gums

THNX Retired for discussing how easy using an AoA indication is.

Sucker shows stall or optimum approach speed or.... regardless of your weight. And we can still use rules-of-thumb to make a comparison - you know, 175 knots plus 5 knots for every thousand pounds above 3,000 ( F-101B VooDoo). If the AoA is really high or low at the textbook speed, check your flaps or fuel gauge or... And that's the OODA I was referring to, PJ, basic instrument crosscheck.

I am having my leading edge flap failure approach digitized, and will make it available in a few days. I flew airspeed and not AoA due to controllability factors, but you can see the AoA bracket and watch what it does just before touchdown. Can also see how valuable the flight path marker is, as it shows you where you are gonna crash.

PJ2

Hi gums;

Re, "THNX, Retired for discussing how easy using an AoA indication is.

Sucker shows stall or optimum approach speed or.... regardless of your weight. And we can still use rules-of-thumb to make a comparison - you know, 175 knots plus 5 knots for every thousand pounds above 3,000 ( F-101B VooDoo). If the AoA is really high or low at the textbook speed, check your flaps or fuel gauge or... And that's the OODA I was referring to, PJ, basic instrument crosscheck. "

Thanks - I learned something again...OODA. We have a slightly different approach but I believe the outcome is the same. Thanks for bringing it up.

I have a question regarding an AoA indicator and how airline crews would use it.

With reference to the following, from the BEA Interim Report #2, pg46, it states:

My question is, how does one use the guage in operations, and how does one stay away from the stall AoA using an AoA guage when the AoA at which the aircraft will stall changes with Mach? Would the guage be calibrated, (using much the same logic as the ECAM does in Alt/Dir Law - a table was posted around Thread 4 or 5 showing this AoA calculation at which the stall warning would sound).

I'm not arguing against installing/using an AoA guage here. I frequently interrogated the ACARS function to watch the flight data which included the AoA among many other interesting parameters.

But, if I may, I think that there is more to an AoA guage than just installing and selectively watching it. Clearly, the AoA at which a transport will stall at high Mach number at cruise altitude is different (due Mach effect) than an approach AoA which is typically 8 to 12deg depending. In cruise, a stall AoA may be as low as 4deg, as it was here when the first blip of the stall warning was heard, and later, at a much-reduced Mach but at FL376 or so, the aircraft was starting to stall at an AoA of around 7deg. I'm trying here to imagine how the guage would get the crew/PF of AF447 out of trouble and how the PF would use an AoA indication and what AoA he would be targeting.

HazelNuts39

Quote HN39: I'm aware that the 'low speed awareness' markings are fundamentally different from Airbus' markings of 'characteristic speeds'.
Quote Clandestino: Not valid for all Airbus characteristic speeds. Valpha max and Wsw on Airbus are very similar to low speed cue. ADC/IR design, flight controls and display architecture are different yet displayed information is similar.

Reply: AFAIK Valpha max is the '1g' stall speed and is not g-sensitive. It is not displayed in alternate law. Although Vsw is g-sensitive, it is calculated for the current weight and moves off-scale as the speed tape moves up to display 60 kts. As I understand low speed awareness (see here), and you concurred in your post 467, it would maintain the 'Vsw' where it is on the display, while the speed tape moves.
Quote Clandestino in post#467: I strongly suspect your further line of thought, about showing wrong stall warning speed when IAS gets unreliable is correct ...
Quote FCOM 3.04.27 p.7: (Alternate Law) ValphaMax disappears ... Unlike VLS, which is stable, VSW (stall warning speed) is g sensitive so as to give additional margin in turns.

Quote HN39: But when airspeed has 'gone south' and in Alternate Law, may be the 'low speed awareness' is better?
Quote Clandestino: If airspeed is lost, how could we have possibly have low speed awareness? High alpha awareness is even better and is provided through aural stall warning.

Reply: The aural stall warning comes 'out of the blue' for the pilots, they do not see it coming, consider it 'inappropriate' and dismiss it as 'false'.

Quote HN39: It would have informed the pilots of the 'validity' of the first brief stall warnings, that they were transient and no reason for concern at that time
Quote Clandestino: At the cruise Mach, they were not 'valid', they were valid. They were transient as the aeroplane was jerked into climb at its max recommended cruise level and were reason to be very, very concerned.

Reply: Yes, they were valid, but partly due to turbulence, and therefore transient, and as such, not calling for immediate nose-down response.

RetiredF4

No, you still dont get it. You have an intention to change the flightpath a given amount, you move the controls and expect the aircraft to respond to the flight control input in relation to the flight control change. The reality of the response shows on the instuments, or you can see and feel it if flying visual and you compare it to the expected outcome and use that one again to fine tune your input. It´s a closed loop system.

is totally off the scope, don´t know where you can find it in my post.


If the input is not done by yourself or cannot be observed (SS issue), you dont know what to expect,´you have no reference, only the real response to an unknown input / unknown malfunction / unknown external input can be observed on the instruments. The source of the unwanted deviation of the desired outcome is not known, only the result. Therfore the correction input to the situation might not be appropriate to the situation as i described extensively in my prior post.

Back to the SS, it could be that PF is already correcting an undesired pitch attitude by SS input, and PNF might add his own input, as he doesn´t know that a correct input is already being made by the PF.

Nearly all performance issues can be observed and judged by instruments to the "what" and "why" is it happening, but as told before, it gets tricky with flight path issues. Those can be (and normally are, but not exclusively) SS or yoke induced. The yoke can give feedback to the PNF in relation to the kind of input, the amount of input, the agressiveness of the input and the duration of the input, the SS in the present configuration dosn´t.

Enough for now.

RetiredF4

Our gauge was a simple round one (posted it before) with aditional tone and indexer lights in the front window for the approach and landing. I don´t know (but assume) that it was somehow calibrated. We could use it over the complete speed range from 0 to Mach 2.2 in all altitudes and under all loads and in all configurations. As our Rhino was not digital and had only an limited capable flight data computer i assume, that it is no rocket science today either to make it usable under all circumstances. With power on the aircraft and the WOW switch pressed by mintenance we hand tested it by turning the vane and observing the gauge, lights and tone at the appropriate units of AOA (don´t know even if the indicated units were equal to degrees).


Good points, but again, i think it is a calibration kind of thing and for sure relevant to the Rhino and other fighters as well. The indicated units could be calibrated to the facts you mentioned and then the numbers would be identical to all situations from T/O to Landing.

First it should keep them out of trouble. There is a fast familirization with the values normal present during cruise (normally close to max range) approach and landing. In a situation where these values start to deviate from the known common values it indcates not only a state, but also a trend. Now you may say, that the speed tape does the same, wether you get fast or slow. The difference being that the AOA and consequently also an AOA indication also puts the loading of the airframe into the equation. During the initial pullup the AOA would have increased from known value to the stall threshold value thus telling the PF to ease of on the controls , reduce pitch and never let this AOA go higher than safe value. While the Stall warning is only an ON / Off indication, the reference to the AOA is a "performance state" indication with hints to the momentary trend. Changes due to loading or unloading are immidiately observable. That could have helped after stall had occurred during the recovery attempts as well. SS aft, AOA stays high, SS forward, AOA reduces, keep SS longer forward, AOA reduces further.... and so on. Even the effects of the power changes would have been seen on the gauge, thus developing a quick learning pattern what will work and what is wrong.

In all my time in the AF i never had or even heard from an AOA malfunction (except birdstrike) and understanding and working with the gauge, the aural tone and the indexer lights was no issue at all.

Just adapt it to the use in the individual airframe and go for it.

HazelNuts39

One could argue that, even without indication of stall and SW AoA's, having an indication of actual AoA is better than nothing. Does anybody know if and how the BUSS addresses that problem?

P.S.
AFAIK the BUSS indication of SW and Stall AoA does not change with Mach. Is is a function of flap/slat configuration only. The stall warning AoA with all three ADR's off is 8.6 degrees in F/S configuration 0/0 . Above FL 250 the pilots are instructed to fly pitch and power and not use the BUSS.

DozyWannabe

We need to be careful here. The automation on the A320 and her widebody sisters is in fact not a great deal more advanced or restrictive than that in your 757, and never was. The difference was that in the 757 the safe limits were coded into the autopilot, whereas on the A320 those same limits were handled by the protections in the FBW logic.

It's very important to separate the automation concepts from the FBW concepts in order to understand them properly. There is some crossover in the case of the protections and autopilot limitations, but it stops there. What I think you're talking about (and correct me if I'm wrong) are the "what's it doing now?" incidents that came with automation - and they were as prevalent on the 757 and 767 as they ever were on the Airbus FBW types. Yes, you can turn it all off on the 757 - but you can turn the automation off on the A320 too.

As I said earlier, there was a greater leap in automation between the 727 and 757 than there was between the 757 and the A320.

If you're talking about the A320's protections - which only go away if there's a significant systems failure - then that's a very distinct aspect of FBW which has nothing to do with automation, and in any case you only encounter them if you try to do something dangerous. Otherwise, as PJ2 and others have testified, the A320 and her sisters hand-fly beautifully. It's not taking control away from you so much as helping you maneouvre safely within the limits of the airframe.

Looks like it works just fine on a two-crew plane as well, going on the evidence - even with yokes, only one of those two pilots should ever be controlling the aircraft, after all. The sidesticks were nothing to do with automation, they were an outgrowth of the fact that big, heavy yokes weren't necessary on fully-hydraulic airliners, and even less necessary with FBW.

I realise that psychologically a connection could be made between the transition from big, heavy yokes to the lighter and less-obtrusive sidestick and the belief that pilots were relinquishing more control to automation, but as I've said above it just isn't the case. FBW/protections and FMS/automation are two very separate and distinct things, and it's important to bear that in mind.

Old Carthusian

Retired F4
Yes I do get it but we were defining expectation pattern differently - we were both focusing on different things. However, we are not talking about visual flying but instrument flying at night with no visual cues. What do you do then? You rely on the instruments to tell you what the aircraft is doing. In fact it's the only thing you can do - relying on any other cue is a recipe for disaster. Your argument becomes irrelevant because the cues or measures you need for visual judgement do not exist. Other sensory judgement is also worthless. You need a a measure. You have to rely on your instruments. A visual appreciation is totally worthless in this sort of situaiton. Hence being able to see the position of the control medium means nothing. Reference to the instruments and what they tell you is what matters.

Owain Glyndwr

Retired F4

OK, I see how an AoA gauge works - it doesn't need any calibration to tell you what the AoA is; it's not until you want it to tell you how close you are to stall that calibration comes into play.

Now AFAIK, you can't do that with any precision unless you know Mach number, since stall AoA is Mach dependent, so my question is how did your F4 AoA gauge work in a UAS situation?

BTW, I'm not trying to argue against fitting AoA gauges, just trying to establish their capabilities.

RetiredF4

Well, last try. It works both ways, also in IMC and dark night and especially there. Your words, ....."You rely on the instruments to tell you what the aircraft is doing"..... and i fully agree and never ever said anything else. But flying does not stop there. A pilot will be and has to be in constant monitoring modus to compare the "what is the aircraft doing" to the "what should the aircraft be doing, which i call the expectation state. To be able to get this comparison you need the input value into the system . If you are flying manual in your 152 and you didn´t deflect any flight controls, because you didn´t want any change of the flightpath, but you observe a sudden bank, you know it is not your input, because you have your hands on the yoke and didn´t move any control surface. There must be another reason causing the input and that again might cause a diferent action from yourself.

Same if you are flying as PF, wether you made the flightcontrols change by manual input or by programming the automatics, you will know that it was your input. Will the PNF know ? He observes the change on the instruments and in a yoke aircraft observes the yoke movement in his lap, but in a non interconected SS aircraft? He has to guess.

Well, that works most of the time, because transport aircraft are and should be operated in a safe and preplanned mannor, so due to CRM it is common knowledge when something should happen in regard to flightpath or performance parameters change, because it is announced by PF, briefed before, or ordered by ATC. Therefore the expectation (we will now start descent, climb, turn..... ) shows as reality on the instruments.

When the sh**t hits the fan really bad like in AF447, the reality on the instruments is no longer nearing the expectation, the aircraft does not behave like expected (i´m in TOGA hehe..........I pulled back for quite a while....) and even both PF and PNF have now different understandig of things and the awareness, what the other guy is doing is lost. The corelation of the aircraft behaviour to the flight control inputs is lost, no valid feedback loop any more and therefore complete loss of situational awareness.

By the way, as far as i understand FBW systems it would be the same. If the system would loose the ability to recognize and measure its own input into the system, it would not be able to maintain normal control, like the dampers then counteracting the flightcontrol deflections.

RetiredF4

I dont know, how it was done, what input it used. Im no techician´and also technically very interested i never questioned the functioning of the AOA. It was there, it was always working, also with a iced up pitot tube (we only had one).

I don´t see the problem there, the information was present in the AF447 aircraft (Stall warning, FDR traces....), it was not displayed.

DozyWannabe

Or, y'know, ask.

("Are you pulling up?"
"Yes. Shouldn't I be?"
"No - we're approaching stall - I have control."
"You have control.")

An exchange for which there was plenty of time between disconnect and apogee/stall. Remember a similar lack of communication did for Birgenair, despite the yoke being in the PNF's lap and - eventually - stick shaker going off.

RetiredF4

Why shold he ask? What would have triggered him? Why should one assume, that the PF is applying backstick in a situation, where it is not appropriate? Even the aircraft talked different to him after the initial pull. From 02:10:29 until 02:10:45 and from 0211:05 until 02:12:00 the loading was below 1g, thats more than 1 minute less than one g against 26 seconds with g at max 1.15. The instruments should have triggered him, absolutely correct. But he didn´t get the clue, otherwise he would have asked or even he would have taken over the aircraft like you mentioned, correct? Maybe he was thinking some kind of updraft, some kind of misreading altitude (speed was gone, why shouldn´t VS and altitude not be affected)? Maybe the unloading gave him the impression, that PF is doing the right thing.

We don´t know what he was thinking, what he was "guessing". But he couldn´t see the SS input and he didn´t ask, and he didn´t get to the right decision, that one we do know.

Old Carthusian

Actually no - as a pilot you should be reading your instruments and taking your cue from them. You might have expectations but these need to be sublimated to the readings on your instruments. They will tell you what is happening. You seem to be describing an expectation state exactly the same as I interpret the phrase and this is dangerous. By allowing yourself to accept this state and to let it influence your actions you are running the risk of deviating from actuallity and ignoring the warning signs. Your definition of expectation is also flawed - it is a belief that a future situation will pertain or a future flow will occur. It is not per se linked to reality. Reality is what happens and rather than focus on expectations one should be focusing on things as they are and trying to predict what will happen based on the reality. Once one gets into expectations one can wander off the path to a very large degree.

RetiredF4

I´m glad, i survived 20 years of flying high performance fighter aircraft and the students who learned flying with my help still are all alive.

jcjeant

A recommendation is not binding
If you refer to motorists not to exceed 120 Km / h on a highway .. what result?
If you ban motorists from exceeding 120 Km / h on a highway .. this is different (constraint) and the results will not be the same

Owain Glyndwr

Retired F4



OK, since I posted my query I’ve found an example of an F4 operational manual on the ‘net.
Short answer is that it wasn’t done with any precision.
The AoA probe doesn’t give incidence in degrees, but an AoA ‘index’. Probe output goes from 0 to 30 arbitrary units for an AoA range of -10 to +40. UAS pages give stall at 27 units, stall warning (pedal shaker) at 21.3~22.3 depending on aileron droop. [But it is said that it may not be recognisable due to heavy buffet!]. There is no mention of any variation of stall or stall warning with Mach number. Values of AoA units are suggested for a range of flight cases, but so far as I can see these simply replace pitch by AoA units as memory items. The weight range for the F4 seems to be small enough for such a simple approach to work. For example the index for approach (19 units) is said to be “adequate” for all loadings and in fact is a simple reflection of approach at 1.3Vs or whatever being close to a unique AoA for all CGs. But the indicated AoA is only valid with gear down – with gear up the aircraft stalls 3 or 4 units earlier. Stall, it is said, is preceded by buffet starting at 12~14 units and stall will “usually be above 25 units” although the actual angle varies considerably with loading. With flaps and gear down the pedal shaker operates about 17 kts above stall and 9 kts before wing rock.


I haven’t found any mention of automatic indication of stall AoA, so presumably you had to set one or more of the ‘bugs’ appropriate to the flight state?


Overall, I have to say that my impression is one of a system that can be made to work well enough on an aircraft operated as military, but which would need some additional sophistication to meet the needs of civil operation, unless one accepts use of memory items for typical AoA in routine day to day operation. Nothing wrong in that, it is already used in pitch, and any indication of AoA would be better than none, but I don’t see it as an all embracing panacea I’m afraid.

gums

I go with Retired about the useful AoA at all mach numbers.

For example, the infamous "pitch up" in the VooDoo had plenty of warning when subsonic, lile medium buffet and wing rock. But we had zero warning when supersonic. So our AoA doofer must have corrected for the change in mach, as it worked like a charm to keep us outta trouble. We flew around with a "limiter" on that keep you from getting within "x" degrees of the stall AoA ( called MCSL). If we pulled thru it ( 60 pounds or so), then the "pusher" moved the stick forward ( 28 more pounds to overcome if you were determined to get out of control).

The AoA steam gauge was large and had the "barber pole" to show the stall AoA and existing AoA.

Viper was the same as far as working supersonic, but we didn't get the AoA bracket until gear was down ( no steam gauge).

I have no problem with relying on the gauges when IFR, as body sensors are no good. Ask Retired about flying formation at night. After a few turns in the WX you were convinced that the flight leader was inverted or had done a roll, heh heh. So a quick glance back inside really helped.

PJ2

RetiredF4;
Thank you for your extended response and for indulging my question. I understand that the technical details aren't at hand...no problem R.F4.

My question isn't about how the guage is read, but what assumptions lie behind its design and whether such assumptions are known and understood by flight crews.

Does such an indication take into account the effect of high Mach Number on the stall AoA?

If not, such a guage or indication is useless for determining when/if an aircraft in cruise at high Mach Number has reached the stall AoA.

If the AoA indication adjusts for high Mach Number, (as the ECAM stall warning does when the Airbus is in Alternate Law), then the indication is useful.

That's all I meant.

Here is the ECAM logic which triggers the stall warning from AoA in Alternate Law - this was posted earlier, (can't recall who to credit), but it may illustrate what is meant by the question:

DozyWannabe

For starters because the aircraft has started to climb, and vertical speed has increased from +1500fpm to +4-5000fpm in a matter of seconds. They had discussed altitude and decided not to go any higher than the level they were at because of the weather. Autopilot is off - who else could be causing the climb?

You ask because communication is a tool that's given to you before you can even pronounce the word "aeroplane". Even if you've got a yoke in front of you, unless you're in a life-and-death situation (which this wasn't at that point), you should take it gently, follow through and verbally confirm what you're feeling before you try to take over. I've never heard of a successful recovery requiring handing over of control where one pilot simply grabbed the controls from the other - except in instances where the other pilot was clearly incapacitated.

He did try to correct the PF verbally, so it's reasonable to assume he realised, or at least felt that something was wrong. I'm not an expert, but his speech pattern seems to indicate indecision. The captain has put the PF in charge - if the PNF takes over before the captain arrives, will it reflect badly on him if it turns out to be unnecessary? Perhaps he felt that correcting the PF verbally until the captain arrived would be the safest path to take

Well, for a start I don't think there's an updraft in existence that could cause a heavy widebody to climb like that, plus an updraft wouldn't necessarily cause the pitch to increase.

If he knew his systems then he'd be aware that altitude and VS rely on a completely different set of sensors (static ports) than airspeed (pitot tubes), and it was unlikely both would have been affected.

I don't think G-loading is as integral to the scan in an airliner is it is in a fighter. It'd definitely be secondary to ADI, altimeter, VS and airspeed.

We also know, as I said above, that he tried to correct the PF verbally several times before the Captain arrived - he clearly didn't think things were going well. The next logical step would have been to take control, and he did try just before the Captain arrived, but didn't follow through - and the PF took back control unannounced a few seconds later

@jcj - The Airbus Service Bulletin was binding in terms of the work being *required*, not recommended - and the work having to be done by a given date. An Airworthiness Directive is the next level up where the type is effectively grounded until the work is done, and that wasn't really necessary in this case. ADs are only used when a fault is so severe that it is likely to result in the loss of the aircraft every time it occurs. The successful recoveries that pre-dated AF447 prove that this was not the case.