Mastering A320 navigation means understanding how the aircraft bridges raw sensor data with automated flight paths. Unlike traditional aircraft where pilots manually cross-reference ground stations for every track change, the Airbus A320 relies on an integrated network of computers, inertial sensors, and global positioning systems to calculate position and guide the airframe from gate to gate.
At the center of this architecture is the Flight Management and Guidance System (FMGS), paired with the Air Data/Inertial Reference System (ADIRS). Knowing how these systems talk to each other, how to initialize them correctly on the ground, and how to manage them when inputs change separates a reactive flight crew from an effective one.
The Foundation: ADIRS and Position Initialization
Before the aircraft moves an inch, the navigation computers need to know precisely where they are. This job falls to the ADIRS, which uses laser gyroscopes and accelerometers to track movement relative to an initial set of coordinates.
[ADIRS Ground Alignment]
│
├── Requires static aircraft on ramp (approx. 10 mins)
├── Pulls coordinates from navigation database or manual entry
└── Blends GPS data once airborne for high-precision "GPS PRIMARY" status
When you power up the flight deck, turning the three IRS mode selectors to NAV initiates the alignment process. This takes roughly ten minutes while the system senses the Earth's rotation to establish true north.
- The alignment error trap: If the aircraft is moved during alignment—such as a tug bumping the nose gear—the IRS will drop its alignment and require a restart. Always verify that the alignment coordinates match your departure gate position on the MCDU INIT page.
- GPS blending: Modern A320s continuously update inertial position calculations using GPS inputs. When the system displays "GPS PRIMARY" on the navigation display, position accuracy is at its highest, virtually eliminating drift.
FMGS and MCDU Interaction
Pilots interact with the FMGS primarily through the Multipurpose Control and Display Unit (MCDU). It's easy to confuse the interface with the computer itself, but the MCDU is simply the keypad and screen. Behind the scenes, two Flight Management Guidance Computers (FMGCs) do the heavy lifting, cross-talking to ensure both sides of the flight deck share identical data.
Programming a route requires following a logical sequence through the MCDU function keys:
- INIT Page: Enter your departure and destination airports, flight number, cost index, and cruising flight level.
- F-PLN Page: Build the lateral route by inserting your departure runway, Standard Instrument Departure (SID), en-route airways, arrival, and approach procedures.
- PERF Page: Input takeoff weights, center of gravity, flap settings, and thrust reduction altitudes to allow the system to compute performance predictions.
When revising a flight plan mid-flight, always check the temporary modification lines before pressing insert. The system creates a temporary path in dashed orange lines, allowing you to review lateral changes before the aircraft commits to a new trajectory.
Managed Versus Selected Guidance
The A320 gives you two distinct ways to fly: managed guidance (where the FMGS computes and flies the path, speeds, and altitudes automatically) and selected guidance (where you override the automation by dialing specific headings, speeds, or vertical speeds on the Flight Control Unit or FCU).
- Managed Speed: The aircraft follows speed constraints built into the flight plan, such as slow-down restrictions near terminal areas. Pulling the speed knob overrides this, switching the aircraft to selected speed.
- Managed Vertical Profile: Pressing the altitude knob in during a climb or descent allows the aircraft to respect altitude constraints and optimization targets calculated by the FMGC. Pulling the knob switches to vertical speed or open climb/descent mode.
Monitoring the Flight Mode Annunciator (FMA) at the top of the Primary Flight Display is mandatory. It tells you precisely what the navigation and guidance systems are currently commanding, preventing surprises during busy transitions.
Backup Navigation and Radio Tuning
While the FMGS handles most routing automatically, regulations require crews to know how to navigate using raw data when automation degrades.
The Radio Management Panels (RMPs) allow manual tuning of VOR, ILS, and ADF stations. Normally, the FMGS auto-tunes navigation aids to support position updating, but you can override this on the RAD NAV page. If both FMGCs fail entirely, the RMPs act as your primary reversionary tool to tune ground stations and fly backup raw-data approaches.
For non-precision approaches, the A320 features the Flight Management System Landing System (FLS). FLS essentially projects an ILS-style glidepath onto procedures like RNAV or VOR approaches that are stored in the navigation database, giving you managed vertical guidance down to minimums without needing physical ground-based glideslope transmitters.
Frequently Asked Questions
What causes a flight plan discontinuity on the MCDU?
A discontinuity occurs when the flight management computer detects a gap between two consecutive waypoints that aren't linked by a recognized airway or procedure. You don't always have to clear them; sometimes they are intentional placeholders for vector segments, but during preflight planning, they must be closed or bridged to ensure smooth lateral navigation.
Why does the navigation display show "NAV ACCUR DOWN"?
This message appears when the estimated position uncertainty exceeds the required navigation performance (RNP) for your current flight phase. It usually happens if GPS signal is lost or degraded while inertial drift accumulates over a long oceanic or remote leg, requiring the crew to transition to alternative navigation sensors.
Can I modify a flight plan while the autopilot is engaged?
Yes. Entering modifications into the MCDU or selecting new targets on the FCU does not disengage the autopilot. The FMGS simply recalculates the trajectory or guidance commands based on your new inputs, which the autopilot then smoothly follows.
What is the difference between RNP and RNAV?
RNAV (Area Navigation) allows an aircraft to fly on any desired flight path within the coverage of station-based navaids or self-contained systems. RNP (Required Navigation Performance) takes RNAV a step further by adding an onboard performance monitoring and alerting system, ensuring the aircraft stays within a strict containment boundary.
