Terrain and Obstacle Clearance Responsibilities

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Terrain and Obstacle Clearance Responsibilities

53RD ANNUAL CONFERENCE, Gran Canaria, Spain, 5-9 May 2014

WP No. 85

Terrain and Obstacle Clearance Responsibilities

Presented by TOC


During the work related to SID- and STAR-phraseology changes and cancellation of level restrictions, fundamental issues arose regarding ICAO standards and procedures for terrain and obstacle clearance. In April 2013 a first working paper was presented at the IFATCA Conference in Bali, and this second paper takes a broader look at terrain and obstacles clearance responsibilities in the ICAO procedures.


1.1. The ICAO PANS and SARPs define the procedures and responsibilities for terrain and obstruction avoidance for ATS Providers, ATCOs, aircrews and Operators.

1.2. Several new developments, particularly in airspace design, make the issue of responsibility for terrain and obstacle avoidance complex. Therefore we must have clear and unambiguous procedures to clarify the roles and responsibilities of ATS providers, controllers, pilots and operators.

1.3. Due to the recent developments on the subject and in reviewing proposed amendments to PANS-ATM, it became evident that ICAO documents do not provide sufficient clarity regarding terrain avoidance.

1.4. Also, there have been several amendments of interest to PANS-ATM, notably Amendment 4 in November 2005, which introduced the “direct to” provision as well as Amendment 5 in 2007, which reorganized chapters and changed radar specific language to more generic ATS-surveillance terminology. These changes unfortunately did not improve the issues and uncertainties.

1.5. CFIT (Controlled Flight into Terrain) should be considered the most avoidable type of aircraft accident. CFIT describes an accident in which an airworthy aircraft, under pilot control, is unintentionally flown into the ground, a mountain, water, or an obstacle. The term was coined by engineers at Boeing in the late 1970s. The pilots are generally unaware of the danger until it is too late. Accidents where an aircraft is damaged and uncontrollable (also known as uncontrolled flight into terrain) are not considered CFIT.

1.6. According to statistics for 2003-2012 by Boeing, CFIT is the #2 killer with 17 out of 75 accidents of the world commercial Jet Fleet. And according to a review of 2012 data by the Flight Safety Foundation, while the general accident rates are hitting all-time lows, CFIT accidents actually have risen (3 of 7 commercial jet accidents and CFIT is #1 fatal reason for commercial turboprops.). During the six years up to 2012, there have been 37 commercial aircraft CFIT accidents (14 jet, 23 turboprop). In the two years up to 2012, more than 50 percent of the commercial jet fatalities have been caused by CFIT accidents. This despite the widespread equipage with TAWS/EGPWS.

1.7. Two recent high profile accidents raising terrain responsibility issues with an ATC component occurred in close succession in 2012. On March 15, 2012 a Norwegian Air Force C130J crashed into Mount Kebnekaise, Sweden on a military mission flying IFR rules under civilian air traffic control. And on 9th of May, a Sukhoi Superjet 100 crashed into Mount Salak, near Jakarta, Indonesia on a demonstration flight, also flying IFR under civilian control.


2.1. In 2012 in Katmandu, Nepal at the 51st IFATCA World Conference, a review of the work and the progress made on the SID-STAR phraseology initiative by ICAO was made by IFATCA Committee B/C combined. During these discussions it became evident that the challenge of the validity and applicability of ATC level-restrictions was very complex, and much wider than initially thought. At the same time the understanding grew among the delegates that not only the level-restriction issue was of concern, but the much more serious issues of terrain and obstacle clearance for IFR-flights was a complicating factor. While restrictions designed for operational, airspace, military and similar reasons may be tactically cancelled without a safety consequence, those for terrain cannot and always need to apply.

2.2. Air traffic control requires an unambiguous definition of who is responsible for what and when. However, in the terrain avoidance case, secondary and “duty of care” responsibilities can blur the lines. The widespread availability of surveillance services where the ATCOs have a chance to observe and advise about deviations may create a false sense of safety among the crews, even when where these services are not available or being provided. On the other hand, pilots are not relieved from monitoring their flight progress in regards to terrain even when under vectors by ATC. Independent safety nets such as MSAW (Minimum Safe Altitude Warning) or EGPWS (Enhanced Ground Proximity Warning System) provide another layer of protection. However, both are clearly backups and should not take the focus away from the primary responsibilities. Those are the focus of this WP.


2.3. ICAO Reference to Terrain and Air Traffic Control

2.3.1. ICAO provisions related to IFR Flight and terrain avoidance are dispersed across several documents, Including:

  • Annex 2 Rules of the Air
  • Annex 4 Charts
  • Annex 6 Commercial Air Transport
  • Annex 11 Air Traffic Services
  • Annex 15 AIS
  • PANS-OPS Doc 8168
  • PANS-ATM Doc 4444

2.3.2. The provisions range from altimetry procedures that were developed in the early days of commercial aviation, to a digital terrain and obstacle dataset to be published for airports regularly used by international commercial air traffic by 2015. Changing and improving provisions without unintended consequences is not easy. However, this should not prevent us from initiating work that can improve the safety of aviation.

2.3.3. The “Note”

In reviewing all ICAO terrain references, the following note appears once in Annex 4 and several times in PANS-ATM, Doc 4444 (Annex 4, chapter 21.1. Note / PANSATM 4444 Chapter 2.1 “Scope and Purpose” Note 2 / Chapter 4.10.3 “Minimum cruising levels for IFR-flights” Note 3 / Chapter 5.9. “Own Separation in VMC” Note 3):

The objectives of the air traffic control service as prescribed in Annex 11 do not include prevention of collision with terrain. The procedures prescribed in this document do not relieve pilots of their responsibility to ensure that any clearances issued by air traffic control units are safe in this respect. When an IFR flight is vectored or is given a direct routing which takes the aircraft off an ATS route, the procedures in Chapter 8, apply.

Without specifying how this should be achieved, the pilot is made responsible to ensure that each ATC clearance provides terrain clearance unless given a vector or direct routing, in which case the following applies.

2.3.4. Terrain Clearance While Vectoring or on a Direct Route: When vectoring an IFR flight and when giving an IFR flight a direct routing which takes the aircraft off an ATS route, the controller shall issue clearances such that the prescribed obstacle clearance will exist at all times until the aircraft reaches the point where the pilot will resume own navigation. When necessary, the relevant minimum vectoring altitude shall include a correction for low temperature effect.

Note 1.— When an IFR flight is being vectored, the pilot may be unable to determine the aircraft’s exact position in respect to obstacles in this area and consequently the altitude which provides the required obstacle clearance. Detailed obstacle clearance criteria are contained in PANS-OPS (Doc 8168), Volumes I and II. See also

Note 2.— It is the responsibility of the ATS authority to provide the controller with minimum altitudes corrected for temperature effect. In terminating vectoring of an aircraft, the controller shall instruct the pilot to resume own navigation, giving the pilot the aircraft’s position and appropriate instructions, as necessary, in the form prescribed in b), if the current instructions had diverted the aircraft from a previously assigned route.

The controller assumes responsibility for terrain clearance for aircraft on vectors or direct routes. This raises the question about the use of the term “ATS Route” and what is considered by being “off” or “on” an ATS route. A direct routing is also not limited to a controller-initiated action as this reference implies. One could argue that any direct routing (even if filed in the original flight plan) would transfer liability for terrain avoidance from the pilot to the controller. While that is not the original intent of the language, advances in aircraft navigation have created conditions where the distinction is no longer clear.

The “ATS-Route” statement also tries to imply that there is a better chance for the crew to be briefed from the preflight planning phase regarding minimum altitudes on their flight and that there is published information for the route available. Nowadays, due to the sheer number of routes and the fact that variations of routes may be cleared on a tactical basis, the crew might find themselves on an ATS-Route they were not prepared/briefed for.

To make things more confusing, this “direct to” provision – by context in PANS-ATM – only applies to areas with surveillance service. However, “directs”, as a quick email survey among IFATCA MAs has shown, are also regularly flown in procedural airspace.

Finally, the point when or where terrain responsibility transfers back from the controller to the crew is open to interpretation. “until the aircraft reaches the point where the pilot will resume own navigation” can either mean at the first point back on the ATS-Route or at whatever point or time where the pilot, assisted by the FMS, resumes the task of navigating by himself, well before being back on route. As a matter of fact, by instructing a “direct to” to any point on the ATS-route, the pilot immediately resumes (respectively stays on) own-navigation, regardless of however off-route that may be.

2.3.5. ICAO Definition of ATC-Service in Annex 11:

Air traffic control service.

A service provided for the purpose of:

a) Preventing collisions:

1) between aircraft, and

2) on the manoeuvring area between aircraft and obstructions; and

b) expediting and maintaining an orderly flow of air traffic.

2.3.6. ICAO Definition of ATS In chapter 2 of Annex 11:

2.2 Objectives of the air traffic services

The objectives of the air traffic services shall be to:

a) prevent collisions between aircraft;

b) prevent collisions between aircraft on the manoeuvring area and obstructions on that area;

c) expedite and maintain an orderly flow of air traffic;

d) provide advice and information useful for the safe and efficient conduct of flights;

e) notify appropriate organizations regarding aircraft in need of search and rescue aid, and assist such organizations as required.

2.3.7. Further definition of ATS-Service, PANS ATM DOC 4444, states:

Air Traffic Service (ATS)

A generic term meaning variously, flight information service, alerting service, air traffic advisory service, air traffic control service (area control service, approach control service or aerodrome control service).

2.3.8. Annex 11 makes the following sub-division of ATS-Service:

a) Air Traffic Control Services (or ATC-service),

b) Air Traffic Advisory Service (ADVS),

c) Flight Information Service (FIS),

d) Alerting Service (ALS).

2.3.9. While air traffic control-service is primarily a service established to prevent collisions between participating flights (and obstacles on the maneuvering area), ATS-service has a broader scope.

2.3.10. It is not our intent to add additional responsibilities for ATCOs and ANSPs, but the statements that ATS does not include the prevention of collisions with obstacles or terrain (except on the maneuvering area) does not accurately reflect the current operating environment.

2.3.11. TOC believes the objective of preventing collisions with obstacles and terrain should be clearly stated in the definition of ATS-Service. Terrain/obstacle safety extends beyond air traffic control and includes charting, flight data processing, information availability and distribution, aeronautical information management, safety nets, support systems and training.

2.3.12. Any change to ICAO documentation would require appropriate consequential amendments to ensure consistency across all references.

2.4. Direct Routing versus ATS Route

2.4.1. The current PANS-ATM provision reads: When vectoring an IFR flight and when giving an IFR flight a direct routing which takes the aircraft off an ATS route, the controller shall issue clearances such that the prescribed obstacle clearance will exist at all times until the aircraft reaches the point where the pilot will resume own navigation. When necessary, the relevant minimum vectoring altitude shall include a correction for low temperature effect.

2.4.2. The statement “and when giving an IFR flight a direct routing which takes the aircraft off an ATS route…” introduces several questions regarding aircraft filing direct routes. The ICAO definition of ATS route is:

ATS route.

A specified route designed for channeling the flow of traffic as necessary for the provision of air traffic services.

Note 1.— The term “ATS route” is used to mean variously, airway, advisory route, controlled or uncontrolled route, arrival or departure route, etc.

Note 2.— An ATS route is defined by route specifications which include an ATS route designator, the track to or from significant points (waypoints), distance between significant points, reporting requirements and, as determined by the appropriate ATS authority, the lowest safe altitude.

2.4.3. Using modern navigation equipment and methods, it is common for aircraft to fly outside an established route structure for at least a portion of the flight. Tying responsibility for terrain avoidance to the specifics of an “ATS route” does not properly reflect the current operating environment.

2.4.4. There is no clear definition of what comprises “on” or “off” route. For example an airway, which may be part of the “ATS Route” is defined as a corridor and not a single line between waypoints.

2.4.5. If the ICAO documents are to reflect the intent of this language, there should be a differentiation made based on whether the vectors or direct routing are controller or pilot initiated. While this type of change would address the ambiguity in the language, it would not address the larger issue of ATS and terrain avoidance.

2.4.6. Further uncertainties exist in relation to weather avoidance deviations by pilots. Both for the enroute phase in procedural airspace as well as at low altitude between the runway and the minimum radar vectoring altitude.

2.5. Visual Approaches

PANS-ATM, (doc 4444) regarding terrain and obstacle avoidance responsibilities during the visual approach portion of an IFR flight states: Controllers shall exercise caution in initiating a visual approach when there is reason to believe that the flight crew concerned is not familiar with the aerodrome and its surrounding terrain. Controllers should also take into consideration the prevailing traffic and meteorological conditions when initiating visual approaches.

Phrases like, “exercise caution”, “when there is reason to believe” and “take into consideration” do not provide adequate clarity with regard to responsibility. This was demonstrated in the legal action following crash of a C500 Citation in Cagliari, Italy during a visual approach. Even in an instance where the procedure was applied properly by the controller and the crew explicitly confirmed maintaining own separation from terrain shows how an ambiguous provision can be used to place both responsibility and liability on the controller.


3.1. Controlled Flight Into Terrain (CFIT) should be regarded as the most avoidable type of aviation accident. At the same time, it is among the top 2 reasons for fatal accidents in the commercial fleet. By bringing terrain avoidance into the objectives of ATS, ATCOs are better able to advocate for appropriate resources for training, charting, and other necessary elements.

3.2. It has been demonstrated that existing provisions do not provide ATCOs with a legal exemption from the responsibility for terrain avoidance. The existing exclusions may provide both ATCOs and ANSPs with a false sense that they are not responsible for areas where they may ultimately be held liable.

3.3. In situations where crews request headings or directs to avoid severe weather, particularly below MVA, ICAO provisions do not provide adequate clarity regarding responsibility for terrain avoidance.

3.4. Terrain avoidance during a visual approach should rest clearly with the pilot when accepting the clearance. Requirements for the controller to make assumptions based on crew familiarity and other circumstances are not acceptable.


It is recommended that:

4.1. IFATCA Policy is:

Responsibility for terrain and obstruction clearance must be clearly defined and always must lie either with the air crew or ATC. There must never be a situation where doubt exists about who is responsible for this task.

and is included in the IFATCA Technical and Professional Manual.

4.2. IFATCA Policy is:

ICAO documentation should provide clear and unambiguous language with regard to responsibility for terrain avoidance, including amendment to the Objectives of air traffic services to include the prevention of collisions between aircraft and terrain.

and is included in the IFATCA Technical and Professional Manual.

4.3. IFATCA Policy is:

IFATCA should encourage ICAO to study the topic of deviations from ATSroutes due to severe weather, particularly when terrain is a factor.

and is included in the IFATCA Technical and Professional Manual.

4.4. It is recommended that:

IFATCA should pursue a course of action to encourage ICAO to remove implied responsibilities for controllers in Doc 4444, Section, regarding evaluating crews in terms of familiarity and judging weather for visual approaches.






ICAO (2007). PANS-ATM Doc 4444.

IFATCA (2010). Technical and Professional Manual.

IFATCA (2010). Punta Cana 2010 – Resolution B9 – WP 88.

Last Update: September 30, 2020  

May 6, 2020   654   Jean-Francois Lepage    2014    

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