Why is AOA information rarely used as a climb control parameter?

[Trim Stab]

Most commercial aircraft that I fly aim to achieve Vymax after the final take-off segment.

Why do manufacturers generally use IAS as their control parameter for the climb? Given that most aircraft have an accurate AOA indicator and powerful ADC, why don't manufacturers certify their aircraft to climb on AOA?

[Pilot DAR]

It's just a guess on my part, but I would expect that the reason lies in the certificaton requirement being expressed in "speed" rather than AoA - but, I'm not the expert at this.

Some pertinent wording from FAR 25.119, as an example:

(a) In non-icing conditions, with a climb speed of VREF determined in accordance with Sec. 25.125(b)(2)(i); and
(b) In icing conditions with the landing ice accretion defined in appendix C, and with a climb speed of VREF determined in accordance with Sec. 25.125(b)(2)(ii).]

[glum]

Speed and angle of attack aren't really linked are they? AOA and pitch are, but you generally see the FD bars giving you a figure like 1.25 stall angle.

The more speed you gain the less likely you are to stall, and coupled with the climb angle being held safely above stall the quicker you'll reach a safe altitude.

[Genghis the Engineer]

Negative Glum. AoA is primary, it's related to speed through weight and normal acceleration, and to incidence through flightpath and movement of the local air mass.

There is AoA instrumentation of some form on many aeroplanes, but the knowledge of how to use it in primary flight guidance is far less mature than for airspeed.

So, IAS is used, probably because it's best understood, not necessarily best.

G

[Trim Stab]

There is AoA instrumentation of some form on many aeroplanes, but the knowledge of how to use it in primary flight guidance is far less mature than for airspeed.


Why has aircraft evolution not gone down this route? AoA is a more precise and efficient control parameter than the crude step changes in IAS/Mach which are typically used as control parameters in different flight regimes, especially during the climb.

I can forgive a certain amount of technology momentum gained from historic use of IAS as a primary flight guidance parameter, but we have had many years of accurate AoA sensors and ADCs.

[glum]

Is it perhaps that speed is used for ATC monitoring - they can tell how fast you're going, but it's virtually impossible for them to calculate your AOA.

Managing your AOA is up to you as a pilot, but they set speed limits for climbs?

[Trim Stab]

Glum, negative again...

[Genghis the Engineer]

Most of the time, if ATC care about anything, it's either groundspeed, or far more likely time to waypoint. In the air, you will get groundspeed from electronic instruments most of the time.

AoA has just never been properly explored for civil use. If it is, and we have certification and calibration strategies, with matching manuals and training material, it might get more use. Until then, I doubt it very much.

Most significant users of AoA are military - for ACM or carrier approaches.

G

[glum]

Glum, negative again...

I'll get my coat...

[BOAC]

if ATC care about anything, it's either groundspeed - I have to say 'negative'! Speed is so much used by ATC - separation on approach and climb out, during arrivals, and it is IAS that is used. ATC would find it hard to demand groundspeeds although they are, indeed, what they want.

I have used AoA (Harrier) and it works well. It would work again. The problem is you are looking at such a major re-work of aviation I don't think it would be possible - take take-off and landing performance? You'll need a speed for V1 and Vr. On final you need an LDA now based weight and on flying the correct Aoa. How will ATC control approach separation without asking for an IAS?

It is a good concept, forward thinking, but a touch too early!

[Genghis the Engineer]

Fair point BOAC.

Declaring an interest, I'm currently seeking funding (and perhaps a few suitably clever collaborators) to put together a research programme into use of AoA in primary flight guidance. A big job, likely to run on for several years, but should be worth doing if we can make it happen.

You'll always need IAS: as you say, for ATC separation (on jets, little interest for smaller aeroplanes), Vr, Vne, Vf. But, it should be able to schedule climb and approach more accurately using IAoA, as well as stall avoidance. Cruise may be able to be accurately scheduled with AoA also, but that's relatively unproven - the maths works, but there's no substitute for actually doing it.


G

[Denti]

There is an option (according to Chris Brady) of AoA indication for the 737 which even has a nice green band for approach AoA values. I do not know if there is any airline out there using it or if it is rather used by military applications.

We do not have that on our 737s, however we had several AoA vane failures about 1.5 to 2 years ago and all of them were pretty confusing to the crews as pretty much every other interesting value (IAS, altitude, even ground speed and wind information) will be way off its real value, often in quite unpredictable fashion. One most probably played part in a runway overrun, all others were in flight.

[ICT_SLB]

Denti said: "There is an option (according to Chris Brady) of AoA indication for the 737 which even has a nice green band for approach AoA values. I do not know if there is any airline out there using it or if it is rather used by military applications. "

That's not an option that's an FAA requirement and known as the Low Speed Cue. It is the electronic equivalent of a painted green line on a simple IAS. It's usually set at 1.3 Vs and is a second order approximation driven from normalized AoA. It gives a reasonable result at low altitudes but it has been known to show up in cruise when it definitely does not match anything - one of our Authority TPs once characterized it as "an abomination unto good airmanship" (guess who's 525 regs DON'T require the green line).

[Denti]

So all airlines flying under FAA rules have to have the extra AoA gauge on their PFD? Didn't know that. There should be more than enough info about AoA flying in commercial jets then.

Remember, i wasn't talking about the normal 1.3g stall margin indication (yellow bar), nor about the stick shaker activation "eye brows" or the usual low speed warnings available on that aircraft (aural alert "AIRSPEED LOW", "BUFFET ALERT" on the CDU-scratchpad, yellow bar, red/black bar etc). I was talking about an additional indication of AoA which has a green band of values, not a line, for approach.

[Trim Stab]

OK, moving away from the certification issues, is there a way to determine experimentally the AoA to obtain Vymax?

One of the types I fly is equipped with an AoA vane (C525) and if I follow the climb profiles recommended by the manufacturer, the AoA is not constant. However, it is actually quite easy to climb at a constant AoA by using the trim wheel. It would be interesting if I could determine the AoA for finesse max, and then try a climb at that AoA.

Glum - ATC do use resulting groundspeed as their own control parameter, but they have to work around whatever groundspeed aircraft are doing. Anyway, it is not the issue here.

[Machdiamond]

If you climb at a constant AoA for finesse max, the wing will be happy but the engine will not. As thrust goes down with altitude, Vymax changes slightly for a good balance between maximum wing and maximum engine performance.

A climb could be scheduled as function of AoA, but it is not going to be more or less effective than scheduling it as function of indicated airspeed.

There is also the accuracy of the measurement/indication that would be involved. AoA is rarely indicated in tenths of a degree (let alone calibrated), which equates to changes of only one KIAS. So when deciding whether AoA or airspeed is the better source of information for climb performance, the latter has an advantage in terms of accuracy. That would particularly be the case if your AFM climb schedule has weight as input parameter.

For stall avoidance, AoA indication is king. But as a tool to get best performance, IMHO speed is best.

[Shawn Coyle]

AoA is only useful when the wing is clean. If anyone wants information on the only device that will tell you the margin remaining to the stall on an iced up or otherwise contaminated wing, please send me a PM.
The Airfoil Performance Monitor works by measuring the ratio of turbulence to smooth airflow and gives remarkably consistent indications of how close to the stall you are regardless of the AoA when the wing is contaminated. Also works on the takeoff roll at a very low airspeed to tell you that your wing is contaminated.