Hi Hazelnuts

You may be right in that, but I'd be willing to bet that the acceleration information is passed through a low pass filter before use in that way, if only to eliminate nuisance signals due to noise. This is going to slug the action I think.

AoA data
 

Re your interest in the AI report of 1995, I only have a PDF that someone kindly sent me. If you PM me with an e-mail address, I will send it to you.

I see that you and AZR have been using the last (data) column of Tome 1 for "INC" (AoA) data.

There are other "INCIDENCE" (AoA) data in Tome 6. On the line for each second are recorded two values, each to one place of decimals; unlike the single column you are using. It is unclear to me at the moment whether the 2 values are from different sources (e.g., FAC1 and FAC2), or if they represent 2 samples per second from a single source. I favour the former explanation, because next to them are twin values of what appear to be sideslip, which is another parameter handled by the FACs.

Using UTC 12:45:39** as the "t" reference, here are the decoded AoA values, from Tomes 1 and 6 respectively, for the last 20 seconds of flight:

t -19 +08 U08.3 U07.2
t -18 +08 U07.6 U08.2
t -17 +09 U09.3 U09.3
t -16 +09 U09.0 U08.9
t -15 +08 U08.4 U07.9
t -14 +09 U08.9 U10.6
t -13 +11 U11.3 U10.8
t -12 +12 U11.6 U11.5
t -11 +11 U10.9 U10.1
t -10 +10 U09.7 U11.4
t -09 +12 U12.1 U13.1
t -08 +14 U13.5 U13.9
t -07 +14 U13.9 U13.1
t -06 +13 U13.1 U12.7
t -05 +14 U14.4 U14.4
t -04 +15 U15.3 U14.4
t -03 +14 U14.3 U13.8
t -02 +14 U13.8 U15.6
t -01 +15 U15.1 U14.9
t -00 +15 U15.3 D73.8
t +01 -00 D00.2 D00.2

** (i.e., elapsed time 335.0 seconds in the second column)

Can anyone inform us of the precise sources of the above AoAs, and if the Tome 6 pairs of data are sequential, or from two different sources simultaneously?

HN39,
I agree that the sequential times in seconds in column 2 are elapsed times from an arbitrary point, which may be the selection of TOGA thrust at the start of the T/O run. (The report states that the a/c was airborne at "12:41", and that the R/H turn was commenced during the second which followed.) :sad:
Perhaps the BEA could supply us with pages 1 - 6 of the DFDR print-outs... ;)

Quote from Owain Glyndwr:
I must admit that the debate has been to a much higher standard than I feared it might be.

Praise indeed... :cool:

Hi Chris,

thanks for the good work. I support your thought that the two columns in Tome 6 probably are from two sources. The values in the first column rounded to the nearest degree correspond to the value in Tome 1.

It is now also apparent what the 1234 in the last column represent. Apparently 4 subframes form one full frame. That permits the number of parameters to be increased, by using a single location in a subframe for multiple alternating parameters, e.g. A and B each sampled every other second, or A, B, C and D once every 4 seconds.

Hi,

As I'm not sure many of the readers of this thread can read French, allow me to quote and (try to) translate some relevant sentences from the BEA report:

(end of § 1.16.1.4.2) :
Other [data-analysis] operations performed also show that if the sidestick had been brought faster or earlier back during the accident flight, an AoA greater than 15° would have been obtained before the impact on the trees. They also show that without an earlier go around [understand: advancing the throttle levers earlier], such a maneuver, which led to increase the instantaneous drag of the aircraft would not have enabled to avoid impact with the trees.


Also noteworthy:

§ 1.16.1.3 (wrongly labelled 1.16.1.13 in the BEA report/PDF) "Expertises effectuées"
The proper functioning of the flight controls has never been questioned by the crew of the accidented A320. The Board of Inquiry, however, found it necessary to check this thoroughly.


§ 1.16.2 "Fonctionnement des moteurs"
In contrast to the flight controls case, engine performance has been questioned by the crew immediately after the accident; the crew indeed declared that after advancing throttle levers, engine thrust had not been delivered.
However, it was established from the early readings of the recorders (CVR and DFDR), that the engines had responded to the request of go around thrust which was performed between 5 and 5.5 seconds before the impact on the trees

Hope this helps.

Thanks AZR,

I think some of us are enjoying deliberately delaying looking at the BEA's conclusions ! ;)

The crew's perception that the two CFM 56-5-A1s were slow to spool up probably indicates more about the effects on their metabolic-rate of the urgency of the situation they suddenly recognised than it does about the performance of the engines.

Why would the two engines, controlled independently by their respective FADECs, accelerate slower than normal; but in perfect unison?

Yes, thanks AZR, that was very helpful.

I would translate it slightly more positively, but essentially those BEA remarks are consistent with what I was trying to say.

The quality of the traces in the published BEA report is so poor that I imagine many people would like to see something better. This is an extract from that AI report cited by Chris Scott

http://i1081.photobucket.com/albums/...ps2f727183.jpg

and this is a somewhat lower quality record of the Airbus flight test replication of Habsheim.
http://i1081.photobucket.com/albums/...ps7e3c62ea.jpg

The crosses are, I think, Habsheim actuals. The AOA trace shows a similar restricted AOA gain to Habsheim over the first few seconds, but goes on to achieve Alphamax a few seconds later.

HN39's Graphs
 

Good evening HN39,

That's a super graph! I was going to ask you which sources you chose for the Alpha1 and Alpha2 curves, and I see that you have now averaged the two values into one curve.

However, although it doesn't affect the overall picture radically, I remain unhappy with the apparent one-second discrepancy between the time bases you and I have been using. (See also my posts of Dec18/0021z and Dec18/2008z.)

In the text accompanying your first graph, you quoted the BEA as follows:
"...the report puts t=0 at 12:45:39 and states that the airplane entered alpha protection mode at 12:45:34 (t-5) at 122 kIAS and alpha=13 degrees."

Why would Alpha-Prot have engaged at an AoA of +13, i.e., 1.5 degrees early? As previously mentioned, the only reference I've found so far is as follows (1.16.1.2):
"A t - 4 s, commutation sur la loi de pilotage en incidence, la valeur de 14.5 deg ayant ete atteinte, cette loi etant ensuite conservee."
[At t - 4 s, switching into the angle-of-attack law, the value of 14.5 deg having been attained, this law being maintained thereafter.]

It also defines "t" as "GMT" 12:45:39; as you have stated. My interpretation of the DFDR print-out is that 12:45:39 is paired with an elapsed time in seconds of 335.0 - not the "334" which you state.

Referring to the first of the many lines annotated "1245", which must represent 12:45:00, the elapsed time is 296.0. So 12:45:30 would be 326.0, and 12:45:39 would be 335.0. That is the last line on each DFDR "Tome", page 8. (BTW, the elapsed times continue at the top of Page 9, but - curiously - the associated GMT times are erroneous.)

So, using your time base, where "t" (12:45:39) is 334.0, t -5 is 329.0, at which the AoA on Tome 1 is indeed +13 (and the Tome 6 values U13.1 and U12.7).

If I am right, however, the time-vs-AoA figures are as follows:
t -06 12:45:33 (329.0) +13 U13.1 U12.7 (Landing mode of Normal Law)
t -05 12:45:34 (330.0) +14 U14.4 U14.4 (Landing mode of Normal Law)
t -04 12:45:35 (331.0) +15 U15.3 U14.4 (Alpha Prot)
t -03 12:45:36 (332.0) +14 U14.3 U13.8 (Alpha Prot)
t -02 12:45:37 (333.0) +14 U13.8 U15.6 (Alpha Prot)
t -01 12:45:38 (334.0) +15 U15.1 U14.9 (Alpha Prot)
t -00 12:45:39 (335.0) +15 U15.3 D73.8 (Alpha Prot)
t +01 12:45:40 (336.0) -00 D00.2 D00.2

I suggest that the numbers annotated on your time axis need to be moved one space to the right. Also, the first two values of "AlphaCmd" need to be deleted, because Alpha Prot did not start until t -4.

Hi Chris,

In reply to your post#107 I already admitted in post #109 that I was in error for the time of entry into alpha-protection mode, which is not t-5 but is t-4. I'm on the move right now but will remove the AlphaCmd point at t-5 as soon as I have retrieved my laptop from my luggage.

At 12:45:34 airspeed is122 kIAS and Alpha is 13 degrees is recorded at 329.0s
Five seconds later at 12:45:39 t=0 so that must be at 329.0s + 5 = 334.0s

Hi Owain Glyndwr,
Thx for your comment (the same goes to you, Chris Scott) :)

Your translation is better than mine, I agree :ok:

Hi HN39,

Sorry to catch you "on the hoof"! When you have time to have a closer look, our one-second discrepancy in timings is identified in your answer.

Quote from your response (my emphasis):
"At 12:45:34 airspeed is122 kIAS and Alpha is 13 degrees is recorded at 329.0s"

My interpretation is that 329.0s = 12:45:33 (see explanation offered in my previous post). But I have little experience in DFDR readings.

Quote:
"Five seconds later at 12:45:39 t=0 so that must be at 329.0s + 5 = 334.0s"

According to my interpretation, that should read "six seconds later". As you can see in my previous post, I have the AoA datum of +13 at t -6.

PS
I now understand what you meant about all the parameters in a one-second time frame not necessarily being recorded at the same instant, but possibly sequentially during the whole second. Do you or anyone else have any news on that?

Re: rudderrat - exchange speed for height
 

You're right. They could have exchanged another 5kt (2.5m/s) of airspeed, from 110 to 105 (~55m/s). for height h (in m) ~ v.Delta-v/g ~ 55(2.5)/10 ~ 14m.

The key question is… did they have that 5kt of airspeed to exchange (FBW doesn't change the physics), while avoiding a stall.

There was also an interesting point about the motion of the aircraft when the elevator changes position - there is indeed an immediate tail-down pitch acceleration, about the center of mass, plus a linear downward acceleration, due to an increase in negative lift from the horizontal stabilizer... the change to the attitude then gives an upward acceleration from the larger, increasing wing lift, traded for forward speed. These changes in response to an instant movement of the elevator would extend over ~ a second.

Hi Chris,

My timeline is based on the information given in the report for 12:45:34 and 12:45:39. That would put frame no. 296 at 12:45:01.

While I agree that one would expect the first 1245 frame at 12:45:00, that doesn't fit with the two points mentioned in the report. Maybe BEA's GMT's are one second off, but who cares? What matters are the seconds to some reference time in the recording. The report on page 10 also puts the impact with the trees between 12:45:39 and 12:45:40. The data clearly show a discontinuity between frames 334 and 335, so that fits also. Another fit is 114 kIAS at 'ts'.

Hi,

If not yet read .. you can read this Airbus report
"The A320 Habcheim Accident
An Airbus Industrie response to allegations made in television programme and other medias"
Rapport Airbus.pdf - Petit Fichier

Possible benefit of "Zoom Climb", using any available airspeed
 

Owain Glyndwr's translation of a BEA conclusion:
"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. They also show that without an earlier thrust increase such a manoeuvre, which would have led to an increase in the instantaneous drag of the aircraft would not have permitted (the aircraft) to avoid impact with the trees."

I notice that OG has so far not specifically disputed rudderrudderrat's calculation that a trade of kinetic energy by zoom climb from an airspeed of 112 kt to 107 kt (assuming a steady wind) would provide an altitude gain of 52 ft. (I assume that the 112 kt was based on the IAS recorded at DFDR time-frame of 333.0 secs, which is only 1 or 2 seconds before the defined point of impact.)

awblain has also taken up the cudgels (my emphasis):
"You're right. They could have exchanged another 5kt (2.5m/s) of airspeed, from 110 to 105 (~55m/s). for height h (in m) ~ v.Delta-v/g ~ 55(2.5)/10 ~ 14m.
"The key question is… did they have that 5kt of airspeed to exchange (FBW doesn't change the physics), while avoiding a stall. "

The short answer is in the negative. The GW was about 59 tonnes, with "Flaps" (config) 3 and L/G down. 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.

My suggestion is that the a/c was in a slight bunt-manoeuvre. The Nz at that stage is around 0.93G, and the pitch has been reducing. Although the IAS had fallen from 116 in the previous second, the ground-speed remained the same (112 kt). That suggests a mini-tailwind shear in that second. The following second shows an IAS of 114 at an AoA of +15 with 1.00G. However, it appears that either the a/c was overperforming slightly, or the ZFW (zero fuel weight) may have been lower than calculated on the loadsheet.

I think it might be more realistic, therefore, to postulate a zoom climb initiated at time frame 329.0 sec; 5 or 6 secs before impact. That is the point at which TOGA thrust had been commanded, at IAS 122. Therefore, the PF was already anxious to expedite his go-around from that point. If rudderrudderrat's energy calculation is good, that should provide at least as much height-gain as he calciulated for his.

As for the zoom climb per-se, I would be very interested to hear the views of Owain Glyndwr, John Farley, and others as to whether it is in any way analogous to the ski-jump concept used operationally, 6 years earlier, to increase the take-off performance of the Hawker/BAC Harrier to permit - in effect - overweight take-offs in STO mode.

I'm wondering if the gain in altitude achieved at the expense of IAS, followed by a semi-ballistic trajectory - maintaining alpha-max at an Nz below 1G - might have provided two advantages, compared with maintaining height/altitude into the treetops.
(1) The initial impact would be avoided (and the engines were rapidly spooling up).
(2) The semi-ballistic segment would delay the stall, providing even more time for the engines to add energy to the total-energy equation. In the case of the Harrier, the engine is already at its TOGA thrust (and, admittedly, with thrust-vectoring). The A320 at Habsheim was light, of course, with very great surplus performance at TOGA, and less than two seconds short of achieving TOGA thrust at the time of impact.

@Chris

Partly because as an energy calculation it is unexceptional, partly because of proccupation with another task in conjunction with a mutual friend and partly because I didn't want to get too involved in a debate about what is actually a complex issue.
The missing parameter is, once again, time. 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 is going to mean that the latter part of any zoom climb is going to be prolonged, and in practice indistinguishable from an (unsustainable) steady climb. If a full zoom is going to take a long time then if the intent were to clear the trees by another 50ft or so the zoom would have to have been initiated well back down the runway. In fact of course any zoom is only needed to give just enough clearance to avoid impact so that only the first part of any pull up is relevant and once again timing of such a manoeuvre is critical.

For me, that issue is covered by the BEA report statement I quoted in earlier posts.

PS I agree with your comments on 'stall' getting in the way.
PPS I'll let John Farley make any comments on ski-jumping.

*Dons the Helmet Of Speculation*

If I were to make an educated guess, I'd say it was because the CAS trend indicated a steady loss of airspeed, and the logic based its pitch limit on the assumption that the trend would continue - which it did right up until t-1.

Hi,

Another thing regarding the zoom climb - the calculation presented assumes that all of the kinetic energy from the speed loss would be converted into height.

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?

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.

Hi C_Star,
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.

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...

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 ...

Totally misleading???

Only in your mind I think

Ok, I have changed it in both posts.

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.

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.

BA had the aircraft back in service shortly
BA?

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

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.

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 phugoïd 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?

Hi roulishollandais,
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?

Semantics
 

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 (...).

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

Hi Hazlenuts

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.

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!

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?

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.]

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.

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 ?


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.