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Habsheim

Old 16th Dec 2013, 21:24
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Dozy
In the event, the crew spotted the airfield late, and noticed that the spectators were lined up on the grass strip, not the paved strip on which they had been briefed. At this point the crew would have been well within their rights to abort the flypast altogether, and press on to the sightseeing trip over the Pyrenees which made up the latter part of their flightplan. Instead, the PIC elected to perform an unauthorised approach maneouvre to match the grass strip, even though they were considerably high and fast to do so.
Wherever you got that information from, it does not look right. The heading difference between the flown grass strip track and the paved strip is 035 degrees, and would have required a total different track from beginning and flight over the houses of Habsheim, which was not permitted for airshow traffic. They would have flown a different track to approach the airfield some miles out with the track of 020 degrees for the paved strip. But the track was flown along the east side of the highway Basel-Straßbourg which is about heading 345 like the grass strip and which is over thin populated areas and offers good visual clues, which makes sense.

The failure was not maintaining the intended altitude of 100 feet AGL. I could go over and measure those trees today after they continued to grow another 25 years, but i doubt that i could find a tree today with a hight of 100 feet.

You may check my points by the maps published under
Habsheim cartes crash

Edit: You might refer to the article in flight international edition 17.april 1990. I think the mentioned obstacle clearance calculation by AirFrance for the paved strip was just that, a calcukation without reference to the intended approach path.

1990 | 1071 | Flight Archive

Last edited by RetiredF4; 16th Dec 2013 at 21:49.
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Old 16th Dec 2013, 21:58
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Originally Posted by RetiredF4 View Post
Wherever you got that information from, it does not look right. The heading difference between the flown grass strip track and the paved strip is 035 degrees, and would have required a total different track from beginning and flight over the houses of Habsheim, which was not permitted for airshow traffic. They would have flown a different track to approach the airfield some miles out with the track of 020 degrees for the paved strip. But the track was flown along the east side of the highway Basel-Straßbourg which is about heading 345 like the grass strip and which is over thin populated areas and offers good visual clues, which makes sense.
Check the BEA report page 19, which confirms what I said. Rather that turn around and make a second and exact approach to the grass strip, the PIC elects to "fudge" an approach from their current position. The reasons behind that decision we will never know - but that decision was nevertheless made and executed.
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Old 16th Dec 2013, 22:05
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Originally Posted by Dozy
I'm referring to the constantly changing variable of airspeed throughout. (...) The aircraft maintained airspeed based on that initial value
????
The airspeed did not increase enough to have a significant impact on calculated maximum alpha
How does airspeed affect the 'calculated maximum alpha', isn't alpha-max equal to 17.5 degrees?
The aircraft maintained airspeed based on that initial value, and maintained pitch attitude equivalent to maximum AoA available.
I don't understand how an airplane can maintain a constant pitch attitude in essentially level flight while it decelerates from 140 kts to 112 kts, and the whole point of the discussion is that the airplane never reached alpha-max.
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Old 16th Dec 2013, 22:15
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Originally Posted by HazelNuts39 View Post
How does airspeed affect the 'calculated maximum alpha', isn't alpha-max equal to 17.5 degrees?
Not sure - but AoA is a function of airspeed and pitch attitude, no?

I don't understand how an airplane can maintain a constant pitch attitude in essentially level flight while it decelerates from 140 kts to 112 kts,
To the best of my recollection, the pitch attitude was not constant.
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Old 16th Dec 2013, 22:30
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Dozy,
could you point to the exact paragraph on page 19?
my french is better than Spain, which i don't speak at all, but not suitable for pinpointing the fact you cite.
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Old 16th Dec 2013, 22:48
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Dozy,

I'm sorry, but we seem to be having some semantic misunderstandings, mainly about 'maintaining' and 'maximum'. In my vocabulary maximum available alpha is alpha-max and according to the accident report that is 17.5 degrees. The AoA that provides a lift equal to the weight is a function of airspeed. In level flight without wind that AoA is equal to the pitch attitude, controlled through sidestick inputs. Yes, the pitch attitude increased during the level deceleration.

The certificated 1g stall speed is obtained at alpha-max, with the sidestick held fully back during 3 seconds, after decelerating at 1 kt/second. But that is in free flight, out of ground effect, i.e. above 100 ft RA.
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Old 16th Dec 2013, 23:23
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Originally Posted by RetiredF4 View Post
Dozy,
could you point to the exact paragraph on page 19?
Section 2.3 on p.19 covers the crew's decision and attempt to define an approach path that would return them to the position they were intending to hit from the outset, offset from the original (incorrect) co-ordinates.

Originally Posted by HazelNuts39 View Post
In my vocabulary maximum available alpha is alpha-max...
Indeed - but Airbus documentation explicitly states that pulling up in High AoA Protection mode does not guarantee a precise application of theoretical Alpha Max.
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Old 16th Dec 2013, 23:36
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Dozy,

IMHO there is nothing 'theoretical' about alpha-max. It is a somewhat arbitrary value chosen by the airplane manufacturer and programmed into the flight control system, just as alpha-prot and alpha-floor. It varies with configuration and (on some airplanes) with Mach number (when M>0.3 typically). And, as I said, it must be obtainable in free air by pulling the sidestick to the aft stop and holding it there until there is no further increase of AoA. I haven't seen any Airbus material 'explicitly' contradicting that.

Last edited by HazelNuts39; 16th Dec 2013 at 23:55.
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Old 16th Dec 2013, 23:58
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Originally Posted by HazelNuts39 View Post
Dozy,

IMHO there is nothing 'theoretical' about alpha-max. It is a somewhat arbitrary value chosen by the airplane manufacturer and programmed into the flight control system, just as alpha-prot and alpha-floor. It varies with configuration and (on some airplanes) with Mach number (when M>0.3 typically).
Agreed - the function that determines the value is programmed into the flight control software as you describe.

And, as I said, it must be obtainable by pulling the sidestick to the aft stop and holding it there.
No - according to the documentation, Alpha Max (as illustrated by the graph in the doc) *may* be achieved by pulling the sidestick full aft, but the documentation clearly states that the theoretical alpha max is not guaranteed to be reached.
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Old 17th Dec 2013, 00:08
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Quote from RetiredF4 (my emphasis):
"Any pilot of my outfit was familiar with low flying down to 100 feet and up to 540 KIAS in Labrador, and we all agreed that flying an airliner at 100 feet or below over a small airfield at minimum speed was suicidal planing.
We discussed, why the crew even went considerably below that planned altitude and came up with the suggestion, that it might not have been the intention of the crew, but happened due to the perspective of the small little airfield. Flying a visual approach to a small little runway / airfield like Habsheim when being used to bigger airfields and buildings like Basel just minutes before gives the visual illusion of being at a higher altitude above ground than actual."

Absolutely. And, on the approach, the distance-to-go would seem greater, once they had "gone visual". (They would initially have been using the FMS with a suitable waypoint constructed and inserted.)

You also make a very relevant point about the routing along the autoroute apparently being impracticable for the stated plan to fly along the paved Rwy 02. That would have involved an awkwardly-tight right turn.

Quote:
the crew... [...] didn't register the descent below 100 feet. Only altitude information comes from the Radar altimeter, Captain and FO did neither acknowledge nor react to those warnings.

...which would be very unusual for an A320 crew, even in those early days. The synthesised voice is clear and loud, calling 100, 50, and 30 ft (etc.). I notice that the BEA report recommended that the calls be included in the pilots' headphones, as well as the cockpit speakers. But I don't remember ever having a problem with our a/c at that time, and we always wore our earphones. Personally, in the L/H seat, I always wore my left earphone and boom mic, but usually kept the right earpiece just behind my right ear to allow conversation without necessarily using the intercom. (We did not get noise-reducing headphones until about a year later.)

HN39,
Bon courage...
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Old 17th Dec 2013, 00:48
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Unusual

What will have been very unusual for the crew will have been the over the nose visibility during the low speed, low altitude pass - the pitch attitude was so high that the trees may well have been hidden just behind the nose - cutoff angle over the nose, if I recall correctly, was 15 degrees down.
Believe it or not, even in an F-16 you have to fly a level, low speed pass on the old fashioned dial altimeter head down, because, sitting at 25 degrees AoA, the outside horizon is 10 degrees below your nose and the Flight Path Marker is way below the bottom of the HUD.
The Habsheim AF crew, in a way, just didn't know what they were really doing.

By the way, the scars in the trees can still be seen when you are on approach to BSL RW 15, an accident takes a long time to heal, in many aspects.
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Old 17th Dec 2013, 01:37
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According to me planning this fly past at 100ft was not a problem but doing so at Valpha Max was an accident waiting to happen. Nobody can fly at dead 100ft at minimumm speed with hardly any elevator authority left for such precision altitude keeping. Such precision with purely thrust management is impossible considering also that the pilot had to look more outside.
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Old 17th Dec 2013, 03:31
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Originally Posted by Chris Scott
The back-stick reached about 17 deg (presumably maximum), but not for a measurable period prior to impact. That's the reason for my observation in (2), above. Why he did not pull harder from t -26 to t -3 is inexplicable, if only because it was delaying the desired attainment of alpha-max. The modest back-stick up to t -4 would, as you know, never achieve alpha-max.
The pilot version is that he was following the altimeter set on the QFE of Habsheim and was indicating 100 ft all along. He also said he has never heard the RA announcements. That can be the reason he did not pull harder earlier during the initial phase of the low pass.

Also, I was having a look at the AF447 data, and the max deflection in pitch for the sidestick was at 16 deg.
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Old 17th Dec 2013, 03:51
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Originally Posted by RF4
In my view all the speculation about possible technical aspects is noise to cover up the fact, that the crew f** ed up badly and didn't register the descent below 100 feet.
I think we all agree on the crew responsibility in that event, but the technical aspect is still of interest.
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Old 17th Dec 2013, 03:57
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Originally Posted by dozy
I'm referring to the constantly changing variable of airspeed throughout.
?
The airspeed did not increase enough to have a significant impact on calculated maximum alpha
?

The more you write on alpha max, the more you look confused.
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Old 17th Dec 2013, 09:10
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Just for accuracy

Originally Posted by DozyWanabee
At this point the crew would have been well within their rights to abort the flypast altogether, and press on to the sightseeing trip over the Pyrenees which made up the latter part of their flightplan.
Pyrenees were a little far from Mulhouse-Bâle Airport. The possible trip was over Swiss Alps not far away.
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Old 17th Dec 2013, 09:26
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Dozy, please read again in your post #26 your quote from the second (A320 series) reference, then look at the graph immediately below the quoted text, in particular the legend which says "ɑMax Full aftstick - VɑMax". Then read the reply by Tester78 in post #27.

The FCOM says that VɑMax is another name for Vs1g.
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Old 17th Dec 2013, 12:16
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Pilot's visibility, perspective, and eye-height versus RA

Quote from EMIT:
"What will have been very unusual for the crew will have been the over the nose visibility during the low speed, low altitude pass - the pitch attitude was so high that the trees may well have been hidden just behind the nose - cutoff angle over the nose, if I recall correctly, was 15 degrees down."

Yes, as I commented in an earlier post:
"Pitch rose through +10 deg at about t -13. No particular significance in +10, but my guess is that, assuming the pilot's eye height was adjusted (using the seat-height adjustment) to the recommended level, the trees would not have been visible through the windshields after that. Also, not many pilots are used to the perspective at that kind of attitude - except on T/O. At t -7 pitch was about +14. Unfortunately I don't have an A320 AWOPS guide to hand, which might help with the relevant geometry."

I added:
At about t -8 the co-pilot warned the captain of some pylons ahead. They were very much further away than the trees, so one can assume that either the co-pilot could not see the trees, or that the steep deck-angle caused him to think that the a/c was higher than the treetops.

EMIT is right. At that deck-angle, the PF would inevitably have been flying on instruments. The A320 RA readings are presented (like most EIS cockpits) in purely digital form - at the bottom of the PFD.The trend of the readings is therefore more difficult to interpret than would be the case with an analogue (dial) presentation. The synthetic voice normally calls the transits of 100, 50, 30, 20, and 10 on a landing approach. If the RA reading hesitates (due premature flare, or passing over a gully), the value can be repeated. The former happened, in effect, during this fly-past, which is why the CVR records the "Thirty" announcement several times.

Quote from CONF_iture (my emphasis):
The pilot version is that he was following the altimeter set on the QFE of Habsheim and was indicating 100 ft all along. He also said he has never heard the RA announcements. That can be the reason he did not pull harder earlier during the initial phase of the low pass.

Thanks for that. The DFDR trace shows the a/c consistently lower, eventually below 30 ft QFE. No pilot expects a baro reading to be that accurate, which is the main reason that, AFAIK, radio-altimetry is mandatory for any AWOPS procedure with a decision height below Category 1 minima (~ 200ft).

The geometry of the pilots' ability to see the trees would benefit from the diagrams in an AWOPS guide. However, there is another aspect of geometry which is interesting: the difference in the pilots' eye-height above the ground, and the RA reading.

The four rad-alt TRx antennae on the A320 were mounted on the bottom of the rear fuselage, just forward of the fuselage tapering area. Examination of the a/c profile diagram shows that, at a pitch angle of +13.5 degrees, the pilot eye height above flat ground would be about 25 ft higher than the RA reading. At the final pitch angle of about +15, the tail-cone and tapering part of the rear fuselage would have been the first to touch any horizontal series of treetops. At that stage, the engine nacelles would have been between 5 and 10 feet clear. (However, we cannot assume the treetops were all the same height.)

So the pilot's eye would have been over 25 ft above the tree-top that the a/c first struck...

Also, FWIW, the static ports that sense the static pressure for the ADCs that feed the altimeters are not far below the pilots.

Last edited by Chris Scott; 17th Dec 2013 at 12:27. Reason: Last sentence added.
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Old 17th Dec 2013, 14:02
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In AoA protection mode the sidestick commands an angle of attack. Attainment of the commanded angle of attack will not be instantaneous. A nose-up movement of the sidestick will request a change of the elevator position to pitch the airplane nose-up until the actual AoA is equal to the commanded value. That will take a certain time, depending on the magnitude of the change, the airspeed, the gains in the control loop and the pitch inertia of the airplane. If the airplane is decelerating or accelerating, the phugoid damping comes into play. The accident report on an A320 hard landing at Bilbao in 2001 describes it thus:

AoA protection (high angle of attack protection) ELAC, L80

(...) in these high angle of attack situations, with a tendency toward phugoid movement, studied by longitudinal dynamic stability, in which the aeroplane oscillates between two kinetic and potential energy levels, the EFCS behaves as a damper of the oscillations, commanding appropriate variations of angle of attack in a way that, when the aircraft is slowing down, makes it pitch downward and vice versa.
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Old 17th Dec 2013, 14:27
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Originally Posted by Chris Scott
The DFDR trace shows the a/c consistently lower, eventually below 30 ft QFE.
I don't see any QFE altimeter reading in the DFDR traces, do you ?

Originally Posted by HN39
A nose-up movement of the sidestick will request a change of the elevator position to pitch the airplane nose-up until the actual AoA is equal to the commanded value.
Where is that change to expect 17.5 deg of AoA ?
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