AoA Indicators.

[Garfly]

[This continues a discussion started in 'Swan river incident' in the forum "Aircraft Incidents and Accidents".]

 

Designers of Angle of Attack indicators did work out that eyes-down and low-level maneuvering ain't such a great combo. That's why most are heads-up readable - and/or beep at you when they need to.

 

[ATTACH=full]48054[/ATTACH]

 

Actually, I've never actually seen one in use, myself. I'm just passing on what I've read; air safety experts urging their uptake in small planes - now that they're relatively cheap.

 

Like Yenn, I was wondering, though, if you had your AoA sensor on say, the left wing, would it be preferable to turn in that direction in a critical situation (i.e, to better protect the inside wing first).

 

Anyway, they say it's all about maintaining awareness, keeping clear of the critical angle and thus of stall/spin scenarios. Or, put another way, if max performance is needed, to be able to fly the wing to its optimum - and not beyond. (Presumably a few thousand hours of ag flying would render one redundant.)

 

Some advocate them as an aid to landing as well as maneuvering flight.

 

If I ever got one, I'd regard it as a learning tool as much as a safety feature.

 

There are several kinds of sensors and displays available: some sense differential pitot inputs, some are based on software algorithms crunched in PFDs and some use a vane arrangement - similar to what airliners have.

 

One of the cheapest is the new one from Belite in the US. The sensor and display for only USD600.

 

Angle of Attack - Belite Aircraft

 

I've seen on YouTube that they even suggest a way to get a heads-up display.

 

For those with pusher props it can mount it on the forward fuselage but otherwise the vane has to stick out from the leading edge or lift strut. A big drawback might be the dodgy aesthetics (esp. on the ground). But then, if your aircraft is pretty agricultural looking to start with. ;-)

 

Related stuff:

 

FAA video:

 

 

 

From the FAA's "Pilot's Handbook of Aeronautical Knowledge" 2016

 

Angle of Attack Indicators

 

The purpose of an AOA indicator is to give the pilot better situational awareness pertaining to the aerodynamic health of the airfoil.

 

This can also be referred to as stall margin awareness. More simply explained, it is the margin that exists between the current AOA that the airfoil is operating at, and the AOA at which the airfoil will stall (critical AOA).

 

Speed by itself is not a reliable parameter to avoid a stall. An airplane can stall at any speed. Angle of attack is a better parameter to use to avoid a stall. For a given configuration, the airplane always stalls at the same AOA, referred to as critical AOA. This critical AOA does not change with: Weight, 
Bank Angle, Temperature, Density Altitude, Center of Gravity

 

An
 AOA indicator can have several benefits when installed in a General Aviation aircraft, not the least of which is increased situational awareness. Without an AOA indicator, the AOA is “invisible” to pilots. These devices measure several parameters simultaneously and determine the current AOA providing a visual image to the pilot of the current AOA along with representations of the proximity to the critical AOA. These devices can give a visual representation of the energy management state of the airplane. The energy state of an airplane is the balance between airspeed, altitude, drag, and thrust and represents how efficiently the airfoil is operating.

 

Flying AOA - Aviation Safety Article.pdf

 

Flying AOA - Aviation Safety Article.pdf

 

Flying AOA - Aviation Safety Article.pdf

[Garfly]

For anyone else interested, I've since turned up some detailed naysaying/yeasaying on the subject here:

 

Update 2014 : Your Recommendations for AOA Indicators - Backcountry Pilot

 

and here:

 

http://airfactsjournal.com/2013/08/angle-of-attack-isnt-a-miracle-cure.

 

I'm a bit persuaded by this comment on the AirFacts article, in that it sees AoA as teacher as much as guardian.

 

"An AOA indicator that provides constant indication of the actual angle of attack of the wing under the actual conditions of flight, placed in the pilot’s field of view (adjacent to the six-pack, or better yet, on a HUD) would over time help pilots develop a much higher level of AWARENESS of how well the wing is flying. Awareness of AOA won’t make airplanes idiot-proof, but over time good awareness provides an opportunity for pilots to learn to develop the proper stick and rudder skills … because of the immediate feedback that the AOA indicator provides that cannot be seen from merely tracking airspeed."

 

 

[dsam]

Here is a screenshot of my Dynon Skyview display during some slow speed performance testing I did. I've labeled the AoA indicator and an "Airspeed Low" alert that also generates a verbal (audible) warning in my headset. Under these conditions (9900 density altitude, no flaps, TAS 69 knots, IAS 60 knots) I was cruising rather close to a stall, and control inputs were naturally a bit mushy. I don't have a photo of the AoA during a full stall (understandably, I'm a bit busy to take photos), but rest assured the AoA indicator loses all the green and yellow range, and red remains. Audible alerts (voice and beeps) continue until stall recovery. At a TAS of 110 knots in the cruise, all green bars are displayed, as one would expect.

 

Despite all this technology, In practice, I rarely give the AoA much visual attention, because with my aircraft, the stick & rudder feedback gives me ample "feel" when I'm getting slow, well before I get the audible warnings. Other pilots unfamiliar with the Eurofox might not be as perceptive, I suppose, and be more reliant on such electronic warnings.

 

 

 

[Yenn]

Nice looking Dynon display, but I notice your wings are level. It would be nice to see in a steep turn, first one way and then the other. Am I correct in assuming that the A of A sensor is in the pitot head. If I am then it would only be correct for the area where the pitot is installed, so the other wing could be unstalled or deeply stalled with no indication.

 

 

[djpacro]

[Garfly]

Yenn, I don't know where the Dynon gets its info from but the Aspen AoA indication is done entirely artificially - that is in software. It's nothing more than a software upgrade for their PFD; no sensors at all. The attached slide from an Aspen presentation explains the data inputs that they feed into their algorithm.

 

Paul Bertorelli of AVweb did a review (and video) of the Aspen device here:

 

What AoA Indicators Don't Do That They Should - AVweb Insider Article

 

He's far from being a fan of safety-through-gadgetry but he goes on to say:

 

"That's not to suggest that I think AoA indicators are a gimmick or not worth the investment. They're not especially expensive and they have the merit of reprogramming pilot understanding of stalls as an airspeed thing to an angle-of-attack thing, where it should have been all along.

 

 

 

And that's why I think the marketing of these devices is off base. They're being sold as stall awareness devices when in fact, they're really performance-measuring instruments that happen to include stall warning and awareness capability.

 

 

 

.... So we set up some slow flight and an approach flying on the AoA indications ... I found that moving the needles well up into the yellow band still gives—at least for me—comfortable stall margin but also four to five knots slower speed, making the touchdown less floaty and a lot shorter. Approaches flown too fast—very often way too fast because so many pilots are terrified of stalls—are a common scenario leading to excursions and overruns. And even if they don't, they use more runway than necessary and chew up tires and brakes. Further, the AoA flags are nicely damped and thus don't jump around nervously like an airspeed digital display or even an analog needle tends to do. It's thus a little easier to fly. It could also be used to index best angle or best rate for best performance departures.

 

 

 

Ultimately, I'd like see AoA systems evolve in just the way I've described here—as speed indexers used on every approach to get the most out of aircraft performance and to wrap the sticker puller's head permanently around the concept of AoA. Otherwise, installing an AoA indicator is a little like having a live-in Cordon Bleu chef fix you spam sandwiches for lunch."

 

 

 

[facthunter]

If you have a pusher or glider a tuft of wool will do it. for you.

 

The airspeed indicator certainly isn't the full picture and on close final approach all you can afford is a quick glance if the conditions are demanding.

 

I've done most of my flying hours with a SCAT system. Speed Command Attitude system which is A o A corrected for configuration. It's a great feature to check at a glance that your speed is adequate, but I would spend 90% of the time referring only to instruments and it's prominently displayed in the artificial horizon which is your prime reference of about 5 instruments you scan.

 

For Visual attitude flying the equivalent would be a head up display (HUD) on the windscreen That would be the optimum IF it was available and IF it's that critical to have a reserve lift indicator or whatever you wish to call it.

 

Mushy controls are a function of airspeed and stalls are not. You can stall at quite a high airspeed when the plane is dynamically loaded. Ie you are pulling "G" .You will stall when your wing AoA reaches a critical angle That's it.

 

OK what does the pilot do to make that happen? {It's the pilot's fault unless there's something wrong with the aircraft, (like tail heavy)} The PILOT increases the AoA by pulling the stick back. SO... we get the concept of stall stick position. A position where it won't stall UNTIL the stick gets that far back PROVIDED you don't change anything else about the aeroplane. Like lower flap, move the Cof G or trim the moving horizontal stabiliser, (which is the common way of trimming jet airliners).

 

You should be aware whenever the stick is well back that you are pushing your luck with proximity to the stall. An aural warning might alert you but do you want it to keep blaring while you try to get out of the spin you are in?

 

Cancellable Maybe? I must admit I've flown on it "beep" in Cessna's when in a critical position, once or twice, but you didn't get that from me. Nev

 

 

[Garfly]

Thanks DJP, that's exactly the information I've not found anywhere else.

 

So, since you'd only have it on one wing (and at one position) I suppose the key is in the calibration - none of them give useful info without being set up for the particular aircraft - and in understanding what the indication is - and isn't - telling you.

 

Interestingly the little Belite sensor is nothing but a piece of plastic swinging in the breeze ahead of the leading edge. But then, I understand that's more or less how an airliner AoA vane - a trigger for the stick-shaker - works.

 

 

[Oscar]

I would absolutely agree with everybody who says that you should NOT treat an AoA indicator as a 'silver bullet' - and I'd go further, to suggest that IF you install one, you should take the time / ask respected knowledgeable people, as to what the specific characteristics of the instrument you have installed are likely to be.

 

I believe that Yenn has a valid point: a single measurement at a specific location can tell you no more than the performance at that location.

 

FWIW: we bought two AoA kits ( in case we stuffed up the making of one), from: AOA_kit Following our enquiry re an audio take-off capability, Barker not only added that but sent us piezo horns!. Really good service!.

 

The instrument is small - very easily mounted on the top of the panel, in eye-line, without blocking forward vision. A pair of them - one driven from each wing strut location - would take about 40mm high x 20mm wide display area. Either could drive the one piezo horn. You wouldn't need to LOOK at them to be warned that A wing was on the point of stalling: if the horn is blaring, you're getting into the danger zone.

 

For a total cost of less than $200.

 

Mind you - even that is HUGE, compared to the solid-state, mechanical, HUD turn coordinator that gliders have used since time immemorial: 125mm or so of wool twist, taped to the centre of the canopy dead in front of your eyes.

 

 

[SDQDI]

Maybe an audible alarm for being out of balance would be more prudent than an Aoa alarm? That would do more for stopping a huge out of balance stall spin than an aoa alarm. Why hasn't that been developed?

 

 

[facthunter]

Be annoying when sideslipping or kicking straight. Have a warning horn when you think about going flying in a U/L?. Nev

 

 

[SDQDI]

Well how different would that be to planes which currently have a stall warning alarm? They are annoying just before touchdown too (some buzz the whole way down final approach) also in rough air.

 

With a balance alarm of course it would be annoying in a side slip but you generally only sideslip on finals so wouldn't be any different to the stall alarm but would definitely be a benefit in normal manoeuvres. (Maybe it would actually help more of us to be more aware of how feet lazy we get in the air?

 

 

[Garfly]

Even if we resist the AoA propaganda in this ad., is it possible to resist lusting after one of these:

 

 

 

[Nobody]

Despite all this technology, In practice, I rarely give the AoA much visual attention, because with my aircraft, the stick & rudder feedback gives me ample "feel" when I'm getting slow, well before I get the audible warnings.

dsam,

 

I agree with you that the aircraft will give you feedback that it is slow when straight and level but when banked the stall occurs at a much higher airspeed. The "feel" that you get through the controls is really the force required to deflect the control stick. This means that when banked the controls have the "feel" of a much higher airspeed rather than the light feel of being slow. Some gliders demonstrate this principle very well. If stalled straight and level the ailerons almost go limp in your hand, banked over at 60 degrees they still have plenty of "feel" as it enters a spin. This principle is one that I think gets a lot of pilots into trouble....

 

 

[dsam]

Nice looking Dynon display, but I notice your wings are level. It would be nice to see in a steep turn, first one way and then the other. Am I correct in assuming that the A of A sensor is in the pitot head. If I am then it would only be correct for the area where the pitot is installed, so the other wing could be unstalled or deeply stalled with no indication.

Hi Yenn, yes, I like the Dynon very much.... one can never have too much visual/audible situational awareness IMHO.

My AoA info is via differential pressure measurement on a purpose-designed Dynon AoA pitot head, mounted to the right wing. AoA is calculated via the Dynon electronics (not sure of algorithm). AoA calibration is done in-flight via a special set-up routine (this was done by my LAME).

 

Yes, Yenn, wings were level, as I was measuring/calibrating other parameters for my performance testing (fuel flow & TAS vs. IAS with different Density Altitudes). I wasn't trying to check AoA performance, so I've got no photos at different flight attitudes, just my recollection of audible alerts & visual displays during stall practice (and landings).

 

Dave

 

 

[KRviator]

Even if we resist the AoA propaganda in this ad., is it possible to resist lusting after one of these:

It certainly is when you see the price, see the purchase agreement and see the performance (or lack thereof...) and see the payload - or lack thereof...Put two average Aussie blokes in it, and you don't have enough fuel for VFR reserves!

[dsam]

dsam,I agree with you that the aircraft will give you feedback that it is slow when straight and level but when banked the stall occurs at a much higher airspeed. The "feel" that you get through the controls is really the force required to deflect the control stick. This means that when banked the controls have the "feel" of a much higher airspeed rather than the light feel of being slow. Some gliders demonstrate this principle very well. If stalled straight and level the ailerons almost go limp in your hand, banked over at 60 degrees they still have plenty of "feel" as it enters a spin. This principle is one that I think gets a lot of pilots into trouble....

Qualified agreement, Nobody. I suppose the "feel" I mentioned should have included the "seat of the pants" G forces, and/or cross-control attitudes. A 60 degree bank with no loss of altitude results in higher G forces and needs a fair bit of back-stick pressure - easily noticed without AoA display/alerts IMHO.

I might also say when I'm landing, I give considerable attention to coordinated - shallow banked & descending turns, hitting accurate airspeed targets. My AoA never seems to talk to me - which is a good thing, I reckon!

 

 

[Garfly]

Thanks KR, I feel a lot better for knowing how pointless desiring an Icon really is.

 

I just hope my Skyranger will have me back now. ;-)

 

By the way, dsam (and others) do you find the wind read-out on your Dynon accurate and useful?

 

 

[dsam]

Thanks KR, I feel a lot better for knowing how pointless desiring an Icon really is.I just hope my Skyranger will have me back now. ;-)

 

By the way, dsam (and others) do you find the wind read-out on your Dynon accurate and useful?

I find Dynon's wind direction & speed indications/calculations are quite helpful. Based on these winds, in real time, Dynon updates my "fuel-range" info on the display, as well as a wind-offset "glide circle" for engine failure. As you would expect, I can glide downwind further than upwind (and it allows for elevated terrain too!)

 

Typically on a cross-country flight, I get the GRIB winds via OzRunways just prior to take off. Then on departure during climb-out, I make a note of actual wind (via Dynon) at each altitude interval. By the time I get to my intended altitude, I have my own "local" winds-aloft profile, and sometimes descend or climb again to a more favorable altitude-wind combination, where terrain and VMC permit.

 

Gotta love technology for situational awareness, safety & efficiency!!

 

 

[rgmwa]

By the way, dsam (and others) do you find the wind read-out on your Dynon accurate and useful?

Yes, very useful. The amount of information that these modern EFIS screens can display is remarkable.

 

rgmwa

 

 

[Kyle Communications]

My MGL Xtreme has the "winds aloft" as well and I use it all the time for flying at the best level. 1000 ft can make a difference in GS of 20 kts at times. Always wanted to fiddle with AOA...I like that kit thing Oscar posted..its very interesting

 

 

[Garfly]

Nev, the red thread ain't dead.

 

 

Finally, AoA for the masses!

 

Or, in the words of the good professor as he concludes his video (below):

 

"Of course, the best way for everybody to acquire a proper appreciation of the approach to the stall

 

and coping with unusual positions, is for all trainees to have a few hours in a Pitts Special. But given

 

that this is probably not practical, the routine use of a leading edge angle-of-attack indicator will go

 

some way towards inculcating an intuitive grasp of the relationship between the elevator position and

 

the angle-of-attack in the approach to the stall. And I would also suggest that a bit of bent wire attached

 

to the leading edge of the wing with a length of red knitting wool tied to it, is the paradigm against which

 

all other angle-of-attack indicators should be judged."

 

(He even has a dime-store heads-up system sorted. LOL)

 

 

 

[Yenn]

I have got a little ball running in a tube on the panel, that and active feet seems to keep me out of trouble.

 

Stalling out of the turn on to final seems to be the place where an AofA meter is wanted and I think my set up is suitable.

 

It does seem from all these and other posts that more attention to the use of the rudder isa needed.

 

 

[Kyle Communications]

Thanks Garfly this has been a really interesting thread. Keep finding out more and post it here. Me thinks I am gunna get me something or make something to do with this

 

 

[Garfly]

Yes, Yenn, I'd have to agree. We've come this far with the ball and ASI to guide us and if we just fly balanced and avoid distractions at crucial moments, we should all be okay.

 

And yet I'm spooked by the thought that most of those pilots who've fallen to stall/spin would've agreed too.

 

In this age of wall-to-wall video evidence, we've heaps of tragedies to pore over grimly. In each of them we see the fatal situation seemingly come out of nowhere, a moments misjudgement and down she goes.

 

Not having been born birds our instinct - when startled - to yank and bank seems to prove fatal every time.

 

None of that's to say that AOA would've saved the day.

 

But maybe AoA awareness is not so much about stall avoidance - making us ever more timid at the controls. It's maybe more like the opposite. I, for one, tend to be taken-aback when instructors I'm flying with take control. I'm impressed by how much more positively they manoeuvre the aircraft around the sky; almost aggressively by comparison. To an outsider it might seem that their flying is actually more 'dangerous' than my sedate (not to say, nervy ;-) style of toodling along. But, of course, we all know otherwise. And this is where I, for one, feel most in need of edjucatin'.

 

It's often suggested in base-to-final accident reports that the pilot seemed to have reached his or her comfort limit of bank angle but then, in dread fear of the stall (perhaps unconsciously) starts helping the nose around with 'pro-turn' rudder.

 

Timidity, it seems, can kill at least as much as heedless disregard.

 

If I ever installed an AOA indicator (even a red-thread gizmo) I'd want to find an instructor (maybe one who's not totally gadget-phobic ;-) and go do lots of different manoeuvering practice, including up around the wing's limits (finally readily observable).

 

I'm sure that this would not tend to make me bold beyond my means, only just bold - and savvy - enough to save the day in a scrape. And a better pilot, all round, including in the landing phase (as Bertorelli argues in his Aspen video). Sure, it'd be great to do a proper upset-recovery course, too, but I'm talking here about getting a feel for what my own wings really can - and can't - do.

 

I noticed (or did I? ;-) one of our venerable pros here recalling that at times in his career, in tight spots, he's needed to fly right at the edge of the stall horn's alarming song. That put me in mind of a YouTube video I'd seen. It seems to me that this pilot saved the lives of all on board by instinctively knowing - feeling - precisely how much he could ask of his wing in extremis. The debate raged, and raved on - back and forth - among the YouTube crowd.

 

I wonder what our own brains trust makes of it.