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Cactus521

Recent crash in Netherlands, focusses on altimeter

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That's interesting. I certainly hope the investigators get to the bottom of this accident, as it is quite a disturbing scenario to have three crew on the flight deck and come down in such a curious way.It seems odd to imagine that the crew would not be closely monitoring speed, altitude and rate of descent when in low visibility. Kind of makes me wonder if the P1 and monitoring P2 were distracted by having a third person on the flight deck and possibly placed too much trust in the autopilot whilst chatting. I understand the third person on the FD was a trainee, so it seems reasonable to assume they'd be briefing the trainee about stuff, and if that proved a distraction from monitoring the instruments, then they wouldn't be the first crew to have an accident because of that.But that's speculation of course, and I guess you'd have to hear the cockpit voice recording to determine the possibility of such a scenario.Al

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Briefly, this is what I gathered from the initial report:The radio altimeters went from 1950 to -8 due to some fault, the A/T followed this and went into retard mode as if the aircraft was in flare. The aircraft stalled and the crew couldn't recover from it. It seems that the aircraft was in autoland mode.I didn't know that the A/T would follow the radio altimeter. BTW, it has also been said that this same fault was seen recently on this aircraft.EDIT: The first thing that came to my mind was the lack of monitoring. How could the crew have missed the discrepancy?

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""According to conversation recorded between the plane's captain, first officer and an extra first officer on the flight, the pilots noticed the faulty altimeter but didn't consider it a problem and didn't react, Van Vollenhoven said.""Being Dutch, I analysed the official announcent by Mr. van Vollenhove, but IMO it's a bit ambigous whether the pilots actually noticed the faulty RadioAlt meter or not. That is: when listening to his statement.What he said was (free translation):"... that the crew noticed the faulty left RadioAlt meter, namely because they got a Gear warning ...."and"... that the crew didn't consider this signalling to be a problem ...."What puzzles me is the second statement, where he says "... that the crew didn't consider this SIGNALLING to be a problem ..."Instead of "....the crew didn't consider the faulty RadioAlt meter to be a problem..."Which is a big difference IMO.Usually (because of possible legal consequences), such announcements are very well and accurately formulated.So I wonder whether the crew actually did notice the faulty Altmeter or not.Time will tell ....But whatever happened exactly, I find it very hard to understand why the crew didn't notice the decrease in airspeed (way below normal), and high pitch angle (because the aircraft was trying to follow glideslope with too low airspeed), untill it was too late.I don't want to pass judgement on the crew at all, but at least you can conclude it's another example of aircraft crew relying on 'automation' too much c.q. the crew 'procedures' to check if the 'automation' is making sence, isn't strict enough.In short, another classic example of where the intuitive human brain and human observation could have corrected faulty 'automation'.Which was programmed by other humans, by the way. (I'm referring to the coupling of AutoThrottle to the left RadioAlt meter). Rob

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Excellent post Rob. I agree with what you say on the crew not noticing the anomalies. I wonder if that altimeter fault was on the MEL list since it was also reported before.I hope we, the general public, will get more information. This initial report has raised more questions than it has answered.

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I didn't know that the A/T would follow the radio altimeter. BTW, it has also been said that this same fault was seen recently on this aircraft.
It seems to make sense (theoretically) that the A/T would respond to the radio altimeter. The FMC database, though it has airport elevation, wouldn't be as accurate as a real-time reading from the altimeter, so the autoland would use the radio altimeter to coordinate the flare.
The radio altimeters went from 1950 to -8 due to some fault...
Damn birds...On a more serious note, maybe the radio altimeter was briefly confused by something which it wasn't programmed to deal with well; I don't imagine a 738's altimeter reads -8 as a matter of course.

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Being Dutch, I analysed the official announcent
Rob,Perhaps you can answer a question for us. Was this aircraft ever cleared for landing?I can only find a clearance down to 4,000 feet, and from then on, no communication between the tower and this aircraft until after it has crashed very close to the runway.Cheers,

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I don't imagine a 738's altimeter reads -8 as a matter of course.
I would have to dig again into Boeing manuals but if I recall the negative altitude is a valid reading of the RA during autoland when aircraft reaches the phase of FLARE and then ROLLOUT which is annunciated on the PFD. Obviously it should never happen the way it happened here when aircraft was still hundreds of feet above ground.

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I assume the "signalling" might have meant that the pilots looking at their Baro alt knew that the gear warning wasn't a problem per se, but the question is did they troubleshoot what caused the alarm?Interesting in that we often hear that the bus FBW throttle is inferior because of its lack of movement, while if this idea of A/T retard (if not flare) is correct, the throttle levers must have moved to idle without crew recognizing it. scott s..

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the throttle levers must have moved to idle without crew recognizing it.
If (as alleged) their heads were turned back towards the trainee sitting on the jumpseat whom they were instructing on use of the checklist then it is easy to understand how they missed everything. Not only they missed the throttle movement but they missed all the annunciations on the PFD. It is not the first time that a minor technical issue leads to major crash becasue attention of the whole crew is diverted to something else.

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I wouldn't wish to second guess the accident investigators, but I've just had a look at the B737 manual for stuff relating to the radio altimeter on approach and some related stuff. I thought it might be of interest to people reading this thread:If you have a dual AP approach set up on the MCP, the automatic flare commences when the radio altimeter reads 50 feet and the autothrottle then retards at 27 feet. I can't find any mention of a negative value on the RA (at least not in the section I read). But based on those values, it doesn't need to get near a negative value to commence flaring and retarding the throttle.The autothrottle disengages 2 seconds after touchdown, but whether touchdown is read as a parameter from the RA, or the weight on the wheels, is not made clear in the manual, although one suspects it is from the RA based on what has occurred in this accident and the fact that the process begins when airborne.If you hit TOGA when in a dual autopilot mode, the FD commands a 15 degree nose pitch up, in addition to the thrust increase, which is obviously likely to precipitate a stall if the speed is very low and the nose goes up before the engines spool up properly and succeed in getting some speed on the clock. Which could explain the tail striking first in the case of this accident, if the crew did hit the TOGA switch. (see the note below)The B737 manual warns that at low airspeeds, such as when windshear might be encountered on an ILS approach - a situation presumably not dissimilar to that which the crew of the Turkish 737 found themselves in, with minimal airspeed on approach (albeit for different reasons) - both the FD and the autopilot will still try and hold the airplane on the glideslope without regard to either angle of attack or stick shaker limitations, and the manual further warns that the airspeed could decrease into a stall if the pilot does not intervene by either hitting the TOGA switch, or disconnecting the autopilot and manually flying the aircraft.It goes on to state that: 'the flight crew must, if necessary to avoid ground contact, be prepared to disconnect the autothrottle, advance the thrust levers to the forward stop, disconnect the autopilot, and manually fly the airplane.' Which of course means that if you hit TOGA when there's 'nothing on the clock but the maker's name', it is likely to make matters worse in comparison to disconnecting everything, ramming the thrust levers to the stops and manually keeping the nose down.This - other than the factual stuff quoted from the manual - is fairly speculative with regard to what occurred in this incident, and I think without hearing the comments on the flight deck it's not really going to be possible to say what happened. Naturally, it is easy to be wise about such things when sitting reading a manual drinking a cup of coffee. Quite different of course when you have seconds to make a choice.Al

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if the crew did hit the TOGA switch.
They never did, based on press reports.

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Which of course means that if you hit TOGA when there's 'nothing on the clock but the maker's name', it is likely to make matters worse in comparison to disconnecting everything, ramming the thrust levers to the stops and manually keeping the nose down.
Aside from the press reports, I find it a bit dubious that the crew would have thought to hit the TOGA switch. This is nothing but speculation on my part (disclaimer: I am not a pilot, have never taken flight lessons, and have never read airplane manuals, but have read a fair bit of theory), but I'm led to believe that the training the crew would have received would have ground into them the procedure for near-stall and stall recovery, and that they would have known that hitting the TOGA switch, even if they didn't specifically remember the section in the manual you quoted, would not produce a desirable reaction (pitching the nose up rather than down).

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Rob,Perhaps you can answer a question for us. Was this aircraft ever cleared for landing?I can only find a clearance down to 4,000 feet, and from then on, no communication between the tower and this aircraft until after it has crashed very close to the runway.Cheers,
Hi Kevin,Although that wasn't explicitely said in that 22 min. statement, I can only assume it was.Because of the statement "everything appeared to be normal".If it hadn't be cleared for landing, I'm sure that would have been reported now.But as said: this statement probably generated more questions than that it answered.Like:- SHOULD the crew have noticed the fatal decrease in airspeed according to 'procedures' ? (I.o.w.: do procedures require them to monitor airspeed in an automated landing? And how often ? )- And if so, WHY didn't they ? Or, if they did notice it, why didn't they react earlier ? - According to the black boxes, this Altmeter problem happened already twice in the previous 10 flights. But the statement doesn't clarify whether this was actually noticed by the other crews or maintenance personel, and if so, what their reaction has been or if/what was done about it.In short, this statement was just a preliminary result of the investigation commitee 's findings, and one can doubt whether it was wise to come with such a statement when all those questions are still unanswered, other than the actual technical problem that lead to the crash.Because it already has unfortunately lead to different "opinions" in Holland and Turkey about the accident. Like, in Turkey, the crew have been presented in the press as "heroes", because they managed to "crash" the aircraft into such a (relatively) good location (muddy clay), thereby saving a lot of lives.The least you can conclude from the committee's statement, that there was nothing deliberate about the location of the crash, and that they were actually trying to save the aircraft and avoid the crash at all.IMO it was wise though, since such a disaster could happen at any time again if pilots aren't aware what the consequences of a faulty left Altmeter can be (even IF they are aware of the Altmeter being faulty). Hence the warning to Boeing (about the A/T-coupling) made by the committee.So it will take a lot of time longer to establish ALL the facts leading/contributing to this crash.And even when all the "facts" are known, there will probably still remain discussions on how to weigh/interprete them; as in many air crash investigations.Because it undoubtedly (and IMO unfortunately) usually leads to the question "who's to blame" (because of legal/financial consequences).Like, even todate, there are still a different opinions on "who's to blame" in the "Tenerife" crash (where a landed, taxiing 747 collided with a departing 747).I've seen 3 documentories on that (US, Dutch, Spanish made), and they all come to a different interpretation/weighing of the "facts".Rob

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