Wednesday, July 18, 2018
Transport Canada - Aviation Safety Letter See, Hear, Comply and Avoid—Maintaining Separation at Uncontrolled Aerodromes

See, Hear, Comply and Avoid—Maintaining Separation at Uncontrolled Aerodromes

by Mike Paddon, Civil Aviation Safety Inspector, System Safety, Atlantic Region, Civil Aviation,Transport Canada

A search of the aviation investigation reports published by the Transportation Safety Board of Canada (TSB) indicates that there have been several instances of in-flight collision and risk of collision in Canada in recent years. The consequence of aircraft occupying the same location in time and space rarely yields results that are less than tragic. Many in the aviation community may harbour vivid recollections of close encounters with other aircraft in otherwise unremarkable or outright benign circumstances.

The range of potential scenarios is extensive and, on occasion, the occurrence venue is airspace in close proximity to uncontrolled aerodromes. There are recorded instances of aircraft departing from or arriving at uncontrolled aerodromes under VFR, or under IFR in visual meteorological conditions (VMC), and unexpectedly finding themselves in close quarters with other traffic. Aircraft in the VFR traffic circuit have been known to conflict with one another even when operating with the benefit of air traffic services (ATS). Fixed-wing and rotary-wing traffic operating in relatively remote settings have historically found themselves in close quarters, both in the field and at nearby community aerodromes typically served by aerodrome traffic frequency (ATF) communication procedures. Examples of periods of heightened activity in terms of traffic volume would include major forest fire fighting efforts and the initial stages of natural resource development projects.

The predominant failing that arises in the vast majority of in-flight collisions is the failure to see and be seen as well as hear and be heard (i.e. avoid). So, what can be done to alleviate or mitigate the risk of collision? Does the answer lie in maintaining vigilance in our visual scanning, being alert to rapid and unacceptable loss of separation, and reacting well in advance of a deteriorating traffic situation; or in adherence to established regulations and procedures and communicating with other aircraft that we share airspace with? The answer likely rests in a combination of each of these defences.

In general, pilots will agree that visually detecting other aircraft can sometimes be very difficult. Most cockpits present challenges to effective visual scanning and the ability to search for and detect other aircraft. Impediments to the process include vision-obstructing struts, posts, doorframes, glareshields, and perhaps a fellow pilot or passenger. In addition, dirty, fogged, scratched andbug-splattered windshields, as well as flight in reduced visibility due to weather, or other obscuring phenomena such as smoke, can further complicate the task, as can vibration, fatigue and workload. Increased attention to cockpit automation and instrumentation can take away from time spent scanning the surrounding airspace for threats to safe separation. Accessibility of sunglasses to combat glare and choice of headgear are also part of the equation. A peaked ball cap may shade the eyes, but it might also restrict peripheral vision in the vertical plane; a factor of particular relevance when operating aircraft that, by virtue of cockpit design, would otherwise provide for enhanced peripheral vision in the vertical axis. Even detection of aircraft in a clear sky can be hindered by what is known as “empty-field myopia.” Shari Stamford Krause, PhD1, herself a pilot, describes this as a condition whereby, in the absence of a visual stimulus (for example, empty space), the muscles in the eye relax, preventing the eye from focusing. This creates a problem for a pilot who is attempting to scan for traffic in a clear, featureless sky. Because the eye cannot focus on empty space, it remains in a state of unfocused, or blurred, vision.

Maintaining vigilance in our visual scanning is critical to the see and be seen (or see and avoid) concept.
Maintaining vigilance in our visual scanning is critical to the see and be seen (or see and avoid) concept.

In an unrelated but pertinent study conducted by the Lincoln Laboratory2 during traffic alert and collision avoidance system (TCAS) flight testing, data showed that a pilot alerted to the presence of other aircraft visually acquired the other aircraft in 57 of 66 cases; the median range of visual acquisition was 1.7 NM. In cases where the pilot was not alerted to the presence of the other aircraft, visual acquisition of the other aircraft was achieved in only 36 of 64 encounters. In the successful encounters, the median acquisition range dropped to 0.99 NM. These studies showed that verbal guidance as to where to look increased the acquisition probability for the pilots, and found that a pilot who had been alerted to the presence of another aircraft was eight times more likely to see the aircraft than the pilot who had not been alerted. The test aircraft involved in the study were light twinengine aeroplanes. Radio advisory calls, TCAS, if fitted, and strobe/landing lights are all means of communicating an aircraft’s position to other traffic.

As pilots, we have a responsibility to read and know the Canadian Aviation Regulations (CARs). Procedures in place for effectively maintaining separation around uncontrolled aerodromes can be found in the Transport Canada Aeronautical Information Manual (TC AIM). This publication is available in print and on-line at: The published procedures are, in fact, regulations and adherence is required. Traffic that complies with the prescribed procedures will have the expectation that other aircraft are acting in a similar manner. It should be noted that the Aeronautics Act defines an aerodrome as:

Any area of land, water (including the frozen surface thereof) or other supporting surface used, designed, prepared, equipped or set apart for use either in whole or in part for the arrival, departure, movement or servicing of aircraft and includes any buildings, installations and equipment situated thereon or associated therewith.

CAR 602.19(10) states that:

No person shall conduct or attempt to conduct a take-off or landing in an aircraft until there is no apparent risk of collision with any aircraft, person, vessel, vehicle or structure in the take-off or landing path.

Mandatory frequency (MF) vs. aerodrome traffic frequency (ATF)… What’s the difference?
The following extract is taken from the RAC section of the TC AIM and is summarized for easy reference in the General section of the Canada Flight Supplement (CFS) under the Communications (COMM) sub-section.

Note: Although lengthy in content, it is considered to be in the interests of aviation safety to reproduce the pertinent references.

4.5.4 Mandatory Frequency

Transport Canada has designated [an MF] for use at selected uncontrolled aerodromes, or aerodromes that are uncontrolled between certain hours. Aircraft operating within the area in which the MF is applicable (MF area), on the ground or in the air, shall be equipped with a functioning radio capable of maintaining two-way communication. Reporting procedures shall be followed, as specified in CARs 602.97 to 602.103 inclusive.

An MF area will be established at an aerodrome if the traffic volume and mix of aircraft traffic at that aerodrome is such that there would be a safety benefit derived from implementing MF procedures. There may or may not be a ground station in operation at the aerodrome for which the MF area has been established. When a ground station is in operation, for example, an FSS [flight service station], an RCO [remote communications outlet] through which RAAS [remote aerodrome advisory service] is provided, a CARS [community aerodrome radio station], or an approach UNICOM, then all aircraft reports that are required for operating within, and prior to entering an MF area, shall be directed to the ground station. However, when the ground station is not in operation, then all aircraft reports that are required for operating within and prior to entering an MF area shall be broadcast. The MF will normally be the frequency of the ground station which provides the air traffic advisory services for the aerodrome….

4.5.5 Aerodrome Traffic Frequency

An [ATF] is normally designated for active uncontrolled aerodromes that do not meet the criteria listed in RAC 4.5.4 for an MF. The ATF is established to ensure that all radioequipped aircraft operating on the ground or within the area are listening on a common frequency and following common reporting procedures. The ATF will normally be the frequency of the UNICOM where one exists or 123.2 MHz where a UNICOM does not exist….The designation of an ATF is not limited to aerodromes only. An ATF may also be designated for use in certain areas other than the area immediately surrounding an aerodrome, where VFR traffic activity is high, and there is a safety benefit to ensuring that all traffic monitor the same frequency. For example, an ATF area could be established along a frequently flown corridor between two uncontrolled aerodromes....

4.5.7 VFR Communication Procedures at Uncontrolled Aerodromes with MF and ATF Areas

  1. Radio-equipped Aircraft: The following reporting procedures shall be followed by the pilot-in-command of radioequipped aircraft at uncontrolled aerodromes within an MF area and should also be followed by the pilot-in-command at aerodromes with an ATF:
    1. Listening Watch and Local Flying [CAR 602.97(2)] Maintain a listening watch on the mandatory frequency specified for use in the MF area. This should apply to ATF areas as well.
    2. Before Entering Manoeuvring Area (CAR 602.99) Report the pilot-in-command’s intentions before entering the manoeuvring area.
    3. Departure (CAR 602.100)
      1. Before moving onto the take-off surface, report the pilot-in-command’s departure intentions on the MF or ATF frequency. If a delay is encountered, broadcast intentions and expected length of delay, then rebroadcast departure intentions prior to moving onto the take-off surface;
      2. Before takeoff, ascertain by radio on the MF or ATF frequency and by visual observation that there is no likelihood of collision with another aircraft or a vehicle during takeoff; and,
      3. After takeoff, report departing from the aerodrome traffic circuit, and maintain a listening watch on the MF or ATF frequency until clear of the area.
    4. Arrival (CAR 602.101)
      1. Report before entering the MF area and, where circumstances permit, shall do so at least five minutes before entering the area, giving the aircraft’s position, altitude and estimated time of landing and the pilotin- command’s arrival procedure intentions;
      2. Report when joining the aerodrome traffic circuit, giving the aircraft’s position in the circuit;
      3. Report when on downwind leg, if applicable;
      4. Report when on final approach; and,
      5. Report when clear of the surface on which the aircraft has landed.
    5. Continuous Circuits (CAR 602.102)
      1. Report when joining the downwind leg of the circuit;
      2. Report when on final approach; stating the pilotin- command’s intentions; and,
      3. Report when clear of the surface on which the aircraft has landed.
    6. Flying Through an MF Area (CAR 602.103)
      1. Report before entering the MF or ATF area and, where circumstances permit, shall do so at least five minutes before entering the area, giving the aircraft’s position and altitude and the pilot-in-command’s intentions; and,
      2. Report when clear of the MF or ATF area.

        NOTE: In the interest of minimizing possible conflict with local traffic and minimizing radio congestion on the MF or ATF, pilots of en-route VFR aircraft should avoid passing through MF or ATF areas.

  2. NORDO: NORDO [no radio] aircraft will only be included as traffic to other aircraft and ground traffic as follows:
    1. Arrival: from five minutes before the ETA [estimated time of arrival] until ten minutes after the ETA, and
    2. Departure: from just prior to the aircraft departing until ten minutes after the departure, or until the aircraft is observed/reported clear of the MF area.

Carrying and referring to updated charts and a current copy of the CFS will help to ensure that correct frequencies for flight in the vicinity of uncontrolled aerodromes are selected on the aircraft radio. Relying on memory to recall MFs and ATFs for specific uncontrolled aerodromes can be problematic, especially at times of increased workload, and in light of the fact that frequencies may be subject to change. Global positioning system (GPS) data cards can provide a great deal of information at the push of a button, but that information can be a contributing factor to disaster if it is not accurate, hence the need for a current data card.

In conclusion, it is perhaps realistic to note that operational and self-imposed pressures to meet timelines and objectives can sometimes influence and cloud our perception of the airspace environment around us and how we fit into it. Risk factors associated with flight in the vicinity of uncontrolled aerodromes can be greatly reduced with the application of acute visual and aural awareness combined with familiarity with, and adherence to, the established rules and procedures. Used in conjunction with timely position reports and the communication of intentions between aircraft, these defences build and reinforce situational awareness and, ultimately, serve to assist aircraft in their avoidance of one another.


  1. Shari Stamford Krause, PhD, Flight Safety Digest, May 1997.
  2. J.W. Andrews, “Modeling of Air-to-Air Visual Acquisition,” The Lincoln Laboratory Journal, Volume 2, Number 3, 1989, p.478.


This article was published by Transport Canada in TP 185 Issue 3/2008 -. Reprinted with permission

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