc etc. How it flies without them is a complete mystery, that it can still do it 60 years on is a miracle.;)

[mm_flynn]

I recently had to replace a battery due to the fact a cell or two was bad.

The symptom was with only a small amount of ground running of electrics I didn't have enough power to turn the engine over. Interestingly, I could tell if I was going to have a problem on my walk around. The lights would come on (strobes, beacon, nav, landing) but when I gave the stall warner a flick - no noise! So at least on my plane it fails at a higher voltage than almost anything else in the plane. But as others have said, there are lots of other warnings of stall than just the electrickey one - so potential lack of stall warning wasn't the issue that got me to replace the battery post haste!

My Bonanza has a 15.5 amp-hr 24 volt battery (70 amp hour seems like a decent size car battery). If FF has similar and threw the belt right as he started his take off roll, with a full suite of avionics and the draw from the gear retraction motor, he could well have had only a little time left. However, 10 does sound like the battery has a weak cell or possibly the belt was dropped at engine start.

[Blues&twos]

My initial thoughts were that a warning system should be fail safe (loss of power brings the warning on) as with most of the electrical gear I deal with.

However, my next thought was that if it failed/lost power etc and switched on the buzzer, there would be no way to switch the buzzer off until you landed. I reckon that would be pretty distracting - a worse potential hazard than not having a stall warning, especially if the power loss was a widespread multi-instrument power loss as you'd be busy enough dealing with that.

Just a thought.

[BackPacker]

My personal opinion is that you should know the POH of your aircraft to the extent that you know by instinct what systems fail if one of your base systems fails.

Here's what generally fails in a small aircraft, fixed gear:
Electric system failure: Internal and external lights, electric fuel quantity indicators, all avionics (Intercom, COM, NAV, ADF, DME, autopilot, transponder), electric flaps, electric stall warner, CDI, electric trim and some engine instruments (T&Ps most likely yes, MAP/RPM most likely not, but it does depend on the exact type)
Vacuum system failure: AI, DI
Pitot/static system failure: ALT, VSI, ASI

For a retractable, you have down & locked indicators which rely on electrics, and the gear itself might be electric or hydraulic. Warnings along the line of "MAP below 15" with the gear up and flaps down" (or other combinations of these) will most likely also be based on electrics.

Now imagine all of the systems that use electricity, to have their own failure indication if the electrics fail. Not only would this require a small backup battery for almost each instrument, but it would also lead to a cacophony of warning signals in the cockpit, all of which need to be cancelled/acknowledged individually...

[Final 3 Greens]

Bookworm

Having stated that airmanship should protect a pilot against stall warner failure, let me now play devils advocate when you say "The failure that you're concerned about (stall warner fails to alert pilot to imminent disaster after electrical failure) would require two highly unlikely events: both the failure of the pilot to maintain a proper AoA and the failure of the electrical system."

I could make a strong argument that electrical failure is just the sort of distraction that could cause a single pilot to miss a change in AoA and get into trouble, especially if the pilot was inexperienced - so perhaps the holes in the cheese might align.

[david viewing]

MM Flynn is right to say 70 amp hour seems like a decent size car battery, so it was a bad example. The 12v battery in the Warrior -161 is about 30AH according to Concord battery, but even 10% (3AH) remaining would still require 18A continuous drain for a discharge in 10mins.

However I was being very conservative in assuming 10% capacity left. That would only really happen if the battery was already in trouble due to a cell fault (as above), overcranking, master left on, etc. all of which ought to merit some consideration before flight. I suppose it's possible that a discharged battery might cause the belt to throw but that's wild speculation.

More realistically the battery ought to be at 50% capacity after starting and it's very hard to explain where all that current went in 10mins without starting a fire. So my point stands - there must be more to it than just belt failure.

PS Interesting about the stall warner being the first thing to fail. You'd expect the designers to require 6v operation in a 12v aircraft for all the reasons we are discussing. I'm sure a replacement would only be £100 (aviation grade), or £0.30 from Maplins, so maybe a time for a new one?