Aircraft Electrical Systems: A320/A321 Briefing

 

When Your GPS Lies: 5 Essential Lessons for the Modern A320 Flight Deck

1. Introduction: The New Frontier of Electronic Warfare

In the modern era of aviation, the cockpit is no longer a sanctuary from global conflict. As pilots fly through or near sensitive regions, they are entering a high-stakes environment where GPS signals—once the bedrock of precision navigation—are no longer guaranteed. In these zones, electronic warfare has turned the sky into a landscape of digital deception.

If the world’s most advanced navigation systems can be "tricked" into believing the aircraft is miles from its actual position, how does a pilot maintain control? Staying safe requires more than just monitoring automation; it requires understanding the hidden logic of the Flight Management and Guidance Computer (FMGC). This post distills expert insights into managing degraded navigation on the A320, ensuring you stay ahead of the aircraft when the satellites start lying.

2. The "Bias" Secret—How Your Plane Actually Knows Where It Is

To manage a navigation failure, you must first understand how the A320 determines its location. The aircraft does not rely on a single data point; instead, the FMGC calculates a "MIX IRS" position, which is the mean-weighted average of the three independent Inertial Reference Systems (IRS).

Because inertial data naturally drifts over time, the system applies a corrective layer known as a "Bias" vector. Think of the Bias like the "tare" or offset on a digital scale that hasn't been zeroed—it is the difference between the drifting IRS data and the most accurate available navigation source. To maintain the "FM Position" (the position used for navigation), the system follows a strict hierarchy of sources to update this Bias:

1. GPIRS (GPS): The primary and highest-priority source.
2. Radio (DME/DME): Triangulation using distance from multiple ground stations.
3. Radio (VOR/DME): Bearing and distance from a single co-located station.
4. IRS Only: If all external references are lost, the system uses "Inertial Coasting."

As an instructor, I emphasize that the system is designed to "cascade" automatically. If the FMGC loses its GPS lock, it will jump down to the next best source (DME/DME) without pilot intervention. However, this logic assumes the data it receives is honest.

3. Jamming vs. Spoofing—Why Only One Requires a "Firewall"

Not all Radio Frequency Interference (RFI) is created equal. Understanding the distinction between jamming and spoofing is critical for proper cockpit management.

• Jamming (Signal Loss): High-power noise blocks the GPS receivers, triggering a "GPS PRIMARY LOST" message. The aircraft is designed to handle this robustly, reverting to Radio or IRS coasting automatically. No pilot intervention is required to protect the flight path, and you will not experience a map shift.
• Spoofing (Deception Attack): This is far more dangerous. Spoofing provides the aircraft with a false but believable signal. The FMGC may not realize it is being lied to, potentially leading to map shifts or—most critically—uncommanded turns.

If the aircraft begins an uncommanded turn while in NAV mode due to spoofing, you must act immediately: disconnect NAV and pull the HDG/TRK knob to regain positive control of the flight path. To stop the "poisoned" data from further corrupting the FM position, you must create a manual "procedural firewall" by executing a DESELECT GPS command. Use the following MCDU sequence: DATA > POSITION MONITOR > SEL NAVAIDS, then select the DESELECT GPS prompt (LSK 5L).

4. The "Poisoned Bias" Trap

A critical risk during interference is the timing of your intervention. If you deselect the GPS after spoofing has already begun, the FMGC will have already incorporated the false data into its calculations. When GPS is deselected, the FMGC enters "coasting mode" and freezes the last calculated Bias vector.

"I see these insidious risks and common traps in the simulator; if the FMS was spoofed prior to deselection, it memorizes a corrupted bias, and the map shift persists."

To detect this, perform a Navigation Accuracy Check: On the PROG page, enter a known VOR/DME in the BRG/DIST field and compare the FMGC’s computed bearing and distance to the raw data displayed on your Navigation Display (ND). If they disagree, you must "flush" the false bias. This is done by performing a manual position update on the PROG page, but it must be done using a known, verified radio fix overhead to ensure accuracy.

5. Don't Panic During an EGPWS "PULL UP"

Spoofing affects more than just your map; it can compromise the Enhanced Ground Proximity Warning System (EGPWS), which uses GPS for its look-ahead Terrain Awareness Display (TAD). If spoofed data suggests you are low or off-course, it may trigger a terrifying and spurious "PULL UP" warning.

In this scenario, do not disable the entire GPWS system. Instead, locate the TERR pushbutton on the center pedestal and switch it to OFF. This action inhibits the predictive terrain functions being fed by corrupted data. When you do this, expect to see a NAV GPWS TERR DET FAULT caution on the ECAM. This is normal. Crucially, this action keeps basic GPWS modes 1 through 5 (based on the Radio Altimeter) fully active, providing a vital safety net while silencing the false alarm.

6. The Surprising Reason to Kill GPS Before Takeoff

A professional "pro-tip" for operating in known RFI environments is to consider deselecting GPS on the ground. This is because RFI at the airport can cause the initial IRS alignment to be degraded before you even leave the gate.

Normally, when you apply takeoff thrust (TOGA or FLX), the A320 performs a "Takeoff Update," resetting the FM position to the runway threshold coordinates to correct taxi drift. However, this update is inhibited if "GPS PRIMARY" is active. If the GPS is being spoofed on the ground, the aircraft will believe its position is perfect and skip the update. By deselecting the GPS before takeoff, you force the FMGC to execute that vital runway position update, ensuring you start your flight with a clean, accurate navigational state.

7. Conclusion: Navigating the Future

The modern flight deck requires a shift from "automated monitoring" to "active management" of navigation sources. As we navigate an environment where satellite signals can be weaponized, our fundamental training remains our strongest asset.

This challenge brings us back to the core of our profession. Adhering to Airbus Golden Rule #1: Fly, navigate, and communicate is paramount, but so is Golden Rule #2: Use the appropriate level of automation at all times. When the magenta line becomes unreliable, reverting to selected guidance and raw data is not a step backward—it is the mark of a master pilot. As technology becomes more complex, our traditional "raw data" skills are proving to be the most high-tech tools we have.