Terrain Clearance and Airspace Design

  • Home 2013 Terrain Clearance and Airspace....

Terrain Clearance and Airspace Design

52ND ANNUAL CONFERENCE, Bali, Indonesia, 24-28 April 2013

WP No. 89

Terrain Clearance and Airspace Design

Presented by TOC


The increasing usage of structured departure and arrival procedures, including PBN operations, has brought to light existing ambiguities in current ICAO SARPs regarding terrain clearance responsibilities. For example, according to ICAO SARPs, Air Traffic Services “do not include prevention of collision with terrain” (ICAO 2007). However, this blanket exclusion does not reflect the reality of the current operational environment in many regions of the world. This paper reviews the current situation, identifies existing ambiguities and recommends additional work in formulating specific recommendations for clarifying terrain and obstruction clearance responsibilities in common traffic scenarios.


1.1 ICAO SARPs defining pilot and ATCO responsibilities for terrain and obstruction avoidance have not kept pace with operational changes in the area of airspace and procedural design, resulting in conflicting or ambiguous language in existing provisions.

1.2 Current modernization projects increasingly focus on precise, structured departure and arrival routings. As these procedures become more common, the need for standardization and clearly defined pilot/controller responsibilities has become more critical.

1.3 Controversial legal findings, such as the decision in the Controlled Flight into Terrain (CFIT) accident, involving an aircraft on a visual approach at Cagliari, Italy, have worked to exacerbate the uncertainty surrounding pilot/controller responsibilities.

1.4 Recent proposals concerning SID/STAR phraseology have generated concerns regarding pilot and ATCO terrain clearance responsibilities. As a result TOC has been tasked to study this issue.


2.1 Several issues regarding responsibility for terrain avoidance were highlighted at the IFATCA conference in Kathmandu in response to the ICAO proposed change related to level clearances on SID and STAR operations. Through this discussion it became apparent that terrain avoidance responsibilities are becoming increasingly unclear, raising significant safety concerns.

2.2 ICAO

ICAO SARPs pertaining to terrain clearance responsibilities can be found in several different documents and the language found in each is often ambiguous and occasionally appears to be contradictory. The lack of a clear, comprehensive set of provisions leads to confusion and uncertainty surrounding terrain clearance responsibilities.

2.2.1 For example, in current ATM systems, in a surveillance environment, aircraft are routinely issued vectors or direct routings with ATCOs accepting the responsibility for terrain clearance. ICAO SARPs stipulate that “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” (ICAO 2007, Doc 4444 paragraph In the case of a direct routing, there is some question concerning exactly when the pilot resumes “own navigation.” In today’s global satellite navigation environment, pilots are often independently navigating to the next fix within seconds. This leaves some question concerning the point at which terrain responsibility transfers from ATCO to pilot. We should also note that paragraph exists in chapter 8 of Doc 4444-ATS Surveillance Services, without providing direction for direct routings in a procedural (non-surveillance) environment.

For clarity paragraph is included below:

Doc 4444 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.


2.2.2 The following note is found in the Scope and Purpose section (page vii) of PANS-ATM Doc 4444:

Note 2.— 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.


This note implies that terrain avoidance responsibility lies with the pilot. However, as mentioned above, flights are routinely issued vectors or direct routings, placing terrain responsibilities with the ATCO. The growing usage of Performance Based Navigation (PBN) procedures in the departure and arrival phases of flight further complicates the environment. Aircraft are increasingly being transitioned between waypoints or intersections on published routings; many of these routes contain altitude restrictions which may or may not be applicable in any given instance. In some instances controllers may delete the altitude restrictions because they are not required for separation from aircraft or airspace. Terrain avoidance responsibility and level restriction compliance in these cases remains ambiguous.

2.2.3 Existing regulations concerning terrain avoidance responsibilities do not always present a consistent, clearly defined view of pilot/controller responsibilities. ICAO Doc 4444 paragraph appears to contradict the above mentioned Note 2 by placing responsibility on ATCOs for issuing altitude assignments that comply with state determined minimums:

“Except when specifically authorized by the appropriate authority, cruising levels below the minimum flight altitudes established by the State shall not be assigned.”


While it is clear that ATCO’s must assign altitudes that comply with state determined minimums, the assignment of an appropriate minimum altitude does not always ensure terrain clearance. For example, upon acceptance of an altitude clearance, pilots are often required to determine the necessary climb profile to clear all obstructions. However, structured vertical profiles are commonly affected by several factors, such as airspace capacity constraints, noise abatement, or fuel efficiency considerations which do not have any connection to terrain or obstruction avoidance. As the usage of published routings, such as SIDs, STARs and PBN procedures increases, more aircraft will be transitioned to and from these structured routings, increasing the safety risks associated with terrain responsibility ambiguities.

2.3 The CFIT accident at Cagliari, Italy has brought increased focus to the responsibilities assigned to pilots and controllers concerning visual approaches. In a controversial court decision following the accident, ATCOs were held responsible for not preventing the collision with terrain. Current language contained in ICAO Doc 4444 does not provide the type of clear, consistent message that is necessary to effectively define responsibilities for aircraft on visual approaches.

In reference to visual approaches, ICAO Doc 4444 states:

Paragraph “An IFR flight shall not be cleared for an initial approach below the appropriate minimum altitude as specified by the State concerned nor to descend below that altitude unless: c) the aircraft is conducting a visual approach.

Paragraph “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.”

Paragraph “An IFR flight may be cleared to execute a visual approach provided the pilot can maintain visual reference to the terrain.”


The above mentioned paragraphs indicate that State determined minimum altitudes are not applicable in the case of a visual approach and that the pilot must be able to maintain visual reference to the terrain. (It is important to note that only the pilot can determine whether or not he/she is able to keep the terrain in sight). But, the insertion of the language “Controllers should also take into consideration the prevailing traffic and meteorological conditions when initiating visual approaches” clouds the lines of responsibility by implying that controllers may be responsible for determining whether or not pilots can be expected to maintain visual reference to the terrain and successfully execute the visual approach. Also, the usage of ambiguous phrases such as “exercise caution,” “when there is reason to believe,” and “take into consideration” inhibits the development of a commonly understood set of responsibilities.

2.4 Existing IFATCA Policy addressing obstacle and terrain avoidance is limited in scope and does not address many of the issues discussed in this paper.

2.4.1 Existing IFATCA Policy concerning this subject is:


Route spacing standards should not be reduced below those that would otherwise be required purely because of the use of radar monitoring.

Radar monitoring should not be used as the means of providing separation with obstacles (terrain clearance) where aircraft are on their own navigation and below the Minimum Radar Vector Altitude (MRVA). Any escape procedure shall provide adequate terrain clearance from the point the aircraft is below the MRVA to the lowest defined altitude at which any such procedure can be initiated. States are required to assure this.

Any introduction of Performance Based Navigation PBN routes that are closely spaced should be subjected to safety analysis. Such a safety analysis may result in hazards being identified that require automated monitoring assistance for the controller to adequately mitigate the hazard.

Any introduction of closely spaced routes should ensure that controllers can, upon identification or notification of a deviation, carry out the necessary action so that the required separation minimum is not likely to be infringed.



3.1 Current IFATCA policy on terrain and obstacle avoidance does not adequately address terrain and obstacle avoidance responsibility issues among pilots and ATCOs.

3.2 There is a need to update some of the language in existing ICAO SARPS to reflect the current state of Air Traffic Services and to clarify terrain clearance responsibilities between pilots and controllers.

3.2 The increasing usage of PBN procedures in the arrival and departure phases of flight accentuates the need to clarify pilot/controller responsibilities concerning terrain and obstruction clearance. An area of particular importance involves those instances when the published vertical profile is waived or is otherwise not applicable.

3.3 The current ICAO direction concerning visual approaches, specifically paragraph of Doc 4444, creates confusion and ambiguity concerning pilot and controller responsibilities regarding the issuance of visual approaches.

3.4 Current ICAO provisions lack clear guidance concerning terrain and obstruction clearance responsibilities for aircraft on direct routings in procedural (non-surveillance) operating environments.


It is recommended that:

4.1 TOC continues its study of terrain clearance responsibility issues, resulting in the drafting of:

1) IFATCA policy statements as needed to mitigate any safety risks associated with current terrain and obstruction clearance practices.

2) Specific language recommendations to ICAO for establishing a clear, common understanding of pilot and controller terrain and obstruction avoidance responsibilities.


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  

December 22, 2019   134   Jean-Francois Lepage    2013    

Comments are closed.

  • Search Knowledgebase