Aircraft Electrical Systems: A320/A321 Briefing

 

Technical & Operational Training Report: A320 Air Data Reference (ADR) & Unreliable Speed Management

1. Executive Summary

In the sophisticated architecture of Airbus Fly-By-Wire (FBW) aircraft, air data serves as the critical "nervous system." Precise measurements of airspeed, altitude, and Angle of Attack (AoA) are not merely for pilot display; they are the fundamental inputs driving flight control laws and envelope protections. For the modern flight crew, a deep understanding of the "Median Value" logic—the method by which the aircraft’s computers validate this data—is a primary safety barrier against automation surprises.

The objective of this report is to move the training focus beyond the rote memorization of emergency checklists toward a deep technical understanding of ADR voting logic, fault isolation, and the Back-Up Speed Scale (BUSS). By evolving from a "button-pusher" to a "system-thinker," pilots can effectively manage high-workload scenarios where the automation may fail to differentiate between valid and corrupted data. This mastery is essential for maintaining safety in the most demanding operational environments.

2. Operational Context

Air data discrepancies typically manifest in challenging environments such as severe icing conditions or high-altitude cruise, where aerodynamic margins are narrow. In these contexts, pitot or static probe contamination—often caused by ice, volcanic ash, or foreign objects—leads to inconsistent or frozen speed and altitude indications.

This knowledge is applied across three primary contexts:

• Line Operations: Identifying and managing unalerted failures during routine flight.
• Simulator Training: Developing the manual flight skills and muscle memory required for initial and recurrent proficiency.
• Check-ride Assessments: Demonstrating cognitive resilience and systematic troubleshooting under the scrutiny of an examiner.

A successful outcome in these scenarios begins with a firm grasp of the technical architecture that supports flight control logic.

3. Technical Foundation: Architecture and Voting Logic

Effective troubleshooting in the cockpit is predicated on understanding system architecture. Without this foundation, a crew may misinterpret displays or isolate the wrong system during a high-stress event.

ADIRU Architecture The A320 utilizes three independent Air Data Inertial Reference Units (ADIRUs). Each unit consists of an Air Data Reference (ADR) part and an Inertial Reference (IR) part.

• ADR 1 supplies the Captain’s PFD 1.
• ADR 2 supplies the First Officer’s PFD 2.
• ADR 3 supplies the Integrated Standby Instrument System (ISIS) and acts as a backup for either PFD.

The Median Selection Rule To ensure data integrity, the ELAC and SEC flight control computers employ a 3-channel median selector. When all three ADRs are active and valid, the computers calculate the "middle value" among the three sources for airspeed and AoA. This median value drives the flight control laws and protections, allowing the system to automatically reject a single outlier that diverges beyond a set tolerance.

The Dual Erroneous Agreement Trap (OEB 48/49) A critical vulnerability exists when two sensors provide identical corrupted data—such as two AoA probes freezing at the same angle due to icing. In this "Agreement Trap," the median logic treats the two corrupted values as the majority and erroneously rejects the single "good" ADR. This specific threat is a primary focus of OEB 48/49.

Law Degradation The system requires a "tie-breaker" to remain in Normal Law. If a disagreement occurs between only two active ADRs, the system triggers a NAV ADR DISAGREE and NAV IAS DISCREPANCY caution. Because the computers cannot determine a valid median without a third source, the aircraft reverts from Normal Law to Alternate Law. These internal logic gates directly dictate the indications presented to the pilot.

4. Cockpit Recognition: Alerts and Indications

Rapid pattern recognition is essential to differentiate between a localized hardware fault and a systemic unreliable speed event.

Indication/Alert	System Significance
NAV ADR DISCREPANCY	A disagreement exists between ADR sources; the system cannot determine a median.
NAV IAS DISCREPANCY	Specific speed disagreement detected between ADR sources.
NAV ADR 1(2)(3) FAULT	A specific ADIRU has experienced an internal hardware failure.
SPD LIM Flag	Loss of speed-based envelope protection calculations.
CHECK SPEED Flag	PFD comparison logic detects a significant difference between PFD 1 and PFD 2.
Speed Tape Flags	Red or amber flags indicating speed data is invalid or outside limits.

Manual control is exercised through the ADR pushbuttons on the overhead panel to isolate units, and the AIR DATA switching selector located on the center pedestal to restore data to the PFDs. These indications must be monitored diligently during all routine flight phases.

5. Normal Operation and Monitoring

The TRI/TRE philosophy centers on "active monitoring," where the crew constantly validates system output against expected performance. In normal operations, while data distribution is automated, the crew is responsible for detecting subtle inconsistencies before they trigger a hard ECAM alert. Monitoring ensures that any discrepancy is managed before it escalates into a degraded flight state.

6. Abnormal & Degraded Operation

Abnormal operations require a transition from "managing the automation" to "manual flight path stabilization."

ADR Rejection and Reversion If an ADR parameter diverges beyond tolerance for a specified time, the flight control computers automatically reject it. However, if all sources are suspect, the crew must take manual control.

The Back-Up Speed Scale (BUSS) The BUSS is a degraded mode available when all three ADRs are unreliable.

• Logic: The BUSS replaces the conventional Indicated Airspeed (IAS) scale with a color-coded Angle of Attack (AoA) scale (Green, Amber, and Red bands). Barometric static altitude is replaced by GPS Altitude.
• Configuration: The BUSS is triggered only when the crew selects all three ADR pushbuttons to OFF.
• Flight Law: Operating with all ADRs OFF forces the flight controls into Alternate Law (without protections) or Direct Law (depending on the landing gear configuration). All automation (AP, FD, A-THR) is lost.

This transition requires immediate and precise crew actions to maintain the flight path.

7. Crew Priorities and Actions: The "Golden Rules"

Management of air data failures follows the "Golden Rules": Fly, Navigate, Communicate. The core mantra is Pitch + Thrust = Performance.

Immediate Actions (Memory Items) To prevent automation from following corrupted data, the crew must:

1. Disconnect AP, FD, and A-THR.
2. Set Initial Pitch and Thrust: Establish a safe, known attitude and power setting for the current flight phase (Takeoff, Climb, or Cruise/Level-off).

Majority Voting / Standby Cross-Check Logic

1. Stabilize: Maintain pitch and thrust first.
2. Compare: Check PFD 1 (ADR 1), PFD 2 (ADR 2), and ISIS (ADR 3).
3. Identify the Dissenter: Compare indications against expected performance for the set pitch/thrust.
4. Isolate: Select the confirmed faulty ADR to OFF on the overhead panel.

This procedure ensures the crew remains ahead of the aircraft during high-pressure transitions.

8. Threat and Error Management (TEM)

The psychological challenge of air data failure includes the "startle factor" and confirmation bias.

• The Identification Trap: Attempting to troubleshoot or diagnose ECAM messages before stabilizing pitch and thrust.
• The Agreement Trap: Trusting two ADRs that agree but are both wrong due to identical icing (OEB 48/49).
• The BUSS Chase: Over-controlling the aircraft by trying to "chase" the green band rather than using smooth, small pitch adjustments.
• Automation Bias: Delaying the disconnection of AP/A-THR, allowing the aircraft to follow false data into a dangerous state.

9. Scenario Analysis: Simulator Training Applications

Scenario A: Single Fault (Pitot Blockage on ADR 1 during Climb)

• Situation: ADR 1 pitot tube is blocked.
• Cues: PFD 1 speed erroneously increases or freezes; PFD 2 and ISIS remain normal.
• Interpretation: Crew identifies a discrepancy between PFD 1 and other sources.
• Action: Stabilize, identify ADR 1 as the dissenter, switch ADR 1 OFF, and use AIR DATA switching on the center pedestal to select ADR 3 for the Captain.

Scenario B: Total Blockage/BUSS (Triple Pitot Icing in Cruise)

• Situation: All three pitot tubes corrupted by icing.
• Cues: All speed tapes suspect. Note: If pressures stay within physical thresholds, there may be no immediate ECAM caution (unalerted failure).
• Interpretation: Recognition that no speed indication can be trusted via "Pitch + Thrust" cross-check.
• Action: Apply memory items (Pitch/Thrust), execute QRH, and select all ADRs OFF. Fly the Green Band on the BUSS.

10. TRI/TRE Instructor Notes

The instructor’s objective is to foster a "system-thinker" rather than a "button-pusher."

Teachable Moments & Competency Observations

• Does the PF prioritize the "Initial Action" phase (Memory Pitch/Thrust) before moving to the "Level-off" or troubleshooting phase?
• Are manual flight skills smooth, or is there evidence of "chasing" the instruments?
• How effective is the task-sharing during the ADR identification cross-check?

Debriefing Questions

1. Why is a cross-check against pitch/thrust tables mandatory even if two speed tapes agree?
2. What aerodynamic parameter is the BUSS actually displaying?
3. How does landing gear configuration affect our flight control law when all ADRs are OFF?
4. Why must we use GPS Altitude for terrain awareness when the BUSS is active?

11. Key Takeaways: Operational Memory Anchors

• Pitch + Thrust = Performance: Your only guarantee of safety when the "nervous system" fails.
• Stabilize First: Never attempt to identify a faulty ADR until the flight path is under manual control.
• The Median Rule is Not Infallible: Two agreeing indicators can both be wrong (OEB 48/49).
• BUSS = Angle of Attack: You are flying an AoA scale, not knots. Maintain the Green Band with smooth inputs.
• GPS Altitude & Terrain: Barometric static altitude is lost on the BUSS; use GPS altitude and maintain high awareness of terrain.