Dozy,

Are you arguing that he levelled off 40 ft lower than he intended, despite that being a potentially-hangable offence, and did absolutely nothing about it for nearly 10 seconds?

Or are you suggesting his barometric altimeter was over-reading by at least 40 ft, due to an error in an ADC or DMC?

It's kind of annoying that the report leaves those calculations as an exercise to the reader (I used this Pressure Altitude Calculator), but yes, it shows that the QFE value received from Habsheim must have been correct.

But Asseline seems honestly upset about the investigators not believing him ("Do they think that both of us are too stupid to read an altimeter?").

To reconcile the DFDR values with an indication of 100 ft on the captain's PFD, one could assume that the crew had erroneously entered a QFE value of 985.5. Now that seems totally unlikely. On the CVR, they repeat the correct value three times (and "cinq" and "quatre" don't even sound similar).

Asseline has another explanation, that the reference value for his altimeter must have shifted somehow, and he has indeed a hair-raising story to tell, where this happened to him on a flight from Paris to Geneva, and he ended up 1000 ft too low, almost (as in "too low, terrain") striking some mountains.

While I have some sympathy for this particular theory of his, it still leads back to the basic problem: the flight crew being unfamiliar with the airfield. Had they been aware that Habsheim control tower is not a good 100-ft-reference, and had they known about these trees that are just about as tall as the tower, it wouldn't have mattered if their altimeter reading was off by a lousy 40 ft.

Hah. Don't be daft, you're always welcome and your input is always thought-provoking in a good way.

No doubt, though on occasion it can be about other things - for example, a tendency to stick to the plan even though some aspects have already gone awry.

An interesting point and one with which I'd agree, with the caveat that in today's cut-throat business world I'd be surprised if most airline captains haven't been concerned at one time or another about the potential fallout from disgruntled pax complaining to the firm when deciding whether to go-around and/or divert. With airline management consisting mostly of MBAs, many of whom don't appreciate the risks involved, the worry that making a safe call could harm or even end your career might be palpable.

In this case though, I'd be surprised if the schedule wasn't more flexible than the majority of airline ops.

Yup, and the 'setup' was poor all the way back to the airline's briefing. However, at the same time there were opportunities to make a more stable approach which were not taken, so some responsibility for the need to "save" the aircraft remains with the captain.

As you well know from your FBW fighter experience, such systems are a result of hard-nosed engineering practice - far from "magic" on closer inspection! Whatever one's views on the technology, the laws of physics remain immutable and if you can't avoid solid objects in your path you are going to hit them...

One of the outcomes was a move to ban pax from the general public on display flights.

I'm not arguing or suggesting anything, because it's outside my area of expertise!

I think it's not a simplistic either/or proposition though. It could be that he moved to level off at 100ft, but the inertia of the rapid, improvised descent profile coupled with an abnormally low thrust setting caused the aircraft to continue to sink while he was heads-up, looking out of the windscreen. The rapid increase in workload due in part to continued improvisation could have lead to confirmation bias and misreading of the altimeter when he looked heads-down again. I'm not saying that's what happened as much as asking you if you think it plausible.

And in my personal view, that - namely taking things personally and reflexively defending with counter-attacks - has always been his Achilles' heel when it comes to the credibility of some of his claims. For their part, the investigators certainly believed that *he* believed the baro alt was showing 100ft - but every bit of hard evidence staring them in the face made it unlikely.

Interesting use of "both of us" as well. To the best of my knowledge, the FO has never spoken up publicly on the subject. I may try hunting for material there.

Which rather begs the question of why, that issue being known to and experienced by him personally, he neither used the RA display as a backup himself, nor - given his claim that the RA display was difficult to read - asked his FO, as pilot monitoring, to do his best to cross-check both baro and radalt.

It's one fundamental issue certainly, but there seems to be a general sense of creeping complacency about the operation as a whole.

20/20 hindsight is a wonderful thing.

Rules of computer art 1988
 

So we do agree about the passage of more than 50 FT RA to the unexpected less than 30 FT RA before the runway threshold.

Being particularly concerned in flying systems as a former manager of mathematics and computational methods, the comment of the expert Max Venet indicates to me a lack of computational methods. From memory, he said the following sentence: "We do not really know why the aircraft did not set in flare passing below thirty FT RA, or how long he should stay less than thirty feet. "
This sentence shows that Venet did not found in Airbus a person who could tell him that or a document with this information. He could not access either to a history of the evolution of the software before its certification, and he did not find the information in the certification documents.

I believe that these serious gaps do not correspond to the state of the art computer at that time (or today!). If indeed at that time certain documents required to decode a specific 0 and 1 know-how it was even more to accompany these documents and comments in everyday language. In any case it is so that AS or AB (or any other designer of the system) should proceed.

The absence of such documents can only be accompanied by operational documents being incomplete and unsafe to the user (airlines, pilots, mechanics).

I say that as a computer specialist who had to put up this kind of working methods among researchers and engineers. And it had nothing difficult, but it is of course a constraint.

I do not see the point either to keep secret the software description. The fight against the copy is much easier to exercise if everyone puts their workon the table in daylight in a reasonable time. Only that can serve as engineers who wish to sell their know-how and pilots who wish to return alive from flight when they flew as we taught them

We've covered a lot of this ground before, and I believe you had a look at my old Prof's overview from his 1993 visit, where he saw use of advanced software engineering and reliability techniques in person.

Would this be the same Max Venet who was President of the SNPL from 1979 to 1982? If so, in which area of expertise was he qualified to comment?

No it doesn't, it shows that he didn't have the information - whether this was because he didn't try hard enough to get it or because Airbus were wary of supplying (at the time) very commercially-sensitive data to a representative of a hostile organisation is unknown.

M. Venet stating that he didn't have those documents is not the same thing as those documents not existing. Again, we're back to Prof. Mellor's visit where he saw contemporary best practice in action.

And I respond as a professional software engineer and aviation nerd who had the good fortune to be educated by someone who saw behind the curtain.

These days it's clear to those with an interest that some of the techniques pioneered with the Airbus FBW systems have percolated down into more mainstream software practice (test-driven development and regression testing in particular). The kind of source-code sharing you describe has also become well-known, spawning the Open Source movement. However in 1988 the legalities surrounding software copyright and ownership were still something of a grey area, and the people dictating commercial policy guarded all aspects ferociously.

Getting back to the point, we've already discussed that Capt. Bechet organised an accurate real-world replication (minus the trees that were hit) of the flight at Toulouse. Flare mode was not activated, but Alpha Prot was.

Based on my understanding of how the systems were designed, it's likely the reason Flare Mode was not activated was because the brief RA reductions were not consistent with the overall trend of the values. M. Venet appears to have been working on the assumption that a value of 30ft RA will immediately trigger Flare Mode under all circumstances. This in turn suggests an overly simplistic understanding of software in general and indicates that his expertise was not in software, real-time or otherwise.

If Flare Mode had been (completely - h/t HN39) activated by overflying the copse, the DFDR would have indicated the elevators down shortly after crossing the strip threshold. Instead, the EFCS complied with the sidestick commands until shortly before impact, where Alpha Prot mediated the pitch-up demand to avoid stall.

And as rudderrudderrat said all the way back in post #45, Flare Mode can be overridden at any point:

I was referring to Asseline's book, p. 35/36. Scattered all over that book, there are a few passages where I can imagine Mazières putting down his copy, muttering "Will you please just speak for yourself, Michel?", and this is one of them. But all in all, Asseline was reasonably cautious about that issue.

From the FCTM: "When reaching 50 ft, auto-trim ceases and the pitch law is modified to flare law.
(...) The system memorizes the attitude at 50 ft, and that attitude becomes the initial reference for pitch attitude control. As the aircraft descends through 30 ft, the system begins to reduce the pitch attitude at a predetermined rate of 2 ° down in 8 s."

This aligns with paragraph 1.16.1.2 of the accident report where it says:
"Pendant la phase de descente entre 50 et 30 pieds, la loi de pilotage est modifiée et prend progressivement en compte (1), au lieu du terme en facteur de charge, un terme en assiette longitudinale (écart entre l'assiette constatée au passage à 50 pieds dite assiette de consigne et l'assiette réelle instantanée) ; Au passage à 30 pieds, un ordre supplémentaire destiné à simuler un effet de sol conventionnel est introduit en complément à la loi de pilotage en assiette précédente : cet ordre ramène progressivement l'assiette de consigne (constatée à 50 pieds) à - 2° en 8 secondes, créant un moment à piquer que le pilote doit contrer (« dérotation »)".


According to the footnote to this paragraph, the transition to flare law is progressive and takes one second to complete. In the same paragraph the report states that flare law (loi de pilotage en assiette) was active between t-21s and t-20s (i.e. when the airplane was overflying the 'bosquet').

RA, sidestick and elevator position are recorded once per second. Even if 'loi de dérotation' was momentarily activated, the time below 30 ft was probably too short (considering the ramps of 1 second and 8 seconds) to result in a movement of the elevator that would have been observable in the DFDR data.

@HN39 - That's interesting. So the transition began, but the 'drop' below 30ft was probably too brief to complete it? OK - that makes sense (and is in keeping with the 'trends and deltas' nature of the systems as I was taught).

Sometimes I really wish I'd taken French alongside German - it'd make this so much easier. It also means that the question of Flare Mode was in fact asked and answered in the report, which somewhat dents Asseline's assertion that they didn't check for it/someone was hiding something.

Effects of Flare Mode
 

Hi HN39,

Thanks for recapping the information from the post-1988 FCTM.

My English-language FCOM from 1988 is in agreement that the first stage of what it calls Landing Mode involves two things passing 50R (50 ft on the radio altimeter): memorising of the pitch attitude; and freezing of the THS setting.

The BEA report indicates that this first stage would have been engaged as a result of the ramp of height to 32R at t -21 (TGEN 313), as the a/c passed over the copse ("bosquet") before the airfield boundary.

As you know, the second stage of what is now called Flare Mode, which begins crossing 30R, introduces a slow bias of pitch that modifies the zero-stick pitch progressively from what it was at 50R to minus-2 degrees - this during a period of 8 seconds. That sub-mode would probably have been engaged by the recorded height of 24R at t -20.

The BEA states that, because the rad-alt height in the following 2 seconds was over 50 ft, flare mode was disengaged back to Normal Law for that period. AFAIK, there is nothing in the recorded data to suggest otherwise.

By t -17, however, the height had once again fallen below 50R, which would have re-engaged the first stage of Flare Mode, freezing the THS at its current setting of U04 (4 deg nose-up trim). The second stage of Flare Mode would never have engaged, because TOGA thrust had been selected by t -4 (see Tome 4), at which point the height had not crossed 30R. (A second later, the AoA exceeded 14.5 deg, and Alpha-Prot Law would have engaged.)

In summary, the only adverse effect of the regime of the first stage of Flare Mode between t -17 and t -5 would have been the freezing of the THS at 4 deg nose-up trim. After levelling off at t -13, the PF had to pull back slightly and increasingly on the sidestick as the a/c decelerated. However, in my experience, pilots of a/c with conventional flight controls flying deliberate approaches to the stall do not continue pitch-trimming below Vref. At level-off, the IAS was already below the Vref for Flaps 3.

Bon soir, roulishollandais,

Does the above interpretation address any of your concerns?

Sometimes Iself really wish I'd taken English alongside German as first foreign language - it'd make this so much easier.
Specialy here, where we are walking on eggs.
I shall take some time to write my answer in English.
Already Thank you to Hazelnuts39 and Chris Scott for analysing these sequence over the bosquet, to gums for global analyse of the flight, to Dozy for the interest he finds now in that forum who could never take place since 25 years, for any reason. Thank you to Conf_iture who shows us that not retired pilots are not all ectoplasms, to John who allowed that forum to exis, and to all the excellent contributors, and to the followers.

My last posts don't want to comment the crash itself (I removed my pilot's cap) but what did me wonder as computer guy, as it may be a chapter of factors in the lessons to learn from Habsheim. But we need a good analysis of the facts.

Just a word about Venet :
He did the mentioned declaration during the first Trial, answering to the President of the Court (dec 1997) . When he says "We" it means himself and Belotti.
Retired AF airline pilot, and former (elected by the members) President of the SNPL the most important pilots Union in France. That does not make him the Devil! Pilots are never on the ground... and sleep (after night flight) when ground People are working hard in Office, etc. We know that old story between Navigants and Rampants So pilots need to be represented in Unions when they are not on the ground, despite someone don't like existence of Unions.

Four enquiries have been done :
1. 27. june 1988 BEA : President Bechet, Gonin, Auffret, Farthouat, Arslanian, Gourguechon, Villeneuve
2. Judiciary expertise from the Juge d'instruction Mrs Marchiani : Auffray and Bourgeois
3. 11.january1990, from Mrs Marchiani, two supplementary experts : Venet and Belotti
4. 28. june 1988, Air France investigation Commission :
Richer the President, Vuillard, Gauthier, Merloz, Bourbon, Conrozier and prof. Org. (Chatelain, Scattolin, Thiébaut, Brédy, Edelbloude)

Didn't say it did. However he is not an expert in the field of computing, and the statement showed a lack of understanding of how such a feature is implemented in terms of real-time safety-critical software disciplines.

The issue with the SNPL regarding any action against Airbus is that the union has held a long-standing animosity against the company ever since the A300 went into service as the first long-haul widebody to do away with the Flight Engineer's station. An evolution of design which their members fought tooth-and-nail, and seemingly never forgave.

As such, there will be an understandable suspicion of bias against Airbus in such representations. In Asseline's case, it seems his union-supplied legal representatives steered him in the direction of pursuing claims of technical issues against the aircraft despite a lack of supporting evidence, when he would have been better served by pursuing the airline's poor operations practices which put him in a bad position in the first place, and for which there was significant evidence and potentially a much stronger case when it came to mitigating factors.

I have to say my personal opinion is that all this political nonsense has served only to hinder understanding of the incident and the issues involved, and furthermore has led to a significant number of people developing entrenched opinions that are, in some cases, still held with an almost religious fervour. This in turn means that there's a degree of personal animosity and ad hominem to the discussion that saddens me immensely.

Well, we get to replay the incident over and over......

Before I return to the "monitor mode", I think Doze summarized the whole thing, and it wasn't "flare mode" or altimeter settings or anything else except:

I have personally lost friends that did stupid things like "pressing" when they should have done a "go around" or broken off the attack ( fighter stuff). Pride is a player, and "get the job done" is also there on most of the losses. Then there is loss of situational awareness and basic failure to use all the displayed data of your flight path and such. On the civilian side, I knew the Cali pilot and the guy at Little Rock ( not like Cali, but still sad to lose a student of mine). Didn't cross paths with Sully, but might have, and he did it right. I got tired of going to memorial services my last two years in the "business" - 4 pilots that were of my age and experience that screwed up, and were all pilot error.

Sorry for the personal stuff.

Let's face it, the accident/crash happened due to poor judgement, and the jet did everything it was designed to do. Not enough to compensate for a sorry execution of a simple flyby, but you play the cards you are dealt.

I am not sure about the purpose of a low, slow flyby at a ridiculous altitude for a big jet and at a high AoA. But the captain tried it. Poor planning and practice. Then getting behind the jet's capabilities. Sad, sad.


I relish the info on the flight control laws and such. Also like the insight I see from professional "heavy" pilots and a few engineers.

Don't be - it's all good. There's no shortage of examples where otherwise excellent pilots paid dearly for a single call that turned out to be poor with 20/20 hindsight, and it goes without saying I'm sorry for your loss.

Because no other jet of that kind at the time could do it safely. I have little doubt that had the crew been properly briefed on the terrain surrounding the airfield we wouldn't be having this discussion.

Quote:
I am not sure about the purpose of a low, slow flyby at a ridiculous altitude for a big jet and at a high AoA.
Without A/T engaged and Alpha Floor, the A320 couldn't do it safely either.
So what were they thinking?

Surely it could do that perfectly safely without either as long as there were no obstacles in the way! A caveat which was firmly in the hands of the operator and crew...

(referring strictly to the aircraft capabilities, not the advisability of the sortie itself)

I concur with Machinbird. The demonstration as planned: a fly-past at Valphamax and alpha-max, therefore with thrust control the only means for controlling the height of the flight path, could not be done safely at 100 ft above terrain, let alone 40 ft. In any case it could not have been done with the level of safety required for a passenger-carrying flight.

Just imagine: To achieve alpha-max you have to pull the sidestick to the rear stop and keep it there. To maintain level flight at alpha-max you have to slow down to Valphamax. The only way to 'pull up' in that situation is to increase thrust to accelerate to a speed greater than Valphamax.

Thanks 'bird and Nuts

We pilots must remember that rate of climb is the delta between thrust available and thrust required for the AoA and weight and...... ( power required)

It's why the nasal radiators tend to use throttle for glide path rate of descent to land on a boat. They trim for an AoA/speed and control the flight path with power. Just look as the Asiana fiasco, much less this one we are re-hashing.

Besides going well below the planned altitude, and a screwed up approach, the power side of the equation was not even close to what was required to "save" the screwed up flyby. Sheesh. I don't like being harsh about a fellow pilot, but this one really bugs me, and I am thankful that so many folks walked away from it.

Lastly, who in the world on the sidelines would have appreciated a high AoA, slow speed, extremely low altitude flyby? I'll bet 99% of the witnesses didn't have a clue about what the pilot was trying to demonstrate.

I still have a vivid memory of the demonstration at the Farnborough airshow of the high angle-of-attack protection by Gordon Corps on the A300 serving as a flying testbed for the FBW system developed for the A320. The speaker didn't inform the audience about the altitude observed in the demonstration but, from memory, it must have been closer to 1000 ft than to 100 ft.

As I said,

Theoretically speaking, at 100ft over a flat landscape and with the power set correctly the A320 would be capable of safely maintaining Alpha Max indefinitely until it ran out of fuel.

With 20/20 hindsight the sortie shaved safety margins to the bone, but the responsibility for that didn't lay with the aircraft.

TREES
 

Farnborough had NO trees exceeding 500 ft. ( QFE ) along the extended centre line of the R/Ws ( IIRC ).
THAT factor alone, would have made a lot of difference for Gordon Corps' demonstration, if done at 1000ft.

Sure, but with the longitudinal control at the stop it doesn't have the controllability required to maintain 100 ft terrain clearance safely in anything but an absolutely 'dead' atmosphere.

Flight on an energy knife-edge
 

Quote from Dozy Wannabe (my emphasis):
"Theoretically speaking, at 100ft over a flat landscape and with the power set correctly the A320 would be capable of safely maintaining Alpha Max indefinitely until it ran out of fuel."

Theoretically, perhaps... Let's try to examine that in practice.

Quote from gums:
"...the nasal radiators [naval aviators] tend to use throttle for glide path rate of descent to land on a boat. They trim for an AoA/speed and control the flight path with power."

That's rather like most of us are taught to do before our first ever solo-flight on a light, propeller-driven aeroplane - albeit without an AoA gauge. We maintain the correct approach speed by adjusting the pitch-attitude, and adjust rate/angle of descent with throttle/power. (With a tractor-propeller, admittedly, the varying slipstream over the wing itself affects the wing lift slightly.)

Later, in jets, we are taught to adjust the rate/angle of descent with pitch-attitude, while maintaining the correct approach speed with power (thrust) - again without an AoA gauge. The two techniques are, however, essentially the same - achieving a suitable, stable AoA, while varying the flight-path angle (FPA) through the air. Although I've cited the approach case, it's worth pointing out that - in either technique - the FPA does not have to be negative.

How does this relate to Habsheim?

Quote from HN39:
"To achieve alpha-max you have to pull the sidestick to the rear stop and keep it there. To maintain level flight at alpha-max you have to slow down to Valphamax. The only way to 'pull up' in that situation is to increase thrust to accelerate to a speed greater than Valphamax."

So it's clear that the Habsheim scenario - once the stick is fully back, and the AoA is at alpha-max - is critically different from an approach at Vref. In fact, any change of the FPA through the air, as HN39 points out, becomes completely dependent on IAS (strictly speaking, CAS). Any change of FPA requires a change in "normal" acceleration (Nz). In a steady airstream, if that FPA change is to be upwards, the necessary increase in lift can only be achieved by an increase of IAS. That, in turn, can only be achieved by increasing the thrust. On the other hand, descent is not a problem...

Clearly, low flight at alpha-max is hazardous, for at least one reason: wind-shear, which is inevitable over and near trees and buildings, unless the wind at all levels is flat-calm (a rare event). Any loss of headwind or increase of tailwind leads to a loss of IAS. Recovery of IAS requires an increase in GS, i.e., kinetic energy. With no surplus of potential energy to convert, that increase in kinetic can only be supplied by an increase in thrust.

A jet engine will respond fairly quickly when already at a medium-to-high thrust setting (hopefully both engines, symmetrically...), but the pilot's reaction time also has to be taken into account. Arresting the inevitable descent caused by any loss of IAS would take time. There is little or no height to trade for IAS recovery.

To quote again from HN39:
"...with the longitudinal control at the stop it doesn't have the controllability required to maintain 100 ft terrain clearance safely."

C'mon, Chris.

The potential rate of climb is a simple function of power available versus power required for your configuration.

My light plane training was the pitch attitude and then power for speed. In later planes with constant speed props, this was even better. My first jets were straight wings and the same applied. Then I went to the high aspect ratio bent wings and things were different. No FBW, so trim was still AoA, or speed for those not used to the great indication of your actual lift capabilities and stall.

So the dude at the flyby needed more power to get the jet climbing, even at max AoA that the control laws allowed. A "conventional" system would have allowed him to pull further back on the stick and then get way behind the "power curve". You know, the area of reverse command.

Let's face it. Wasn't flare mode or Alpha protect or any of the limiters. It was low power setting and a poorly executed demo. And then the 6 "pees".

Hello gums,
"The potential rate of climb is a simple function of power available versus power required for your configuration."

Agreed! I was addressing the problems of flying level and VERY low, stabilised at alpha-max - as they intended, but never achieved. I think that's what HN39 also had in mind.

The TOGA go-around is another matter, although they didn't initiate it in time. Pity (the seventh P)...

Yeah, Chris, we are on the same page.

I did not mean to challenge techniques and such, just point out that "power" was the problem at the flyby, and not the flight control laws and such.

Staying out of the coffin corner near the ground.
 

Back at the dawn of the commercial jet age, when piston pilots were transitioning to jets, there was a rather horrific accident caused by a poorly planned approach and failure to allow sufficient time to spool the engines up. United Airlines Flight 227 - Wikipedia, the free encyclopedia The Captain had 334 hours of jet time at the time of the accident having transitioned from the DC-6. Again, this was early in the jet age, and many of the safety initiatives that you might consider standard had not yet been developed.

Since I was then undergoing basic jet training and was soon to go to the boat, I understood the reasons for the accident and it made an impression on me that still remains. You do not ever want to be caught near the ground on the wrong side of the jet engine acceleration curve. Too bad that lesson was forgotten/not known by the Habsheim crew.

More recently we have had a couple of accidents that resulted from pilots expecting A/T to manage their thrust while they flew the nose of their aircraft. When the A/T did something unexpected, they were caught with insufficient time to spool up before smiting the ground.

By this point in the jet age, pilots should universally understand that jet engines have a significant spool up time from idle (although it is much better now than when I was carrier qualifying in aircraft powered by a centrifugal flow jet engines and early axial flow engines.)

Since we seem to design modern cockpits where you must stare at a PFD to get your critical flight data, do we need some simple thrust indications on the PFD? Do we need a "bitching Betty" when your thrust vs altitude doesn't make sense? (Something that would not be disabled by the Radalt locking on zero as happened in Amsterdam.) I think we still have room for improvements in thrust awareness.

No need for acceleration, sufficient thrust alone will do.

It was not erroneously omitted, it was simply not necessary.

If he had, I do suspect the BEA would not have kept quiet about it.

Or he could have got what was still in the aerodynamics to attempt to survive the tree tops and head back to BSL ...

Thrust alone will do?
 

Yes, if you're flying a fighter, yes it will, although it might not be the fastest way to achieve the desired height gain.

With an A320 full of people, weighing something like 130,000lb and with about 50,000lb of thrust, it'd be hard to climb without getting more speed to add some lift to help.

Some basic aerodynamics and flight mechanics: Changing the direction of the flightpath upwards requires that the lift force plus the vertical component of thrust is greater than the weight. The only way to increase lift at alpha-max is to increase airspeed. At alpha-max of 17.5 degrees roughly 30% of the gross thrust force contributes to the lift, while 95% contributes to acceleration. So yes, increasing thrust sufficiently to cause the airplane to accelerate will change the flight path upwards.

Power required versus power available
 

Guess I had a different aero course and pilot training courses than those here.

I can't find a single reference to the effect that we must increase lift to climb, with the notable exception of being so far behind the "power curve" that we are virtually stalling.

See: http://www.dept.aoe.vt.edu/~lutze/AOE3104/climb.pdf

See also discussions right here on pPrune forums:

BEST ANGLE vs BEST RATE of climb [Archive] - PPRuNe Forums

So I stand behind the Vermont aero department statement referenced above:

Further, as Nuts pointed out, at an attitude of 17.5 degrees +/-, a significant amount of thrust should contribute to a climb. No need to increase the trimmed AoA or even the negligible amount of gee command to the 'bus computers.

Doggone it, the guy was behind "the curve" at his power setting - no climb capability without more thrust, and it appears he had plenty but got it applied way too late.

Hi gums,

strange that you never felt the 'gee' when you pulled up to increase your rate of climb. The point is that with the stick at the stop and the airplane at alpha-max in level flight you can't pull up.

gums,

To repeat myself, the tricky scenario HN39 and I were discussing was not the go-around, but the planned fly-past at an AoA of alpha-max, using manual thrust to maintain height by keeping the IAS at Valpha-max. This would involve a delicate balancing act of thrust that doesn't apply in the case of the go-around.

As HN39 first pointed out: with the stick already fully back, the only way to arrest a descent (perhaps due to a tailwind shear) would be to add thrust. You are wrong to say that no extra lift is required to initiate recovery from a descent (or, for that matter, start a climb from level flight). As I stated in a previous post, any increase of FPA requires a temporary increase in Nz (normal acceleration), which can only be achieved by increasing the lift. With the AoA already at the maximum permitted, the extra lift is dependent on extra IAS or an increased vertical component of thrust.

Once the new FPA is established, the lift requirement reduces somewhat, but the higher pitch-attitude reduces the vertical component of the wing lift.

Hope that makes sense! When compared with what we airline guys are used to, it's worth remembering that in this case we are dealing with the very edge of the flight anvelope.

Gums,

Are you thinking of having rather more thrust to weight in hand to gain height?

I think you're right in general, you can use increased thrust to climb without changing speed, but perhaps not usefully in the special situation that the A320 was holding itself at near the maximum AoA, and lift, that it could achieve at that speed. I think that adding height from more thrust in that state would be possible, but gradual.

Potatoes are better with salt, tea is worse with salt !
So is Any SOP's design, depending on the case, better or worse with automation.

Designers of a new aircraft must know the limit between the two domains in SOPs of that aircraft.

Human activities use mostly closed loops and wait the feedback .
Go around is nearly the only SOP that pilots learn to do in direct loops. That is taught before the first solo flight. Instructors and pilots know that it ALWAYS works. But automation and systems never claim probability=1.

Reading Machinbird's, gums' and others' experience, Go Around are much safer, better understood, better realized when it is strictly handflown rather with magic A/T TOGA button, despite we must learn the lesson from late thrust decision from Capt Gale C. Kehmeier and Michel Asseline , in United Airlines flight 227 and Habsheim.

Pilots must respect what their old and wise instructors taught hundreds times.
Then the SOP will be safe and not confusing. Designers must respect pilots' brain training.

TOGA button provides royalties on every sold aircraft l, would it crash or not.

Another issue for Asseline was that he has been surprised TWO times by the landing gear warnings. He was not waiting these alarms, he was behind his aircraft already at that moment. He was totaly in trouble, did not undestand the systems, the runways, the landing gear, the warning, and surely was no more able to resolve the energy, drag and
lift reckoning, angle and Vs climb, he was living in a schyzophrenic -Classic and Airbus - system. So was Mazières too : during the trial 9 years after Habsheim, he still said to the Court he thougt the plane could not stall, and seemed to still think it. Pierre Baud laughed and said the energy decided.

Thank you, Okie.

From Okie's posts previously, I assume he had experience in a FBW system before the AB320 came on line. The Block 10 comment is the clue, and I flew Bk1,5,10 and 15.

The problem I have seen in the 'bus laws was the implementation of an AoA input, primarily with the gear down. In short, the only way to establish a constant AoA gear up or down is to use the Okie technique and pull full back, achieve whatever AoA the FCS allows and go from there. Don't try this at home, but if you have sim time, then go for it.

Our primitive FBW law with the gear down had an AoA bias mixed with the gee command. And recall that ours used a gee command like the 'bus, but we could trim for a certain gee if we wished. So with gear down we had the "feel" of what all of us were used to - speed stability due to AoA. On my leading edge flap failure approach, I was able to control flight path using power versus increasing or decreasing AoA/speed. This was important because the guy before me pranged by increasing the AoA in a "flare".

@ Chris , et al.....As far as feeling a Nz change when using power to increase or decrease rate of descent, it's negligible. 100 or 200 feet per minute is about 0.07 gee for a second or two until stabilized. up or down. Even my sensitive butt could not feel that.

So one more time: The 'bus FCS control law is for a one gee command corrected for pitch attitude. Without being at one of the AoA limits, you won't see any change in rate of climb with a power change. You just keep going faster or slower on the existing flight path. But once at an aero limit like AoA, you perform just like all the books say.

That is entirely correct but not relevant. With the SS full aft the pilot has no control over the elevator position, the FCS in alpha-protection mode controls it. Holding the SS full aft sufficiently long results in AoA equal to alpha-max. With the thrust correctly set the airplane can fly around all day at Valphamax with the SS on the aft stop, provided it stays clear of terra firma. To stay clear of the ground it is not sufficient to 'set the correct thrust'. The airplane must be actively controlled to counteract disturbances. It requires a certain 'room to maneuver', depending on the magnitude of disturbances and how easily and accurately it can be controlled.

Sure, thrust changes will produce changes in vertical speed, but in my judgement this mode of flight path control is less responsive, less accurate, and less intuitive than pitch control.

As to the magnitude of disturbances, the accident occurred on a sunny summer afternoon. The wind was northerly at 5 - 10 knots. The presence of 1/8 cumulus at 780 m indicates that the lower atmosphere was slightly unstable, and surface wind therefore likely to be variable in magnitude and direction.

A change in headwind component results in a change of IAS. A loss of airspeed results in a loss of lift at constant AoA. But we do not know how accurately the FCS maintains a constant AoA because there is also the phugoid damping feature to consider.

So given these factors, what would have been a safe height above terrain to conduct the planned demonstration?

Quotes from OK465:

"Having the SS full aft doesn't necessarily equate to full ANU elevator or any specific elevator position at all."

I don't recall anyone arguing here that it does.

"Though I don't have the luxury of an AOA gage, as long as there exists some residual elevator authority (i.e. elevator not pegged full ANU), the FCS can 'move' the nose resulting in a flight path change commensurate with thrust applied."

To "move the nose" up = instantaneous increase of AoA.

"The range of AOA that's available to the FCS to generate a pitch rate, I can't see on a cockpit gage unfortunately..."

Yes. BTW, is there a read-out on the sim-instructor panel?

"...but observed on FFS flight package diagnostics, ostensibly from flight test data, overshoots and undershoots from the stabilized Alphamax value appear to occur during vertical transitions due to thrust changes....but no more than a couple knots IAS variation depending on rate of thrust application or reduction."

Now that is interesting... And reminiscent of the Gordon Corps video, in which the AoA does vary noticeably - despite his steady application of full back-stick (as discussed on this thread from January 2nd).

It suggests that the EFCS may keep a degree or two in hand - short of alpha-max - to allow the pilot a small increase in pitch to deliver an increase in AoA. That would provide an increase in lift to deliver the increase in Nz involved in any flare manoeuvre.

If by pulling up you mean climbing, why couldn't you ?
Or are you already full thrust to barely maintain the altitude ?

If you increase speed, you're not at alpha max any more.
If you maintain full back stick you maintain alpha max, therefore you maintain Valphamax - No speed increase.
What makes you climb in this case is a sufficient thrust increase alone.

How would you want the pilot to go and get 'a small increase in pitch' as he's already full back stick in order to obtain and maintain alpha max ... ?

CONF iture
With full backstick flight path change due to elevators is taken out of equation so you can only climb due to thrust vector but it has to develop first then overcome the momentum so there will be noticeable lag. If you are doing this at 100ft or 50ft above ground I don't think you have a chance. If thrust was idle then it takes almost 3 to 4 seconds for the thrust to come out of idle.

" If you maintain full back stick you maintain alpha max" - correct. The lift at alpha max is then proportional to airspeed squared. If the speed drops(*) you get nz < 1 and start to descend, if the speed increases you get nz > 1 and start to climb.

(*) Due to a change in headwind component, for example.

(To keep it simple I left out the thrust vector and phugoid damping).