TAM A320 crash at Congonhas, Brazil
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I´m curious.....
Has Boeing fixed the non auto retraction of spoilers upon thrust levers advancement on the 757??? And have they fixed the non retardation of the thrust levers/thrust upon inflight deployment of thrust reverser on the 767???(AA 757 in Colombia and Lauda Air 767)
Has Boeing fixed the non auto retraction of spoilers upon thrust levers advancement on the 757??? And have they fixed the non retardation of the thrust levers/thrust upon inflight deployment of thrust reverser on the 767???(AA 757 in Colombia and Lauda Air 767)
The GROUND SPOILERS on the 757 will retract if the thrust levers are advanced after landing (i.e a baulked landing / go-around situation).
The flight spoilers will not retract in flight if the thrust levers are opened, adn nor should they - why would you want that - there are situations where you want thrust and spoilers (burning fuel off in a hold to get down to max landing weight being just one of many)
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I'm quite amazed about the level of "my airplane system may be bad, but your's isn't perfect as well!" that I'm seeing here.
Is there any ground rule that we can talk about all human factors we want but not criticize the technology involved?
Actually, naive as I am, I would have thought that specifically pilots of the system involved in this accident should have a vested interest to understand if and what system on the airplane has failed, as they could find themselves in the same pr a similar situation at some point.
pj
Is there any ground rule that we can talk about all human factors we want but not criticize the technology involved?
Actually, naive as I am, I would have thought that specifically pilots of the system involved in this accident should have a vested interest to understand if and what system on the airplane has failed, as they could find themselves in the same pr a similar situation at some point.
pj
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What a bunch of hypocrats. The brazilian authorities had no problems jumping to conclusions when they arrested two American pilots without a proper investigation.
I know TAM had nothing to do with it, but it is pathetic to see what a change in attitude Brazil has had in general when it is one of their own.
That might be the impression you’re getting out wherever you are – but TAM is experiencing a sound financial trouncing over here, with ticket sales down by 30% or more. And the local politicians aren’t being too finicky in hammering TAM to the stake and lighting a bonfire – if it serves their purpose.
I hope they learn something about propper investigation and implementing changes to prevent further accidents rather than just throwing out blame.
Sorry to disappoint you but up until the Gol/ExcelAire midair collision, Brazil’s CENIPA was far and away the best aircraft accident investigation board in Latin America, earning recognition from its peers in the US, UK and elsewhere for the manner in which it conducted its investigations – especially those involving foreign flag carriers. Needless to say, these two major mishaps (Gol/ExcelAire & TAM) have seriously tarnished the CENIPA’s reputation – and through no fault of its own.
Unfortunatey, judging by the way they are handling the whole ATC fiasco, I don't think they will.
Brazil’s ATC problems are far more complex than what meets the eye and it would be best to open up a an entirely new thread to discuss it.
Regards
Tail Chase
the deadly (mind) set
Going through the pages of David Beatys excellent book, the "naked pilot", a possible scenario is the following:
- pilots discuss the reverser inop several times during approach
- When landing, the PF simply discards this engine with the inop reverser from his mind when retarding (since the engine does not add to the braking)
- "retard" warning shuts up after one engine is idle
- the rest is pretty well known know
- pilots discuss the reverser inop several times during approach
- When landing, the PF simply discards this engine with the inop reverser from his mind when retarding (since the engine does not add to the braking)
- "retard" warning shuts up after one engine is idle
- the rest is pretty well known know
Last edited by clearedtocross; 3rd Aug 2007 at 11:30.
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flyingnewbie10
It seems like the word 'malfeasance' is tacked on to anything that has corporate involvement in your mind.
Yes, aircraft manufacturers have done some pretty silly things in the past to protect the bottom line, Airbus in not taking criticism of the early A320 interface (pre-A330 incident and subsequent improvement) on board early enough, Boeing maintaining that the 737 rudder actuator did not have a potentially dangerous failure mode despite 2 unexplained hull losses and many more incidents and even Douglas with their DC10 cargo door design (1 serious incident and 1 hull loss).
On the other hand, they do occasionally do the right thing. Boeing admitted responsibility for the botched repair that brought down JAL123, Airbus did eventually improve the systems interface of their FBW family and Douglas (by then MD) fixed their cargo door.
I think that a software change that continues the "RETARD RETARD" warning when either of the levers is forward of the IDLE detent would be worth pursuing (subject to a thorough HF report), but at the end of the day the levers were not retarded. I saw a post earlier stating that AB changed a procedure saying that both levers should be pulled into reverse, as opposed to the only the one with the functioning reverser, but even the previous procedure would have the lever to the engine with the non-functioning reverser at IDLE, not forward of it!
Yes, aircraft manufacturers have done some pretty silly things in the past to protect the bottom line, Airbus in not taking criticism of the early A320 interface (pre-A330 incident and subsequent improvement) on board early enough, Boeing maintaining that the 737 rudder actuator did not have a potentially dangerous failure mode despite 2 unexplained hull losses and many more incidents and even Douglas with their DC10 cargo door design (1 serious incident and 1 hull loss).
On the other hand, they do occasionally do the right thing. Boeing admitted responsibility for the botched repair that brought down JAL123, Airbus did eventually improve the systems interface of their FBW family and Douglas (by then MD) fixed their cargo door.
I think that a software change that continues the "RETARD RETARD" warning when either of the levers is forward of the IDLE detent would be worth pursuing (subject to a thorough HF report), but at the end of the day the levers were not retarded. I saw a post earlier stating that AB changed a procedure saying that both levers should be pulled into reverse, as opposed to the only the one with the functioning reverser, but even the previous procedure would have the lever to the engine with the non-functioning reverser at IDLE, not forward of it!
I know similar incidents have been raised elsewhere on this thread but since there seems to be a danger of drifting towards a pilot error scenario which leaves the living happily off the hook, I make no apology for raising again. According to ASN database, there have been 16 A320 hull losses, but 4 of these were from ground damage due to fire, so really only 12.
Of these 12, it seems "one thrust reverser locked out, throttle mishandled" features in 3 (Bacolod,Phoenix & Congonhas) plus in one other serious overrun (Taipei). That's surely more than just a coincidence of the same human error in managing the throttles in such an apparently innocuous defect situation...is it replicated on other types, and if not, why on the AB?
There was also a "thrust reverser locked,throttle mishandled" scenario in one fatal A310 overrun (Irkutsk).
It would of course be interesting to know whether other incidents have occurred but been remedied in time & without serious consequences - I guess embarrassment could be an understandable deterrent to reporting.
Nevertheless, the reported advisory Airbus mod to prevent throttle mismatch seems to lack urgency given the apparent similarities.
Incidentally the Phoenix scenario is interesting in that by applying reverse to both (incl the locked out engine) there seems to have been a loss of directional control, so would this have been the best idea at slippery CGH?
Bacolod
http://aviation-safety.net/database/...?id=19980322-0
Irkutsk
http://aviation-safety.net/database/...?id=20060709-0
Phoenix
http://aviation-safety.net/database/...?id=20020828-1
Taipei
http://aviation-safety.net/database/...?id=20041018-0
Of these 12, it seems "one thrust reverser locked out, throttle mishandled" features in 3 (Bacolod,Phoenix & Congonhas) plus in one other serious overrun (Taipei). That's surely more than just a coincidence of the same human error in managing the throttles in such an apparently innocuous defect situation...is it replicated on other types, and if not, why on the AB?
There was also a "thrust reverser locked,throttle mishandled" scenario in one fatal A310 overrun (Irkutsk).
It would of course be interesting to know whether other incidents have occurred but been remedied in time & without serious consequences - I guess embarrassment could be an understandable deterrent to reporting.
Nevertheless, the reported advisory Airbus mod to prevent throttle mismatch seems to lack urgency given the apparent similarities.
Incidentally the Phoenix scenario is interesting in that by applying reverse to both (incl the locked out engine) there seems to have been a loss of directional control, so would this have been the best idea at slippery CGH?
Bacolod
http://aviation-safety.net/database/...?id=19980322-0
Irkutsk
http://aviation-safety.net/database/...?id=20060709-0
Phoenix
http://aviation-safety.net/database/...?id=20020828-1
Taipei
http://aviation-safety.net/database/...?id=20041018-0
Last edited by Max Tow; 3rd Aug 2007 at 09:44.
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to maxtow
just a small correction max. TAipei over run by TransAsia A320 was not a fatal accident. IN fact there were no injuries and the plane is still in service. TransAsia had a mishap in Tainan with an A321 where the plane ran over a truck on landing. That plane was written off but no injuries to crew or passengers. Two people in the truck suffered injuries from which they later recovered .
The Taipei over run could easily have been a Congonhas type accident had the runway not been long enough for them to come to a safe stop in the grass after the end of the runway. The scenario was almost identical.
The Taipei over run could easily have been a Congonhas type accident had the runway not been long enough for them to come to a safe stop in the grass after the end of the runway. The scenario was almost identical.
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Max Tow,
Correct, but I think it is wise to include the one A321 hull loss as well as the one A319 hull loss, for a total of 18.
I think also that hitting a ground vehicle on landing is nothing really to do with the airplane type (Transasia, Tainan 2003) so "really" only 11.
Bacolod is not clear to me; people suggest a "technical problem". At Phoenix, the thrust on one engine was mishandled during rollout.
The overrun at Taipei was not fatal, neither was the aircraft taken out of service. It is the incident which bears most resemblance to the most recent information about Congonhas. The pilots left one thrust lever at 22.5 degrees throughout the landing (flare and rollout). The report goes into extensive detail about everything except what I would call the engineering psychology: *why* the pilots, who were experienced by any standards, did not retard that thrust lever. And that is, of course, the most crucial insight that could have been obtained and shared.
I think twice in some 50+ million flights stands a very good chance of being coincidence, yes. Unfortunately, we will likely never be able to find any common reasons if there happen to be any, since the Taiwanese passed up the chance and the Brazilian pilots are no longer around to interview.
The phenomenon of failures with similarities is replicated on other types. For the other similar aircraft type introduced in the 80's, the B737 2nd generation, one can note the problem with rudder hard-over.
Apropos of "not fixing what ain't broke", one should be very wary of modifying something which has shown itself to work well in all but a handful of cases in 50 million. Every change buys you a new system possibly with new quirks that will take you another few million flights to exhibit. Best first to understand *exactly why* you might want to change
system, which means understanding *exactly how* it went wrong in the couple of cases that are worrying you.
BTW, I do not agree with patrickal's comparison of digital flight control systems with Windows operating systems, cell phones other consumer electronics. Safety-critical digital systems, especially in aerospace, are designed and built to different standards and criteria from other digital systems, using for the most part different tools. And they are very highly scrutinised at very high cost. However, we still don't seem to be able to guarantee performance better than one fault in one hundred thousand hours of operation (these are mathematical limits on evidence about complex-system behavior; note that I said "guarantee", I did not say that there are not systems around with better reliability, only that we do not know). Compare that with two instances of thrust not reduced to idle at flare in 50 million landings and notice a factor of 500 difference! And think about it next time you fly in your favorite FBW airplane.
PBL
According to ASN database, 16 A320 hull losses, but 4 of these ground damage due fire, so really only 12.
I think also that hitting a ground vehicle on landing is nothing really to do with the airplane type (Transasia, Tainan 2003) so "really" only 11.
Of these, it seems one thrust reverser locked out/throttle mishandled in 3 (Bacolod,Phoenix & Congonhas) plus in one other fatal overrun (Taipei).
The overrun at Taipei was not fatal, neither was the aircraft taken out of service. It is the incident which bears most resemblance to the most recent information about Congonhas. The pilots left one thrust lever at 22.5 degrees throughout the landing (flare and rollout). The report goes into extensive detail about everything except what I would call the engineering psychology: *why* the pilots, who were experienced by any standards, did not retard that thrust lever. And that is, of course, the most crucial insight that could have been obtained and shared.
That's surely more than just a coincidence of human errors in managing the throttles in such a defect situation...is it replicated on other types?
The phenomenon of failures with similarities is replicated on other types. For the other similar aircraft type introduced in the 80's, the B737 2nd generation, one can note the problem with rudder hard-over.
Apropos of "not fixing what ain't broke", one should be very wary of modifying something which has shown itself to work well in all but a handful of cases in 50 million. Every change buys you a new system possibly with new quirks that will take you another few million flights to exhibit. Best first to understand *exactly why* you might want to change
system, which means understanding *exactly how* it went wrong in the couple of cases that are worrying you.
BTW, I do not agree with patrickal's comparison of digital flight control systems with Windows operating systems, cell phones other consumer electronics. Safety-critical digital systems, especially in aerospace, are designed and built to different standards and criteria from other digital systems, using for the most part different tools. And they are very highly scrutinised at very high cost. However, we still don't seem to be able to guarantee performance better than one fault in one hundred thousand hours of operation (these are mathematical limits on evidence about complex-system behavior; note that I said "guarantee", I did not say that there are not systems around with better reliability, only that we do not know). Compare that with two instances of thrust not reduced to idle at flare in 50 million landings and notice a factor of 500 difference! And think about it next time you fly in your favorite FBW airplane.
PBL
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Originally Posted by Dream Land
This accident could have been prevented a few different ways, let's see:
.
2. An auto thrust system similar to the Boeing's that retards thrust automatically at a certain height,
.
4. Another idea by Tyro, whereby T/R deployment is dependent on spoilers being deployed, thereby giving you the go around option.
.
2. An auto thrust system similar to the Boeing's that retards thrust automatically at a certain height,
.
4. Another idea by Tyro, whereby T/R deployment is dependent on spoilers being deployed, thereby giving you the go around option.
In a Boeing, you only get the thrust to go back to idle automatically in an autoland, just like an Airbus. The approach was flown landed manually.
The "T/R deployment" is commanded to the FADEC from the spoilers (SEC), in this case they did not deploy as with a thrust lever angle greater than 20 degrees above IDLE the logic is for a touch and go or rejected landing, it has been reported that the right thrust lever was left in the approach setting, therefore the split in the thrust levers in this accident is something like 15-20cm (6-8").
An attempt to move them BOTH to idle like on any aircraft when landing, even with one at IDLE and the other at 15 degrees above IDLE (less than a 10 degree rearward shift in the thrust lever) of thrust lever angle the spoilers would have deployed (20 to 35 degree range in the diagram below).
Nothing stops a Boeing from going down a runway with a thrust levers at 70% N1 or 1.20 EPR and the other at idle, on some Boeing types you can also have on in reverse and one above reverse.
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PBL, I agree with you that the statistical probability of this specific accident to happen has shown to be rather low.
I would, however, like some spotlight on the spoiler inhibiting logic, where the Warsaw accident bears some similarity.
Basically, there's a chance that the system goes into the "logical sitting duck" mode. Meaning that the logic can't come to a decision on the state change.
As others have pointed out, in the given configuration, wheels spinning, squat switches (supposedly) depressed, one engine in reverse, pilots applying manual brakes, we're certainly committed to the landing.
Even if it was only relatively few lives lost, it should be considered to create an override mode for the ground spoiler system.
Also, the state "one engine in TOGA/CLB, the other in REV" seems to be not very meaningful and it should be investigated if there could be a logical lockout for that.
Of course, this is all for the final report to show, but I really hope, it will go to that detail rather than just stating a pilot error.
pj
I would, however, like some spotlight on the spoiler inhibiting logic, where the Warsaw accident bears some similarity.
Basically, there's a chance that the system goes into the "logical sitting duck" mode. Meaning that the logic can't come to a decision on the state change.
As others have pointed out, in the given configuration, wheels spinning, squat switches (supposedly) depressed, one engine in reverse, pilots applying manual brakes, we're certainly committed to the landing.
Even if it was only relatively few lives lost, it should be considered to create an override mode for the ground spoiler system.
Also, the state "one engine in TOGA/CLB, the other in REV" seems to be not very meaningful and it should be investigated if there could be a logical lockout for that.
Of course, this is all for the final report to show, but I really hope, it will go to that detail rather than just stating a pilot error.
pj
PBL
It would seem you are at odds with the ASN Bacolod synopsis.
I don't buy the 4 similar accidents in "x" million flying hours = acceptable coincidence argument. Given your reduced figure of only 11 A320 operational hull losses, I think it's entirely reasonable to look at those for consistent factors. The purpose of accident investigation, after all, is to ensure as far as possible that we don't fall into the same hole twice, and to answer the question (for at least the benefit of relatives and liability lawyers) as to whether with the technical and historical information available, the accident chain of events could have been broken.
Let me declare an interest here: one of the saddest accident enquiries in which I took part involved a mishandling of an apparently innocuous defect, with near catastrophic consequences. Within 24 hours, two other pilots had the courage to come forward and admit that they had done the same thing (but recovered the situation) but in each case had not reported because they felt it was their unique stupidity rather than a procedure easily amended.
It would seem you are at odds with the ASN Bacolod synopsis.
I don't buy the 4 similar accidents in "x" million flying hours = acceptable coincidence argument. Given your reduced figure of only 11 A320 operational hull losses, I think it's entirely reasonable to look at those for consistent factors. The purpose of accident investigation, after all, is to ensure as far as possible that we don't fall into the same hole twice, and to answer the question (for at least the benefit of relatives and liability lawyers) as to whether with the technical and historical information available, the accident chain of events could have been broken.
Let me declare an interest here: one of the saddest accident enquiries in which I took part involved a mishandling of an apparently innocuous defect, with near catastrophic consequences. Within 24 hours, two other pilots had the courage to come forward and admit that they had done the same thing (but recovered the situation) but in each case had not reported because they felt it was their unique stupidity rather than a procedure easily amended.
Last edited by Max Tow; 3rd Aug 2007 at 10:40.
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BTW,
If I may (and I realise this is off-topic), I would like to put in my two cents on the polity, because I think a few points are in danger of being missed.
First, it is obviously very important for Brazilians, especially those living and flying out of Congonhas, to be able to figure out if and how operations at Congonhas might have to change, and to figure this out as soon as possible. That is unavoidably a political decision, since it affects large numbers of people. From this point of view, there is an advantage to knowing what went on in this accident as soon as possible, even if the information is incomplete or not 100% accurate. Because at least that helps you decide whether you should focus on changing operations at airlines flying in there, whether you should raze a city block or two to build 600 m overrun and an EMAS, or whether you should just send all planes other than small ones to Guarulhos and erase a few more buildings to build an express railway to get all the passengers there quickly.
I disagree with those Brazilians who are embarrassed by political behavior in light of the accident. It seems to me that through processes which are usual in any democratic country your press and your politicians have succeeded in getting crucial, accurate information about the accident into the public domain within two weeks, in circumstances in which other administrative bodies would have wished to have kept it secret for a long while. Since the decision about what to do about Congonhas (if anything) is properly an urgent public decision, and not necessarily one to be left to an elite (professional or otherwise), one could argue (I will leave it to others to do so if they wish) that this agitation and release of information is ultimately to the public good.
I have also been impressed at the background information which inquisitive journalists have garnered. Whatever they ultimately say or write for whatever reason, many of them have done their homework, indeed rather better than some do around here!
PBL
If I may (and I realise this is off-topic), I would like to put in my two cents on the polity, because I think a few points are in danger of being missed.
First, it is obviously very important for Brazilians, especially those living and flying out of Congonhas, to be able to figure out if and how operations at Congonhas might have to change, and to figure this out as soon as possible. That is unavoidably a political decision, since it affects large numbers of people. From this point of view, there is an advantage to knowing what went on in this accident as soon as possible, even if the information is incomplete or not 100% accurate. Because at least that helps you decide whether you should focus on changing operations at airlines flying in there, whether you should raze a city block or two to build 600 m overrun and an EMAS, or whether you should just send all planes other than small ones to Guarulhos and erase a few more buildings to build an express railway to get all the passengers there quickly.
I disagree with those Brazilians who are embarrassed by political behavior in light of the accident. It seems to me that through processes which are usual in any democratic country your press and your politicians have succeeded in getting crucial, accurate information about the accident into the public domain within two weeks, in circumstances in which other administrative bodies would have wished to have kept it secret for a long while. Since the decision about what to do about Congonhas (if anything) is properly an urgent public decision, and not necessarily one to be left to an elite (professional or otherwise), one could argue (I will leave it to others to do so if they wish) that this agitation and release of information is ultimately to the public good.
I have also been impressed at the background information which inquisitive journalists have garnered. Whatever they ultimately say or write for whatever reason, many of them have done their homework, indeed rather better than some do around here!
PBL
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In a Boeing, you only get the thrust to go back to idle automatically in an autoland, just like an Airbus. by Zeke
D.L.
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STS,
Just to be clear, I think along with you that all possible contributing phenomena should be scrutinised as closely as possible. I wouldn't wish you to imagine that I think otherwise.
The "logical sitting duck" mode to which you refer is known in other technical contexts (namely, harware arbitration) as a "metastable state." Maybe we could call it a Buridan state (cf. Buridan's ass), in recognition of the paper of Lamport and Palais which pointed out the theoretical existence of such states in digital systems some thirty years ago, and of Lamport's more popular-style paper recalling Buridan which he wrote on it a decade later.
There is a related issue with so-called Byzantine failures (also named and investigated by Lamport), which remained theoretical objects of interest until more recently. Kevin Driscoll's 2003 SAFECOMP paper points out that a major FBW aircraft type came within days of having its airworthiness certificate withdrawn because of a series of Byzantine failures in its FCS. A Byzantine failure occurs when two different processors take the same sensor reading to have two different values (how can that happen? I suggest reading Driscoll et al's account for a very good explanation).
There are also other kinds of failures which worry me quite a lot, such as the 2005 incident over Perth with a B777 in which a latent fault with the FCS manifested itself and almost caused the crew to lose control over pitching motions. That latent fault had been there in the FCS since the beginning.
I must admit that those kinds of things worry me, and others, really quite a lot. Because you cannot deal with them simply by keeping up to date with your SOPs.
PBL
I would, however, like some spotlight on the spoiler inhibiting logic, where the Warsaw accident bears some similarity.
Basically, there's a chance that the system goes into the "logical sitting duck" mode. Meaning that the logic can't come to a decision on the state change.
Basically, there's a chance that the system goes into the "logical sitting duck" mode. Meaning that the logic can't come to a decision on the state change.
The "logical sitting duck" mode to which you refer is known in other technical contexts (namely, harware arbitration) as a "metastable state." Maybe we could call it a Buridan state (cf. Buridan's ass), in recognition of the paper of Lamport and Palais which pointed out the theoretical existence of such states in digital systems some thirty years ago, and of Lamport's more popular-style paper recalling Buridan which he wrote on it a decade later.
There is a related issue with so-called Byzantine failures (also named and investigated by Lamport), which remained theoretical objects of interest until more recently. Kevin Driscoll's 2003 SAFECOMP paper points out that a major FBW aircraft type came within days of having its airworthiness certificate withdrawn because of a series of Byzantine failures in its FCS. A Byzantine failure occurs when two different processors take the same sensor reading to have two different values (how can that happen? I suggest reading Driscoll et al's account for a very good explanation).
There are also other kinds of failures which worry me quite a lot, such as the 2005 incident over Perth with a B777 in which a latent fault with the FCS manifested itself and almost caused the crew to lose control over pitching motions. That latent fault had been there in the FCS since the beginning.
I must admit that those kinds of things worry me, and others, really quite a lot. Because you cannot deal with them simply by keeping up to date with your SOPs.
PBL
Paxing All Over The World
AlexL
That may be true but what has been stated in this thread on several occasions, is that: with a manual landing - in any manufacturer's machine - if you leave one TL forward, then it will give the thrust commensurate with that position.
When the report comes out, we may understand more of the other statement that seems to be well made in this thread:- 'That which seemed logical to the pilots at the time'.
I am not a pilot, merely someone who has read every single post in this thread.
However I still make the point, that in normal ops the 757 (and all other boeings?) the thrust lever matches the actual thrust delivered at any point n time, whereas on the airbus it doesn't.
When the report comes out, we may understand more of the other statement that seems to be well made in this thread:- 'That which seemed logical to the pilots at the time'.
I am not a pilot, merely someone who has read every single post in this thread.
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Max Tow,
No, not necessarily, I am being agnostic. Reason is that I have part of the report, FAXed by a Philippino correspondent, sitting around somewhere and I seem to remember what it says differently from what I read in ASN.
I also think you are interpreting me a little too far. We appear to judge similarity differently. I also did not say that Taipei-Sungshan and Congonhas were "acceptable coincidence". I said that they could well be coincidence. They might also not be coincidence, but I doubt we will ever find out unless (let's hope not) a couple more similar accidents occur, as with the B737 2nd gen rudder hard-overs. And I don't consider any accident, or any accident rate except zero, "acceptable".
PBL
It would seem you are at odds with the ASN Bacolod synopsis.
I also think you are interpreting me a little too far. We appear to judge similarity differently. I also did not say that Taipei-Sungshan and Congonhas were "acceptable coincidence". I said that they could well be coincidence. They might also not be coincidence, but I doubt we will ever find out unless (let's hope not) a couple more similar accidents occur, as with the B737 2nd gen rudder hard-overs. And I don't consider any accident, or any accident rate except zero, "acceptable".
PBL