Stall Warning

Safe Flight lift detector

FAA certification rules require that an airplane provide pilots with an unmistakable warning of an impending stall. The warning can be an airframe buffet, or buffeting of the flight controls, but the warning must be totally obvious and must occur enough in advance of the aerodynamic stall that a pilot of normal abilities can avoid the actual stall.

This is an almost impossible requirement to meet even though its safety objective is so essential. But thanks to Dr. Leonard Greene every airplane can meet the rule and give pilots enough warning to avoid stalling. You see, Dr. Greene invented the stall warning system near the end of World War II and founded Safe Flight Instrument Corp. to build the equipment so every airplane can warn its pilot in time to avoid stalling.

Although there are several methods to measure angle of attack and thus warn of an impending stall, Safe Flight’s most popular system uses that small vane mounted on the leading edge to detect a change in lift. Safe Flight has manufactured hundreds of thousands of the stall warning lift detector systems and they can be found on airplanes all over the world.

I believe Dr. Greene and his stall warning system did more to advance aviation safety than any other single technology. The stall induced accident has decreased in number dramatically in the decades since stall warning became common. There are still some stall related accidents but those usually occur when a pilot loses control in the clouds, or when the airplane simply can’t do what the pilot needs it to do such as climb over an obstruction at the end of a short runway.

But experimental amateur-built airplanes have not seen the same decrease in stall accidents. In fact, stall-spin accidents are the single leading cause of fatalities in E-AB. And E-AB have absolutely no requirement for stall warning, either natural buffet, or an artificial warning. Pilots of standard airplanes have benefited greatly from Dr. Greene’s invention, but E-AB pilots mostly have not.

While the stall warning system gets most of the credit for general aviation’s overall stall accident improvement manufactures have employed other stall taming techniques, too. On many production airplanes cuffs on the leading edge, or vortex generators manage airflow so that the outer wing continues to fly after the wing root has stalled. That maintains aileron effectiveness so wing roll off at the stall is minimized.

Another method used to manage stall behavior is to limit up elevator travel. Without a lot of elevator authority a pilot cannot pull an airplane as rapidly and aggressively into a stall so the maximum angle of attack achieved will be lower and the abruptness of the stall minimized.

And airplane manufacturers make tradeoffs between the lowest drag, highest performing possible airfoil shape in favor of wing sections that behave acceptably when they stall.

The homebuilder, on the other hand, has the freedom to ignore artificial stall warning systems, and to opt for wings and control systems that could never meet certification rules. And that is as it should be. That’s why we put “experimental” prominently near the entrance of any E-AB to warn anyone boarding that the behavior of the airplane is unknown, at least not known in the certification sense.

E-AB is all about innovation and the freedom to tryout your own designs, concepts and craftsmanship. But I think innovation is what Dr. Greene did almost 70 years ago. While he was in the Army during the war he witnessed airplanes stalling and crashing and vowed to do something about it. And he did and all of aviation is so much safer for it.

What we need in E-AB is someone like Dr. Greene who has seen enough stall-spin accidents and will do something the help prevent the crashes. Dr. Greene didn’t ask for new regulations, and neither am I. Instead he created a technology that addressed the problem and helped prevent countless accidents in all manner of airplanes. Is there among homebuilders another Leonard Greene who can show the way to improve the disastrous stall-spin accident rate? I truly hope so.

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35 Responses to Stall Warning

  1. Paul Richardson says:

    Excellent article.

  2. Kayak Jack says:

    While an ability to experiment is a great boon to aviation, experimenting with stalls without warning does not seem like a wise decision. Isn’t a stall warning horn & switch even an OPTION on E-AB aircraft?

  3. Cary Alburn says:

    What is most definitely “an option” for all aircraft, certified or experimental, is an Angle of Attack indicator. I had an Alpha Systems mechanical AOA indicator installed on my P172D about 3 years ago, and with roughly 185 hours or so of flying with it, I’m sold on the idea that all airplanes should have AOA indicators, not as an option, but as standard equipment. It gives a much better idea of how close to losing lift than the normal Greene stall warner, which is really just a switch that is either on or off, set to go off at a few knots above the stall speed. Once activated, it doesn’t tell the pilot how much slower the airplane will fly before the wing stalls.

    Properly installed and calibrated, an AOA indicator allows flight with no risk of loss of control, generally much slower than the POH approach speeds, simply because it reacts based on angle of attack, not airspeed. Fundamentally, in normal operations the airplane’s stall speed will vary greatly depending on the weight carried, because the wing’s angle of attack will be different at different weights, assuming the same airspeed. So by measuring angle of attack, the pilot can slow down at lighter weights and still maintain sufficient lift safely. Obviously, that’s a huge benefit in an airplane where the weights vary more than mine, but even in mine it’s quite a benefit, where one flight may be at gross (and the POH speeds work well), and the next may be with quarter tanks and just me, so it’s several hundred pounds under gross.

    The FAA now allows AOA indicators to be installed in all aircraft, without an STC or even a 337, unless a heated probe is installed, in which case tying into the aircraft’s electrical system requires a 337 for certified aircraft.

    No doubt the Greene invention has saved lives, but I think an AOA indicator in every airplane would do so, even better. BTW, I have 2 friends with high performance experimentals, Lancairs of different sizes (one is a 2 place with an O320 Lycoming, the other is a 4 place with an IO540 Lycoming) who both say that they would not fly without their AOA indicators.


  4. Jim Butler says:

    Cary is spot on. I have an angle of attack sensor in my E-AB and it not only provides continuous feedback with regards to angle of attack, but it announces “Angle, angle, push” over my intercom if preset values are exceeded. It is actually more instructive than a blaring stall warning.


  5. Mac says:

    Hi Jim,
    Actually, a stall warning system is an angle of attack sensing system. It is by measuring the angle of attack (alpha) that a stall warning system detects approach to a stall. In common pilot talk we say it is an angle of attack system if it shows a range of alpha, but is a stall warning if the system has only a single threshold. An angle of attack system could give an earlier warning than a basic stall warning, but if the angle of attack system is going to issue aural or visual warnings, it must have preset calibrated thresholds so in that situation there is no difference. In a typical jet there are angle of attack sensors–usually dual–and those sensors activate the stick shaker stall warning if the airplane is approaching a stall, but those same sensors can present continuous angle of attack informaiton, too. We’re all talking about the same information, just different presentation.
    Mac Mc

  6. Roger M. Derby says:

    Somebody missed the point. Sure, AOA displays are wonderful and I wish I had one, but “experimental” says build it your way.
    There was a fellow in a T-hanger in El Paso when I was an airport bum there and he was just whittling away at sheets of aluminum and riveting them together with intent to fly.
    People marveled.

  7. Earl Schroeder says:

    As others have mentioned, AOA devices are available for Experimental aircraft. My Lancair has one and it is calibrated to my aircraft. If it does not ‘light up’, we do not fly until it is fixed. I hear ‘push angle push’ during every lift off which is a good indication that it is still in calibration.
    It is difficult to ‘convince’ a non AOA owner to invest in a life saving device.
    One possible solution might be to have instructor pilots teach AOA instead of airspeed only to prevent stalls.

    • Peter Dankelman says:

      I’ve been flying my RV 7A since Decembeer 08 that I built with no warning. I pay close attention to my airspeed indicator at all times especially at landing. Remember that ground effect kicks in just before touchdown.

  8. Johnny White says:

    No Electrical or electronic stall warning system is accurate. Only the Stick tells the truth.
    Stick to Neutral not stall. Period!Visit my web site and see why.

    • Walt says:

      Johnny, I can tell you personally that though you may be generally right, some of us have been in situations where you are dead wrong. Visit last weeks AVwebFlash discussion on this same issue for some insights.

  9. Jon Carlson says:

    I think you are missing where the innovation lies in the vastly improved safety record of certified aircraft. Yes, the technology was true innovation and should be commended. But the regulation which requires its installation is what saves most people.

    E-AB does not lack the technology as nothing prevents installation of the technology available. It simply lacks the regulation requiring it for *everybody*.

  10. Kelly Patterson says:

    I’m one of the E-AB aircraft owners that have this stall warning on my RV-6A. I wanted to have the ability to see when I was close to the edge of flight and set this ‘Greene’ device in my leading edge, hooked to a bright red light just under the glare shield, straight with my line of sight. No blaring Cessna horn that gets everyone off kilter, just a visual warning that 5 knots slower and I’ll feel the buffet on the wings. Works great and I KNOW when I’m close to the edge.

    Took all of $20 to eBay, one wire and light to hook it up. That should be easy for even ‘cheap’ builders to afford. Would do it again if I build another. Why mess with your life for $20?

  11. Dick Frederick says:

    I believe a device called a “Lift Reserve Indicator” is still available and it provides the kind of information to avoid a stall.

  12. Cary Alburn says:

    The “Life Reserve Indicator” actually is an Angle of Attack indicator. Alpha Systems mechanical AOA is a more sophisticated and more accurate version, and of course, Alpha Systems also makes much more sophisticated electronic versions with LED lights to show the AOA–I happen to like the analog face of the mechanical version, and it’s a lot less expensive. The LRI wasn’t being manufactured for awhile, but in the last year or so, it’s become available again. The manufacturer was hawking it at OSH last summer.

    I agree that the Greene stall vane is an AOA indicator, but it only knows when it’s been tripped, i.e., off/on–it doesn’t show the relationship of the current angle of attack to the angle of attack at the stall. For sure it’s better than no indicator at all, but after flying with my AOA for sometime now, I would not own another airplane without one. Frankly, I thought I was being a bit extravagant to have it installed–it was one of those things that I wondered if I could ever justify it. But at this point, I have no doubt whatsoever.


  13. Josh Johnson says:

    Probably the biggest thing I’ve noticed in some of the experimental/light sport aircraft is lack of predictability in a stall. Some give nice warning in the form of a buffet, others give nothing and just break. Not sure I think it should be the law in experimental, but it sure should be in an LSA. I’ve flown LSA’s that are more wicked in a stall than a Twin Cessna, and we’re putting studnts in them!
    Does the stall behavior result in more e/ab accidents – I would sure think so.

    The guy that says stick neutral and the plane will never stall – insane and dangerous. An improper trim setting (i.e. runawaw autopilot) configuration changes or unusual attitudes can result in a stall with the yoke stabilized. A couple prime instances are go-around in a single Cessna before you work the flaps out, and a Bonanza where the pilot forgets to reset trim after landing. In both cases, you will be pushing big time until the plane is reconfigured.

    • Josh Johnson says:

      I should say the yoke or stick neutralized. Sorry, my ipad finger pecking is not what it should be!

  14. SkyGuy says:

    - When the stall waring system fails in flight.
    - You probaly won’t know it has failed.
    - During decent and on final….keep the wings as close to level as can be…and the NOSE ALWAYS POINTED DOWN.

  15. DEL says:

    The most effective means of stall prevention is a stall-resistant aerodynamic design. There are such designs — my ICP Savannah Classic (similar to Zenith 750) has one. I can slow it down gradually, attempting to maintain altitude, t0 zero IAS (not CAS, of course!) and all it does is gently sinking, without significantly changing nose attitude. There’s a down side to this: it gives me no warning at all that something’s wrong. But, then, that something is not critical. My Savannah is equipped with full-span fixed leading edge (LE) slats, resulting in a stall AoA so large even my big elevator cannot produce. (The zero IAS results from the pitot tube intake located just ahead and below the slat, at mid left halfspan, so that the flow through the slat at very high AoA is nearly perpendicular to the tube.)

    There are several more design tricks that prevent stall or make it hard to accidentally occur. the most famous, though radical one, is the well-designed canard configuration: if the canard surfaces incidence is set so as to make them stall first, the nose would drop and the wing would never get to stall. But there’s a catch: the wing lifting potential cannot then be fully utilized.

    Another trick, for empennage configurations, is to place the horizontal stabilizer in such a vertical position that the rough wing wake at near-stall AoAs would impinge on it. That would reduce the effectiveness of the stabilizer, reduce the negative lift it must produce to hold the nose up, and consequently make the nose drop. A necessary condition: the wing must be designed so as to stall root-first, which is imperative anyway. Root LE spoilers, that spoil only at high AoAs, is one of several ways to ensure this behavior.

    A horizontal stabilizer with a relatively sharp LE is another trick. In straight-and-level flight, a small negative AoA of the stabilizer (relative to the wing downwash) produces the small negative lift required and the sharpness of the LE poses no difficulty. But as the global AoA approaches that of stall, the stabilizer AoA becomes positive, and large enough for the LE sharpness to spoil its effectiveness.

    All these tricks, and others, are good for tame airplanes and sedate pilots. They are out of the question, however, for combat or aerobatic aircraft and for answering the call of the wild, as they would kill performance and/or maneuverability. For these, and for poorly designed non-sporty E-ABs, an electronic warning device coupled to an AoA indicator seems a simple-enough solution.

  16. Bill Tomlinson says:

    Re-introduce spin-training as part of the PPL syllabus – better still, make aerobatics compulsory – and it won’t matter whether you have a stall warning or not.

    Until you’ve stalled off the top of a loop you haven’t really stalled at all.

  17. Steve says:

    AOA systems currently on the market are way better than stall indicators. Properly calibrated,They present (light,sound) indication of reserve lift (or not) in any configuration and at any weight. The best tool for safe slow flight for any aircraft. They should be approved ( not required) for any certified as well as Experimental.

  18. James Butler says:

    This is slightly off topic, but I think the E-AB community is being done a disservice by the way accident statistics are being compared. If your going to do a true comparison, you need to include the test flights for the certified aircraft when they were first being developed. As the statistics are now presented, they are comparing the mature designs of the certified aircraft with the test flights of the E-AB plus the flight hours of the E-AB after the test flight hours are flown off.

    Even with a mature design of an E-AB, there is always the very high probability that the builder made some modification to the design to put the aircraft back into the test flight phase. Therefore, the way the statistics are being presented now will always skew the results in favor of the certified aircraft. This same bias was exhibited when comparing the early flights of the V-22 Osprey with the flight record of the UH-1 Huey that had long since completed its test flight stage. That comparison was typically done by the news media, so it sort of could be understood. However, one would think the NTSB and FAA would know better.


  19. Tony Fletcher says:

    I too am curious about adding an AOA indicator to a Grumman Cheetah. How does the device handle different levels of flap settings? With flaps set, is the indicator’s information valid in landing configuration?

    • James Butler says:

      Tony, on my angle of attack indicator there is a switch that senses when the flaps have been deployed and causes the angle of attack indicator to operate with a different set of parameters. My angle of attack indicator works by measuring the difference in pressure on the top and bottom of the wing. It isn’t the vane type. After installation, you have to fly a routine to calibrate the unit to he aircraft in both the clean and flaps/gear deployed condition.


  20. Andy Schmidt says:

    I am not going to argue against having a stall warning device. They are wonderful safety devices. However, after flying a Champ that had no such device, or, for that matter, instruments beyond the bare minimum, I discovered that one could fly,safely, without an attitude indicator (But I did note that my attitude usually improved after a flight) as well as without a stall warning device. I believe that it is important to have the safety devices, but it is also as important, and perhaps more so, to learn to feel and listen to what the airplane is telling you. And not exceed its limitations. There seems to be too much emphasis placed on technology and not enough on “flying skills”. Perhaps a balance of the two would be appropriate. Besides, if the horn is sounding and you are too low, it may already be too late.

    • Josh Johnson says:

      Andy, what you’re saying is very very true. Too many pilots fly based upon the numbers, and their safety devices – e.g. stall warning horn – and ignore the other cues that the airplane is literally screaming that things are going badly. Some designs don’t offer near the amount of other cues that a Champ does either and are far more wicked in the stall – good to know your airplane.

  21. Thomas Boyle says:

    I loved the stall warning in the C152 and have never encountered a better system. It was (is) a suction-driven reed, requiring no electrical power to operate. It could be tested preflight if you could get manage to suck (gently) on the opening in the wing leading edge. As the AoA increased, the pilot would first hear a “sucking” noise that was quite subtle, then a sort of moan as the reed began to vibrate, then a solid note as an “official” stall warning, ascending into a high-pitched wail as the wing finally transitioned to actual stall. The system provided actual AoA information over a range of angles, did not require the pilot to look at an indicator, and did it all without requiring electrical power. I have often thought this system should have been far more widely adopted.

  22. Mitch Velickovich says:

    Mac. I trully enjoyed your articles when you were employed by that other magazine you used to work for.

    I noticed you have tried several times to bridge into our world of E-AB at EAA with ideas and suggestions from the certified world of aviation.

    Not trying to be ugly, but can you please stop now?

    Ps. Bill Tomlison….spot on!

  23. Thomas Boyle says:

    Mitch, Bill Tomlinson,

    All the spin training in the world won’t help you, with a spin entry at 500 ft. That’s what stall warning / alpha alert is for.

    • Mitch Velickovich says:

      You are incorrect Sir. Mac is not talking about AOA,s at all. The thread might have morphed from the original subject which is Stall Warning Indicators.

      I noticed you mentioned a C152. May I ask your experience in E-AB aircraft? Have you ever built or owned one? Do you have any clue what it takes to pass operational approval on a stall warning system on an E-AB? Since you mentioned spins now as related to stalls indicators, What is your spinning experience?

      If you want one a Stall Warning in your E-AB, right now as the regulations stand, you have the option to design and install one on your own and get it through your certification process if you choose to do so.

      Using Mac,s paragraph….”E-AB is about innovation to try your own designs, ideas……”

      • Thomas Boyle says:

        Hi Mitch,

        I wasn’t, in this case, referring to what Mac said – just responding to Bill Tomlinson’s remark, that spin training should be required for the PPL.

        I presume, in context, that Bill meant this as a way to reduce fatal stall-spin accidents in E-AB. I had to do spin training for my original PPL (in fact, I had to do it pre-solo), and I have enjoyed aerobatics (although not enough to make a habit of it).

        I understand that very few stall-spin fatalities are caused by spinning in from altitude (they happen, but they’re rare). Most stall-spin fatalities are fatal because they happen near the ground, often resulting in impact in less than one turn. Spin training won’t help there: only stall prevention will. That was my point. Do you have a different view?

        Since you ask, I am a sport pilot but I have never owned or built an E-AB. I do have about 40 hours PIC in one. I have owned an E-R&E sailplane, a certificated sailplane, and an experimental ultralight sailplane. My spin experience is limited: I have spun a C-150 (maybe 10 times, including twice by accident, while practicing stalls), a CAP-10, the ultralight sailplane – and the E-AB. Again, I spun the E-AB by accident, with the owner aboard, while practicing stalls for the checkout in it – and when he tried it, he spun too (“snap rolled” would be a better description). It turned out that the airplane needed some re-aligning.

        I am generally familiar with the requirements for certification of an E-AB aircraft. I am not familiar with – nor was I aware there were – specific regulations about certification of an E-AB aircraft with a stall warning device.

        Turning to Mac’s piece, he explicitly said that he is not calling for regulations, but that he would like to see someone come up with a way (or ways) to reduce the number of stall/spin accidents in E-AB. I’m sure we can all agree that would be a good thing, and it sounds like exactly the sort of innovation E-AB was intended to facilitate – no?


        • Mitch Velickovich says:

          For an effective stall warning system to work accordingly it requires design, extensive and proper testing in different flying conditions as it is done in every single certified aircraft. Add the number of different available e-AB kits, plans or homemade designs and as a result, we have an impossible task not to add more accidents in the process from inexperienced builders.

          How about just having a slow speed alerting or warning device? A big bright red index line in yor ASI is the cheapest safety device you can have…Or if someone is to lazy to look at their instruments once in a while the E avionics manufacturers can add a dumb light or an additional aural horn if the aircraft is flown below certain speeds as required in some transport category aircraft now days. . It worked for me well. I placed a BIG RED index line at the 100 mph marking on my Pitts Special….fly below 100 and expect anything…..that was the easiest reminder I ever needed when slowing down.

          • DEL says:

            1. Insufficient airspeed is not the direct cause of stall — AoA is. In principle, stall can occur even at higher that usual speeds, provided one of the following conditions exist: a high transient AoA; unusually high weight; unusually low density altitude; and, of course, during even a moderate-g turn or pull-up.

            2. Unintended low airspeed may catch a pilot by surprise, especially when flying relatively slow, as in the pattern, and being distracted by secondary tasks such as navigation, communication, attending to a passenger, etc. Looking at the ASI “once in a while” is no guarantee against lossing airspeed a second later. But an audible warning is.

  24. mitch velickovich says:

    Thanks Del. Yup! you are correct!

    • DEL says:

      I’ve been correct, but not entirely. I should have written high, instead of low, density altitude. Low density altitude implies the density is high.

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