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Stand Entry Guidance Systems
Stand Entry Guidance (SEG) Systems
There is a huge variety of stand entry guidance systems ranging from a marshaller with bats or lighted wands to a wide range of fully or semi automated systems. Each method may provide either or both of azimuth (centreline) guidance and stop position information. Systems may differ between different terminals at the same airport or even within terminals. Although flight crew guides normally contain the relevant information for any particular gate, late allocation of gates or late changes to initial allocations may make it impracticable for flight crew to pre-brief themselves on unfamiliar systems without stopping the aircraft prior to proceeding onto stand for that specific purpose.
Airport procedures should specify which aircraft types are permitted to use which gates. Flight Crew on board documentation should provide equivalent information. Allocation of an appropriate gate will depend upon the Flight Plan having been filed with the correct aircraft type and variant. Errors in this area are particularly common when a flight operates on a stored RPL and an aircraft substitution, even just a variant, is made.
Responsibility for aircraft safety
Procedures for ground staff for the activation of automatic or semi automatic SEG for arriving aircraft should require that the full marked gate area for aircraft use is clear of all obstructions and surface contamination or FOD. However, neither this nor the correct application of SEG activation procedures can be assumed. The aircraft commander (or other authorized person with clearance to conduct out of service aircraft ground re-positioning of an aircraft under its own power) is left with sole responsibility for the safety of the aircraft as it arrives at the gate. The difficulty of establishing the fitness of the gate area for use from the flight deck is clear, especially when it is dark. The majority of guidance systems except a marshaller are ‘calibrated’ to be viewed from the LH pilot seat so they can only be interpreted accurately by the pilot occupying that seat. Pilots occupying the RH seat can not therefore use most currently available aids of this sort with confidence.
This may be achieved simply by following the painted/lighted taxiway line with or without the guidance of a marshaller. Alternatively, It may be provided by various forms of` left / right indications on an illuminated head-of-gate display. These are generically described as AGNIS (Azimuth Guidance for Nose-In Stands). One system in common use has two vertical, parallel light bars. Both appear green when the aircraft is on the stand centreline but, if it is displaced to the left or right, the light on that side turns red, inviting the pilot to turn towards the green light to regain the centreline.
The painted lead-in centerline is likely to have painted cross markers for NLG position according to aircraft type groups. A marshaller can be expected to signal when the required position has been reached. One common form of eye-height display which shows the correct stop position is called PAPA - Parallax Aircraft Parking Aid. This has a horizontally slotted black marker board mounted to one side of the head of stand behind which a vertically oriented white fluorescent light shines through the slot and appears to traverse the slot as the aircraft moves forward. Marks on the board identify the correct alignment of the light according to aircraft type. The operating principle of PAPA requires that the board is offset from the gate centreline and so the pilot must alternate his view between it and whatever system is provided for azimuth guidance.
Alternatively, the stop position may be indicated (independently of azimuth guidance) by means of a stand alone ‘traffic light’ system, which is often operated by remote control from the ramp, or by use of the Mirror Guidance System which relays to the flight crew the stop mark on the taxiway centerline and, as it reaches the vicinity, the NLG position.
Azimuth and Stop Guidance combined
Systems which have a composite display above the head of the gate which shows stop position and the reducing distance to go are usually described as APIS - Aircraft Positioning and Information Systems. Various displays are provided.
Emergency Stop Systems
All light-display stop position indicator systems are provided with emergency stop buttons at both ramp level and on the air bridge platform. However, care must be taken to distinguish these from potentially similar stop buttons which apply to the moveable airbridge and cut its electrical power supply, since, if pressed in error, electrical power to the SEG system control and display may also be lost, including the emergency stop element.
Hazards and their avoidance
Fixed rather than moveable air bridges are less common than they used to be. When they are encountered, they are likely to result in closer proximity to moving aircraft than for the more common movable bridges. Movable bridges should be parked away from the ‘attached-to-aircraft-position’ when not in use and no aircraft is on the gate. The closeness of the fixed bridge reduces the margin for error by the person taxying an aircraft onto the gate. Whenever an aircraft is taxyed onto a gate, it should be done at a very slow speed and the aircraft braking system status should be checked upon arrival at the turn-in point to ensure that this system is functioning normally. Flight crew operating more than one aircraft type variant or who may have recently undergone type conversion need to be absolutely sure they cross check the type/variant they are operating before proceeding so as to follow any SEG system correctly. These systems will be marked / displayed with the aircraft type set on the SEG System and flight crew need to check that the marking/display does correspond to their aircraft type.
- The UK CAA ‘Visual Aids Handbook’ (CAP 637) Chapter 4 - Visual Docking Guidance Systems provides illustrations of the most common systems.