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AF447

Old 19th Sep 2009, 20:24
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The Energy Wasn't Dissipated
That's the problem. With enough elevator control power and no computer between you and it to restrict G, you can yank the nose up without changing the velocity vector much at first. The pitot won't read much airspeed because it's looking forward and not down, but you're still going down at almost the same rate of descent, which is consistent with a high angle of attack.
T-Turtle, I have participated in a few accident investigations including one where the pilot yanked the nose up to avoid flying into the ground (to no good effect) and I can state pretty conclusively that is not what happened with AF447. Further, although I may be going out on a limb because BEA has not made a statement in this regard, the character of the damaged components I have seen pictures of and the reported condition of the human remains indicates a low speed collision with the water below 200 knots. Believe me, that energy was dissipated. If you would like to discuss further, drop me a PM.
Further, on an A330, there is no direct connection between the side stick controller and the control surfaces. You only give the computer "hints" at what you want it to do and it processes those hints in accordance with the current control law it is operating in. And if there is no power to the computers-there is no control.
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Old 19th Sep 2009, 23:00
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the character of the damaged components I have seen pictures of and the reported condition of the human remains indicates a low speed collision with the water below 200 knots.

Very interesting comment, particularly with regard to condition of the bodies recoverd. You seem to have a firm idea on airspeed, however, do you care to post an opinion on vertical speed?
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Old 19th Sep 2009, 23:10
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Smile

however, do you care to post an opinion on vertical speed?
... I think he already did, since the aircraft was moving vertically at the time?
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Old 19th Sep 2009, 23:56
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Vertical velocity estimate

Hi Wes-wall. I posted the answer to your question some time ago but with the volume of posts, I can well understand that you might not have seen it. http://www.pprune.org/5085433-post3947.html. But maybe as high as 9000 fpm down.
Since the rudder was thrown forward according to the BEA preliminary report, it is probable that there was comparable forward velocity too , but at this point I'm just guessing.

Last edited by Machinbird; 20th Sep 2009 at 00:09.
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Old 19th Sep 2009, 23:57
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JD-EE,
Thanks! Notwithstanding your comments, I did generate -
Someone might like to comment on how effective the rudder would be in direct law at near stall speed pulling TOGA with only one engine. Remember, the BEA stated the vertical stabilizer damage showed the tail was rotating to port on impact and I couldn't help but wonder if only No.1 was operational.
and later I also remarked -
One and a half days after the impact the floating debris pattern would still have been relatively compact - say a radius of 0.5NM, as I construe the BEA's description of the impact as having little or no horizontal momentum - more a tail heavy "flat spin".

In the cruise, I assume the trim tank was in use - not helpful later on.
My supposition was that the a/c had had a 2 x flame-out at around 02:13:10z (loss of supply to the SATCOM) and the subsequent re-powering of the SATCOM at 02:13:40z was either on a restart of No.1 or the APU. A deep stall and high sink rate was already established, and the attitude could have resulted in the descriptions alluded to in the above quotes.

To put it bluntly, the crew/pax that were not belted in could well have compounded the maximum aft center of gravity problem.

The BEA's preliminary report provides -
02:14:14 - .1/WRN/WN0906010214 341036006MAINTENANCE STATUS ADR 2
02:14:20 - .1/FLR/FR0906010213 22833406AFS 1,,,,,,,FMGEC1(1CA1),INTERMITTENT
02:14:26 - .1/WRN/WN0906010214 213100206ADVISORY CABIN VERTICAL SPEED
These final 3 ACARS/ATSU/ACOM messages were probably initiated at 02:14:10z and their transmission time and sequencing provide for the reception times noted by the BEA. The Cabin Vertical Speed advisory tells me that passing through say 8,000 feet (various docs say 7350 or 9550ft) the a/c's vertical speed was in excess of 1,800 feet/min, or more succinctly that the cabin air pressure controllers ability to compensate had been exceeded, i.e. the cabin pressure was less than the external pressure. The earliest the CVS advisory could have been generated is 02:14:10z and the latest at 02:14:22z which provide descent/sink rates of 2,700 or 6,700 feet/min if impact is assumed at 02:14:30z. I have assumed a GS at impact of around 120KTS, which combined with the descent/sink rates mentioned of 2,700 feet/min (24KTS) or 6,700 feet/min (61KTS) is quite a substantial impact velocity. Put another way, the a/c's attitude was pitched up +10/15 degrees and on a glide slope (misnomer) of between 14 to 40 degrees.

The damage noted in various photos would probably support those speeds when combined with the impact square area and a/c mass, along with the resulting sequential break-up. In the case of the v/s the combination of the forward velocity and the tail yaw (to port) have obviously been used by the BEA in making their initial conclusion.

So in summary, the track flown from the Last Known Position at 0210z to the possible impact positions previously deduced could well have been direct and for about 1 minute prior to 02:14:30z the deep stall/high sink rate was established.

mm43


Last edited by mm43; 20th Sep 2009 at 01:33. Reason: the glide slope (misnomer)
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Old 20th Sep 2009, 01:47
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Probably as Close as We'll ever get to the Truth?

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Part ONE- A little hard to follow
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Part TWO - Quite close to the mark (IMHO)
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Old 20th Sep 2009, 05:51
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Probably as Close as We'll ever get to the Truth?
Part ONE- A little hard to follow.
Part TWO - Quite close to the mark (IMHO)
Hi Shadow, Yes, there is a lot of good information in those links, but the author intimates that the resulting loss of control was in the overspeed direction. Frankly, I would expect an overspeed loss of control to end up in a pointy end down high speed water entry or an inflight breakup. It doesn't quite fit the observed data.
I like the Boeing article here: http://www.boeing.com/commercial/aeromagazine/aero_08/erroneous_textonly.html and believe that some of the accidents involving loss of airspeed data reported at the end of the article in the block titled, "ACCIDENT AND INCIDENT CASE STUDIES" are a better model for what happened to AF447 (IMHO).
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Old 20th Sep 2009, 07:30
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...the author intimates that the resulting loss of control was in the overspeed direction.
I think he's straight out saying just that. A Mach encounter due to speed sneaking up to the high side unobserved could lead to a pilot mistaking its characteristics for stall buffet and lowering the nose, further embedding in a Mach tuck pitch-down. Some of those Boeing examples demonstrate how easy it is to mistake symptoms. Disorientation after a Mach Crit encounter inducing a loss-of-control could easily lead to a nose high/stall entry type situation.

Personally not sure about the plausibility of a double flame-out (from a post-disorientation stall/spin scenario) and failure to relight - culminating in an attempted engines-off ditching (as an explanation for the assumed wings level water-entry attitude, high RoD and low speed). The 4 minutes (only) from height could be explained away by the high speed/high RoD required for relight attempts OR that those 4 minutes just represented the time from height to losing all useful electrics (to the ACARS) due to a LOC induced double flame-out.

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Old 21st Sep 2009, 01:59
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Transonic Transitions

I think he's straight out saying just that. A Mach encounter due to speed sneaking up to the high side unobserved could lead to a pilot mistaking its characteristics for stall buffet and lowering the nose, further embedding in a Mach tuck pitch-down. Some of those Boeing examples demonstrate how easy it is to mistake symptoms. Disorientation after a Mach Crit encounter inducing a loss-of-control could easily lead to a nose high/stall entry type situation.
Ok, lets follow that line of thought a bit further. The nose starts to tuck as trim limits are reached (because of the shifting center of pressure on the wing as you go transonic) and the nose starts to fall, altitude starts to unwind quickly and the crew reacts by reducing power and deploying speed brakes. Assuming they are successful in arresting the plunge, what is the next thing they would encounter? It would be a transonic pitch up as they decelerate (caused by the center of pressure moving back to its normal subsonic positon) as all the nose up trim makes itself felt. Say the aircraft bottomed at FL 250 while pulling maximum permitted g, and just below M Crit. In an F-4 for example, this type of transition to subsonic could cause a 50% 'g' overshoot because it happens very quickly. Can the Airbus G protection mitigate this 'g' spike quickly enough to keep the wings from breaking (while in alternate law and with an aft cg)?
Would the wings stay on? I don't know since I don't have enough aircraft data, but if the wings did stay on, then you would probably soon find the nose pretty high in the air since the crew would be unlikely to have the presence of mind to drop a wing. Then you could get into a deep stall very quickly. But, can the critical Mach recovery even be made in alternate law?
On the face of it, the foregoing scenario doesn't pass the Occam's razor test.http://en.wikipedia.org/wiki/Occam%27s_razor Why not a simple deceleration into a stall with heavy turbulence and a cockpit full of warning lights as a distraction? It seems to fit the event time line better.
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Old 21st Sep 2009, 05:03
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AF447 was going Faster or Slower? (than indicated)

Machinbird says: (of post 4426/4427 - link)
"....Why not a simple deceleration into a stall with heavy turbulence and a cockpit full of warning lights as a distraction? It seems to fit the event time line better."
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Seems to fail the logic test for Occam's Razor. Up until the A/P disconnect, as far as the ADIRS and PRIM were concerned, everything was nominal. The auto-thrust would have been doing its job in maintaining the sched speed. ........ as the three pitots iced up internally and indicated, to a gullible system, that the aircraft was tending to slow (just as the autothrust would respond, say, if it was picking up a load of wing and fuselage ice).

That means that the speed observed on both the L&R PFD's was (misleadingly), and continued to be, exactly what the pilots expected to see, but in actuality the aircraft speed through the air was somewhat in excess of that (and increasing as the pitot blockage increased towards, but not necessarily TO, a total blockage). Then the autopilot disconnected and the TCAS dropped out - because the two different sources of static pressure were now in total disagreement. .... and parametrically unacceptable for RVSM flight. Why the TCAS? It needs the static pressure to be in agreement (and good) for a highly accurate RVSM flight level maintenance.

That's what I'm getting from those two links. Suggest you re-read them both.

So the case for it having been a Mach Crit encounter is there (IMHO - and unless some of the cognoscenti have a contrary argument).
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Old 21st Sep 2009, 05:58
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OVERTALK,
Why the TCAS? It needs the static pressure to be in agreement (and good) for a highly accurate RVSM flight level maintenance.
Why was the ACARS TACS FLT msg not explained by the BEA? Your logic is not in question, and the scenario you have promoted fits well with the assumed outcome.

mm43
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Old 21st Sep 2009, 06:28
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Sorry Overtalk, I understood the point made in the two parts of the referenced article from the beginning, however I am having great difficulty in understanding how an overspeed induced departure will lead to other than a sky full of confetti.or a high speed impact with the water. The fact that AF447 arrived at the surface apparently essentially intact and apparently at low speed high angle of attack, high sink rate and perhaps in as little as 5 minutes requires an involved process if one assumes an initial overspeed departure from controlled flight. Until such time as additional information arrives supporting that view, IMHO it might be more productive to consider how a low speed departure from controlled flight might have happened.
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Old 21st Sep 2009, 10:42
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In explnation of the TCAS outage ACARS MSG

OVERTALK,
Why the TCAS? It needs the static pressure to be in agreement (and good) for a highly accurate RVSM flight level maintenance.
Why was the ACARS TCAS FLT msg not explained by the BEA? Your logic is not in question, and the scenario you have promoted fits well with the assumed outcome.

mm43
TCAS derives its altitude information from the aircraft altimeter (i.e. its mode Charlie squawk that's continually being punched out in response to ATC and TCAS interrogation via the transponder). If the ADIRS is suddenly in WTF? rejection mode for increasingly divergent derived static pressures (due to the pitot blockage rate increasing), then two things must happen:

a. Autopilot baro hold will be corrupted and so the autopilot will kick out and....

b. TCAS will throw in the towel (and ACARS will be stimulated to tattle-tale that info also)

Same thing (essentially) happened when the BIZJet copilot placed the laptop on the center console over the Amazon jungle and its lid cancelled their transponder - effectively crippling their TCAS (which then showed a non-flashing and bland TCAS message on-screen)- for quite a while before their connecting with the GOL 737....
But then again, ACARS wasn't part of their bizjet repertoire. In their case their baro hold was good, but it was still the TCAS that had been fatally disabled. In Af447's case their TCAS merely lost that valid mode C input. ...and quit.
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and then Machinbird said:
The fact that AF447 arrived at the surface apparently essentially intact and apparently at low speed, high angle of attack, high sink rate and perhaps in as little as 5 minutes requires an involved process if one assumes an initial overspeed departure from controlled flight.
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Hmmm
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For a non T-tail, a sustained deep stall is not really on the cards. A flat spin maybe? Not really. The A330 aerodynamics don't support either proposition. A double flame-out due to a nose-high departure and auto-rotation following a Mach Crit encounter and loss of control? YES, most affirmatively. WHY?
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Well Airbus test-pilots don't test for any flame-out proclivities during stall or coffin corner auto-rotation, however the A330's engines would be quite vulnerable to that at cruise height (see recent Pinnacle Airline's example). My guess is that the AF447 crew were burning off height at a great rate attempting relights all the way down and then, logically, were eventually forced to give up on the relight attempts for an engine-off, best configured/best attitude/best speed arrival at ditching station "terra oceana". That's what could have happened to Air Transat's A330 - if the Azores hadn't been in their sights all the way down.

That explains it all via Occam's Razor first principles - as modified by arody logic (IMHO).

The links at:
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Part ONE: Air France Flt 447

Part TWO: The AF447, QF72 and 9M-MRG comparison

contain a quite convincing version of the likely AF447 event.
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Old 21st Sep 2009, 13:08
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TCAS

The TCAS gets its altitude from the selected transponder. The transponder gets its altitude from its selected ADC. ADC altitude is separate from ADC airspeed, whose source was apparently flawed. There is no reason for the ADC to fail its altitude output if its airspeed input has failed.

Hence, the TCAS Fail is unrelated to pitot problems, and still a mystery, in my alleged mind. The BEA report said the same.

GB
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Old 21st Sep 2009, 14:37
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GrayBeard says
The TCAS gets its altitude from the selected transponder. The transponder gets its altitude from its selected ADC. ADC altitude is separate from ADC airspeed, whose source was apparently flawed. There is no reason for the ADC to fail its altitude output if its airspeed input has failed. Hence, the TCAS Fail is unrelated to pitot problems..

Perhaps drop to another earlier level of air data processing (i.e. the ADM)
and reflect that the pitot takes in BOTH RAM (dynamic) and static pressure (the latter being deducted by static port sourced static pressure to derive the CAS). It's the fact that the two sources of static received by their respective ADM's slip outside allowable minor differences that creates the "reject".
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per.....
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Each ADIRU comprises an Air Data Reference (ADR) and an Inertial Reference (IR) component.
An ADR (Air Data Reference) fault will cause the loss of airspeed and altitude information on the affected display.
Air Data Reference

The ADR component of an ADIRU provides airspeed, Mach, angle of attack, temperature and barometric altitude data. Ram air pressure and static pressures used in calculating airspeed are measured by small Air data modules (ADM) located as close as possible to the respective pitot and static pressure sensors. The ADMs transmit their pressures to the ADIRUs through ARINC 429 data buses.
Complexity in redundancy
Analysis of complex systems is itself so difficult as to be subject to errors in the certification process. Complex interactions between flight computers and ADIRU's can lead to counter-intuitive behaviour for the crew in the event of a failure. In the case of Qantas Flight 72, the captain switched the source of IR data from ADIRU1 to ADIRU3 following a failure of ADIRU1; however ADIRU1 continued to supply ADR data to the captain's primary flight display. In addition, the master flight control computer (PRIM1) was switched from PRIM1 to PRIM2, then PRIM2 back to PRIM1, thereby creating a situation of uncertainty for the crew who did not know which redundant systems they were relying upon.
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Reliance on redundancy of aircraft systems can also lead to delays in executing needed repairs as airline operators rely on the redundancy to keep the aircraft system working without having to repair faults immediately (the MAS 777 case - 9M-MRG had graceful degradation failures dating back many years that were intentionally "hidden". A further failure brought that failed accelerometer back into play - and precipitated QF72's wild ride).
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Precedents:
a. This pre QF72 incident to a QANTAS A330 (QF68 on 12 Sep 06) (see: this link ) was quite probably an incipient AF447 scenario. No fault was ever found. Why? The pitot ice had melted well prior to landing and the CirroStratus encounter/exposure was likely to have been a mild one (AND the crew took prompt and luckily, correct action).

b. On 07 Feb 08 another QANTAS aircraft (VH-EBC) suffered an identical event while conducting the JQ7 service from Sydney to Ho Chi Minh City, Vietnam.

c. 27 December 2008, Qantas Flight 71 from Perth to Singapore, the same A330-300 registration VH-QPA and the same ADIRU as involved in the Qantas Flight 68 incident, was involved in an incident at 36,000 feet approximately 260*nautical miles (480*km) north-west of Perth and 350*nautical miles (650*km) south of Learmonth Airport at 1729 WST.

d. While examining possibly related events of weather-related loss of ADIRS, the NTSB decided to investigate two similar cases on cruising A330s. On a 21 May 2009 Miami-Sao Paulo TAM Flight 8091 registered as PT-MVB, and on a 23 June 2009 Hong Kong-Tokyo Northwest Airlines Flight 8 registered as N805NW each saw sudden loss of airspeed data at cruise altitude and consequent loss of ADIRS control.

Whether or not the Autopilot and TCAS drop out probably depends upon how fast the situation onsets - and to what extent the differently derived static pressures eventually disagree.... prior to crew intervention.
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Old 21st Sep 2009, 15:06
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Belgique:
Perhaps drop to another earlier level of air data processing (i.e. the ADM) and reflect that the pitot takes in BOTH RAM (dynamic) and static pressure (the latter being deducted by static port sourced static pressure to derive the CAS). It's the fact that the two sources of static received by their respective ADM's slip outside allowable minor differences that creates the "reject".
From the ISIS block diagram I copied from a much earlier post, each pitot ADM is a separate module from each static ADM. The static ports are a good distance from the pitot probes, of course. Are you saying each pitot probe has its own static port in addition to the primary static ports? Otherwise, CAS is calculated inside the ADR, whereas altitude at flight levels is more or less raw data, unaffected by airspeed or lack of.

Thanks for the info on the QF anomalies being related to the pitot problems.

GB
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Old 21st Sep 2009, 17:19
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Too Much Static?

Are you saying each pitot probe has its own static port in addition to the primary static ports?
No, of course not. Forget ADIRS. Think Cessna GA. What comes down the pitot tube is not just the dynamic pressure of airspeed. Yes it's a dynamic pressure, but comprising BOTH airspeed(A) + static(x). That's why a static line delivers static port pressure to an ASI, now isn't it?...... to offset the pitot-derived static pressure.

To deduce the airspeed (A), the (baro)static pressure(y) from the static ports is deducted from x - in any conventional analogue Airspeed indicator. It's just that, in an ADIRS, it's done using digitized data.

A more confusing way of putting it is: "Airspeed indicators work by measuring the difference between static pressure, captured through one or more static ports; and stagnation pressure due to "ram air", captured through a pitot tube. This difference in pressure due to ram air is called impact pressure."

But in an ADIRU, when the pitot ADM's (measuring x) start showing a significant difference from the static ADM's (measuring y), then there's the increasing possibility of a rejection by various systems (baro hold and TCAS being two of them and the more sensitive).

A case study perhaps? Take the case of a static line containing water. You climb through freezing level and it turns to ice. You continue to climb. What happens to the airspeed? Because the static port derived (baro)static pressure is then trapped at the higher value of a lower altitude, the IAS winds back towards zero. In fact at 220kts it's back to zero within a further 2400ft of climb (been there, done that, got the guernsey). Downward sloped static port with a bung inserted upwards into it. You'd swear it'd never allow water into the lines while parked?

Wrong! Bung had hole in its centre to allow pressures to equalize. Rainwater dripped down over hole and got drawn through bung into static lines as local atmospheric pressure increased with the passage of a front. Airborne passing FZLVL in the climb, you have now lost the altimeter, the VSI zeroes out and your airspeed winds back to zero. Quite frightening when you're in the thick gloop.

In a descent of course the ASI increases as the pitot tube's contribution of static pressure eventually equalizes (and exceeds) at the same height that the static lines froze..... at which point it's over-reading in that descent (until the ice melts of course).

I was quite young when it happened to me and didn't know that the solution was to depressurize and get another cockpit source of static by smashing the face of the VSI. It was the reason why DC4's and Neptunes etc had an ALTERNATE static source switch to tap cockpit air pressure as a fallback static source.



There's a much higher resolution different cutaway diagram of an ASI/machmeter at this link
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Rudimentary but hopefully helpful in understanding why clogged pitots can subvert system resolution of static pressures.
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For a description of operation of an ASI (see this link)
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Old 22nd Sep 2009, 13:31
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Another thought to icing

I follow this thread since the beginning, i read every single post.

Machinbird says: (of post 4426/4427 - link)
Quote:
"....Why not a simple deceleration into a stall with heavy turbulence and a cockpit full of warning lights as a distraction? It seems to fit the event time line better."

Overtalks answer to that:
Quote:
So the case for it having been a Mach Crit encounter is there (IMHO - and unless some of the cognoscenti have a contrary argument).

Why do we assume, that the icing up of the pitot-tubes and the subsequent automatic system-reactions contributet to an drastic increase in IAS followed by desaster?

A few thoughts to augment my question.
Would it be logical to assume, that icing took place not only within the pitot-tubes but as well on other critical aircraft parts/ surfaces as Flight-controls?

Would it also be logical, that the heated pitot-tubes would be the last parts being affected by icing due to the heating of those?

What would that weight increase be, how would it effect aerodynamic lift and how much would it increase aerodynamic drag?

How much elevator trim would be necessarya to counter those effects given a stable speed?

How much would the AOA increase to maintain level flight?

How much power is availabel at the given altitude to maintain straight and level under such icing conditions, or even accelerate, and if that is possible, how long would an acceleration take?

If my thinking concerning those questions above are correct (and forgive me, i have no practical expierience on heavies) AF447 might have picked up icing, autopilot compensates for loss of lift and increase of weight, thrust increases to maintain not only speed (the wrong one) but also to maintain altitude, and finally within short time autopilot and autothrust reach their limits to maintain controlled flight. Icing up of the tubes leads to the final desaster of droping an aircraft into the lap of the crew, which just has departed controlled flight.

I dont know the timeframe in which this could have happened to AF447 (if at all), in my F4 2 decades ago it came out of nowhere in a climbout in military power at around Fl 250. Even selected afterburner didnt help in maintainig level flight. It took me 10.000 feet down to regain full authority over my aircraft and being able to get rid of the ice.
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Old 22nd Sep 2009, 22:51
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Another thought to icing

RetiredF4 asked ...

Would it be logical to assume, that icing took place not only within the pitot-tubes but as well on other critical aircraft parts/ surfaces as Flight-controls?

Would it also be logical, that the heated pitot-tubes would be the last parts being affected by icing due to the heating of those?


I believe that the suggestion is that AF477 was in abnormal icing conditions, where ice would only build up on heated surfaces. [See references to engine core icing earlier in the thread.]

Regards, Peter

I'm still struck by the apparent near-simultaneous failure of the pitot-tubes. It might be irrelevant to the eventual outcome, but it seems surprising there wasn't an observable gap between the failures.
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Old 23rd Sep 2009, 01:42
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The Ultimate Occam's Razor Edge

The contents of those links is the nearest I've seen to a credible explanation for AF447. To understand how a competent well-trained crew, used to avoiding enroute severe weather, could be sucked into a scenario, you need to entertain an insidious technological circumstance having sneaked up on them - for a TOTAL surprise - full of uncertainty and confusion. This scenario at those links paints just such a picture.

What's more, flying through layered thick CirroStratus and quietly accumulating ice crystal build-ups (uniformly) in the pitot heads isn't in any way beyond belief. In fact it accords with exactly what was known about the major deficiency of that mark of Thales pitot head. Its pitot heat was unable to cope with prolonged exposure to thick Cs cloud - which is composed of super-cooled ice crystals. The pitot heaters were thermally overrun and the Airbus automation disguised that fact, sufficient to allow an autopilot diisconnect, several system alerts and a flight control degrade at speed and height. Sometimes that's all it takes to induce an LOC. It's not as if there weren't numerous precedents with exactly the same type build-up, even though lacking the eventual onset of a terminal development. At night, above the thick thunderstorm clouds of the ITCZ made the fatal difference.

So I'd urge you to disregard this input below as a total red herring (for all the above reasons)
Why do we assume, that the icing up of the pitot-tubes and the subsequent automatic system-reactions contributet to an drastic increase in IAS followed by desaster?

A few thoughts to augment my question.
Would it be logical to assume, that icing took place not only within the pitot-tubes but as well on other critical aircraft parts/ surfaces as Flight-controls?

Would it also be logical, that the heated pitot-tubes would be the last parts being affected by icing due to the heating of those?

What would that weight increase be, how would it effect aerodynamic lift and how much would it increase aerodynamic drag?

How much elevator trim would be necessary to counter those effects given a stable speed?

How much would the AOA increase to maintain level flight?

How much power is availabel at the given altitude to maintain straight and level under such icing conditions, or even accelerate, and if that is possible, how long would an acceleration take?
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