Unreliable Speed: When Your Instruments Can't Be Trusted

Imagine climbing through 13 000 ft when your airspeed indicators suddenly show conflicting readings. The captain's display shows 280 kt, the first officer's shows 320 kt, and the standby instrument reads 250 kt. Which one is correct? This scenario represents one of aviation's most challenging situations: unreliable speed indications. Understanding how your A320/A321 detects, manages, and helps you fly through these events could be the difference between a manageable situation and a critical emergency.

The Problem: When Speed Becomes Uncertain

Your aircraft relies on accurate airspeed data for everything from autopilot commands to stall protection. The Air Data Reference (ADR) systems continuously monitor three pitot probes and six static ports, feeding critical information to flight computers, displays, and protection systems. But what happens when this data becomes corrupted?

The most common culprit is obstruction of pitot or static probes, whether from ice, water, volcanic ash, mud, or even insects. Unlike total system failures that trigger obvious warnings, these obstructions often create subtle, misleading indications that can fool both pilots and computers. The aircraft might show normal readings during taxi and takeoff, only to display erroneous data once airborne.

How Your Aircraft Fights Back: The Voting System

The A320/A321 employs sophisticated fault accommodation logic based on a voting principle. Think of it like having three witnesses to an accident—if two witnesses tell the same story and one tells a different story, you'd probably trust the majority. Similarly, when one ADR provides data that significantly differs from the other two, the aircraft systems automatically reject the outlier and continue operating with the remaining sources.

This voting system protects flight control computers (ELACs), Flight Augmentation Computers (FACs), and the Flight Management and Guidance Computer (FMGC). Each system continuously compares airspeed data from all three ADRs, ready to exclude any source that appears unreliable.

When the System Recognizes Problems

The aircraft provides several levels of alerts when speed data becomes questionable:

Single ADR Failure: When one ADR provides erroneous data, you might see ECAM cautions like "NAV IAS DISCREPANCY" or "NAV ADR DISAGREE." The faulty ADR is automatically excluded, but your autoland capability drops to CAT 3 SINGLE as a precaution.

Multiple ADR Problems: When two or more ADRs disagree, the situation becomes more serious. Both autopilot and autothrust disconnect automatically, flight controls revert to alternate law (losing high and low-speed protections), and you'll see the SPD LIM flag appear on both Primary Flight Displays (PFDs). The aircraft tells you, "I can't trust my speed data enough to provide normal protections."

The Dangerous Scenario: Sometimes, all three ADRs provide consistent but incorrect data—imagine all three pitot tubes blocked by volcanic ash in exactly the same way. The voting system fails because there's no disagreement to detect, yet all the data is wrong. This is when the speed monitoring function becomes critical, using backup calculations, if installed.

Reading the Warning Signs

Unreliable speed events don't always announce themselves with ECAM alerts. You need to recognize the subtle signs:

Flight Parameter Mismatches: Watch for unusual relationships between pitch, thrust, airspeed, altitude, and vertical speed. If you're climbing with high thrust and nose-up pitch but altitude isn't increasing, or if indicated airspeed increases despite significant nose-up pitch, your speed data is likely corrupted.

Autopilot Behavior: The autopilot might pitch excessively up in climb mode or down in descent mode, trying to maintain a target speed that doesn't match reality. These seemingly erratic commands often indicate the autopilot is responding to false speed data.

Environmental Clues: Pay attention to aerodynamic noise that doesn't match the indicated airspeed or "rain" appearing on windshields at temperatures too low for liquid water—this could indicate ice crystal encounters that affect pitot probes.

Your Response: The Unreliable Speed Procedure

When you suspect unreliable speed, your first priority is maintaining aircraft control. The memory items focus on stabilizing the flight path before troubleshooting:

Immediate Actions: Disconnect automation to prevent erroneous commands from autopilot or autothrust. If autothrust disconnects, the thrust lock function engages, holding the current thrust until you move the levers.

Troubleshooting: Cross-check speed and altitude on the captain's PFD, first officer's PFD, and standby instruments. Use (e)QRH Pitch/Thrust tables to verify expected speeds for your weight and flight level. Don't dismiss an outlier ADR too quickly—multiple ADRs can show identical but incorrect data.

System Management: Once you identify faulty ADRs, turn them off. This triggers ECAM alerts but prevents flight control laws from using unreliable data. If you can't identify which ADRs are faulty, turn off two ADRs above FL250 (keeping one active) or all ADRs below FL250, relying on the backup speed scale, if installed.

Flying with Unreliable Indications

When your normal speed references fail, the aircraft provides further backup systems, if installed. The Digital Backup Speed (DBUS) system replaces normal airspeed with backup speed calculated from angle of attack, load factor, aircraft configuration, and GPS altitude. GPS altitude replaces barometric altitude, giving you reliable references for continued flight.

The Backup Speed Scale (BUSS) appears when all ADRs are turned off, showing speed ranges rather than precise values. The green area represents safe operating speeds, with amber and red zones warning of excessive or insufficient speed. This system relies on angle of attack data, which remains reliable even when pitot-static systems fail.

Flying Techniques for Unreliable Speed

Change only one parameter at a time—speed, altitude, or configuration. This prevents compounding errors and helps you understand cause-and-effect relationships.

Use GPS altitude and flight path vector when available: These provide reliable references for maintaining level flight. Adjust thrust while monitoring pitch attitude, comparing observed pitch with (e)QRH targets. If pitch is above target, you're slow—add thrust. If below target, you're fast—reduce thrust.

For approaches: Plan early stabilization and use backup characteristic speeds displayed on the PFD. VMAX corresponds to VFE/VMO, while VMIN corresponds to the stall warning speed. Conduct configuration changes with wings level, staying in the upper part of the green band before retracting flaps and the lower part before extending them, using the BUSS.

The Bigger Picture: Why This Matters

Understanding unreliable speed isn't just about following procedures—it’s about maintaining situational awareness when your primary references fail. The A320/A321's multiple backup systems and sophisticated monitoring provide layers of protection, but they require adequate pilot understanding.

Remember that stall warnings remain functional even when speed data is corrupted, because they use angle of attack sensors rather than pitot-static data. Trust these warnings absolutely, even if they seem to conflict with indicated airspeed.

The key to managing unreliable speed events lies in recognizing them early, understanding your aircraft's backup systems, and maintaining disciplined flying techniques when normal references fail. Your A320/A321 provides multiple ways to continue safe flight even when speed data becomes unreliable—but only if you understand how to use these systems effectively.

This knowledge transforms a potentially catastrophic situation into a manageable emergency, demonstrating why understanding your aircraft's systems deeply matters more than simply memorizing procedures.

What to do when A320 airspeed can't be trusted – how the ADR voting system works, how to recognize corrupted data, and how to fly with backup speed references.