Disclaimer:
▪︎This content is Not an official document and does not represent the views of Airbus or any other aviation authority.
▪︎The information provided may be incorrect or misinterpreted and should not be relied upon for decision-making.
▪︎Always refer to official documents and consult with a qualified aviation professional before making any decisions based on the information provided in this blog post.
▪︎The information provided in this blog post is based on personal study and review.
There exists an unreliable airspeed memory item procedure for the Airbus A320.
I recall an instance from years ago when I engaged in an "unreliable airspeed" scenario in the simulator session.
Despite diligently adhering to the manual's memory item procedure,
I found myself unable to maintain control of the aircraft, prompting the instructor to halt the simulator session.
I was curious about the underlying issue during that situation.
Have you recently practiced an unreliable airspeed scenario in the simulator?
(By the way, the FCOM designates the memory item callout as "unreliable Speed.")
Unreliable Speed memory item
It could be different depending on the airplane. Basically, when dealing with unreliable speed, the memory item is to turn off the autopilot, switch off the autothrust, and deactivate the flight directors. This depends on the plane's altitude, but the idea is to control the pitch and thrust manually by flying without the help of flight directors and autothrust.
The memory item for dealing with unreliable speed is built on the assumption that the pitot tube or static ports might give inccorect information for different reasons. This can cause the autopilot and autothrust to behave strangely. That's why we have a memory item to fix this unusual behavior.
Protection in Normal Law
Before we proceed further, we need to discuss Airbus protection within the realm of normal law.
Within the Airbus A320 family's normal law, several forms of protection are established:
Load Factor Protection
Pitch Attitude Protection
High Angle of Attack Protection
High-Speed Protection
HIGH SPEED PROTECTION
High speed protection is activated at/or above VMO/MMO. When high speed protection is activated,
As the speed increases above VMO/MMO, the sidestick nose down authority is progressively reduced,
and a permanent nose-up order is applied to aid recovery to normal flight conditions.
HIGH ANGLE-OF-ATTACK PROTECTION
while high speed protection causes a nose-up order, High angle-of-attack protection is opposite.
In normal law, When the current AOA becomes greater than αPROT, the high AOA protection activates.
The system inhibits further nose-up trim beyond the point already reached. The nose-down trim remains available.
and this This High AOA protection has priority over all other protections.
In short, during normal law, the High-Speed Protection triggers a permanent nose-up pitch command when an overspeed situation occurs. Additionally, the High Angle of Attack Protection prevents nose-up trim when the aircraft is at a high angle of attack attitude.
Unrealistic Simulator scenarios
In 2019, Airbus published on safety megazine. "The Adverse Effects of Unrealistic Simulator Scenarios"
In this scenario, the two corresponding ADRs deliver incorrect yet consistent airspeed readings. This leads to a false increase in computed airspeed and an unwarranted activation of the High-Speed protection, lasting until the High Angle of Attack protection comes into play. Eventually, the high-speed protection and the Angle of Attack (AoA) protection activate in alternation. This leads to significant pitch oscillations.
I believe this scenario is the one I experienced years ago
(two pitot bloced at the same time with same degree) ,
where I was supposed to apply the unreliable speed memory item. However, I couldn't control the aircraft because it stayed in normal law. All these protections made it impossible to control the aircraft unless I switched it to alternate law.
In this document, Aibus confirmed that this is unrealistic scenario and said it must not be used for the training scenario.
Abnormal V-alpha prot OEB 48/49
The Unreliable Speed procedure is founded on the assumption that when the pitot tube or static ports give incorrect information, but what if AOA probe is giving wrong information ?
There is an Operational Engineering Bulletin (OEB) 48/49 concerning the AOA probe. If the AOA provides erroneous information, it could wrongly trigger excessive High Angle of Attack protection, preventing a proper pitch-up. Thus, there was a OEB procedure that involves turning off the pushbuttons for two ADRs.
But if your airline doen't have this proceure, it means they upgraded the software.
Even if the OEB48/49 is canceled after the upgrade, a note persists within the memory item in the Flight Crew Operating Manual (FCOM) and the Airbus Aircraft Flight Manual (AFM) under the emergency procedures section, specifically addressing the ADR 1+2+3 fault in the navigation system.
"If the BUSS does not respond to longitudinal stick input while flying within the safe speed range indicated by the green area on the speed scale, the flight crew must disregard the BUSS input. Instead, they should make adjustments to pitch attitude and thrust based on the current flight phase and aircraft configuration, aiming to achieve and sustain the target parameters."
This procedure delineates the necessary actions to be taken in the event of an AOA problem.
Remain in Alternate law
Airbus stated in the safety magazine that the simultaneous blockage of 2 pitot tubes at the same degree is considered unrealistic, leading to the removal of this training scenario.
Apart from these exceptional situations, when an 'ADR disagree' condition occurs, the ECAM system assists the airplane by transitioning it to alternate law. This transition prevents unintended pitch protection during instances of incorrect ADR information or in unreliable speed situaion.
Disclaimer:
▪︎This content is Not an official document and does not represent the views of Airbus or any other aviation authority.
▪︎The information provided may be incorrect or misinterpreted and should not be relied upon for decision-making.
▪︎Always refer to official documents and consult with a qualified aviation professional before making any decisions based on the information provided in this blog post.
▪︎The information provided in this blog post is based on personal study and review.