Hi,
PJ2
The airplane was fine moments before the loss of airspeed indication, and such loss means nothing to the actual airplane itself...
Means
nothing? Sir, respectfully i
disagree; here is (the beginning of)
WHY:
With the Synergy of a highly motivated group we can touch some "sensitives points" in our effort to understand what occurred with F-GZCP that night.
The diversity of the posts show the agenda of the posters is diversified. I think the more constructive one is to allow us to be prepared to avoid problems when dealing with similar equipment.
This can be important not just to pilots.
A Feedback System is EXACTLY what his name implies:
Something that receives "feed" back (to itself).
Tacoma narrows bridge received energy (from wind turbulence), OSCILLATED to the structural limits and was destroyed.
This was a typical POSITIVE feedback (the one that presents GROWING oscillations)
Positive feedback can be dangerous. But when controlled (by lossy components) are very useful and common in our lives.
Example: HF radio equipment used during Titanic accident. (A coiled wire, a capacitor, a switch, a spark gap and an antenna and a power source)
Another positive Feedback System: An A/C subject to an UNDAMPED oscillation when PF apply larger inputs (larger than a given threshold). This may led to the destruction of the equipment. The loss of the F4 posted by
Machinbird perhaps is a good example.
During some initial discussion on the (clearly obsolete) AS probes still today used by the aviation industry we had the opportunity to understand the advantage to have a better one using a closed loop. The feedback WOULD REDUCE the time required to the Pitot to "recover" from the "cold" it frequently suffer.
In the 30+ cases of UAS they briefly (seconds) failed to work properly.
In F-GZCP last flight the System (A/C + crew) received an "input" (heard in CVR) so important that affected the stability of the System.
The ice crystals WERE NOT ADEQUATELY PROCESSED BY AN IMPORTANT SENSOR (so important is MULTIPLE) that as per
A33Zab diagram
has not a TEMPERATURE CONTROL. It seems (the heater) "less intelligent" than the appliance we use to press our clothes. The pressing iron has a thermostat, i.e. a feedback system to control it's temperature.
This kind of NEGATIVE FEEDBACK improves the operation of the equipment and allows a high quality work. Our clothes are preserved, the pressing iron can operate IN THE BEST range, etc.
Another example of a feedback system is the yaw damper. "With the help" of the rudder the A/C has the directional stability greatly improved.
At that night the System (A/C) put the crew (PF) in the "feedback loop". The pilot had to act immediately. Doing so he corrected the roll. A roll we may understand started by LARGE AMPLITUDE STIMULI from the environment. A harsh one they had to do a "last minute" deviation. PF started to apply LARGE AMPLITUDE stimuli to the new System (A/C + PF) and this could led to UNSTABILITY.
Machinbird and others suspect of PIO (roll) after this PF action.
Up to this point the System received 'inputs" in some degree affecting the Stability of the entire System (System+PF):
1) Ice crystals.
2) Loss of important feedback loops: A/P and A/THR
3) Turbulence
4) Large PF inputs
This facts generated almost immediately 10 other facts:
1) System was automatically reconfigured (NL to ALTN)
2) System degraded (to "another" A/C with less available resources)
3) ISIS "suffered"
4) A/C broadcasted anomalies (ACARS)
5) Crew not was informed on reason of A/P and A/THR quit
6) Turbulence became POTENTIALLY more relevant
7) HF "surfaced" (by lack of training, etc.)
8) Not use of SOPs and adequate CRM
9) Lack of understanding ("uncharted waters") by PF AND PNF
10) A persistent climb with high rate
Clearly after this
facts the System was less robust than before.
A safe (and stable) System MUST BE:
FAULT TOLERANT
And show:
GRACEFUL DEGRADATION.
And i will add:
A safe System must show RESILIENCE (System + crew). The transient (perhaps up to the apogee) dominated the scene. And After "inflexion point" put the plane in a "steady state" condition lasting roughly 4 minutes falling (and circling) at near terminal speed.
For me it is clear the System at this moment (PF starting to act IN THE NEW LOOP) was showing less capability to "resist" to the initial "input" and subsequent consequences:
tiny ice crystals.
(to be continued in part B)
In the meantime we must think looking to the factual information we have so far.
PS
Questions to think and take into account in our analysis:
The man machine interface became essential at this critical moments?
We will know what PF saw? (RH was not recorded and ISIS also had "impairment")
Some remarks related to AS probes: Open loop (no feedback): LONGER RECOVERY TIME (to recover from the "cold")
A33Zab
Thanks for the diagram. Amazingly the probes are just heated and their temperature is not controlled*. This fact alone may explain part of the observed problems related to air speed measurement.
As
Machinbird pointed there is room for improvement. With a closed loop temperature control the time to reheat the critical internal regions of the probe would be reduced.
Anyway i am deeply surprised (negatively) with the approach used. Sensors capable to "disable" important functions of the A/C more simple than the necessary.
Much of my rationale in some posts were based in a better (closed loop) approach. when in AUTO mode. In both AUTO and ON (what i called MANUAL) the power applied IS THE SAME.
(*) And we really don't know if a ballast wire is used.
Bottom line:
The thermal "recovery" of probes takes too long (in ~20 seconds after hitting ice crystals A/P and A/THR quit). The recovery time WAS NOT COMPATIBLE with System requirements of GIGO.
The recovery time MUST BE REDUCED and i'm sure can be reduced.
The different probe characteristics of FR (now obsolete) and US (with limitations) may be explained by thermal inertia plus the factors mentioned by
Owain Glyndwr
Also the materials used certainly are important like mentioned by Lyman
Up to this point (in this sensitive issue and related posts) thanks for comments/questions from: HN39, PJ2, Lyman, roulishollandais, and contributions from Machinbird, A33Zab, Owain Glyndwr and others including PM channels.
I will try to comment and answer questions made before, when possible. (asap)