Why isn't AOA on the panel?

[Mad (Flt) Scientist]

Originally Posted by safetypee

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.

It's the use of normalised AoA; we use it too. It depends on having a very good understanding of your lift-curve slope, as you note. It (invariably) assumes this to be linear (which it usually ISNT) which means its an approximation. Because the prime use of normalised AoA on the airspeed scale is to get the "low speed cue" aligned with shaker activation, it's "tuned" for that case; it can be CONSIDERABLY in error under other conditions.

(What you do is define an AoA for "zero lift" and an AoA for "CLmax" for each configuration, then you assume the relationship between CL and AoA is linear, and that the AoA for CLmax corresponds to stall speed.
So, "normalised" AoA is then defined as:

AoA(n) = [ AoA - AoA (zero lift) ] divided by [ AoA (CLmax) - AoA (zero lift) ]

With the assumption of linear CL variation, you can show that the square root of normalised AoA is proportional to the inverse of your stall speed ratio, that is, for
AoA (n) = 1, V/Vs = 1/sqrt(1) = 1.0 (naturally)
AoA (n) = 0.5, V/Vs = 1/sqrt(0.5)= 1.41 Vs
and so on.
If you know your shaker stall speed ratio (and hence AoA(n) for shaker) you can use this to display shaker info.

To put it onto an airspeed tape involves a bit more messing around, to account for current conditions, but not much.)

Normalised AoA would work exactly the same as flying to a dimensional AoA in terms of weight - you end up at the "right V/Vs" but the "wrong Vref" compared to what you work out from the 'book'. As I suggested, that's good for stall speed margin, bad for landing distance margin.

[BizJetJock]

Every Citation flying has AoA displayed, both as a guage and a reference on the ADI or PFD depending on age. Similarly the L1011 has it, and upon selection of landing flap the speed selector on the MCP is hashed out and the AP/FD targets AoA whether you like it or not (If I am recalling correctly - it was 25 years ago!). And as has been pointed out Concorde had it.
Therefore all this discussion of why it can't be done is some 35 years behind the times - it can be done, but as usual cost comes before mere usefulness until there is an accident.

[Mad (Flt) Scientist]

Is it primary, secondary or advisory on the Citations? (sounds like it's primary for a phase of flight for L1011).

[BizJetJock]

The manual says it's secondary, however on the older ones the fast/slow pointer on the left of the ADI is purely referenced to 0.6AOA which is Vref in all configs as the system is compensated for flap position etc. In the newer ones the same info is displayed on the speed tape on the PFD as a white circle.

[CL300]

Originally Posted by Mad (Flt) Scientist

Is it primary, secondary or advisory on the Citations? (sounds like it's primary for a phase of flight for L1011).

dunno about the new citation but on the old one the "AOA" was calibrated for clean and full flap. On full flap and gear down you would have the Red / Green / Amber Light repeaters on the glareshield. On clean it shows the best L/D at .55

On the Falcons AOA are used to trigger the stall protection ie extending slats at 17° and retracting the inboards at 19° for example (depending on model of course)

Safe Flight manufacturer of those kits got a lot of info handy if you ask them.

[John Farley]

Mad (Flt) Scientist

I always look to your posts to sort out most threads and have never disagreed with your many positions…...until here.

While I do not disagree with your technical comments on detailed aspects of AOA I do feel you are rather missing the main point behind the debate.

Surely it is about whether it is easier to glance at an AOA indication and at once have a reasonable feel for your margin from the stall or whether you would do better to direct the glance towards the ASI?

Looked at that way it is no contest as far as I am concerned as the ASI reading is no help without in some way taking into account the current weight, bank angle and g loading – the effects of which on the ASI indication at the stall as we all know can be large.

By way of light relief I was one day flying the final approach in a CN-235 twin engine transport on behalf of the Captain who was busy trying to calculate the speed we should use. He eventually announced a speed and I suggested he should check his sums. This he did and after quite a while came up with a new number 11 kts faster. He was extremely apologetic (we were less than two miles out by now) looked at the ASI and saw I was already flying at the higher speed. ‘How did you know the correct speed John?’ I was not qualified on type but the aircraft had a really excellent AOA gauge as standard fit which of course I was using. To my undying shame in response to his question I merely gave the wheel a twitch and said ‘Don’t worry X -after a while one develops a feel for such things’.

JF

[Mad (Flt) Scientist]

Originally Posted by John Farley

Surely it is about whether it is easier to glance at an AOA indication and at once have a reasonable feel for your margin from the stall or whether you would do better to direct the glance towards the ASI?
Looked at that way it is no contest as far as I am concerned as the ASI reading is no help without in some way taking into account the current weight, bank angle and g loading – the effects of which on the ASI indication at the stall as we all know can be large.

...and if the pilots are briefed properly on where the real limits are, and if the gauge is reliable, that's fine. We routinely flight test with a boom (as does most everyone) and the AoA is displayed prominently in the cockpit, and limits are briefed for various tests. I'm sure our TPs wouldn't fly without it.

BUT, for routine line use, unless you generate the same level of briefing knowledge in line crews, then it's not so useful. The fear - to use that word again - among the design/engineering community is that an AoA gauge will either mislead or be abused or both.

"raw" AoA would be difficult to justify in a line operation - stall angles are too dependent on config and on speed to expect anyone to remember the values. So some form of indexing/normalisation is required. But even that can't be made "right" and sometimes is downright misleading. For example....

Suppose the stall AoA is 10 deg at M0.6, and the zero lift AoA is 0, to make life simple. A plane dawdling along at 8 deg AoA at M0.6 will displane a normalised AoA of 0.80, and would appear to show a stall speed of sqrt(AoA-n)*0.60=M0.54. But as you slow down, let's say by M0.55 the stall AoA has increased to 11 deg; the plane will be somewhere near 9.5AoA at this slower speed, but AoA-n will now be 0.86, and the stall speed will now appear to be sqrt(0.86)*0.55=M0.51.

This Mach-dependency effect is real and confusing, especially to crews who've never seen it before. So predicting what our hypothetical crew might do with this imperfect information is hard. Similar examples can be shown for things like contamination/anti-ice failures, OEI operations, sideslip conditions. There's a lot of compensation going on the stall computers to address all these kinds of things, but since AoA itself isn't being displayed it's possible to be conservative in applying corrections (one example might be deliberately mis-compensating for sideslip to provide more stall protection in sideslip cases); if it were displayed we'd have to make it "right", whatever that means.

[Itswindyout]

Citation had AoA indicator

Many years ago I flew a Citation 500 / 501, and this has an AoA gauge on panel, this was coupled to flap setting.
Part of the test flight was to calibrate this gismo.

Later in life: the Gulfstream 550, has an eyebrow indicator that shows the stall barrier on the attitude indicator, this I hope is also config adjusted.

Windy

[Dan Winterland]

It appears that Boeing offer it as an option on 737NG, 767-400 and 777 aircraft. A document explaining the philosophy and the system is at www.wingfiles.com/files/instruments/aoa.pdf

[alexban]

The NG has it as a option and some companies use it .Pegasus was one of them,on the 738 (last I know of).It appears on the EADI instead of RA indication(upper right side) ,which is displayed then on the lower part of the EADI .
It's a nice tool,but you have to learn to use it,and most of the experienced pilots will find it difficult to accomodate with.
The same with the NG FPV. When we asked about it's use,we were told to forget about it,just a new gadget.Anyway,without a cage... good in wxr radar use,though.

[FE Hoppy]

The new Embraer Jets have a Pitch Limit Indicator. Its a mustach type symbol that is displayed with reference to the aircraft pitch symbol and uses the adi pitch scale to show margin in degrees to the AOA limit. Its colour matches that of the Low speed awareness tape and when you reach the AOA limit it would be in line with the pitch symbol and the stick shaker will fire. This will also trigger AOA limiting in the fly by wire system to prevent any increase in AOA. The nornal flight referance symbol is a Flight Path Angle used in all phases of flight except take off. so AOA can be seen as the diffrence between FPA and Pitch.

Honeywell EFIS.

[Mad (Flt) Scientist]

There's a big conceptual difference between providing an indication which is driven from AoA (in whole or part) such as 'eyebrows' or a 'low speed cue', and actually providing an AoA indication itself.

With the former options, it can be pretty accurately determined how these will be used, and they can be presented - or not presented! - in the appropriate circumstances, and even manipulated in some circumstances. (On one of our types the 'eyebrow' feature appears during windshear alerts, but not otherwise, IIRC)

A 'pure' AoA display is far more prone to abuse, and makes the designers far more nervous about its presentation, reliability, veracity. And raises more certification issues, too.

[stilton]

I was a passenger on the jumpseat of a Delta 767-400 a few years ago and noticed that an AOA indication is provided on the PFD.

We operate the same aircraft but do not have this option.

It would appear quite a handy piece of kit though.

[Ignition Override]

For the (US) FAA certification, were cost and maintenance complexity the original reasons for the avoidance of designing an accurate indicator in the first generation and later jets etc? Possibly the older generation of pilots from WW2 and Korea preferred what they were familiar with? The T-38 had a fairly good system and could not have been very complex. Of course you could feel a high angle of attack in the short, stubby wings when "rocks" then feel like "bigger rocks".

Our 60's generation jet has them on the right side of the ADI, but there are no accurate markings and no training or limitations for them. A good system would probably have saved hundreds of lives in the US, i.e. New Orleans, DFW and at other places.

[formulaben]

Originally Posted by BizJetJock

Every Citation flying has AoA displayed...

Sadly, the one Citation that arguably needed it the most, the Citation X, (C750) offered the panel mount as an option. The airspeed indicator has a "low speed awareness tape" which is, in effect, an AOA but only displays AOA information at low speeds. It was quite disappointing to be up high trying to determine max endurance or estimate climb performance and having to either get the book out or make an educated guess. A simple AOA guage would have been quite useful. IMHO, all aircraft should have full-time AOA info available to the pilot for certification.

[Mad (Flt) Scientist]

Originally Posted by formulaben

It was quite disappointing to be up high trying to determine max endurance or estimate climb performance and having to either get the book out or make an educated guess. A simple AOA guage would have been quite useful. IMHO, all aircraft should have full-time AOA info available to the pilot for certification.

Perhaps Cessna are denying you the gauge by default because they WANT you to use the data from the manual, perhaps because it isn't just a simple function of AoA.....

[deagles]

Originally Posted by Mad (Flt) Scientist

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)

We used ADD (Airstream Direction Detector) to land Sea Vixen and Buccaneer aboard ships. ADD removed the need to adjust approach airspeed to cater for different landing weights. It was always sensible to check that the target approach ADD against the ASI/AUW was as expected. Both Vixen and Bucc had audio ADD which allowed the pilot to concentrate visually on the centre-line and mirror info, which were as vitally important as the speed. The audio signal was: short high interrupted tone for too fast, (peep peep peep) long, low, interrupted tone for too slow (burp, burp, burp) and a steady note for on target ADD. I only once met an ADD system that was sticky and therefore gave an innacurate reading,--and that caused loss of the aircraft.
So I remember being unimpressed later at Warton, with the recommended take off procedure for a heavy Jaguar, which was to rotate to, and climb out at, a high ADD reading, for guaranteed best performance. I remained uneasy about putting reliance on a simplex instrument that couldn't be properly checked on the ground before each flight. (You could twiddle the probe and see the gauge move, but you couldn't assess stiction).
In the military, returning to land with underwing stores, (in some cases with hangups which the pilot didn't know were there), use of ADD guaranteed stall margin on the approach.
When later, watching daughter flying 767s for BA, I was surprised that they didn't use AOA for the approach, with such a huge variation in possible landing weights. But she assured me that AOA was indeed measured and was contributing to the safety systems, such as stall warning.

[formulaben]

Originally Posted by Mad (Flt) Scientist

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)

I guess it depends on where your priorities lie. Given worst-case scenarios, would you rather stall on final, or overrun the runway? I'm pretty certain of my answer.

Originally Posted by Mad (Flt) Scientist

Perhaps Cessna are denying you the gauge by default because they WANT you to use the data from the manual, perhaps because it isn't just a simple function of AoA.....

Perhaps. But perhaps not. But pray tell, if it's not a simple function of AoA, then just what exactly else do you suppose is figured into a climb calculation when you're already at max power?! Last time I checked, there's only 2 things can determine climb performance: pitch and power.

[Double Zero]

Alpha display

Given that, how about a reliably informed & programmed ( ie. by Test Pilot / Flt. Test gen', not avionics types with computers in bedrooms ) - a decent Stores - ( weight / fuel Mangagement System, with civilian equivalent ?

Along with suitable decent sensors.

Seems to a pleb' like me a much more user - friendly way to do things, after previous users & systems required a lot of heads-down deciphering...

DZ

[Mad (Flt) Scientist]

Originally Posted by formulaben

I guess it depends on where your priorities lie. Given worst-case scenarios, would you rather stall on final, or overrun the runway? I'm pretty certain of my answer.

The problem being, that the existing certification margins for both apparoach speed and landing distance are designed, however empirically, around the CURRENT design assumptions i.e. that the consequences of a weight error are equally distributed between risk of stall and risk of overrun. (If anything, service experience would seem to indicate that risk of overrun is actually far greater, but it is as you note generally (but not always) more survivable).

Changing to a system where all the risk is loaded onto the overrun side of the equation would represent a net REDUCTION in safety unless the conservatism in the current landing distance calculations were increased to account for the increased risk.

Also, give the 'worst case scenario' of being grossly overweight, there are warning systems or characteristics which will alert the crew to a too-slow approach; there's nothing to warn you that your calculated landing distance is too short (that I know of) except indirect clues. So to stall on approach requires the weight error and failure to respond to warnings. With an AoA approach, what's going to protect against overrun?

Is it not significant that many of the military types which use AoA for approach are CARRIER aircraft - a case where landing distance considerations DONT have much significance.

Perhaps. But perhaps not. But pray tell, if it's not a simple function of AoA, then just what exactly else do you suppose is figured into a climb calculation when you're already at max power?! Last time I checked, there's only 2 things can determine climb performance: pitch and power.

Scheduled speeds can also be affected by, amongst others, VMC (a, l or g), VMU or variations in stall AoA with conditions; none of which will be captured by a simple AoA target.