What is the azimuth guidance principle in a VDGS?
The lateral half of docking guidance. The display shows which way to steer to reach the stand centreline, then tells the pilot when to hold it.
Azimuth guidance answers one question for the pilot: where am I relative to the stand centreline? It is the lateral half of a visual docking guidance system. The other half, the stopping position indicator, answers how far there is still to go. Together they bring the aircraft to the same spot on the same line every time.
1Measure the offset, show the correction
The system finds how far the aircraft's centreline lies to the left or right of the stand centreline and shows the pilot which way to steer. The pilot steers, the offset shrinks, and the display changes to centred. It is a closed loop with the pilot as the controller, so the display must be clear and steady.
2Where the display sits
The azimuth unit sits on or close to the extension of the stand centreline, ahead of the aircraft, so it can be seen from the cockpit throughout the docking manoeuvre. Annex 14 asks for it to be aligned for use at least by the pilot in the left seat. The pilot's eye is not on the aircraft's centreline, so the system is set up so that “centred” means the aircraft is on the line, and how that is done differs between optical and sensor-based designs.
3Green on the line, red off it
The guidance must give unambiguous left and right information that lets the pilot acquire and hold the lead-in line without over-controlling. Where colour is used, green shows the centreline and red shows deviation from it. The system must work in the weather, visibility and background light it is intended for, by day and night, without dazzling the pilot.
4What goes wrong
A display that is not aligned to the surveyed stand centreline guides the pilot faithfully to the wrong line, and the aircraft stops off centre. A display that is too sensitive or late makes the pilot chase small errors. Glare, a blocked sensor or poor continuity with the lead-in line and stand markings leaves the pilot without a clear signal at the moment it matters.
| Item | Principle | Why |
|---|---|---|
| What it shows | Which way to correct, and when on the line | Unambiguous left and right |
| Colour, where used | Green on the centreline, red for deviation | Read at a glance |
| Location | On or near the stand centreline extension, ahead of the aircraft | Visible throughout the docking |
| Viewing | Aligned for at least the left-seat pilot | Covers the left-seat pilot as a minimum |
| Continuity | With stand markings and any manoeuvring guidance lights | One line, no hand-off gap |
| Behaviour | Acquire and hold the line without over-controlling | Stable, not weaving |
Frequently Asked Questions
Q1What is azimuth guidance in a VDGS?
It is the lateral half of a visual docking guidance system. It tells the pilot of an aircraft taxiing in to a nose-in stand whether the aircraft is on the stand centreline and, if not, which way to steer. The other half, the stopping position indicator, tells the pilot how far to go.
Q2Why are green and red used?
ICAO Annex 14 says that where azimuth guidance is shown by a colour change, green indicates the centreline and red indicates deviation from it. The pilot can read the state at a glance without interpreting a symbol while also controlling the aircraft.
Q3Where is the azimuth unit placed?
On or close to the extension of the stand centreline, ahead of the aircraft, so its signals are visible from the cockpit throughout the docking manoeuvre. Annex 14 asks for it to be aligned for use at least by the pilot in the left seat.
Q4How do optical and sensor-based azimuth guidance differ?
Older optical azimuth units, for example AGNIS, depend on what the pilot's eye sees from the cockpit, so the view changes with the pilot's position. (PAPA, a different optical aid, is a stopping-position device and belongs to the next entry.) Modern sensor-based displays detect the aircraft, work out its offset from the centreline, and show the result as an arrow or centre indication. The pilot's task is the same: steer until the display shows centred.
Q5What does 'without over-controlling' mean?
The guidance must let the pilot acquire and maintain the lead-in line without swinging from side to side. A display that is too sensitive, jumpy or late makes the pilot chase small errors, so the aircraft weaves instead of settling on the line.
Q6What happens if the display is not aligned to the stand centreline?
The pilot will follow it faithfully to the wrong line. The aircraft stops off centre, and the boarding bridge or ground equipment may not reach the doors. That is why the unit's alignment is surveyed against the stand centreline at installation and checked afterwards.
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