Discussion:
Air France 447: Second interim report.
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JF Mezei
2009-12-18 00:01:10 UTC
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This is about the Air France A330 Between Rio and Paris that crashed in
the ocean on June 1 with unrecovered FDR/CVR.


http://www.bea.aero/fr/enquetes/vol.af.447/vol.af.447.php

English version of the 2nd interim report:
http://www.bea.aero/docspa/2009/f-cp090601e2.en/pdf/f-cp090601e2.en.pdf


A new search campaign for the flight recorders is hoped by Fefruary 2010.

There is a graphic on page 14 highlighting where recovered parts
belonged on the intact aircraft. They have recovers from the Radome to
the rudder in the back.

##
1.12.3.1. Cabin parts
A high degree of vertical compression can be seen on the cabin parts
such as the galleys, stowage, partitions and toilet doors. This vertical
compression is observable from the front (stowage and toilet at the
level of door 1) to the rear of the aircraft (Galley G5), and from the
right- to the left-hand sides.

Certain overhead luggage racks were found with their fuselage attachment
fittings. Besides the damage due to the vertical compression, these
fittings have deformations that are due to a forward movement of the
overhead luggage racks.
##


29 passenger O2 containers were recovered. (The box in the overhead bins
with the trap door that opens to deploy the masks). 3 of them had the
door still on, in closed position. (This would indicate that the
emrgency O2 system was not deployed).

The actual mechanism to trigger delivery of O2 was not triggered.

They could determine that 3 of them were in closed position. The
system is designed so that all O2 containers are triggered at the same
time. Depressurisation means pressure inside the cabin corresponding to
an altitude of more than 14,000 ft.

Flaps were retracted at time of impact.


The observations made on the debris (toilet doors, partitions, galleys,
cabin crew rest module, spoiler, aileron, vertical stabiliser) evidenced
high rates of compression resulting from a high rate of descent at the
time of impact with the water.

This high rate of compression can be seen all over the aircraft and
symmetrically on the right- and left-hand sides.


The deformations of the fuselage frames at the root of the vertical
stabiliser
were not consistent with an aircraft nose-down attitude at the moment of
impact.
From these observations it can be deduced that:
-The aircraft was probably intact on impact.
-The aircraft struck the surface of the water with a positive attitude,
a low bank and a high rate of descent.
-There was no depressurisation.




50 were identified. 45 pax, 4 FAs and the Captain.


Eight were seated in business class between doors 1 and 2;
-Three were seated in business class aft of door 2;
-Twelve were seated in economy forward of the over-wing exits;
-Twenty-two were seated at the rear of the airplane, between the
overwing exits and the number 3 doors.

Forty-three of the victims had fractures of the spinal column, the
thorax and the pelvis. The fractures described were located mainly at
the level of the transition vertebrae.



Ar about page 36, they start to analyse and explain each of the ACARS
messages that were received. Very informative read.



At this stage of the investigation, analysis of the messages makes it
possible to highlight an inconsistency in the speeds measured just after
2 h 10 which in that minute generated ten of the twenty-four maintenance
messages. Eleven other messages generated between 2 h 10 and 2 h 14 can
also be linked to anemometric problems (inconsistencies in the speeds,
low speeds and/or erratic speed values).

The aircraft switched to alternate 2 law in the minute at 2 h 10 and
remained in that law until the end of the flight..

No message present in the CFR indicates the loss of displays or of
inertial information (attitudes).



Page 47 in the pDF descripes the different FBW limitations (normal law,
alternate law and direct law).

This seems to be a very good explanation of the various FBW control
laws, what triggers changes etc.

In alternate law, stall protection is not available, but stall warnings
are still issued. (aka: you can use the controls to stall the aircraft).



One aspect mentioned a couple of times is the case where 1 ADR is
rejected because it is too different, but the remaining 2 are the ones
that are wrong, but because they are wrong by the same amount, the
difference between the is small enough to gate the computer to decide
that those values are the right ones.


Summary: Much time is spent analysing the air probed/pitots. Basically,
the previous cases of failure result in autopilot/autothrust disengaging
until the potos thaw again and proper data ia available to the computers
again. In cases where 2 pitos have similar errors, the computers
believe them and ay generate stall warnings because they report a speed
that is too slow.

None of the report tries to explain how the aircraft can go from level
flight to falling flat onto the ocean because of lopss of
airspeed/altitude indicators.


I find it interesting that such aircraft would not have GPS ground-speed
and altitude as inputs to the computers. Seems to me that a GPS
information would provide a sanity check for the computers. ground speed
and GPS altitude may not match airspeed and barometric altitude, but the
computer can see monitor CHANGES in the GPS data to see how it relates
to the air based sensors.


The other aspect not mentioned in the report is whether the many
airprobe failure incidents recorded for the 330/340 have their
equivalents in Boeing aircraft.

Since there are no wind tunnels to test pitots under all flight
conditions, one would think that design problems resulting in some
pitots not behaving under certain circumstances would not be specific to
one aircraft type.
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Uwe Klein
2009-12-18 08:49:42 UTC
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Post by JF Mezei
I find it interesting that such aircraft would not have GPS ground-speed
and altitude as inputs to the computers. Seems to me that a GPS
information would provide a sanity check for the computers. ground speed
and GPS altitude may not match airspeed and barometric altitude, but the
computer can see monitor CHANGES in the GPS data to see how it relates
to the air based sensors.
Mixing in more data has more axis of freedom to make errors.
i.e. you gain maybe plausibility in select cases but the
resulting complete system is less reliable. it is a tradeoff.
Post by JF Mezei
The other aspect not mentioned in the report is whether the many
airprobe failure incidents recorded for the 330/340 have their
equivalents in Boeing aircraft.
Toward the end where they talk about getting data sets from similar
events some Boeing related events are mentioned.
They talk about events reclaimed from recorded data that were
not noticed/reported by the flightcrew at time of relevant flight.

Low mention on Boeing craft may be completely due to low
observability ( and not to incidenceless ).

Observed events seem to cover all types, all manufacturers of pitot tubes
with all available types having functionality well beyond certification
requirements and only slight differences in coverage.

The up to now assumption was that pitot tubes have no _functional_
limitations in the working environment. i.e. their only mode of
fault is a material defect ( with all the acompanying probability
effects of single defect versus 2 simultaneous defects.)
But fault via physical limitations of the sensor will with good probability
be simultaneous!

Synopsis from the report is:
that the certification requirements are no longer sufficient.
that the extended (over cert) manufacturer specs are similarly insufficient.
that there is not sufficient knowledge on the relevant meterological effects.

Open:
Have there been recent atmospheric changes ( Global Warming )
or is it just due to airlines moving to different routing (path/flightlevel)
in the last decade?
( Compare to the 777 ice slush in fuel oil heat exchanger )
Post by JF Mezei
Since there are no wind tunnels to test pitots under all flight
conditions, one would think that design problems resulting in some
pitots not behaving under certain circumstances would not be specific to
one aircraft type.
At this moment we have lost quite a bit of certainty.
Territory that will have to be reclaimed.

uwe
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JF Mezei
2009-12-18 19:58:35 UTC
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Post by Uwe Klein
Observed events seem to cover all types, all manufacturers of pitot tubes
with all available types having functionality well beyond certification
requirements and only slight differences in coverage.
While it *appears* that pitots would have failed, it does not explain
why the aircraft dropped out of the sky.

What was not said in the report is whether the aircraft had significant
forward speed when it impacted water. There is an indication that there
was *some* but they don't seem to have quantified it. They have been
able to say there was significant vertical speed though.

But they know that the aircraft impacted water in an almost level
attitude. This would *seem* to point to a controlled flight as opposed
to a plane falling out of the sky, or the pilot aiming the nose down
towards the ocean.

I would need to reread the report. But if the pitots are faulty, does
the main computer rule out all data from the ADR including altitude, or
would it rule out only airspeed ?

In other words, would pilots have still been informed of their altitude ?

If they had to bring nose down because of a stall indication, then
Shirley they wouldn't have kept it down long enough drop 30k feet ?

What if some freak weather resulted in a thick coat of ice applied to
the wings and fuselage. Not only would the aircraft have lost lift, but
have also been weighted down by ice. Could that ice have survived all
the way to sealevel, preventing aircraft from flying ? (and explaining
high vertical speed despite level attitude at contact with water).

Exactly how does an aircraft measure stall ? Does it have sensors on
wings to measure lift ? Or does it merely calculate maximum angle of
attack for the current altutude/airspeed and aircraft weight ?

For instance, if you are on descending, and you have ice on wings, does
the computer know you have ice on wings and recalculate your minimum
speed to avoid stall ?

or does it declare stall only when the ice accumulation has increased
drag so much that your speed has reduced to a point where you are near
stall ?
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Miles Bader
2009-12-19 03:16:48 UTC
Permalink
Post by JF Mezei
While it *appears* that pitots would have failed, it does not explain
why the aircraft dropped out of the sky.
From what I recall (it's been a while since all this stuff was in the
news), many people were saying that at that altitude/speed/etc, there's
not a huge safety margin, and it's actually fairly easy to stall or
damage the plane if you're not in the slot.

Presumably having an incorrect indication of air-speed might have caused
them to exceed the margin, and... maybe something important just tore
off...

-Miles
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Uwe Klein
2009-12-18 23:46:57 UTC
Permalink
Post by JF Mezei
Post by Uwe Klein
Observed events seem to cover all types, all manufacturers of pitot tubes
with all available types having functionality well beyond certification
requirements and only slight differences in coverage.
While it *appears* that pitots would have failed, it does not explain
why the aircraft dropped out of the sky.
No, or "right".
BEA seems to not report on any events they have no data on.
Which is correct from my point of view.
Post by JF Mezei
What was not said in the report is whether the aircraft had significant
forward speed when it impacted water. There is an indication that there
was *some* but they don't seem to have quantified it. They have been
able to say there was significant vertical speed though.
The piece of hardware that fixes the rudder against the vert. stabiliser
against vertical forces ( a diagonal strap, see image in report ) seems
to have been broken @ 120.000N ~= 36g deceleration.
elsewhere a forward moment of deceleration was mentioned.
_my_ guess is that forward motion was a lot slower than vertical motion.
This would indicate a stall in near perfect attitude?
Lets do a bit of math:
v = sqrt( 2l * a) ; initial velocity, distance for braking available, acceleration
a = 36*9.81m/s^2
l = 2m ( my assumption is that a sparse structure like a widebody will brake
to near zero velocity after 2m of immersion )
then :
v ~= 26m/s ~= 4500ft/min ~= 94km/h which would jibe with terminal velocity
for a falling airliner.
( 36 g would jibe with bone destruction observed on body found imho,
the destructive effect seems to have come from an upward deceleration )
( and just to have an idea about the timeframe: 30000ft / 4500ft/min ~= 6.66 minutes )
can one do a meaningfull test on adrenaline levels on bodies?
This could maybe give an indication on the time lapse from start of stress to crash.
Post by JF Mezei
But they know that the aircraft impacted water in an almost level
attitude. This would *seem* to point to a controlled flight as opposed
to a plane falling out of the sky, or the pilot aiming the nose down
towards the ocean.
NO, see above.
Post by JF Mezei
I would need to reread the report. But if the pitots are faulty, does
the main computer rule out all data from the ADR including altitude, or
would it rule out only airspeed ?
as written before and afaiu: majority rules. if 2 pitots give the same wrong values
those are assumed to be right. This seems to be a distinct Airbus problem.
Post by JF Mezei
In other words, would pilots have still been informed of their altitude ?
altitude is detected by the static ports!?
Post by JF Mezei
If they had to bring nose down because of a stall indication, then
Shirley they wouldn't have kept it down long enough drop 30k feet ?
no information.
Post by JF Mezei
What if some freak weather resulted in a thick coat of ice applied to
the wings and fuselage. Not only would the aircraft have lost lift, but
have also been weighted down by ice. Could that ice have survived all
the way to sealevel, preventing aircraft from flying ? (and explaining
high vertical speed despite level attitude at contact with water).
my guess : yes.
Post by JF Mezei
Exactly how does an aircraft measure stall ? Does it have sensors on
wings to measure lift ? Or does it merely calculate maximum angle of
attack for the current altutude/airspeed and aircraft weight ?
the a330 has Angle of Attack sensors ( newer models may be combined
with pitot? its a turnable vane )
ha found an image:
Loading Image...
Post by JF Mezei
For instance, if you are on descending, and you have ice on wings, does
the computer know you have ice on wings and recalculate your minimum
speed to avoid stall ?
or does it declare stall only when the ice accumulation has increased
drag so much that your speed has reduced to a point where you are near
stall ?
tentative answer: stall is declared on AOA being out of (over) allowable range.

Thinking about your answers/questions.

It could well be that a large amount of ice formed very fast
stuffing all pitots and static ports, freezing fixed all AOA sensors
and maybe ruining airfoile profile.
full darkness outside, pilots drowsy, ..
could it be possible that with indications frozen and
while the aircraft was completely stalled the pilots thought
to be a "correct for flying" attitude and speed?

it still looks very much like a freak accident.

uwe
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