Why isn't AOA on the panel?
 

Reading through Adrian's post on Vx and Vy speeds got me thinking.

A whole lot of the numbers that we try to remember when flying a new type are much more to do with angle-of-attack than they are to do with airspeed.
Vs, Vx, Vy, Vref are all functions of load, power and load factor though if represented on an AoA instrument they would be (almost) constant.

I spoke to someone who had put a modern AoA gauge on a homebuilt RV-8, and he now flies all his approaches and climbs using AoA, and also uses it for reference during aerobatics.

I guess that a stagnation point type sensor wouldn't compensate for flap position, though that could be easily accommodated.

Would you like one on your panel?

An angle of attack guage would be useful and I wish I had one.
But most pilots are not concerned with AOA as they shoud be, rather they think in terms of airspeed. They would need to learn (or remember) more advanced facts. The fact that a wing stalls at a lower angle of attack with the flaps or aileron down is forgotten by most pilots.

Please correct me if I'm wrong.

I've always thought a wing would stall at the same angle of attack (give or take) regardless of flap.

AoA is the angle between the freestream airflow and the chord line (effectively a straight line between LE and TE). By lowering flaps and maintaing the aircraft pitch you are automatically increasing the AoA. And hence it is at a lower pitch attitude that an aircraft will stall.

If, as has been mentioned, they can compensate for flap settings then yes it could be useful.

An airfoil will always stall at the same AoA. That does not mean any airfoil; different airfoils will stall at different AoA. Adding flaps alters the chord, camber and size of the wing; it may alter the flow over the wing (as in slots energizing the boundary layer etc), and will definitely change the critical AoA.

Reference any decent principles of flight book; it will have graphs of CL plotted for different AOAs in different configurations. Without having access to any at the moment; I believe the general tendency for extending flaps is moving the graph up and to the left; ie increasing CL for any given AOA, but dereasing AOAcrit. Adding slots, either by slats or slottet flaps, will extend the graph up and to the right; ie not provide any significant increase in CL for a given AoA, but increase the AoAcrit. These two used together, as in a flap+slat combo or even a decent fowler flap, will increase CL, and may increase or decrease AoAcrit.

But don't shoot me if I got it wrong either. 11 months ago I had never even touched a yoke; now I'm a CFI, so there might be some gaps here and there ;)

If a pilot flying in a turn about on the edge of stalling and close to the ground gets scared and suddenly applies full aileron control to level the wings, the down going aileron may stall that wing and instantly put him in a spin.
The pilot is trying to level his level wings but the opposite occurs. The aileron control reverses when it is stalled.
An AOA instrument might help, but the angle of attack of the aileron can be changed very abruptly. The pilot should be looking out instead of looking at the panel, so an audio AOA device might be good. I have an audio variometer that works well.

Wings stall at about 15 degrees angle of attack without flaps.
Wings stall at about 12 degrees with the flap down. The aileron is the same as a flap with regard to how it changes the angle at which stall occurs. It would be more difficult to monitor the ailerons rapid movements with a gauge.

Which angle of attack shall we present? The local one sensed by the probe, or a corrected value to fuselage AoA based on flight test with a noseboom, itself subject to corrections. Shall we correct for pitch rate induced alpha, or not?

Then things which you think are "almost constant" often are not. What about variations with altitude, or Mach number? Or cases where scheduled speeds are determined by other criteria - like VMC, for example. Your Vref-alpha chart would be no simpler than the Vref-speed chart.

It's also common to have more redundancy in the airspeed system than in AoA systems - 2 AoA vanes, but 4 airspeed systems, for example.

And to reinforce the previous point - trailing edge flaps (including ailerons) do influence the stall AoA - using a constant fuselage reference for AoA, not accounting for theoretical chordline changes.

In Serial 25 of http://www.pprune.org/forums/showthr...=223467&page=2 , Mr Farley sums it up very well.

Unfortunately, there is no single AoA for a pilot to remember. For a useful display of 'AoA' for a pilot you'd have to provide some kind of normalisation to account for configuration, Mach number, altitude, load factor, all of which CAN affect stall or similar behaviour.

To a reasonable extent thats what the 'low speed cue' and 'green line' on airspeed tape aircraft displays - speed adjusted for AoA and config, which is much the same as AoA adjusted for speed and config - but even these can present misleading info and there's no practical way to make them perfect.

It would be pretty simple for an AOA display to be biased appropriately by flap position, so that the indicated value for Vso remains constant.

I can't help disagreeing with M(F)S just a little bit.

Whilst it's true enough that stalling AoA, and by relation the preferred AoA for approach, manoeuvre entry, etc. is affected by altitude, configuration and so-on, it should be less affected than airspeed since for a start weight and g are removed from the equation. PEC also shouldn't be as big an issue one might hope.

So, it should for most aeroplane types be possible to schedule (for example) a single approach I_alpha per configuration which may lead to significant simplification of operating limitations and advice.

It'll never replace airspeed - Vne and VMC for example are inavoidably functions of EAS, but it might help make life a little simpler and easier.
Maybe.

It would certainly be an interesting thing to look at seriously in regard to an example aeroplane for which both IAS and I_AoA correction and operating data are available.

G

Actually, it wouldn't be (and indeed, isn't!)

The first question is: what stall AoA do you want to display - the one you would achieve if you were to conduct a 'standard' 1 kn/sec slowdown from your current speed, or the one you'd achieve if you were to pull a wind-up-turn at your current speed? Because they won't be the same, and the latter may be noticeably less than the former. If you display the 1'g' stall AoA then you may be giving the pilot a false sense of confidence in manoeuvring flight; if you display the manoevring stall AoA you will find that the displayed AoA moves around as you speed up and slow down (anyone who's watched their "low speed cue" or "green line" has probably seen it moving around on some aircraft, where the input data are Mach-dependent).

There are also issues with the accuracy of AoA sensing at low angles/high speeds, compared to high angles/low speed - as one gets faster the intrinsic accuracy of the airspeed system improves relative to the AoA sensors - for things like stall or shaker speeds the AoA system is generally more accurate than the airspeed system, when the two are compared, but the AoA resolver accuracy becomes more and more significant as the AoA drops. You'd have better luck trying to fly a precise cruise speed than a cruise AoA I expect.

John Farley's remark that automated systems expect AoA as an input is correct, but FBW systems also can have gains and such as a function of AoA; most autopilot systems don't, to my knowledge, use AoA as a control parameter.

Let's not forget that there are many aircraft - high performance jets, notably, that fly their approach using AOA as the primary reference.

If we had our choice I imagine that we would choose to fly solely AOA on approach, as it is the most appropriate measure of both aircraft performance and safety margin. Conventional aircraft are AOA stable, and as such we choose an airspeed that results in a given AOA - important point here...in a given configuration, at a given weight, the AOA is known, constant, and stable for any given airspeed (and vice-versa). The weakness that we have today is in the method of displaying AOA, and the smoothness of the data.

Previous posters are quite correct in that the AOA can vary with different wing configurations at a given airspeed; however, this can be easily compensated for procedurally. If standard procedure is to fly an approach with full flap at 9 degrees AOA on the guage one can easily accomplish this. Adjustments can be made for different configurations, either by aircraft systems or procedures. The important point is that the pilot has an appropriate target to fly to, and that he or she is capable of maintaining the target.

As an aside, when flying the Alpha Jet I would always cross reference my approach AOA to an airspeed once configured. Flying AOA was great until there was turbulence or gusty wind conditions, at which point the airspeed indicator would tend to jump around less than the AOA guage, and was therefore a better instrument to use.

Several autoland systems, to the best of my knowledge, use AOA as a control parameter and adjust descent rate using direct lift control methods. Carrier based systems (present and future) are predicated on AOA, although I suspect I'm diverging from the 'civil' nature of the thread... :>

The military flew indexed ‘AOA’ which accommodated configuration changes – the doughnut chevron system. In later years indexed systems were produced for civil aircraft, mainly in corporate operations. Both of these systems focussed on safety, providing approach speed / speed margin and guidance for keeping out of trouble, stall, windshear etc.

If the full value of AOA is to be realised then more sophisticated calculation is used, but then the dominate problem in civil operations is the acceptability of a display. A standalone advisory system can be certificated on the basis that it is not misleading. Using AOA as primary the primary ‘speed’ reference would present significant problems in certification, but probably the issues of training and standardisation costs to the operators would prohibit this option. A counter example is the use of HUD, where the costs have been offset against the Cat3 capability.

On a point of standardisation, speed or AOA, I was always impressed by the US Navy system of colour coding ASIs. This provided speed bands for flap (gear) speed and the range of approach speeds; each aircraft type was calibrated / marked accordingly. Thus it didn’t matter which type you were current on – flying the approach in the ‘green’ band provided a significant degree of error proofing.

Civil flight tests (1970’s) experimented with AOA for auto thrust control with little success, but the military IIRC, were more successful – A7.

AOA on the panel.
 

Lest we forget??

CONCORDE.

The Rocket had it and the crew used it all the time.

Most flight data recorders log AOA as a parameter and AOA features in a lot of accident investigations. All airliners have a AOA vane as part of the stall warning system and certain FBW aircraft use it's input as part of the control law configuration. Accident reports frequently mention high AOA, but wht don't we get a display on the flight deck?

Cost! My A320 FCOMs show an optional AOA guage on the instrument panel, but to my knowledge no operator has ever fitted one. And until enough reports state that one factor of an accident was an increasing AOA which the pilot wasn't aware of and they become mandatory, they won't be fitted.

The Tucano in RAF service has an AOA guage and an AOA indexer. Unfortunarely, the indexer is not calibrated to the approach speed of the Tucano in RAF service and is not much use. And use of the guage was not taught. But after losing my ASI during an inverted spin one day (a regular problem until a restrictor was fitted in the pitot line) I was forced to use AOA against power as an approach reference. This made me realise the value of the instrument and I used it regularly from that point.

Some airliners have the ability to display a Flight Path Vector on the Primary Flying Display. The difference between your attitude and the FPV is you angle of attack. The A320 SOPs for my airline requires use of the FPV for visual flying and Non Precision Approaches. It's a very useful tool.

Cost might a reason why an operator might not choose an option. But the reason why it's not even offered as an option is FEAR - fear of what it will be used for by pilots, fear by the engineering and design people. And the second part of your post shows why we FEAR installing AOA gauges. You had a system that you know isn't much good and which you were never taught to use, but now you've decided to "use it regularly".

One of the biggest hurdles to implementing ANY new display device - not just AOA - is a fear that it will be misleading, or misused. Even if we don't take cert credit for a display, it has to pass the "causes no hazard" test, and agencies will generally assume that if its there and misleading, people will be misled.

Turn things the other way round for a moment; suppose you had an aircraft which was flown with AOA as primary, and then we decide to add airspeed as a "nice to know" secondary display. Without knowing about PEs and SSECs and reversionary systems and redundancies and all the other things we have to do right now for airspeed displays - would you just fly based on an unapproved display?

For those advocating use of AOA for approach instead of airspeed. If the aircraft weight is correctly known, they amount to the same thing. Consider, though, what happens if the weight is wrong.

Assume the aircraft is 5% heavier than calculated (a pretty gross error, but its just a number...)

For an aircraft flying airspeed, it will be flying at a higher AOA than it should (effectively flying at 1.20 Vsr instead of 1.23Vsr) and it will also have 5% more energy/moment to bring to rest - say 5% more landing roll required (though in fact the brakes will work better, so it'll be less in practice)

If instead the aircraft flies AoA, it'll be at the 'correct' 1.23vsr, and consequently 2.5% faster than the book says for the weight. Therefore it'll now have 10% more enegry/moment to bring to a halt.

Flying AoA protects more in terms of stall speed margins, but puts all the error into the landing distance, unless you recalculate based on the actual speed flown (which would seem a bit of an imposition)

There already is an audio angle of attack indicator and it's in just about every fixed wing airplane. It's the stall warning. If you're approaching the critical angle of attack, it kicks in. Even better, it works no matter your weight, balance, wing loading, airspeed or configuration. Is is as effective or complete as a nifty visual indicator that displays in all regimes of the flight envelope? No, but it does tell you what's most important - when you're running out of angle. 'Member, stalls aren't a function of airspeed, just AoA. Go slow enough and there isn't an AoA that won't stall you, but it's still angle of attack.



I would imagine also that a visual angle of attack indicator would also introduce an unnecessary incentive to keep one's eyes on the panel.



Or I could be talking out of my ass. I haven't slept for thirty hours.

What Traffic Several RN aircraft used ‘indexed’ audio AOA for Carrier approaches.

MFS IIRC there is a clever method of using AOA to display (overlay) AoA related information such as stall margin or approach speed on EFIS airspeed scales.
I believe that the technique required knowledge of CL polars etc, and on some aircraft a trim input as an approximation of cg. As I understand this overcomes the problems of weight; are we discussing similar concepts?
I am confident that this was used on the Avro RJ – it used a Honeywell algorithm, which may have originated from the MD11, as both these aircraft had Honeywell EFIS speed computation. The Avro RJ did not provide any FMS derived aircraft weight input to the system.