35TH ANNUAL CONFERENCE, Tunis, Tunisia, 15-19 April 1996
WP No. 129
Responsibilities & Functions of Aerodrome Controllers with Regard to Surface Movement
The subject of the responsibility and functions of Aerodrome Controllers, with regard to surface movements, was placed on the work programme of SC4 for 1995/1996 at the 34th Annual Conference in Jerusalem.
Consideration is given to the responsibilities and functions of Aerodrome Control, with respect to the visual monitoring and control of aircraft and vehicle traffic on Airport areas; and also to the issue of appropriate ATC clearances and instructions in order that controllers can establish and maintain a safe and orderly flow of air traffic.
The subject matter addresses the concerns of several M.A.’s. to the introduction of types of Apron Management Service, staffed with unqualified and unlicensed personnel. In addition, consideration is given to the effect of Low Visibility Operations and the existing requirements of Surface Movement Guidance and Control Systems (SMGCS) and proposals to introduce Advanced-SMGCS.
This working paper also seeks to justify that whenever a control function is provided to aircraft operating on any ground area of an airport, that function must be provided by a qualified air traffic controller.
Within the content of this working paper, when reference is made to ‘aerodrome controllers‘, it takes into consideration the job task and responsibilities of Air and Ground Movement controllers.
Unless stated, it can be assumed that when reference is made to ‘aircraft and vehicles‘ in this working paper, it refers to the interaction between aircraft and vehicles, and also to the interaction between aircraft and aircraft.
The term ‘manoeuvring area‘ is used to describe the surface areas of an airport used for the movement of aircraft and vehicles, normally the runway and taxiways, but not including the apron or aircraft parking areas.
The term ‘movement area‘ is used to describe the manoeuvring area and the apron areas on an airport.
The term “direct visual observation” refers to the actual visual perspective from a control tower and not one obtained from the use of television or surface monitoring equipment.
The role, responsibilities and functions of Aerodrome Controllers:
It is generally accepted that air traffic controllers are responsible for the issue of clearances and instructions, in order to provide safe separation between aircraft and aircraft, and aircraft and vehicles, operating on the surface of an airport. By the very nature of their function, aerodrome controllers are expected to maintain direct visual observation of traffic movements and to assist aircraft under their control. They are also expected to provide information and advice to aircraft and to liaise with ground agencies in the event of any emergency or incident occurring.
|“Aerodrome control towers shall issue information and clearances to aircraft under their control to achieve a safe, orderly and expeditious flow of air traffic on and in the vicinity of an aerodrome with the object of preventing collision(s) between:
1. aircraft flying in the aerodrome traffic circuits around an aerodrome;
2. aircraft operating on the manoeuvring area;
3. aircraft landing and taking off;
4. aircraft and vehicles operating on the manoeuvring area;
5. aircraft on the manoeuvring area and obstructions on that area.”
(ICAO Doc 4444 – Part V – Aerodrome Control Service)
|“Aerodrome Control Towers are also responsible for alerting the safety services […]”|
(ICAO Doc 4444 – Part V – Aerodrome Control Service)
|“The movement of pedestrians or vehicles on the manoeuvring area shall be subject to authorisation by the aerodrome control tower […] Notwithstanding such an authorisation, entry to a runway or runway strip or change in the operation authorised shall be subject to a further specific authorisation by the aerodrome control tower.”|
(ICAO Doc 4444 – Part V – Aerodrome Control Service)
The issue of ATC clearances and instructions are based on the knowledge and awareness of the exact position and movement of aircraft and vehicles along taxiways and runways. Aerodrome controllers are expected to ensure that any movement on the manoeuvring area can be accommodated and is safe.
Where the functions and responsibilities for aerodrome control are split between runway controllers and ground movement controllers, the effective use of co-ordination and communication between these controllers is essential for the safe integration of aircraft and vehicles.
Because of the siting of apron and aircraft parking areas, and because of the unrestricted movement of ground serving vehicles, it is not generally assumed that controllers can effectively be responsible for the safe separation of aircraft and vehicles on these areas of an airport. However, with increased needs to provide a safe separation service for aircraft moving on these areas, the role of controllers to provide a ‘control‘ service will need to be extended and supported by suitable equipment, and procedures, in order for them to undertake their duties effectively.
(The role of personnel engaged in an “Apron Management Service” is discussed later in this working paper)
When appropriate to do so, aerodrome controllers are expected to issue advice and information on matters related to the safety of movement on the airport surface, with particular reference to the exact nature of any irregularity that might affect each aircraft or vehicle. Such information may include actual surface conditions, details of any work in progress or notified obstructions, failure of any lighting systems, the closure of any taxiways, etc.
Factors affecting the responsibilities and functions of Aerodrome Controllers
The correct position and siting of control towers is vitally important, as the aerodrome controller must be able to view, determine and monitor all aircraft and vehicle movements on the surface of an airport. A control tower must therefore afford an aerodrome controller with direct visual observation of the manoeuvring area. It must enable the controller to determine that runways and taxiways are clear of obstructions, in order that safe clearances and instructions can be issued. The development of airport buildings and aircraft parking areas needs to be carefully considered and should take into account their effect on the overall visual perspective from the control tower.
The overall task of the aerodrome controller is based upon an awareness of the movement of aircraft and vehicles by direct visual observation. The task of the aerodrome controller to control aircraft movements is a defined safety function and one that requires unobstructed visual observation from the control tower. The use of televisions and other aids to overcome deficiencies or obstructions is a short term measure to alleviate difficulties in directly visually observing aircraft movements. But, in themselves, can provide an additional distraction to the controller from actually visually observing other movements of aircraft and vehicles on other parts of the manoeuvring area. The use of televisions limits the overall field of view and television displays are often subject to their own particular limitations. For example, a monochrome display cannot afford a controller with an adequate interpretation whether smoke from the undercarriage of an aircraft is also associated with a fire. Televisions are not very useful at night and rely on sensitive cameras to be able to record events in low light conditions. The emphasis should always be on providing the aerodrome controller with a total, unrestricted view of the manoeuvring area from the control tower itself.
Even in good weather conditions, the complex layout of airports requires greater vigilance in monitoring aircraft movements at night. With many aircraft taxiing, the position of individual aircraft can easily be confused and mistaken. With little more than visual reference to go by, the controller is often faced with assuming responsibility for the safe control of aircraft and vehicles in conditions that render the accurate position of traffic difficult, if not impossible, to determine. This is especially difficult at airports where the control tower may be situated some distance from the operational runway threshold or take-off point.
The view over the apron and aircraft parking areas are often obstructed by building developments and controllers are not always able to determine the exact location of aircraft, or their immediate effect to other aircraft movements. In restricted parking areas, such as cul-desacs, the issue of push-back and taxi clearances require greater controller awareness and monitoring. The issue of “push-back” clearances to aircraft on apron areas is a common function at most airports. However, the legal liability for the issue of such ATC clearances must be clearly defined to uphold the ICAO position, that controllers only have responsibility for the safe separation of aircraft and vehicles on the manoeuvring areas of an airport. Where such liability has not been suitably determined, controllers should expect their local or national ATS authorities to provide them with all necessary exemptions from liability to avoid any undesirable legal consequences.
The layout of runways and taxiways is not always conducive to simple and easily understood instructions for movement on an airport surface. The layout of an airport‘s taxiways can be extremely complex, especially to visiting pilots who are unfamiliar with the airport. Therefore, it is important to be able to provide aircraft with well defined routes along taxiways in order to overcome confusion and inhibit difficulties associated with complicated taxy instructions.
The accurate designation and identification of taxiways, holding points, and routes to the operational runways, is of major assistance to aircraft following the instructions given by an aerodrome controller. The use of non-confusing taxiway lighting is of great assistance to the movement of aircraft on an airport at night. The use of clearly identifiable holding points can assist the controller in determining the position of aircraft and ensuring compliance with clearance limits. Where taxiways cross or enter runways, the use of switchable stop bar lights across the taxiway can ensure that unwarranted runway incursions are prevented.
The use of communication in the issue of instructions and clearances by R/T is of prime importance and becomes increasingly acute at busy airports. Incorrectly read back clearances and missed instructions can lead to greater workload and the possible loss of separation between aircraft and vehicles. At airports where taxiways are similar, or are often used as ‘emergency‘ runways, the need for correct observance of taxi instructions is paramount. The use at some airfields of the phrase “clear to land” when the runway is still obstructed, is often seen as a means of providing a necessary clearance at an opportune moment, but still requires the controller to ensure that the area is safe for the landing to actually take place. The frequent use of this type of procedure could reduce alertness and, when combined with distractions, can impinge on a controller‘s awareness of actual traffic situations and the safety over the movement of aircraft.
The need to control or eliminate unnecessary distractions is a key human factor element in maintaining a controller‘s alertness to traffic situations on the manoeuvring area. A disciplined use of clear data displays, be it flight strips or radar displays, are essential elements to enable a controller to maintain an effective, safe and efficient level of monitoring and control over the surface movements of aircraft and vehicles.
A study undertaken in the USA, to determine causal factors for unwarranted runway incursions, found that aircrew are easily ‘confused‘ at major airports when taxiing their aircraft. This unfamiliarity with taxiway routes occurs even at airports used frequently and is compounded in difficult weather conditions and at night. It would also appear that taxi instructions issued by ATC are a key element in understanding correct taxi routes. When ATC clearances are issued, it is a normal requirement that aircrew will not only understand the clearance, but will also comply with it. The ability for aircrew to understand an ATC clearance, therefore, becomes an essential factor in ensuring aircraft follow the correct taxi routes. Language difficulties, or confusing routes, will add to any misunderstandings and require controllers to be even more vigilant in monitoring aircraft moving on the ground.
ATC clearances and instructions can be misinterpreted and aircrew are often reluctant to question ground clearances, especially when the controller sounds busy. The number of runway incursions occurring may also indicate a lack of close monitoring of aircraft movements by ATC. Having issued an appropriate, and sometimes complex taxy instruction, the controller may assume that the aircraft will act in accordance with its clearance. The controller then continues with other tasks and does not adequately supervise the subsequent movement of this aircraft.
This is particularly apparent at the busier airports where 20 or 30 aircraft, or more, may be taxying at any one given time. And this on a complex airport layout that may involve runway crossing and parallel taxiway routes.
An alternative solution might be the use of ‘datalink‘, but there is no guarantee that clearances issued this way will be understood any better. The objectivity of any ATC clearance is that not only will it be understood, but that it is also adhered to. The removal of air-ground radio telecommunications between pilots and controllers may well remove the opportunity to question clearances that are received or to ensure that a correct read-back is acknowledged. Datalink only assists the speed of transmission of clearances and data and does not necessarily improve the understanding of clearances affecting safety and separation. Correct and safe procedures therefore need to be developed before automation is used to bypass any existing safeguards that protect the integrity of any ATC clearances issued.
In the period 1989 to 1993, there were 1106 recorded runway incursions in the USA; an average of 221 per year. Although the highest number in one year was 281 in 1990, this total had reduced to 141 in 1993 and steps taken by the FAA to address this matter will no doubt reduce this total even further, as new procedures and equipment are introduced.
The aircraft ground collision between a landing B737 and a Metro lined up for takeoff at Los Angeles in 1991 involved factors identified in paragraphs 2.2.3, 2.2.6 and 2.2.7 above.
(See the report on the runway collision at Los Angeles: “A case history of human error in ATC and flight operations” by James W Danaher, Chief, Operational Factors Division, Office of Aviation Safety, NTSB, as presented to the Professional Panel at IFATCA 94 in Ottawa.)
When an Apron Management Service has been introduced, the division of responsibilities for the safe control and movement of aircraft to and from the apron areas, as well on the apron area itself, may be delegated to a separate department of the airport authority, and not necessarily part of any ATC organisation. To enable the apron management service to be provided, additional visual cabs may be needed in order to afford the relevant personnel an unrestricted view over the apron and parking areas.
Although commanders of aircraft are deemed to retain overall responsibility for the safe operation of their aircraft, the extension of a controller‘s responsibility must be considered and the legal liabilities determined. This is especially important where an interaction of controlled aircraft and uncontrolled vehicles and personnel exists. An alternative solution, would be for the ground based aircraft handling companies to assume total responsibility for the safe movement of aircraft on aprons, during push-back, coincident with the authority from ATC to the commander of the aircraft to push-back from the stand.
Low Visibility Procedures
In periods of poor weather and low visibility, the visual outlook from a control tower can be impaired to such a degree that aerodrome controllers can no longer determine the position of aircraft and vehicles on the manoeuvring area, by direct visual observation. Low visibility procedures are introduced to afford additional protection and safe-guarding to runway areas, in order to control surface movements of aircraft and vehicles when aircraft are making ILS Category II and III approaches, or using the ILS Localiser course for guidance on take-off. The implementation of low visibility procedures is usually governed by reported surface horizontal visibility and lowest cloud base. However, once introduced, low visibility procedures are applicable to all aircraft movements and also to all vehicles on the manoeuvring area, regardless of the type of approach being flown.
In circumstances of low visibility, a controllers‘ awareness of the movement of aircraft and vehicles becomes totally dependent on position reports and the flow of traffic becomes effectively restricted as the controller concentrates on attention to safety. Despite the actual ‘slowing‘ down of the traffic flow, the workload of controllers increases as attention is focused on ensuring aircraft follow the correct instructions and clearances.
With regard to workload, IFATCA has the following policy:
|“Research should be carried out in each country to determine the capacity of the ATC system and the workload to be carried by each air traffic controller”|
(Page 4122 of IFATCA Manual, para. 2.4.1)
Even with restricted clearances, controllers are not always able to accurately determine the position of aircraft and vehicles. In low visibility it is quite possible for aircraft and vehicles to follow an incorrect route along a taxiway or runway. The ability of an aerodrome controller to accurately assess the actual traffic situation and respond quickly to it, is limited in poor visibility. The responsibility to alert the emergency services is also limited, unless some other form of monitoring is used. Although involving misunderstood radio communications, the tragic accident at Tenerife in 1977 was an incident that occurred in poor visibility and in circumstances where the aerodrome controller was not fully alert to the sequence of events that contributed to the accident taking place. The need, therefore, for a monitoring device to detect the movement of aircraft and vehicles on the manoeuvring area is paramount.
With regard to the availability and use of monitoring devices to assist the controller, IFATCA has the following policy:
|“Air traffic controllers should be provided with ATC equipment commensurate with their operational requirements so as to promote an optimum level of safety.”|
(Page 4122 of IFATCA Manual, para. 2.4.2)
With the increasing upgrading of airborne and ground-based navigation systems, the presence of low visibility conditions no longer impedes the safe movement of arriving or departing aircraft. Even at the smaller airports, aircraft are now able to land in a runway visibility of 100 metres, or less. The increase in aircraft so equipped to make landings in poor weather conditions places a requirement on airports to accommodate near ‘normal‘ traffic flows. Although procedures are employed to safeguard aircraft landing in low visibility, which has the effect of slowing traffic movement on an airport, considerable workload results in the control of aircraft movements during these low visibility conditions. The use of surface movement guidance systems can assist the controller, but does not necessarily reduce the additional workload and stress which results from an overall lack of visual reference to the control of aircraft moving on the ground.
Consideration needs to be given to the mandatory use of Surface Movement Radar when aircraft movements are occurring in low visibility conditions. Although the aircraft may be fully equipped, and the aircrew trained, to operate to Category 2 and 3 minima; the controller, who is ultimately responsible for controlling the movement of aircraft on the manoeuvring area and for alerting the emergency services in the event of an incident, may not be able to see the aircraft by direct visual observation, at all. All necessary ground based equipment and safeguards need to be in place before aircraft can carry out ILS Category 2 and 3 approaches. However, there is no such mandatory requirement for aerodrome controllers to have at their disposal surface monitoring equipment that will enable them to safely monitor the movement of aircraft, in these conditions. The provision of a serviceable SMR should be seen as a mandatory operational and safety requirement to permit Category 2 and 3 movements.
Surface Movement Guidance Control Systems
In Annex 14, ICAO recommends:
|“A surface movement guidance and control system shall be provided at an aerodrome” (para 8.8.1)
“A surface movement guidance and control system should be designed to assist in the prevention of inadvertent incursions of aircraft and vehicles onto an active runway” (para 8.8.4)
” The (surface movement guidance and control) system should be designed to assist in the prevention of collisions between aircraft, and between aircraft and vehicles or objects, on any part of the manoeuvring area” (para 8.8.5)
“Surface movement radar for the manoeuvring area should be provided at an aerodrome intended for use in runway visual range conditions less than a value of the order of 400m” (para 8.8.6)
IFATCA has the following policy on Surface Movement Guidance and Control Systems (SMGCS):
|“At their simplest these may consist of ground markings and stop & go lights. More sophisticated systems will incorporate taxiway centre line lighting and stopbars which can delineate the cleared route of aircraft. Where a need exists surface movement radar or some other form of aircraft position sensing may be installed. Whatever the sophistication of the system the essential requirement is that some means exist of ensuring the safety of aircraft while moving on the manoeuvring area”
“The appropriate ATC authority should institute an SMGCS which includes procedures for avoidance of collision between aircraft, and between aircraft and vehicles on the ground”.
“ICAO should not specify the separation standards necessary to achieve the above objective”.
“Surface movement radar should be used as a monitoring device and should not be used for the provision of a control service unless procedures are available”.
“The boundary between apron and manoeuvring area should be clearly defined”.
(Page 3223 of IFATCA Manual, para. 2.3)
IFATCA has the following policy on Surface Movement Radar (SMR):
|“Before SMR can be used in other than a monitoring or information role procedures for identifying, and subsequently maintaining the identification must be established”.
“Identification procedures for use of SMR, with or without labelled displays, should be established. Until such procedures are available SMR should not be used to :
6. issue instructions to hold clear of intersections to avoid traffic conflicts;
7. provide assistance in timing runway utilisation while avoiding conflicts with departing and arriving aircraft;
8. provide guidance information to an aircraft uncertain of its position.
In order to maintain identification of traffic displayed on an SMR a system to display identification labels should be developed. The SMR displays must be capable of being viewed without a hood in full daylight from the normal working position of the controller.”
(Page 3222 of IFATCA Manual, para. 2.2)
SMR is available at many airports, although the world-wide introduction – even in countries like the USA – has not been seen as a priority. The number of runway incursions and ground incidents underlines the need for a surface monitoring device. SMR is therefore a valuable and essential piece of equipment in the control of aircraft movements on the ground areas. The procedures for the operational use of SMR are very much dictated by the relevant ATS regulatory authorities and some ATC Units are able to make use of such radar to a greater extent than others. The use of labelled displays, using SSR or GPS derived information, is not so common but affords the controller with identified radar data that could be used for control and separation functions.
The use of available SMGCS equipment enhances the ability of controllers to monitor the ground movement of aircraft, particularly at night and in low visibility conditions. It also provides an additional safety device for ensuring runways and taxiways are clear of traffic, in all weather conditions.
An alternative to ground based surface movement guidance systems would be to engage the services of marshaller/follow me vehicles to lead aircraft around the manoeuvring area. However, it has to be acknowledged that at the busier airports the provision of such vehicle would be considerable and would themselves add to the numbers of vehicles operating on the manoeuvring areas and an increase in ATC/vehicle RT workload in the provision of safe clearances and information.
Advanced Surface Movement Guidance Control Systems
ICAO has already established a working group to consider the parameters of separation minima for aircraft taxying in trail using a control function within the framework of SMGCS. Known as Advanced SMGCS (A-SMGCS), a concept is being developed to determine the longitudinal spacing in metres as part of proposed operational requirements. A-SMGCS is designed to provide capacity and safety when justified by traffic density, aerodrome layout and weather conditions. However, the benefits of A-SMGCS are not entirely limited to low visibility conditions.
The significant difference between the existing recommendations for SMGCS and the proposed A-SMGCS, is that the latter will provide more exacting guidance and control for all aircraft and vehicles on the manoeuvring area, as well as ensuring separation between them, especially when visual separation is not possible, or is limited. It is also anticipated that, at the busier airports, A-SMGCS can function as a surface management system interfacing with the ATM system as a whole. This would then support a gate-to-gate operation on a global scale, rather than just acting as an enhancement to guide aircraft safely and quickly from runways and parking areas in poor weather conditions. It is obvious that although future concept of A-SMGCS interacting with a global CNS/ATM structure is being considered, any application of A-SMGCS will need to adapt to specific airports and be compatible with any other ATS systems in use.
The ICAO working group have already acknowledged that human operators will be an integral part of A-SMGCS operations. The flexibility of humans is acknowledged, especially when dealing with unexpected changes in traffic flow and changing weather conditions that directly affect the movement of aircraft. Although A-SMGCS will require suitable technology to reduce the workload on ATCO’s, it is acknowledged that the human must be involved with the executive functions of this system. A manual control capability will also need to be established to overcome system failures and to provide for greater flexibility in dealing with the day to day use of this system. Aviation operations require systems that are not rigid and are able to react rapidly to unplanned events, and to overcome them.
Although A-SMGCS could be considered desirable and beneficial, providing additional safe separation between aircraft and vehicles on the manoeuvring area, the emphasis on enhancing the flow of traffic, in low visibility conditions, will impact on controller workload should any automated A-SMGCS system fail, or become unusable.
Consideration is also needed for the impact of A-SMGCS on the control of the movement of aircraft on the apron areas of an airport, and the divisions of responsibility for any control function over these movements. The present ICAO definition of a manoeuvring area excludes the apron areas. However, the concept of A-SMGCS to provide a control and separation service between the runway and parking stands will bring the apron areas into a wider framework of ATC operations and responsibilities. ICAO acknowledges that responsibility for the apron areas may be delegated to different control authorities which are not exclusively part of any ATC operation. The responsibility for overall control may therefore vary as a result of the actual weather conditions. But, with A-SMGCS it is proposed that there is a need to maintain the level of the ‘control‘ service, regardless of who is actually responsible for providing it.
From the controller point of view, if A-SMGCS is implemented, the concept and responsibility for the control of aircraft and vehicles on the movement area (manoeuvring area and aprons), must be seen as an ATC function. The provision of separation, the monitoring of separation using any automated aids, the issue of clearances and instructions, and the responsibility for alerting the emergency services must also be considered ATC functions. This is endorsed by the working group who consider that the operation of A-SMGCS meets the objectives of an ATC service. They also consider that suitable training must be provided to personnel engaged in SMGCS and to continue that training to maintain standards of competency.
It also has to be noted that the options for deviation from any separation provided by A-SMGCS is a different risk from that experienced in airspace terms. Whereas, there is the availability for deviation in 3 dimensional terms to avoid collision in the air, this is not so when operating on the ground. The taxiway routes may well be correctly designated, marked and lit, but do not offer anything like the same options for deviation to avoid collision. The taxiway and runway areas are significantly limited by size and dimension and any deviation from the paved surface may lead to an excursion onto unprepared ground. This in itself is likely to result in a further incident or accident impacting on the continuing flow of traffic and increasing ATC workload significantly. Whatever safe separation is proposed, factors that will have to be taken into account are the speed of the moving aircraft, the braking/safe stopping distance required and the safe minimum distance that can be accurately determined between them. These specified minima must also have a consideration for any manual intervention if an automated system, sufficiently capable of maintaining minimum separation between aircraft, fails or become unusable.
When considering airspace, ATC is nominally responsible for the provision of safe separation of IFR/SVFR flights. However, with A-SMGCS, the responsibility for safe separation is now extended to all aircraft, regardless of weather conditions and their individual flight rules, and also to vehicles. Given that ATC already has an existing and proven responsibility for aircraft and vehicles on the manoeuvring area, it must be accepted that ATC should also remain as the sole authority for the implementation and monitoring of separation of aircraft and vehicles operating within any A-SMGCS area. As the provision of separation is a clearly defined ATC function, it also follows that any personnel providing a control function, within A-SMGCS, should be suitably qualified air traffic controllers.
The use of radar-type equipment, in control towers, is common place, although most applications are for the monitoring of aircraft on the ground or in the circuit. The use of surface monitoring equipment, such as SMR, to provide effective safe separation between IFR aircraft moving on the movement area, requires a consideration for the introduction of additional qualifications to operate this equipment. The identification, control and separation of aircraft on the ground, using radar-type equipment, is an additional control function for which there is no clearly defined qualifications or licensing procedures. IFATCA has already acknowledged the increased use of such equipment in control towers and has approached ICAO with regard to the establishment and implementation of an Aerodrome Radar Control Rating.
Apron Management Service
In Doc. 9476, The Manual of SMGCS, ICAO acknowledges that there are no specific ATC instructions relating to a service covering the apron areas, but states that apron management is required to regulate the activities and movement of aircraft, vehicles and personnel on apron areas.
At present, IFATCA has no clearly defined policy that extends to the provision of an Apron Management System or requires the use of qualified air traffic controllers to provide that service.
ICAO states that apron management is an essential task at any aerodrome but, suggests that there a variety of ways that a dedicated apron management service can be developed and implemented, depending on the individual needs of an aerodrome. ICAO states three operational factors that must be considered, namely:
- the traffic density;
- the complexity of the apron layout; and
- the visibility conditions under which the aerodrome authority plans to maintain operations.
ICAO also states that any decision to implement an apron management service must rest with the aerodrome authority. The reason given is to ensure that flexibility remains with the aerodrome to provide the service they deem necessary and meets their needs. This seems to conflict with ICAO‘s statement that an apron management service is essential to regulate the movement of aircraft and vehicles on apron areas. It also removes any implementation of any standards and recommended practices from this ‘essential‘ service. Which ultimately means that the level and effectiveness of apron management services can vary from airport to airport and would be entirely dependent on individual airport authorities to decide what is most appropriate for them.
Where an apron management service is merely limited to the publishing and observance of rules and regulations for the movement of aircraft and vehicles, and their individual priorities, rights of way and responsibilities for avoiding collision, then there would seem to be no particular conflict with who should exercise authority over that particular apron area. But, ICAO goes on to say that complex apron layouts will need a comprehensive apron management service, including radio communication facilities. ICAO also endorses the need to ensure the safe and efficient operation of aircraft and vehicles in close proximity, especially in low visibility conditions.
ICAO considers the scope of management needed, as well as tackling the issue of who issues clearances and instructions. ICAO alerts aerodrome authorities to the interaction of ATS instructions and clearances, but also allows non-ATS personnel to duplicate a nominated ATC task.
|“If apron management staff are required to exercise control over aircraft and vehicles on the apron area to ensure safe separation, then such staff should be properly trained and licensed and their legal authority clearly established”.|
(ICAO Doc.9476. Manual of SMGCS para 8.2.6 c.)
It therefore follows, that when any function of ground control is delegated to non-ATS or unlicensed personnel, be it clearance delivery or the issue of taxy and parking instructions, clear guidelines must be established by the relevant ATS regulatory authority to ensure that aircrew are fully aware that they are no longer in receipt of a fully qualified ATC service.
The same regulatory authority should also determine the areas of responsibility provided by each organisation effecting responsibilities over the movement of aircraft and vehicles on the ground and to ensure that all relevant procedures and safeguards for the maintenance of the highest standards of safety are implemented. The liability of any organisation using unqualified, or unlicensed, personnel should be clearly identified and notified to airport users. This is particularly important when the responsibility for the provision of an alerting service is accepted by an apron management service. There should be clear guidelines as to the role and responsibility for notifying the emergency services in the event of any aircraft incident on the apron areas. Aircrew on the apron area, whether parked or in transit, should be in no doubt who to contact in the first instance, when an emergency situation occurs.
Where non-ATS or unlicensed personnel are employed to provide a apron management service, methods of training, supervision and competency assessment should equal that of qualified personnel, when the issue of instructions and clearances are included in the provision of that service. This is supported by ICAO‘s considerations in Doc. 9476, para 8.2.6. c. refers, which states that staff should be properly trained and licensed, but is not made a mandatory requirement for the provision of any apron management service. This is particularly important when determining the qualifications and training needed to determine such tasks as the prioritising the inbound and outbound flow of aircraft, and to establish the best possible take-off sequence.
The supervisory functions of personnel engaged in apron management tasks should not be overlooked and their interaction when monitoring aircraft movements, in a ‘controlled‘ environment, calls into question the use of unqualified, or unlicensed, staff in this important service.
In Canada, “Apron Traffic Control Units” have been set up at several major airports to manage “Controlled Aprons”. The term Apron Traffic Control Unit and Controlled Apron is not defined by ICAO and such is the wording, that aircrew have been under the impression that this an ATC function. Clearances issued by the apron traffic control units are defined by the appropriate regulatory authority as “authorisation…..for an aircraft to proceed on a controlled apron”. Again, the lack of a standardised terminology for what is essentially an apron management service has mislead aircrew as to the nature and structure of the service being provided to them and to the division of responsibilities for their movement on the airport. The control function being applied is not subject to any licensing or common standards of training, can be provided by non-ATS or unlicensed personnel and is effectively limited to that particular airport, only.
Apron management service is an ‘essential service‘ that can be provided by any aerodrome authority using whatever personnel they deem necessary. There is no requirement for this personnel to be trained, licensed or assessed competent to carry out their duties, other than an appropriate R/T licence if radio communication is to be used. It is not always clear to aircrew who is providing an ATC service and where the ATC service stops and an unlicensed service commences.
The issue of clearances on the manoeuvring area of an airport is a notified function of air traffic controllers. The inclusion of the apron areas in SMGCS, A-SMGCS and the provision of apron management service that issues clearances and instructions, requires appropriate recommendations to be implemented globally to ensure compatibility and standardisation of procedures and operational practices. The provision of separation and the management of safety is already vested in the ATC function and should now be extended to cover the apron areas, as well. It cannot be assumed that different levels of service exist at an airport, or in different weather conditions. A-SMGCS already proposes the standardisation of the level of service to maintain the safe flow of traffic, irrespective of weather conditions.
ICAO already has clear guidelines for the imposition of SMGCS especially in low visibility and is now considering A-SMGCS in all weather conditions. With A-SMGCS, ATS and Airport authorities will need to consider the implementation of procedures not only for the safe movement of aircraft on the ground but also the introduction of separation standards to maintain a safe, efficient and orderly flow of aircraft between the runways and parking stands. The linking of control over the movement of aircraft from runway to parking stands, effectively extends the area of responsibility within which qualified air traffic controllers presently provide a full ATC function. These ground based clearances are uniquely an ATC function and the use of unqualified staff to provide these clearances should be seen as undermining the authority and purpose of controllers.
If the area of ground based clearances and separation extends to parking areas and aprons, then clearly this becomes an extension of the ATC control function – not an alternative to it. Such personnel engaged in issuing specific ground clearances and instructions, necessary for the maintenance of safety and any separation standards, must be part of a qualified ATC operation and should be trained and licensed, in accordance with appropriate skills and knowledge requirements, to exercise these functions. There is no advantage in providing different levels of service when effectively implementing similar types of clearances and separation standards. The basis for the integration of ground based clearances and separation standards should be to extend the overall effectiveness and control of the ATC service, and to enhance the safe, orderly and efficient movement of aircraft at the airport.
At all airports, the direct visual observation from the control tower is an essential factor that governs the ability of aerodrome controllers to undertake their responsibility, and functions, in the monitoring and control of aircraft and vehicles on the manoeuvring area. The positioning of control towers should be such as to give aerodrome controllers unrestricted views over all the airport surface, with particular emphasis on operational runways and taxiways. The siting of the control tower should also take into account any future or planned developments of runways and taxiways to ensure that direct visual observation can be maintained over all the manoeuvring area.
As the purpose of the control tower is to provide air traffic controllers with an unrestricted view over the manoeuvring area of the airport, it follows that the involvement of ATCO’s in the development and planning of control towers would be both useful and essential in ensuring that the visual control room can meet all the requirements of the Aerodrome Control Service.
Once a control tower has been established, safeguards should be imposed on all other planning processes to prohibit the development of any structure that would impede the direct visual observation from it.
The layout, designation and identification of runways, taxiways and holding points, together with any associated ground lighting, are key elements to enable controllers to provide clear instructions for the surface movement of aircraft and vehicles; and for these clearances and instructions to be fully understood and complied with by aircraft and vehicles.
Because of the nature of the siting and congested nature of aprons and aircraft parking areas, it cannot be accepted that aerodrome controllers can provide effective control over the movement of ground servicing vehicles. It therefore follows that it is not reasonable to expect aerodrome controllers to assume responsibility for the safe movement of aircraft on aprons and aircraft parking areas, unless all appropriate procedures and equipment are introduced in order for them to provide a ‘control‘ service over the apron areas.
Local ATC Management should clearly identify all the restrictions that affect the visual outlook from a control tower and should publish the correct procedures to be adopted by aerodrome controllers for the issue of all necessary ATC clearances and instructions to aircraft and vehicles moving on the manoeuvring area.
In poor weather conditions, and whenever visibility from a control tower is limited, the position of an aircraft and vehicles moving on the runways and taxiways need to be accurately determined, in order that safe clearances and instructions can be issued by aerodrome controllers. Equipment that enables the surface movement of aircraft and vehicles to be identified, such as surface movement radar, should be provided in control towers at airports, in order that aircraft movements on the airport can be determined and monitored at all times. This should be considered mandatory when aircraft movements are approved to Category 2 and 3 minima.
It must be acknowledged that a controller‘s responsibility also includes the alerting of the fire and rescue services and to the provision of all necessary information useful for the safety of flight. It therefore follows, that where the controller‘s visibility over the manoeuvring area is restricted, either by the positioning of the control tower or prevailing weather conditions, the ability of the controller to safely monitor the movement of aircraft and alert the fire and rescue services, may well be limited. ATS and Airport authorities should therefore consider this factor as an essential safety item when considering increasing the movement of aircraft in low visibility conditions and at airports where the view over the layout of the manoeuvring area is restricted.
When considering advanced forms of SMGCS, it must be accepted that the risk of collision on the ground is inherently different from that considered when airborne. It must be understood that any ground based separation standards that are introduced, do not embrace any safe margins for error that airspace based separation standards provide. ATC separation within airspace allows deviation, or avoidance from collision, in the vertical, horizontal and lateral planes. On the ground, with the constraints of runway and taxiway width and direction, there is very little margin for error. Any deviations to avoid collision can only be accommodated within the constraints of the available manoeuvring area. Any deviations from runway or taxiway surfaces will undoubtedly result in a ground based incident, or accident, involving damage to an aircraft which may well be substantial depending on circumstances and may also cause injury to persons on board the aircraft.
Before considering the introduction of SMGCS or A-SMGCS, ATS authorities must be able to ensure that all such separation standards that are required can be safely implemented, and that redundancy factors are established for failure of any related equipment or procedures that directly affect the safe provision of this ground based separation.
When surface monitoring equipment is used for the identification, control and separation of aircraft moving on the ground, appropriate qualifications should be introduced by ICAO to reflect the additional responsibilities and training needed to use radar-type equipment in control towers. The establishment of an Aerodrome Radar Rating is supported by IFATCA.
Before considering the introduction of an apron management service, airport authorities must be able to prove that traffic levels demand this type of service and that all necessary ground based equipment and procedures will be established to enable this service to be safely provided.
When an apron management service is provided, all personnel should receive training appropriate to the functions that they are required to carry out. ICAO suggests that personnel should be licensed. In the interests of standardisation, ICAO should clearly set out the training qualifications needed to exercise these tasks, or establish an appropriate licences for this particular function. Alternatively, ICAO should recognise the importance of this task and endorse qualified air traffic controllers as the most suitable personnel to undertake this function.
When an apron management service is provided by non-ATS staff, or unlicensed personnel, the appropriate airport authority should ensure that the different levels of service being provided is adequately notified to all users and any limitations in the ability of the personnel engaged in these duties, to deal with any event are clearly identified.
The levels of responsibility provided by an apron management service must be clearly defined, especially with regard to the provision of an alerting service and notification to the emergency services.
When an apron management service is provided, any clearances that are issued in a controlled apron environment must be seen as an executive function and, as such, are an extension of the existing ATC service. ICAO clearly states that this service should be provided by personnel suitably trained and licensed to undertake these tasks. However, unless ICAO is willing to establish a specific apron management service licence, it should be considered that the most appropriate personnel to undertake this function are qualified air traffic controllers.
When provided within the framework of A-SMGCS, it must be recognised that the objectives of an apron management service equate to that provided as part of a designated ATC service. It therefore follows that only licensed ATC personnel should provide the tasks within the areas of jurisdiction of that service.
It is recommended that:
ICAO should not specify the separation standards necessary to achieve the objectives until the introduction of appropriate future technology on the flight deck and appropriate visual or electronic aids on the ground.
A control tower must afford an aerodrome controller with direct visual observation of the manoeuvring area. The use of CCTV equipment is not acceptable as a substitute for direct visual observation.
The layout of runways and taxiways and the provision of visual aids, should be such, as to enable simple and easily understood instructions to be issued and complied with.
Where a separate apron management service is established, personnel engaged in issuing specific ground clearances, instructions and clearance delivery should be trained and licensed to exercise these functions.
Surface Movement Surveillance Systems should be installed at all airfields where low visibility operations take place and its operation should be mandatory while these operations are in progress.
Safeguards should be imposed to prohibit the development of any structure that would impede the direct visual observation from the tower.
ICAO Annex 14 – Aerodromes
ICAO Doc. 4444 – Rules of the Air and Air Traffic Services
ICAO Doc. 9476 – Manual of SMGCS (Surface Movement Guidance and Control Systems)
Runway Collision at Los Angeles: A case history of human error in ATC and flight operations. Presented at the Professional Panel during the IFATCA 94 Annual Conference in Ottawa, Canada, by James W Danaher, Chief, Operational Factors Division, Office of Aviation Safety, NTSB. FAA 1993 Runway Incursion Plan. FAA December 1992.
Update briefing on the status of runway incursion initiatives to the NTSB. Michael J Harrison, Airport Surface Systems Manager, FAA. November 1993.
Applied Human Factors in ATC : “Anyfield” – a fictitious case study. Presented at the Professional Panel during the 1993 IFATCA Annual Conference in Christchurch, New Zealand, by Bert Ruitenberg, IFATCA Executive Vice-President Professional.
Last Update: December 24, 2019