Response of FBW aircraft in updraft
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Erroneous response?
In post #14 I wrote that an A320 will not stall when the high-angle-of-attack protection (alphaprot) of FBW system is triggered by an upward gust.
An Emergency Airworthiness Directive was recently issued in response to an occurrence where an Airbus A321 aeroplane encountered a blockage of two Angle of Attack (AoA) probes during climb, leading to activation of the Alpha Protection (Alpha Prot) while the Mach number increased. The EAD requires amendment of the applicable AFM to advise the flightcrew of emergency procedures for abnormal Alpha Prot. The new procedure starts with:
“At any time, with a speed above VLS, if the aircraft goes to a continuous nose down pitch rate that cannot be stopped with backward sidestick inputs, immediately:
Keep on one ADR
Turn off two ADRs”.
Suppose the event mentioned in post #14 occurs, and is erroneously associated with the new emergency procedure introduced by the EAD. Would switching off two ADRs reconfigure the FCS to Alternate Law, and the high-angle-of-attack protection would be lost?
An Emergency Airworthiness Directive was recently issued in response to an occurrence where an Airbus A321 aeroplane encountered a blockage of two Angle of Attack (AoA) probes during climb, leading to activation of the Alpha Protection (Alpha Prot) while the Mach number increased. The EAD requires amendment of the applicable AFM to advise the flightcrew of emergency procedures for abnormal Alpha Prot. The new procedure starts with:
“At any time, with a speed above VLS, if the aircraft goes to a continuous nose down pitch rate that cannot be stopped with backward sidestick inputs, immediately:
Keep on one ADR
Turn off two ADRs”.
Suppose the event mentioned in post #14 occurs, and is erroneously associated with the new emergency procedure introduced by the EAD. Would switching off two ADRs reconfigure the FCS to Alternate Law, and the high-angle-of-attack protection would be lost?
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Gysbreght
The very idea of switching of two ADRs is to get in Alternate law. That is the only way of getting out of the strangle hold of the hard protections as they are now triggered by erroneous AoA data. So you will get reduced protection as is in alternate law which will also be erroneous but can be over ridden.
The very idea of switching of two ADRs is to get in Alternate law. That is the only way of getting out of the strangle hold of the hard protections as they are now triggered by erroneous AoA data. So you will get reduced protection as is in alternate law which will also be erroneous but can be over ridden.
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vilas,
So if your are subjected to up and down drafts and turbulence, and your only working ADR is receiving erroneous info from a frozen pitot (due icing), and the stabiliser trim is still auto trimming, how does the Airbus now fly a safe trimmed AoA (speed)?
So you will get reduced protection as is in alternate law which will also be erroneous but can be over ridden.
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Goldenrivett
As you know stick free Airbus trims for 1g and not for speed. AoA or speed data is not needed for that.The AD is for frozen AoA and not for frozen Pitot. These have different implications. In frozen Pitot case one ADR is kept for stall warning which comes from AoA probe and not for pressure data. However in frozen AoA case stall warning will also be false. This AD is not very comprehensive and doesn't cover frozen Pitot and can mislead the pilot if Pitots are also blocked. A320s which have MOI153528/P12909 the FPV is modified to calculate FPA from hybrid GPIRS(mixed IRS and GPS position). So you can use it in this case. But without this MOD bird can only be used provided Pitot/static is not blocked.
As you know stick free Airbus trims for 1g and not for speed. AoA or speed data is not needed for that.The AD is for frozen AoA and not for frozen Pitot. These have different implications. In frozen Pitot case one ADR is kept for stall warning which comes from AoA probe and not for pressure data. However in frozen AoA case stall warning will also be false. This AD is not very comprehensive and doesn't cover frozen Pitot and can mislead the pilot if Pitots are also blocked. A320s which have MOI153528/P12909 the FPV is modified to calculate FPA from hybrid GPIRS(mixed IRS and GPS position). So you can use it in this case. But without this MOD bird can only be used provided Pitot/static is not blocked.
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Goldenrivett
No problem .. fly level, and from the pitch attitude and config you can deduce whether your IAS is higher or lower than the recommended value, just like every other aircraft on the planet.
So if your are subjected to up and down drafts and turbulence, and your only working ADR is receiving erroneous info from a frozen pitot (due icing), and the stabiliser trim is still auto trimming, how does the Airbus now fly a safe trimmed AoA (speed)?
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Hi TyroPicard,
If you fly level during an updraft, you will initially observe your pitch attitude is lower than recommended and be tempted to reduce power. If the updraft is large, you will be able to fly level when the aircraft's rate of descent matches the updraft; and if you manage to set your recommended pitch attitude, that's only achievable when you're flying slow.
Thanks vilas, you recognised what I was getting at.
No problem .. fly level, and from the pitch attitude and config you can deduce whether your IAS is higher or lower than the recommended value,
Thanks vilas, you recognised what I was getting at.
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Further to an earlier post
here,
I have recently learned from a discussion on another site that the following picture shows one of the last ADS-B transmissions received from Air Asia flight QZ8501 and that the data shown are from GPS. The missing data are all from the relevant ADIRU. Since a total failure of an ADIRU (ADR and IRU) may be considered unlikely, it is possible that it was switched off.
here,
I have recently learned from a discussion on another site that the following picture shows one of the last ADS-B transmissions received from Air Asia flight QZ8501 and that the data shown are from GPS. The missing data are all from the relevant ADIRU. Since a total failure of an ADIRU (ADR and IRU) may be considered unlikely, it is possible that it was switched off.
Last edited by Gysbreght; 12th Jan 2015 at 20:07. Reason: typo
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Gysbreght
It appears to me that two issues are getting mixed up. In the first case the aircraft is in normal law and there is no icing of any AoA sensors or Pitot tubes. But due to change in AoA externally without any action on side stick by the pilot the alpha prot protection is activated. This will cause speed(AoA) to latch and the aircraft will normally descend but if AoA decreases because of outside airflow on the wings the aircraft will climb. This can be unlatched by pushing the stick forward. The second case is AoA probes are frozen at low speed (high AoA). When the aircraft accelerates to higher Mach the alpha prot threshold is decreased as the CL max reduces and that triggers erroneous alpha prot which cannot be unlatched. In this case you put the aircraft into alternate law and override the protection. This case is not going to put the aircraft in serious dive but normal descent at constant speed as it is latched. At lower altitudes at equatorial region temperatures ice will melt and situation should become normal.
It appears to me that two issues are getting mixed up. In the first case the aircraft is in normal law and there is no icing of any AoA sensors or Pitot tubes. But due to change in AoA externally without any action on side stick by the pilot the alpha prot protection is activated. This will cause speed(AoA) to latch and the aircraft will normally descend but if AoA decreases because of outside airflow on the wings the aircraft will climb. This can be unlatched by pushing the stick forward. The second case is AoA probes are frozen at low speed (high AoA). When the aircraft accelerates to higher Mach the alpha prot threshold is decreased as the CL max reduces and that triggers erroneous alpha prot which cannot be unlatched. In this case you put the aircraft into alternate law and override the protection. This case is not going to put the aircraft in serious dive but normal descent at constant speed as it is latched. At lower altitudes at equatorial region temperatures ice will melt and situation should become normal.
