Terrain and Obstacle Charting

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Terrain and Obstacle Charting

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

WP No. 86

Terrain and Obstacle Charting

Presented by TOC


Work on SID- and STAR-phraseology changes and cancellation of level restrictions identified fundamental issues regarding ICAO procedures for terrain and obstacle clearance, primarily whether a restriction is ATC or terrain based. This WP proposes policies to clarify charting.


1.1  Several new procedures, trends and developments in airspace design, and constant technological evolution make policies regarding responsibility for separation from terrain and obstacles more urgent and complex. As advanced airspace design and procedures are developed that allow aircraft to fly in IMC conditions near terrain, the role of the controller in terrain clearance monitoring becomes more important.

1.2  ICAO Document 9613 (Performance Based Navigation) makes an affirmative case for the use of advanced procedure design as a tool for terrain avoidance and states:

From an obstacle clearance perspective, the use of RNP applications may allow or increase access to an airport in terrain-rich environments where such access was limited or not previously possible.

1.3  Therefore we must have clear and unambiguous information available to ATS providers, controllers, pilots and operators with regard to published restrictions incorporated in procedure design.

1.4  CFIT (Controlled Flight into Terrain) accidents are avoidable if pilots and controllers have access to adequate information and a clear understanding of responsibilities.

1.5  Two recent high profile accidents raising terrain responsibility issues both coincidentally 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 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 issue of published level-restrictions was complex without consistent rules for distinguishing between terrain based restrictions and restrictions designed to manage traffic flow. This distinction is necessary because restrictions for operational, airspace, military and similar reasons may be negotiable, those for terrain are not and always need to apply.

2.2  ICAO PANS-ATM states:

“Published minimum levels based on terrain clearance shall always be applied”.

2.3  However there is not a consistent standard on how that information is made available to the pilot and controller. Aeronautical charts are not required to include the purpose of a restriction. (i.e ATC or terrain).

Issue 1: Objectives of ATS do not include terrain

The “Note” – When studying the documents, one comes across the following note several times:

2.4  (Annex 4, chapter 21.1. Note / PANS-ATM 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.

2.5  In the case of level restrictions, the aircraft on an ATS Route may have a level restriction cancelled. Without awareness that the subject restriction is terrain based, the pilot cannot ensure the clearance is safe as described in the note.

2.6  While the “note” states that the provision of air traffic control service does not include prevention of collision with terrain, the definition of an ATS route clearly places the determination of the lowest safe altitude with the ATS Authority. When looking at definitions, according to DOC 4444 (definitions), ICAO states:

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.

Issue 2: Reasons for altitude restrictions in SIDs/STARs and other charts are not distinguishable from terrain

2.7  Existing IFATCA policy concerning Instrument Departures and Arrivals is as follows (ATS 3.28, Page 3 2 3 30):

For aircraft on SIDs and STARs, all level change clearances shall explicitly indicate whether the published vertical profile is to be followed or not, provided that controller workload does not increase beyond an acceptable level.


The possibility that an aircraft will miss a vertical restriction is always of great concern, but it is even more so when terrain is a factor. The greatest issue in the SID/STAR phraseology debate has been the concern that a terrain-based restriction will be unintentionally cancelled. This is why the use of explicit clearances to retain or cancel level restrictions is so vital.

2.8  But today’s SID and STAR charts indicate nothing about the nature of the altitude restrictions. Terrain clearances must never be cancelled, but procedural and airspace clearances can be cancelled at the discretion of the controller depending on airspace and/or traffic. The indication of terrain restrictions would be helpful not only today where there is a lack of clarity, but in the future when routes will become more dynamic, and further automation is introduced.

2.9  The issue is not limited to SID and STAR charts but also applies to enroute publications. There are variations around the world in how charts are published and several different minimum altitude values and definitions are applied. There is a need for a consistent and clear worldwide standard to indicate terrain based restrictions.

2.10  A potential approach could be to bring the old concept of MSA (Minimum sector altitude) around a Navaid into the 21st century, supported by the right tools. Ultimately, modern avionics should take care of validating a 4D-trajectory against a DEM (Digital Elevation Model) of planet earth in realtime. In the meantime, an intermediate improvement step is needed.


3.1  CFIT (Controlled Flight Into Terrain) should be regarded as one of the most preventable categories of aircraft accidents. 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.

3.2  Current charting requirements do not distinguish between terrain based and other restrictions. This does not provide pilots, controllers, and operators with sufficient information whether a given level restriction can be safely cancelled.


It is recommended that:

4.1  IFATCA Policy is:

All published altitude restrictions should indicate whether they apply for reasons of terrain avoidance.

and is included in the IFATCA Technical and Professional Manual.

4.2  IFATCA Policy is:

IFATCA should work with ICAO to facilitate the development of ICAO Standards to require any published altitude restriction to indicate whether it applies for reasons of terrain avoidance.

and is included in the IFATCA Technical and Professional Manual.


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   683   Jean-Francois Lepage    2014    

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