Habsheim

[awblain]

Drag? Skijump?

Drag would increase. But only marginally.
They were descending and slowing gently with the existing settings, so that rate of slowing from drag would have increased, in addition to the slowing from energy exchange.

OG is right. There was probably not time in any case. Being gentle, they'd need a while to make the exchange.

Free falling a distance h=14m takes a time t = sqrt(2h/g)=1.7s. Could they have pulled up to 2g to take this time? If so, they'd surely have enjoyed an accelerated stall, which in this case wouldn't have lead to them doing a Bud Holland/Elmendorf C17, but rather an unavailability of sufficient lift to execute the commanded climb.

If they didn't realize they needed more power, it's unlikely that they'd realize they needed to exchange speed for height either.

Climbing to over tree-top height would also have reduced their ground effect… perhaps leading to a stall? Would flopping onto the trees have been less destructive than settling into them? Almost all the passengers were unshaken enough to climb out. If it had dropped from 30m, that might not have been the case.

A skijump is very different - that points you upwards using the reaction force with the ground through the wheels. Zeus, for whatever reason, forgot to grab under the A320 and push to give them such an advantage.

[rudderrudderrat]

Hi C_Star,

Any idea how that could affect the height gained?

My calculation was for 100% efficient energy swap, which is reasonably close to what an aircraft could do on the front side of the drag curve when thrust> drag with a speed reduction.

However this flight was well below min drag speed on the curve, so any speed reduction results in more drag - hence less time to impact.

The flight was doomed when they attempted Alpha Max with idle power without having either an infinitely long runway ahead of them, or sufficient height to allow for 6 seconds of engine spool up time.
Pretty basic stuff really.

[Chris Scott]

Quote from C_Star:
"I think there would be some loss due to increased drag due to higer AoA required to pull g's and to maintain lower speed afterwards. Any idea how that could affect the height gained?"

Welcome! That may be the nub of the issue. OG implies it above:
"A zoom climb being essentially a pull up manoeuvre one has to consider the 'g' available to execute such a pull up. One might expect the lift curve to be nonlinear up near alphamax, so that is one complication. Ignoring any nonlinearity but allowing for the zero lift AOA, we might expect an available 'g' of about 1.1 at 14.5 deg AOA falling to 1.00 at 17.5 deg."

That's just one of the reasons I'm uneasy about my ski-jump analogy (but thought I'd air it anyway, particularly as it's now the wintersports season). In the Harrier ski-jump, the vertical acceleration results from the reaction between the vehicle and what amounts to terra-firma.
The A320 has to generate it by an increase in AoA (and therefore drag),

(I'm going to be simplistic and empirical here. The Harrier ski-jump looks like a gentle, continuous curve, rather than the level-change ramps at a multi-storey car park, but I'm going to describe the latter in the hope that the principle would remain roughly the same.)

Once the rotation of the A320 has been completed and the climb established, the extra lift associated with the increasing AoA can be used to enable a gain of altitude. Well, not much, because it has lost airspeed in the rotation... The airspeed decay-rate for a given thrust (still negligible) has increased, due to the climb angle. As r-r-rat points out, the a/c is seriously on the wrong side of its drag curve...

The Harrier is supported by the ramp throughout, and does not need any lift from its wing. That must reduce the drag considerably.

Once the two a/c have left their respective "ski-jumps" (in the case of the A320, when it reaches alpha-max), they become semi-ballistic (please pardon that expression, but I can't think of a better one). The A320 follows a quasi-parabolic () trajectory at alpha-max. The Harrier presumably has to rotate to a suitable AoA, perhaps not far short of the stall, and vector its thrust upwards?

The A320 will return rapidly towards its start-altitude, and the resulting VS will have to be arrested before the treetops are reached, The Harrier, on carrier ops, has the luxury of having the extra height of the carrier deck to play with before it hits the water...

[CONF iture]

Originally Posted by Owain Glyndwr

Other calculations show that if the elevator had been brought back faster or earlier during the accident flight, an AoA greater than 15° would have been obtained before the impact on the trees.

Totally misleading translation here, commande de profondeur stands for sidestick not elevator.
Question is why the elevators did the opposite of the sidestick displacement ?
That is the question that all of you should try to find an answer for ... but don't worry both BEA + Airbus have taken great care to avoid the question in the first place ...

[Owain Glyndwr]

Totally misleading???

Only in your mind I think

Ok, I have changed it in both posts.

[awblain]

Skijump ramp shapes - very different to Habsheim

The skijump ramp isn't a flat plane with a sharp change of gradient at the bottom, as that would probably smash the nose wheel as it hit it.

The role of the ramp is simply to rotate the aircraft quickly to the best compromise of attitude and flight path to climb away from the ship. Both are set by the inclination at the top - the profile is just chosen to minimize the stress on the landing gear. The forward speed of the ship then determines the angle of attack off the top of ramp.

This has no relationship with the pre-tree 5 seconds at Habsheim.

[Saint-Ex]

The dreadful Box Film programme on the accident found an accident investigator who also asked why the up elevator command resulted in a pitch down movement. They also asked the opinion of a PPL expert, also ignorant of the 320`s fby philosophy. I did the translation of the initial report and it appears the elevator acted exactly as it was programmed to. To their great credit, BA had the aircraft back in service shortly afterwards and they had a highly respected and very critical flight safety department.

[rogerg]

BA had the aircraft back in service shortly
BA?

[HazelNuts39]

From the NTSB investigation of the A320 ditchng in the Hudson river on 15 January 2009:

NTSB Accident Number DCA09MAS026, Docket Item 86
Aircraft Performance 13 - Factual Report of Group Chairman
url=http://dms.ntsb.gov/pubdms/search/document.cfm?docID=322563&docketID=47230&mkey

Reference 8 documents an Airbus simulation of the last 300 ft of the flight, and indicates that the airplane was performing as designed as was in α-protection mode from 150 ft to touchdown. Per Reference 6 (quoted above), in α-protection mode, “the angle of attack is proportional to side stick deflection. That is, in the αprot range, from αprot to αmax the side stick commands α directly” while keeping α < αmax. However, in α-protection mode, the flight control system incorporates a phugoid-damping feedback term in addition to side stick commands when computing the commanded elevator position (which in turn determines the pitch angle response). As described by Airbus,

… the aircraft was in angle-of attack (AoA) protection from about 150 ft RA.
When in AoA protection law, stick command is AoA objective. Stick neutral commands alpha-prot and
full back stick commands alpha-max.
However, AoA protection shall take care of the A/C trajectory and, thus, looks after phugoid damping
as well as AoA control: there are feedbacks within the AoA protection law aiming at damping the
phugoid mode (low frequency mode). The feedbacks are CAS and pitch attitude variations. Without
these feedbacks, an aircraft upset from its stabilized flight point up to constant high AoA would enter
a phugoid (which is, by definition, a constant AoA oscillation) without possibility to stabilize the
trajectory. As a consequence, commanded AoA is modulated as a function of speed and attitude
variations: for instance, if A/C speed is decreasing and/or pitch attitude is increasing, pilot's
commanded AoA is lowered in order to avoid such a situation to degrade.
On the last 10 sec of the "Hudson" event, it is confirmed that pitch attitude is increasing and CAS
decreasing. Then, the phugoid damping terms are non nul and are acting in the sense to decrease
the finally commanded AoA vs. the stick command, in order to prevent the aircraft from increasing the
phugoid features.

Based on this explanation, it appears that on the accident flight, the nose-up side stick commands from 15:30:36 to 15:30:43 were offset somewhat by the phugoid-damping feedback term, thereby limiting the pitch angle and α increase below 150 ft radio altitude.


E. Conclusions
(…) A phugoid damping feedback term in the flight control laws, that is active in α−protection mode, attenuated the airplane’s nose-up pitch response to progressively larger aft side stick inputs made below 100 ft radio altitude.

[DozyWannabe]

Originally Posted by CONF iture

Question is why the elevators did the opposite of the sidestick displacement ?
That is the question that all of you should try to find an answer for ... but don't worry both BEA + Airbus have taken great care to avoid the question in the first place ...

Now who's being misleading? The answer to that question is summarised right there in the report - the behaviour of the aircraft was consistent with High AoA Protection mode. I suspect that the problem is not that you don't know the answer so much as you don't *like* the answer.

I must admit that despite initial reservations, creating this thread has given me an opportunity to understand the situation in much more detail, and everything new I've seen seems to back up the BEA's summary.

Correct me if I'm wrong, but you seem to be operating on the assumption that pulling up when in High AoA Protection will command the flight control systems to achieve and hold Alpha Max (as it is displayed on the graph) almost immediately based on a snapshot of the aircraft's orientation and configuration at that precise point. If so, this assumption appears to be in error.

For starters, the FCTM material earlier in the thread seems to flatly contradict this assumption - the wording states either that Alpha Max "may" be achieved, or doesn't refer to Alpha Max at all ("a maximum AoA" isn't the same thing). What they state is that the system will maintain a setting which will provide maximum lift based on the current status of the aircraft.

As OG correctly points out, in order for a digital system to work reliably in real-time there needs to be a degree of "filtering" of the data. I'd be very surprised if the systems did not, in addition to this filtering, observe trends over time (as they would have to when, for example, checking that the A/THR disconnect switches were held down for a certain period of time).

Presuming that the FCTMs are correct, what pulling back on the sidestick will do when in High AoA Protection mode is command the systems to configure the flight surfaces to provide maximum lift based on the data over a certain period of time. Airbus spent the best part of a decade, with their experimental FBW A300 and the first A320s off the line, collecting and refining the data used to define the aircraft's behaviour.

As I was saying when I first donned my Speculation Helmet, the systems would have been fed with data indicating that the aircraft was slowing down. Because the laws of physics demand that there must be a delay between the movement of a flight surface and the aircraft responding to that movement, as well as other uncontrollable external factors such as wind speed and direction, it follows that the pitch attitude commanded in a situation where the aircraft is slowing down will need to be somewhat shy of what it is at that precise moment - otherwise it risks encroaching on approach to stall, which would defeat the whole purpose of the protection.

Therefore, as pulling back in that mode cannot be assumed to give Alpha Max (or, at least, not immediately), what that sidestick command actually "tells" the flight control system is "give me the best AoA you can". So, going back to the beginning, the elevators were briefly commanded nose-down because the systems were trying to maintain the optimum (not necessarily maximum) AoA, and that was what was required to maintain it. And if I were to hazard a guess, given the period in which the elevator position was noted, the reason it was required would have been to counter the pitch-up moment from the engines as they spooled up and started producing significant thrust.

[roulishollandais]

Because the laws of physics demand that there must be a delay between the movement of a flight surface and the aircraft responding to that movement, as well as other uncontrollable external factors such as wind speed and direction, it follows that the pitch attitude commanded in a situation where the aircraft is slowing down will need to be somewhat shy of what it is at that precise moment - otherwise it risks encroaching on approach to stall, which would defeat the whole purpose of the protection.

Therefore, as pulling back in that mode cannot be assumed to give Alpha Max, what that sidestick command actually "tells" the flight control system is "give me the best AoA you can". So, going back to the beginning, the elevators were briefly commanded nose-down because the systems were trying to maintain the optimum (not necessarily maximum) AoA, and that was what was required to maintain it.

Laws of physic don't request much time : As freefaller we are used to modify the position, to feel on the body the aerodynamic forces and moments created, and resume the movement in less than 1/20 second. Not the tens of seconds of the phugoďd low period. An aerobatic's plane needs a longer time than /20 second but is reacting very fast. What needs time on your plane is the result of cheap and low sampling (perhaps to avoid high frequency transient response?) or a bad model and algorithm specialy near the stall.

And what is optimized? Cost as usual?

It is often question of "time needed" but no figures are given. The expert Max Venet said during the Habsheim trial - answering to the President of the Court - "we don't know very well how long the aircraft needed to be less than 30 feet RA over the trees (before the runway threeshold, not after the end of the runway..) to get in flare - I listened and
understood that - what is sure is that he did not find in the hard- and software documentation and modification history the answer to that question.

I wonder too to see now the ref of 150 FT RA in the landing algorithm -Hudson- (and phugoid damping) which is much more than the most oftened read 100 FT RA or 50 or 30 FT : How could the crew and the Court know really how the system works?

[rudderrudderrat]

Hi roulishollandais,

How could the crew and the Court know really how the system works?

They didn't know exactly how it works, because FCOM is not that detailed. However, the system did exactly as it says on the tin - by preventing a stall.
I don't know how close to the stall you, as a passenger on that flight, would want to be flown at below 50' radio, but please tell us - how much closer to the stall would you still have been happy?

[HazelNuts39]

Semantics

Originally Posted by DozyWannabe

For starters, the FCTM material earlier in the thread seems to flatly contradict this assumption - the wording states either that Alpha Max "may" be achieved, or doesn't refer to Alpha Max at all ("a maximum AoA" isn't the same thing).

Perhaps we are are dealing with semantics again, but I think Airbus offers a more accurate description in the above reference:

Stick neutral commands alpha-prot and full back stick commands alpha-max.
However,(...) there are feedbacks within the AoA protection law aiming at damping the phugoid mode (low frequency mode). The feedbacks are CAS and pitch attitude variations. (...) if A/C speed is decreasing and/or pitch attitude is increasing, pilot's commanded AoA is lowered (...).

[roulishollandais]

Originally Posted by rudderrruddererrat

I don't know how close to the stall you, as a passenger on that flight, would want to be flown at below 50' radio, but please tell us - how much closer to the stall would you still have been happy?

If it is only protection against stall 150 FT RA is already a little low

[Owain Glyndwr]

Hi Hazlenuts

However,(...) there are feedbacks within the AoA protection law aiming at damping the phugoid mode (low frequency mode). The feedbacks are CAS and pitch attitude variations. (...) if A/C speed is decreasing and/or pitch attitude is increasing, pilot's commanded AoA is lowered (...).

OK, I can go along with that. I was bitching that the actual phugoid mode would not have much effect on the short term motion, but I can see that the phase advance damping terms in the control laws might well do so.

[Chris Scott]

Hudson River accident - FBW protections

roulishollandais

Correct me if I'm wrong, but I think you may be misinterpreting two sentences of the NTSB report into the Hudson Bay ditching:
"… the aircraft was in angle-of attack (AoA) protection from about 150 ft RA."
AND
"...Based on this explanation, it appears that on the accident flight, the nose-up side stick commands from 15:30:36 to 15:30:43 were offset somewhat by the phugoid-damping feedback term, thereby limiting the pitch angle and α increase below 150 ft radio altitude."

So, on the A320's ditching approach, Alpha-Prot Law was activated at 150R, because the AoA happened to reach alpha-prot at that height. It remained engaged thereafter because it takes priority over Normal Law, including the landing mode (which normally commences at 50R, and which at 30R starts to require an increasing amount of back-stick if the pilot wants the pitch-attitude to be maintained).

You state in your post, above:
I wonder too to see now the ref of 150 FT RA in the landing algorithm -Hudson- (and phugoid damping) which is much more than the most oftened read 100 FT RA or 50 or 30 FT : How could the crew and the Court know really how the system works?

So, on the Hudson approach, 150R was an arbitrary height at which Alpha-Prot Law had to take over from C* (Normal Law). In the extract quoted by HN39, the report does not state that the feedbacks used in Alpha-Prot Law - aimed at damping the pitch-phugoid tendency - change at any height. As for 100R, that is simply the height below which Alpha-Floor is inhibited.

However, it may be worth reminding ourselves that the Hudson accident was 20 years after Habsheim.

HN39,

FWIW, I don't regard any of your discussion about phugoid damping as semantic!

[Chris Scott]

Monday morning in Blighty

Quotes from Saint-Ex:

"To their great credit, BA had the aircraft back in service shortly afterwards..."

Just in case that causes any reader to infer that the Habsheim accident a/c was operated by, or owned by, BA: it was neither.

I think we did ground our A320s for about 24 hours. The AF accident took place on a Sunday afternoon...

"...they [BA] had a highly respected and very critical flight safety department."

Yes, although I don't think there were any A320-qualified people specifically on it at that stage. We had only been operating our a/c down at Gatwick for a couple of months, and all of us aircrew and our fleet management were ex-BCAL.

"I did the translation of the initial report..."

Which report was that?

[DozyWannabe]

Is he talking about the A320 that almost landed on Crawley High St?

[EDIT : As discussion later in the thread details, this comment came as a result of reading the Andrew Weir book "The Tombstone Imperative", the first edition of which mentioned this incident in the book, and prominently used the phrase "almost landed on Crawley High Street" on the back cover. In order to verify this, I tracked down a later edition of the book, in which both references were deleted. If this error has caused any upset, I apologise.]

[Saint-Ex]

Hi Chris. I was on an A320 fllght the night before the Air France crash. I didn`t think it necessary to mention the aircraft was not one of BA`s as it was so widely reported.
As far as Flight Safety is concerned you may remember the first BA pilot to be allocated to the Airbus actually headed the Flight Safety department.
I was given the first French report very soon after the crash and it comprised the full CVR recording (unbelieveable) and, if I remember correctly, an analysis of the removeable FDR.

[CONF iture]

Originally Posted by Chris Scott

For an A320-100 (no winglets), the VS1G is a CAS of 114 kt. Given that VS1G is said to be defined on the A320 as the steady airspeed at the alpha-max in this confiuration of 17.5 deg (not the CL-MAX), provided the Nz (normal acceleration) is 1G, that begs the question of why the AoA coincident with 112 kt on the Habsheim fly-past was only +14.

Maybe something does not add up here ...
The comment made by HN39 has its merit but if Vs1g is stalling speed at 1g how V alpha max would be Vs1g ?

Originally Posted by rudderrudderrat

I don't know how close to the stall you, as a passenger on that flight, would want to be flown at below 50' radio, but please tell us - how much closer to the stall would you still have been happy?

Very unhappy to be intentionally flown that way, but considering how badly the situation was engaged ... very happy if the system commanded alpha max as requested by the pilot to give a chance to avoid the crash.