46TH ANNUAL CONFERENCE, Istanbul, Turkey, 16-20 April 2007WP No. 167Review Policy on Visual Observation in a Control Tower EnvironmentPresented by PLC |
Introduction
1.1 This item was put on the Professional & Legal Committee (PLC) work programme after the first joint meeting with the Technical & Operations Committee (TOC) in Cape Town, South Africa, October last.
1.2 During this meeting the TOC draft working paper on the review of the provisional policy on virtual towers was discussed in a joint session with PLC. The representatives decided that although this topic was originally foreseen to be part of that working paper it was preferred that the first two paragraphs be the subject of a separate working paper.
1.3 At the 2006 Kaohsiung joint Committee B/C meeting lively discussion on the “Virtual Tower Concept” ended up with the acceptance of provisional policy as stipulated hereafter:
| “Direct visual observation is defined as: Observation through direct eyesight not supported by means other than optical aids (glasses/lenses) that correct vision.
An aerodrome control unit is a unit established to provide air traffic control service to aerodrome traffic. It shall provide aerodrome controllers the capability to maintain a continuous watch on all flight operations on and in the vicinity of an aerodrome as well as vehicles and personnel on the manoeuvring area. This watch shall be maintained by direct visual observation, possibly augmented by other means. Any future ATM concept for the provision of Aerodrome Control Service, is only acceptable to IFATCA, provided that: – When direct visual observation is replaced by alternative means, it must be as good as, or better than direct visual observation; and – In addition to standard fall back procedures for Aerodrome control units, appropriate fall back procedures must be implemented in case of failure of any technical device influencing the situational awareness of aerodrome controllers.” |
1.4 Note that the last part of the provisional policy starting with “Any future …is subject to agenda item B.5.5.
1.5 The general feeling was that the above mentioned draft recommendations weren’t mature enough to be accepted as policy and that further work on the matter was appropriate.
Discussion
2.1
| “Direct visual observation is defined as: Observation through direct eyesight not supported by means other than optical aids (glasses/lenses) that correct vision”. |
2.1.1 WP 165/2006 stated that:
“Although ICAO doesn’t define the term “visual observation” IFATCA only accepts this as being: “observation through direct eyesight not supported by means other than optical aids (glasses/lenses) that correct vision”.
2.1.2 What is understood by the word “direct” in this context? Direct means that the light reflected by the observed object has travelled in a most direct line before being projected on the eye’s retina. In other words that the observed object is situated “in the line of sight” of the observer.
2.1.3 “Direct” visual observation stops when mirrors are used to overcome obstacles in the line of sight.
2.1.4 Within ATM indirect visual observation does not exist. Pilots are not permitted to use mirrors to observe the vicinity around their aircraft and controllers are not permitted to use this tool when performing control functions. That is the reason why ICAO doesn’t define the term “visual observation”.
2.1.5 What can be understood by the term “optical aids”? In the industry, optical aids are instruments used not only to correct but also to enlarge or reduce a view using solely the properties of light refraction and dispersion through optical lenses. These lenses can be used separately as in glasses or contact-lenses or can be used in combination with other lenses and mirrors as in monocular and binocular systems and telescopes. Please note that ICAO only permits the use of binoculars in a control tower. (ICAO Doc 9426 Requirements of a Control Tower)
2.1.6 PLC is of the opinion that whether the observer is wearing glasses or contact-lenses is of no importance in defining the term visual observation.
2.1.7 The definition of direct visual observation as mentioned in the provisional policy is too restrictive. Using binoculars to enlarge distant objects for better viewing must also be considered as a visual observation of these objects. Furthermore it is PLC’s opinion that the word direct can be deleted in its first use as indirect visual observation is of no existence in this context.
2.1.8 As such, visual observation in ATM could be defined as:
“Observation through direct eyesight of objects situated within the line of sight of the observer possibly enhanced by binoculars”.
2.2 ICAO recommendations referring to visual observation in a TWR environment
2.2.1 ICAO Doc 4444, Air Traffic Management, Chapter 7: Procedures for Aerodrome Control Service, 7.1 Functions of Aerodrome Control Towers:
“Aerodrome controllers shall maintain a continuous watch on all flight operations on and in the vicinity of an aerodrome as well as vehicles and personnel on the manoeuvring area. Watch shall be maintained by visual observation, augmented in low visibility conditions by radar when available.”
ICAO Doc 4444, Air Traffic Management, Chapter 8: Radar Services, 8.10 Use of Radar in the Aerodrome Control Service):
SMR (Surface Movement RADAR) should be used to augment visual observation of traffic on the manoeuvring area and to provide surveillance of traffic of the manoeuvring area which cannot be observed visually.
This is a very important statement by ICAO. It doesn’t leave any room for remote or virtual tower concepts as replacement of an Aerodrome Control Tower. Visual observation is the sole tool to be used to watch flight operation in VMC. Under low visibility conditions this visual observation can be augmented solely by the use of radar. SMR (Surface Movement Radar) for watching ground movements and Surveillance Radar for airborne traffic.
Closed-circuit Television (CCTV) and other vision enhancement systems may not be considered as control tools but must be seen as additional information tools.
Air traffic control instructions based solely on the information derived from CCTV and other non-control tools may have legal implications.
2.2.2 ICAO Doc 4444, Air Traffic Management, Chapter 7: Procedures for Aerodrome Control Service, 7.3 Information to Aircraft by Aerodrome Control Towers:
“In the event the aerodrome controller observes, after a take-off clearance or a landing clearance has been issued, any obstruction on the runway likely to impair the safety of an aircraft taking off or landing, such as a runway incursion by an aircraft or vehicle, or animals or flocks of birds on the runway, appropriate action shall be taken as follows:
a) in all cases inform the aircraft concerned of the obstruction and its location on the runway;
[…]
b) cancel the take-off clearance for an aircraft which has not started to roll; c) instruct a landing aircraft to go around.”“Whenever an abnormal configuration or condition of an aircraft, including conditions such as landing gear not extended or only partly extended, or unusual smoke emissions from any part of the aircraft, is observed by or reported to the aerodrome controller, the aircraft concerned shall be advised without delay.”
These actions are very time critical. Within a second a controller must observe and react. Observation through cameras can be too time consuming and too inaccurate to react promptly. Safety can be jeopardized when the time laps between the event and the reaction of the controller is too long.
2.2.3 ICAO Doc 4444, Air Traffic Management, Chapter 7: Procedures for Aerodrome Control Service, 7.4 Essential information on Aerodrome Conditions:
“When a not previously condition pertaining to the safe use by aircraft of the manoeuvring area is reported to or observed by the controller, the appropriate aerodrome authority shall be informed and operations on that part of the manoeuvring area terminated until otherwise advised by the appropriate aerodrome authority.”
This implies that the complete manoeuvring area should be visible from the tower. Otherwise the controller has to rely solely on reports from others. The use of CCTV can come in handy for this type of information.
2.2.4 ICAO doc 4444 Air Traffic Management, Chapter 7: Procedures for Aerodrome Control Service, 7.5 Control of Aerodrome Traffic:
“All vehicles and pedestrians shall give way to aircraft which are landing, taxiing or taking off, except that emergency vehicles proceeding to the assistance of an aircraft in distress shall be afforded priority over all other surface movement traffic. In the latter case, all movement of surface traffic should, to the extent practicable, be halted until it is determined that the progress of the emergency vehicles will not be impeded.”
This situation can occur at any time without previous notification. Prompt reaction and re-clearance or alternate instruction issuance implies that the movement of emergency vehicles and other traffic can be observed simultaneously and that visual or surveillance derived track prediction is needed to avoid emergency stops of involved traffic movement.
Note that this is a control function. (see 2.2.1)
2.2.5 ICAO doc 4444 Air Traffic Management, Chapter 7: Procedures for Aerodrome Control Service, 7.5 Control of Aerodrome Traffic:
“When communications by a system of visual signals is deemed to be adequate, or in the case of radiocommunication failure, the signals given hereunder shall have the meaning indicated therein: …”
This part of the safety chain is difficult to achieve from a remote position. The visual signals are part of the control functions and cannot be performed by unqualified personnel.
2.2.6 ICAO doc 9426 Air Traffic Services Planning Manual, 3.2.2 Specific Requirements for an Aerodrome Control Tower:
“Surveillance by the aerodrome controller is normally done by visual means (eyesight) alone, mechanically through the use of binoculars to improve eyesight or electronically, through the use of radar or closed-circuit television…
The height of the tower should be such that, at normal eye level (about 1,5m above the floor of the tower cab) the controller is provided with visual surveillance previously described…”
Surveillance in this context means a close observation or supervision over the operational environment. This includes the process of gathering information to develop and maintain an oversight of the situation.
2.3 Human depth perception in a 3-dimensional world
2.3.1 To understand why visual observation is the irreplaceable prime control tool for a Tower Controller it is necessary to understand the subtle interactions between eyes and brain.
2.3.2 If we think about modern traffic or even the simple-looking task of directing a fork with something to eat into our mouth, it is easy to recognize that 3- dimensional perception of our 3-dimentional world is essential for humans. Our eyes only have 2-dimensional retina images and no special third component for depth perception. This requires an interpretation of our physiological cues that leads to useful “perception”. By combining the retinal images of our 2 eyes we get some of the best and most convincing information about the three dimensions of our world. Two binocular sources of depth perception are:
Disparity and Convergence
The 6 cm distance between the two pupils causes us to see two slightly different images of the world. This displacement between the horizontal positions of corresponding images is called binocular disparity. It is probably the most important cue for depth perception, the amount of the displacement depends on the relative distance of the objects from the eye. By looking with both eyes, we are not aware of the fact that most of the objects seen stimulate different regions of the two retinas. Objects with different distances are projected onto different parts of the retinas due to disparity. If disparity of corresponding images is small enough, the visual system can “melt” them to the perception of a three dimensional object.
The performance of the visual system is really remarkable: It takes two different retina images, compares them for their horizontal distortion of corresponding parts and produces a uniform perception of a 3- dimentional object. We really interpret the horizontal distortion between the two images as dept.
We gather other binocular dept information from convergence; this is the converging of the two axes of the eyes when looking at a nearby object. The information of convergence is reported to our brain from the eye muscles, but it can only be used for depth perception up to a distance of about 10 feet.
There are a number of other possibilities for us, not dependent on the existence of two different retina images. A first one is “motion parallax”.
To make this effect clear, try the following: close one eye and bring both index fingers at different distances into a line with a remote object. Keep your eye fixed on this object and do not move your fingers. If you now move your head to one side, the images of the index fingers will move at different speeds, while the fixed object itself will remain still. This information is called relative motion parallax. When moving, the relative distances of objects determine the amount and direction of their relative movements in the retina image.
Even if we close one eye and do not move the head, we can still gather information to enable depth perception. These kinds of depth perception are also found in pictures:
- Overlay (Interposition), gives us the information that an object blocking part of another object from view is nearer than the partially covered object.
- Shadows give information on the three dimensional form of objects as well as on the position of a source of light.
- Other pictorial cues are easily recognized by optical characteristics:
- Relative Size is due to the fact that objects of the same size but in varying distances cast different retinal image sizes. This so-called size- distance relation gives us cues about the distance of objects of known absolute or relative size.
- Linear Perspective is known to most of us from drawing- lessons at school. A consequence of the size-distance relation, perspective is mainly expressed by the fact that lines representing physically parallel structures seem to converge in the distance towards a vanishing point. In painting, this principle was not applied until about 1400; before that only overlay, shadows and relative size were used to create depth effects.
- Texture Gradient is another image factor depending on the size-distance relation. It is mainly applied to textures (structures) of surfaces. A regular structure, e.g. a tiled floor, appears to show smaller structures with increasing distance.
Finally, two other components of perception are to be mentioned, they mainly depend on our experience:
It has been proved that the brighter of two otherwise identical objects is perceived as nearer. There is no satisfactory physical, geometric or photometric explanation for this effect; however, an interpretation is our experience when working with a source of light (e.g. a torch) in dark environment.
Eventually we know (also in our sub-conscious) that light is reduced and refracted by our atmosphere in greater distance, so that mountains on the horizon appear bluish and out of focus. This factor is called Aerial Perspective.
Coherent Interpretation
As a final remark it is to be pointed out that under normal perception conditions all of these different cues to depth perception lead to a single, coherent, three dimensional interpretation of our environment. We regard the perception of depth as a fact but not the various image factors mentioned above. Our visual system uses the cues of information available rather automatically and without employing our consciousness. Natural sight signals take 50 milliseconds – 1/6th of an eye blink – to zip from eye to brain and for the brain to see.
2.4 The necessity of visual observation in a Control Tower
2.4.1 Taking the above into consideration, depth perception when looking to a display is limited to pictorial cues. Only part of the tricks our brain uses to perceive depth is available, creating uncertainties the brain is confronted with taking time to sort out and creating additional fatigue.
2.4.2 Depth perception is not the only advantage the human eye has by looking to the outside world. Inside the skull the eyes have a certain freedom of movement which enables them to keep focus on moving objects and surroundings. Together with the mobility of the neck the human is able to survey a field of view of over 180 degrees in the horizontal plain and a little less in the vertical plain without the necessity of moving the thorax.
2.4.3 Central to our multi-resolution visual perception is the dynamic nature in which the eyes execute movements to scan a scene. Despite a large field of view, the human eyes process only a tiny central region in great detail. In order to build a detailed representation of a scene, the human visual system (HVS) therefore uses a dynamic process of actively scanning the visual environment using discrete fixations linked by ballistic saccadic (jumps) eye movements. The eye gathers most information during the fixations while little information is gathered during the saccades.
2.4.4 Scanning the visual environment is for Tower controllers of the utmost importance to get a global picture of all movements under their control, similar to the scanning of a radar data display for radar controllers.
2.4.5 To obtain something similar with the aid of CCTV or other visual displays a wide range of cameras are required. Preferably these cameras should be manned by experienced cameramen with a very good knowledge of Air Traffic Control. This may sound hilarious but controllers familiar with an aerodrome know exactly where to look at, what to look for and when. Imagine a Formula 1 Grand Prix shown on television with static cameras or with cameras manipulated with remote joysticks by inexperienced people.
2.4.6 A simple sweep to overview the availability of a runway takes less than a second for the human to overview and analyze. A camera sweep of the same duration provides no information at all as there is any opportunity for the human to fix any points during the sweep.
2.4.7 The use of multiple cameras has as disadvantage that the human being is not the centre of all these cameras. He has to move mentally towards those cameras to have a correct perception of the picture produced by these cameras. The more cameras, the more difficult this perception becomes.
2.5
| “An aerodrome control unit is a unit established to provide air traffic control service to aerodrome traffic. It shall provide aerodrome controllers the capability to maintain a continuous watch on all flight operations on and in the vicinity of an aerodrome as well as vehicles and personnel on the manoeuvring area. This watch shall be maintained by direct visual observation, possibly augmented by other means.” |
2.5.1 The first sentence of this provisional policy statement is an amalgamation of ICAO’s definition of an aerodrome control tower (ICAO Doc 4444 Air Traffic Management, Definitions):
Aerodrome control tower. A unit established to provide air traffic control service to aerodrome traffic.
and ICAO’s definition of Air traffic control unit (ICAO Annex 11 Air Traffic Services, definitions):
Air traffic control unit. A generic term meaning variously, area control centre, approach control unit or aerodrome control tower.
Probably this definition finds its origin in the confusion between the control tower as building and the Aerodrome Control Tower as air traffic control unit. However, in the working paper leading to this statement nothing is mentioned about the reasoning behind the statement. ICAO uses different terms to refer to the building, aerodrome control tower, control tower as well as tower. The operational floor is called tower cab. (ICAO doc 9426 Air Traffic Services Planning Manual, Facilities Required by ATS)
2.5.2 PLC is of the opinion that on one hand altering an existing ICAO definition is not solving but adding to the confusion and that on the other hand ICAO should be informed about the confusion as to develop an unambiguous term referring to the control tower building.
2.5.3 The second sentence of the statement makes reference to the operational floor from where Aerodrome Control Service is performed while the first is referring to the unit performing the service. The word “it” doesn’t refer to the correct subject.
2.5.4 The last sentence of this provisional policy statement refers to an existing ICAO statement (Watch shall be maintained by visual observation, augmented in low visibility conditions by radar when available) and should not be altered. To open the scope for the introduction of new tools assisting the controller in his tasks an additional statement is needed. Note that if it consists of new control tools adequate ICAO recommendation should be developed in order to cover the legal side of the use of such tools. Human factors studies should be carried out to ascertain that the introduction of this new technology doesn’t increase neither head-down-time nor workload nor overwhelm the controller with information that could deteriorate his overall situational awareness.
2.5.5 The mentioned provisional policy statement is in contradiction with the next provisional policy statement.
2.5.6 PLC proposes to rephrase the statement as follows:
“An Aerodrome Control Tower is a unit established to provide air traffic control service to aerodrome traffic. The tower cab shall be constructed as to provide aerodrome controllers the capability to maintain a continuous watch on all flight operations on and in the vicinity of the aerodrome as well as vehicles and personnel on the manoeuvring area. Watch shall be maintained by visual observation, augmented by radar or other approved control systems when available”.
2.6
| “Any future ATM concept for the provision of Aerodrome Control Service, is only acceptable to IFATCA, provided that:
– When direct visual observation is replaced by alternative means, it must be as good as, or better than direct visual observation; and – In addition to standard fall back procedures for Aerodrome control units, appropriate fall back procedures must be implemented in case of failure of any technical device influencing the situational awareness of aerodrome controllers.” |
2.6.1 It is the opinion of PLC that visual observation as the prime control tool in Aerodrome Control Service may not be replaced by alternative means unless approved by ICAO as control tools in which case the alternative means should be better than visual observation.
2.6.2 Please note that this provisional policy statement is under review as agenda item B.5.5.
Conclusions
3.1 Human vision is a very complex interaction between the eyes and the brain, enabling the human to perceive depth and detail in a three dimensional world.
3.2 Visual Observation is the prime control tool in Aerodrome Control Service. This tool may be augmented by other surveillance tools such as SMR.
3.3 CCTV and other camera connected displays must be considered as additional information tools to assist the tower controller to observe situations beyond the line of sight.
3.4 PLC is therefore convinced that concepts based on CCTV and other camera connected displays do not coincide with the duties of a tower controller and as such cannot be used for the purposes of providing aerodrome control service.
3.5 The provisional policy needs amending as some statements are too restrictive while others are confusing and contradictory. Therefore following is proposed.
3.6 Visual observation in ATM can be defined as: “Observation through direct eyesight of objects situated within the line of sight of the observer possibly enhanced by binoculars”.
3.7 An Aerodrome Control Tower is a unit established to provide air traffic control service to aerodrome traffic. The tower cab shall be constructed as to provide aerodrome controllers the capability to maintain a continuous watch on all flight operations on and in the vicinity of the aerodrome as well as vehicles and personnel on the manoeuvring area. Watch shall be maintained by visual observation, augmented by radar or other approved control systems when available.
Recommendations
4.1 Liaise with ICAO the ambiguity around the term “Aerodrome Control Tower” meaning both the control unit and the infrastructure for aerodrome control service.
4.2 Amend page 3 2 2 2 to read:
2.2 Visual Observation and New Aerodrome Control Tower Concepts
Visual observation in ATM can be defined as: “Observation through direct eyesight of objects situated within the line of sight of the observer possibly enhanced by binoculars”.
An Aerodrome Control Tower is a unit established to provide air traffic control service to aerodrome traffic. The tower cab shall be constructed as to provide aerodrome controllers the capability to maintain a continuous watch on all flight operations on and in the vicinity of the aerodrome as well as vehicles and personnel on the manoeuvring area. Watch shall be maintained by visual observation, augmented by radar or other approved surveillance systems when available.
References
IFATCA Manual.
WP 90 – Kaohsiung 2006.
WP 165 – Kaohsiung 2006.
ICAO Documents.
Spatial Vision, Russell L. DeValois & Karen K. DeValois, Oxford Science Publications, 1988.
Fundamentals of Sensation & Perception, 2nd Edition, Michael W. Levine & Jeremy M. Shefner, Brooks/Cole Publishing Co. 1991.
Eye, Brain, and Vision, David Hubel. Scientific American Library, 1988.
Natural Image Statistics and Human Eye Fixations; Umesh Rajashekar, Alan C. Bovik, Lawrence K. Cormack (Laboratory for Image & Video Engineering, Texas University).
Last Update: September 29, 2020