Air France A330 accident
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@Handyman - cc:BOAC
The loss of cabin pressure message is incorrect.
According to a former A330 engineer, the ACARS data that was posted on French TV translates to the cabin pressure controller logging a warning/fault to the CMS of the aircraft altitude catching the cabin altitude.
The loss of cabin pressure message is incorrect.
According to a former A330 engineer, the ACARS data that was posted on French TV translates to the cabin pressure controller logging a warning/fault to the CMS of the aircraft altitude catching the cabin altitude.
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hurricane hunters
Quoting: Gergely Varju
I think that impression of hurricane hunters should be set right. It's incorrect to think the planes in the hurricane hunter fleet merrily "go over" the storms of a full hurricane. The planes are limited by physics like any other, even if they might have more power for their weight and so on. They're not trying to avoid the "bad" weather. They go in low to study it. But if you want an account of why only hurricane hunters fly into storms. Forgive me if it's been seen before.
Hunting Hugo : Weather Underground (an account of flying into Hugo)
As Seen It All said, it's not a mission where they go out to take crazy risks. It really is a mission for those pilots and they're not going to go out there to risk the plane or the people anymore than a commercial pilot would like to. (but they will get into close scrapes as per the account above.)
It is quite different to fly above the storms, and measure the pressure in the eye,
Hunting Hugo : Weather Underground (an account of flying into Hugo)
As Seen It All said, it's not a mission where they go out to take crazy risks. It really is a mission for those pilots and they're not going to go out there to risk the plane or the people anymore than a commercial pilot would like to. (but they will get into close scrapes as per the account above.)
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Aircraft altitude catching cabin altitude? Let's see If I understand this right: The airplane was descending so fast it went through cabin altitude before reaching the water?
Whats the max rate for the cabin press system?
I guess you see where we are going.. isn't it?
Whats the max rate for the cabin press system?
I guess you see where we are going.. isn't it?
Tim Vasquez has posted a fantastic weather analysis for the AF744 flight ... At the same time some pilots have posted observations of temps within cells climbing with 30 degrees within seconds ...
"I do not agree that a bubble of warmer air (that is, any warmer than about 5 degrees compared to the environmental air) would have made it up to flight level. This requires exceptionally high equivalent potential temperatures at some..."
"I do not agree that a bubble of warmer air (that is, any warmer than about 5 degrees compared to the environmental air) would have made it up to flight level. This requires exceptionally high equivalent potential temperatures at some..."
The aircraft was on a scheduled domestic passenger service flight from Karratha to Perth at Flight Level 310 (31,000 ft). As the aircraft entered cloud while diverting around a large thunderstorm, there was a sudden and significant rise in the outside air temperature. A short time later, all four engines progressively lost power and the aircraft was unable to maintain altitude. During the next 17 minutes, numerous attempts to restore engine power were made without success until, approaching 10,000 ft altitude, normal engine operation was regained.
The report is here:
http://www.atsb.gov.au/publications/...200286_001.pdf
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It may be of interest to other jet pilots, so I reproduce here a relevant passage from Checkboard's and Yaw String's posts regarding 'warm air pools' - a new concept to me. It does not offer any advance on all the 'computer' glitches, I fear, but may explain the TAT changes quoted on various threads. It is from Appendix 1 to the report quoted.
The phenomenon is almost certainly the result of folding of stratospheric air back into the troposphere with concomitant warming as it descends. The most likely mechanism for this to occur is strong convection (and for the pilot, strong returns on his weather radar). It is reasonable to assume that, where there is strong thunderstorm activity, there is a fair likelihood that an area of anomalous warming will be present somewhere around the storm cell. The vertical extent of the anomaly is difficult to ascertain and is probably dependent on the strength of the outflow from the storm and its interaction with the environmental flow, but in this case, with the tropopause at 41000 feet, observations from the aircraft indicated that the warming stopped abruptly at 28000 feet. The horizontal distribution of the anomaly is most likely to be oncentrated in a zone of confluence between the environmental flow and the thunderstorm outflow, as that would be the area where maximum descent was occurring, associated with the largest perturbation of the tropopause. In this case the anomaly took place on the western flank of the main convective activity. It is unlikely that any anomalous warming would have occurred on the eastern side of the storm as there would have been no confluence and no tropopausal perturbation. The temperature anomaly was observed over a horizontal distance of approximately 60 nm. By comparison, the incident in 1979 took place within a distance of near 40 nm. It is appropriate to relate some of the pilots’ comments here. His normal practice (and it is understood that this is standard practice) when diverting around radar echoes was to ensure a buffer of 10 nm between the radar echo and the aircraft. As an extra safety margin on this occasion he diverted 30 nm from the strongest echo and in doing so encountered the phenomenon. Perhaps this is a reason why it is not encountered regularly by aircraft - most pilots (in maintaining the normal 10 nm buffer) may travel between the anomalous zone and the thunderstorm. It is estimated that in the ncident associated with tropical cyclone Kerry in Queensland, the aircraft encountered the temperature anomaly about 40 nm on the western flank of the main convection area. The BAe 146 pilot also commented that he had been flying in an area of innocuous radar returns - visible as a “green fuzz” on his radar screen. He noticed that the warming commenced as they flew along the boundary between the weak returns and the clear air. It is suggested that that boundary was an indication of the delineation between air masses of differing characteristics. Another interesting comment was that prior to the incident there had been an incredible display of St Elmo’s Fire, the best that he had ever seen. Whether this was a precursor to this type of event or merely coincidence is conjectural at this stage.
Disregarding, of course, the type involved etc etc, it is significant that the apparent rise in SAT caused a large degredation in the altitude/weight capability of the engines.
The phenomenon is almost certainly the result of folding of stratospheric air back into the troposphere with concomitant warming as it descends. The most likely mechanism for this to occur is strong convection (and for the pilot, strong returns on his weather radar). It is reasonable to assume that, where there is strong thunderstorm activity, there is a fair likelihood that an area of anomalous warming will be present somewhere around the storm cell. The vertical extent of the anomaly is difficult to ascertain and is probably dependent on the strength of the outflow from the storm and its interaction with the environmental flow, but in this case, with the tropopause at 41000 feet, observations from the aircraft indicated that the warming stopped abruptly at 28000 feet. The horizontal distribution of the anomaly is most likely to be oncentrated in a zone of confluence between the environmental flow and the thunderstorm outflow, as that would be the area where maximum descent was occurring, associated with the largest perturbation of the tropopause. In this case the anomaly took place on the western flank of the main convective activity. It is unlikely that any anomalous warming would have occurred on the eastern side of the storm as there would have been no confluence and no tropopausal perturbation. The temperature anomaly was observed over a horizontal distance of approximately 60 nm. By comparison, the incident in 1979 took place within a distance of near 40 nm. It is appropriate to relate some of the pilots’ comments here. His normal practice (and it is understood that this is standard practice) when diverting around radar echoes was to ensure a buffer of 10 nm between the radar echo and the aircraft. As an extra safety margin on this occasion he diverted 30 nm from the strongest echo and in doing so encountered the phenomenon. Perhaps this is a reason why it is not encountered regularly by aircraft - most pilots (in maintaining the normal 10 nm buffer) may travel between the anomalous zone and the thunderstorm. It is estimated that in the ncident associated with tropical cyclone Kerry in Queensland, the aircraft encountered the temperature anomaly about 40 nm on the western flank of the main convection area. The BAe 146 pilot also commented that he had been flying in an area of innocuous radar returns - visible as a “green fuzz” on his radar screen. He noticed that the warming commenced as they flew along the boundary between the weak returns and the clear air. It is suggested that that boundary was an indication of the delineation between air masses of differing characteristics. Another interesting comment was that prior to the incident there had been an incredible display of St Elmo’s Fire, the best that he had ever seen. Whether this was a precursor to this type of event or merely coincidence is conjectural at this stage.
Disregarding, of course, the type involved etc etc, it is significant that the apparent rise in SAT caused a large degredation in the altitude/weight capability of the engines.
TAT temp increase
Re: the serious “a sudden and significant rise in the outside air temperature” (#608 R&N) and above.
Subsequent to the Australian report, the majority, if not all of the temperature rise was shown to be an apparent rise due to a frozen TAT probe – the probe indicated the ice temperature and not that of the atmosphere.
Following several other incidents BAe/Honeywell conducted tests in and near large Cbs to investigate the engine icing problem. As a result, the affected engine variant was modified.
The initial flight tests showed that the problem did not per se occur in the tops the Cbs, instead it was the water/ice concentrations in the out flow of these large storms (in the anvil) which presented a threat over a large area. The analogy is of smoke spreading out from a chimney.
The erroneous TAT in BAe146 aircraft was not deemed a significant risk. IIRC there was only an input to the TMS takeoff mode and a temp display in some FMS variants. Thus, the problem was documented and risks agreed with certification authorities; all engines were modified by AD.
At that time the industry did note similar problems with engines and TAT/pitot probes in other aircraft types, but (at that time) the safety concerns were not significant, certainly not in the same category as the multiple engine ‘rollbacks’ with the 146.
The TAT problem persisted, but industry follow-up identified differences between probe manufacturers and probe design, i.e. aspirated / non- aspirated versions.
I suspect that TAT is used in many modern computations and hence there are multiple sensors. Assuming that individual sensors do not ice at the same time / rate then disparities in temperature or in the parameters using TAT could trigger comparator alerts or even system lockout through data disagreement.
Many of the reported ACARs messages appear to represent this type of problem.
Subsequent to the Australian report, the majority, if not all of the temperature rise was shown to be an apparent rise due to a frozen TAT probe – the probe indicated the ice temperature and not that of the atmosphere.
Following several other incidents BAe/Honeywell conducted tests in and near large Cbs to investigate the engine icing problem. As a result, the affected engine variant was modified.
The initial flight tests showed that the problem did not per se occur in the tops the Cbs, instead it was the water/ice concentrations in the out flow of these large storms (in the anvil) which presented a threat over a large area. The analogy is of smoke spreading out from a chimney.
The erroneous TAT in BAe146 aircraft was not deemed a significant risk. IIRC there was only an input to the TMS takeoff mode and a temp display in some FMS variants. Thus, the problem was documented and risks agreed with certification authorities; all engines were modified by AD.
At that time the industry did note similar problems with engines and TAT/pitot probes in other aircraft types, but (at that time) the safety concerns were not significant, certainly not in the same category as the multiple engine ‘rollbacks’ with the 146.
The TAT problem persisted, but industry follow-up identified differences between probe manufacturers and probe design, i.e. aspirated / non- aspirated versions.
I suspect that TAT is used in many modern computations and hence there are multiple sensors. Assuming that individual sensors do not ice at the same time / rate then disparities in temperature or in the parameters using TAT could trigger comparator alerts or even system lockout through data disagreement.
Many of the reported ACARs messages appear to represent this type of problem.
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sp - "the probe indicated the ice temperature"- thus you are saying that ice that forms at -39C will be at a significantly higher temperture? I'm familiar with L Heat release at state change but cannot see how this can be? Surely also the measured TAT will decrease if the probe ices since there will be less airflow to 'halt'? We are looking to explain an overall 20deg rise or so.
Can someone remind me how/where SAT is derived on a/c? I've had a 'blank moment'
Can someone remind me how/where SAT is derived on a/c? I've had a 'blank moment'
Subsequent to the Australian report, the majority, if not all of the temperature rise was shown to be an apparent rise due to a frozen TAT probe – the probe indicated the ice temperature and not that of the atmosphere.
sp - "the probe indicated the ice temperature"- thus you are saying that ice that forms at -39C will be at a significantly higher temperture?
Can someone remind me how/where SAT is derived on a/c?
(The lower sensor is the Rosemount probe housing - the other is an ozone detector, for the MOZAIC programme.)
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Concerning one of the last ACARS messages,
Here's an excerpt of the A332 AMM about the Cabin Pressure System, I shortened some paragraphs:
(1) [...]
The cabin pressure sensor within the pressure controller 311HL (312HL) is of a vibrating cylinder type. The principle is, that a physical body vibrates at its natural frequency with high stability under constant environmental conditions. The frequency of oscillation of this system is a function of materials and shape of the cylinder. The frequency depends on the environment, specifically temperature and pressure surrounding the cylinder. Therefore, the frequency can be used as a reference value of the pressure.
The cabin pressure controllers send the subsequent warnings to the System Data Acquisition Concentrators (SDACs) or the Flight Warning Computers (FWCs):
- excessive cabin altitude (EXCESS CAB ALT),
- outflow valves not open (FWD OFV NOT OPEN, AFT OFV NOT OPEN),
- low delta p (LO DIFF PR),
- safety valves open (SAFETY VALVE OPEN),
- system fault (SYS1 FAULT, SYS2 FAULT or SYS 1+2 FAULT),
- fault of the landing-field elevation selector (LDG ELEV FAULT).
(a) Excessive Cabin Altitude
If the pressure in the fuselage is less than the atmospheric pressure between 9550 ft. (2910.78 m) and 14350 ft. (4373.79 m) (depending on the airport altitude):
- the automatic circuit of the cabin pressure controllers 311HL and 312HL sends a signal to the SDACs and FWCs,
- the manual backup circuit of the cabin pressure controller 311HL also sends a signal to the SDACs and from there to the flight warning computers.
[...]
This shows that the aircraft will have a negative differential pressure if the descent rate is continued and that a negative pressure relief will occur. The signal is sent from the automatic part of the cabin pressure controller 311HL or 312HL 1.5 minutes before the external pressure is the same as the pressure in the fuselage.
This warning indicates that at least one of the safety valves is open. Therefore the differential pressure between cabin and atmosphere is to high and can cause damage to the aircraft structure. The cracking point of the valve (and thus the maximum differential pressure) is at approximately 610 mbar (8.84 psi) .
If there is a general fault in the CPC system, the respective warning (SYS1 FAULT, SYS2 FAULT or SYS 1+2 FAULT) comes on the ECAM.
[...]
WN0906010214 213100206ADVISORY
Here's an excerpt of the A332 AMM about the Cabin Pressure System, I shortened some paragraphs:
B. Cabin Pressure Controller
(1) [...]
(2) Internal Pressure Sensor
(3) Warnings
The cabin pressure controllers send the subsequent warnings to the System Data Acquisition Concentrators (SDACs) or the Flight Warning Computers (FWCs):
- excessive cabin altitude (EXCESS CAB ALT),
- outflow valves not open (FWD OFV NOT OPEN, AFT OFV NOT OPEN),
- low delta p (LO DIFF PR),
- safety valves open (SAFETY VALVE OPEN),
- system fault (SYS1 FAULT, SYS2 FAULT or SYS 1+2 FAULT),
- fault of the landing-field elevation selector (LDG ELEV FAULT).
(a) Excessive Cabin Altitude
If the pressure in the fuselage is less than the atmospheric pressure between 9550 ft. (2910.78 m) and 14350 ft. (4373.79 m) (depending on the airport altitude):
- the automatic circuit of the cabin pressure controllers 311HL and 312HL sends a signal to the SDACs and FWCs,
- the manual backup circuit of the cabin pressure controller 311HL also sends a signal to the SDACs and from there to the flight warning computers.
(b) Outflow Valves Not Open
[...]
(c) Low Delta P/High Descent
This shows that the aircraft will have a negative differential pressure if the descent rate is continued and that a negative pressure relief will occur. The signal is sent from the automatic part of the cabin pressure controller 311HL or 312HL 1.5 minutes before the external pressure is the same as the pressure in the fuselage.
(d) Safety Valves Open
This warning indicates that at least one of the safety valves is open. Therefore the differential pressure between cabin and atmosphere is to high and can cause damage to the aircraft structure. The cracking point of the valve (and thus the maximum differential pressure) is at approximately 610 mbar (8.84 psi) .
(e) System Fault
If there is a general fault in the CPC system, the respective warning (SYS1 FAULT, SYS2 FAULT or SYS 1+2 FAULT) comes on the ECAM.
(f) Set-LFES
[...]
The ATA2131 advisory message is not specified in detail what the problem really was. This creates some possibilities, that are (or are not) discussed up to now:
=> No hard evidence of mid air break-up
=> No hard evidence of steep dive
- excessive altitude of cabin pressure (lead to speculation about fuselage break up)
- excessive aircraft descent rate taking over cabin pressure (lead to speculation about deep dive)
- safety valves open (not yet discussed)
- system component fault (not yet discussed)
- cabin pressure – measured by Cabin Pressure Sensor located in the e-bay below the cockpit
- static outside pressure – measured by the Static Ports and gathered by the ADIRUs
=> No hard evidence of mid air break-up
=> No hard evidence of steep dive
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A further mail from Tim Vasquez
Thanks Bert,
I have posted an update on the AF447 page clarifying my position
on the warming theory based on some new data. You may feel
free to relay this in whole if you wish. Hopefully it helps the
discussion.
Tim
On Mon, 8 Jun 2009 08:36:17 +0200, Bert van Horck wrote:
Dear Tim,
I have posted your comments on the open AF thread on the PPRUNE
forum. Several pilots have nevertheles come forward after this
posting confirming they have had temperature rises of in excess of
20 degrees C within Cb's.
Kind regards,
Bert
I have posted an update on the AF447 page clarifying my position
on the warming theory based on some new data. You may feel
free to relay this in whole if you wish. Hopefully it helps the
discussion.
Tim
On Mon, 8 Jun 2009 08:36:17 +0200, Bert van Horck wrote:
Dear Tim,
I have posted your comments on the open AF thread on the PPRUNE
forum. Several pilots have nevertheles come forward after this
posting confirming they have had temperature rises of in excess of
20 degrees C within Cb's.
Kind regards,
Bert
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Originally Posted by CB
The lower sensor is the Rosemount probe housing
In any of these supposed 'warm air' events, surely the clue would lie with the displayed SAT, not TAT? If that has increased too, then the air is warmer, if only TAT has increased.....?????
Checkboard “…interpolating the Airbus AD backwards…” absolutely not; the BAe/Honeywell work preceded the concerns on the ‘big jets’.
Links to the science relating to the TAT probe freezing are in http://www.pprune.org/tech-log/37578...ml#post4963257 ; most of this relates to engine core freezing, but the mechanism and atmospheric conditions are similar if not identical, for certain TAT probes.
BOAC, et al, as the TAT probe freezes the indicated temperature increases toward zero - ice temperature in a static airflow; because the airflow is blocked. The atmosphere - ambient air temperature is still cold.
In engine rollback incidents the TAT increase was a preceding or confirming characteristic seen on the FDR in the vast majority of events. Conversely there were TAT rises without engine malfunction. ( See slide 8)
In the very unusual conditions associated with large Cbs (probably only those which penetrate into the troposphere), the area in and around the anvil contains large quantities of minute ice crystals and often a high super cooled water content. In these conditions the TAT probe heater is progressively overcome by the high negative heat flow – melting ice crystals and water-flow off, such that further ice crystals accumulate in the probe throat (behind the heater). The melt water acts as glue for the ice crystals until the probe becomes blocked. IIRC the effect has been demonstrated in an icing tunnel.
With respect to the AF accident, it is of interest to establish how the various sensors use ‘temperature’, and particularly where there are multiple TAT inputs, how the systems (ADC/ ADIRS, Cabin, Rudder Lim) might cope with different values.
Assuming that multiple probes do not freeze identically; there will be different TAT values.
In a dual system, comparison can provide a comparator warning (systems still operative). However, with a triple input it may be possible to vote out an erroneous value; except in this instance the odd-one-out might be the true value and the voting system may have capability to shut down a ‘good’ system i.e ADC/ADIRS, etc.
Another issue is pitot icing. If the pitot system is also susceptible to icing in the same conditions, then conventionally the pressures existing at the time of blockage will be trapped. Thus there is no change in any of the input parameters with icing – whereas TAT will change with probe icing.
However, is has been suggested that the A330 pitot systems have a small leak path (drain) which will result in a pressure reduction to ambient (decreasing or fluctuating IAS). Thereafter the issues of value comparison and shut down are as above.
Links to the science relating to the TAT probe freezing are in http://www.pprune.org/tech-log/37578...ml#post4963257 ; most of this relates to engine core freezing, but the mechanism and atmospheric conditions are similar if not identical, for certain TAT probes.
BOAC, et al, as the TAT probe freezes the indicated temperature increases toward zero - ice temperature in a static airflow; because the airflow is blocked. The atmosphere - ambient air temperature is still cold.
In engine rollback incidents the TAT increase was a preceding or confirming characteristic seen on the FDR in the vast majority of events. Conversely there were TAT rises without engine malfunction. ( See slide 8)
In the very unusual conditions associated with large Cbs (probably only those which penetrate into the troposphere), the area in and around the anvil contains large quantities of minute ice crystals and often a high super cooled water content. In these conditions the TAT probe heater is progressively overcome by the high negative heat flow – melting ice crystals and water-flow off, such that further ice crystals accumulate in the probe throat (behind the heater). The melt water acts as glue for the ice crystals until the probe becomes blocked. IIRC the effect has been demonstrated in an icing tunnel.
With respect to the AF accident, it is of interest to establish how the various sensors use ‘temperature’, and particularly where there are multiple TAT inputs, how the systems (ADC/ ADIRS, Cabin, Rudder Lim) might cope with different values.
Assuming that multiple probes do not freeze identically; there will be different TAT values.
In a dual system, comparison can provide a comparator warning (systems still operative). However, with a triple input it may be possible to vote out an erroneous value; except in this instance the odd-one-out might be the true value and the voting system may have capability to shut down a ‘good’ system i.e ADC/ADIRS, etc.
Another issue is pitot icing. If the pitot system is also susceptible to icing in the same conditions, then conventionally the pressures existing at the time of blockage will be trapped. Thus there is no change in any of the input parameters with icing – whereas TAT will change with probe icing.
However, is has been suggested that the A330 pitot systems have a small leak path (drain) which will result in a pressure reduction to ambient (decreasing or fluctuating IAS). Thereafter the issues of value comparison and shut down are as above.
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A bit premature on 447, but with the image of the complete R/VS assembly and the aerodynamic failures associated with its loss, the topic may be timely now. "Donated Failure". We have been addressing this concept for some time at my firm. Consider "crumple zone" where energy is dissipated by sequential failure of linear structural assemblies. "Ablative Cooling", where temps are dropped by sacrificing material in its melting or burning; "anomalous design" where systems have different capabilities but solve the same problems, Airbus and its "three different computers".
In Brief, if AA 587 had suffered a loss of its Rudder instead of the Rudder/Vertical Stabiliser together? Perhaps, though not demonstrable for AF447, the same?
A Rudder is a trimming device. Without it, flight is difficult but survivable. Without a VS, the game is over.
Had 587 been equipped with design limited hinges on its Rudder, well, you get the picture?
MD11 at Narita. Way too strong MLG and NLG caused the last bounce (when the NG failed to break off) and the subsequent "landing" on the mains, which broke both wings (but the gear "survived" on the right).
AF 447 may have had other issues, but if the VS separated first, instead of not at all (with a "donated failure" of the Rudder), ?? Will have to see.
In Brief, if AA 587 had suffered a loss of its Rudder instead of the Rudder/Vertical Stabiliser together? Perhaps, though not demonstrable for AF447, the same?
A Rudder is a trimming device. Without it, flight is difficult but survivable. Without a VS, the game is over.
Had 587 been equipped with design limited hinges on its Rudder, well, you get the picture?
MD11 at Narita. Way too strong MLG and NLG caused the last bounce (when the NG failed to break off) and the subsequent "landing" on the mains, which broke both wings (but the gear "survived" on the right).
AF 447 may have had other issues, but if the VS separated first, instead of not at all (with a "donated failure" of the Rudder), ?? Will have to see.
as the TAT probe freezes the indicated temperature increases toward zero - ice temperature in a static airflow
In any case - I am aware of the icing conditions and engine effects in the links you posted. You appear to think that that is the only possibility for engine rollback, and indeed dismiss high altitude temperature increases as misunderstood icing events. The science for high altitude warming is well understood - and there is no evidence that the JJP 146 event (certainly) was an icing event. Both events occur, both are rare.
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Structural failure - hardly
IMHO,
Structural failure as a cause of this accident is highly unlikely. Especially unfounded speculation(s) regarding the v/stab and rudder on Airbus articles.
A lot more then a mere picture would be needed to provide sufficient data to support this. Everything else is out of scope for the "real world" situations like fleet operations with thousands of cycles.
Cheers
Structural failure as a cause of this accident is highly unlikely. Especially unfounded speculation(s) regarding the v/stab and rudder on Airbus articles.
A lot more then a mere picture would be needed to provide sufficient data to support this. Everything else is out of scope for the "real world" situations like fleet operations with thousands of cycles.
Cheers
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Regarding separation of the vertical fin:
The onboard computers that keep the pilot from 'overpowering' the rudder seem to get the job done while in 'Normal Law' on Airbus aircraft. But, when not in Normal Law for whatever reason, what indication does the pilot have to keep from exerting too much force through the rudder pedals to the rudder to keep it from departing the aircraft ?
The onboard computers that keep the pilot from 'overpowering' the rudder seem to get the job done while in 'Normal Law' on Airbus aircraft. But, when not in Normal Law for whatever reason, what indication does the pilot have to keep from exerting too much force through the rudder pedals to the rudder to keep it from departing the aircraft ?