57TH ANNUAL CONFERENCE, Accra, Ghana, 19-23 March 2018
WP No. 86
Phraseology on Ground Based Safety Nets
Presented by TOC
Proposals on implementation of standard phraseology and common procedures for ground based safety nets in conjunction with approach path monitoring (APM).
1.1 This topic is currently being developed by IFATCA as a response to an IFALPA discussion paper on “Phraseology for Safety Nets”. It is known that ICAO is currently working on this subject through its ATMOPS Panel. It was suggested by IFALPA that the following safety nets should have appropriate phraseology and working procedures: APM (Approach Path Monitoring), MSAW (Minimum Safe Altitude Warning), and E-GPWS (Enhanced Ground Proximity Warning System).
1.2 As MSAW (PANS-ATM, 16th edition, 2016, chapter 12, 12.6, page 12-41 ICAO’s Manual of Radiotelephony Doc 9432, 4th edition, 2007, chapter 6, 6.7, page 6-7) already has both phraseology and procedures in force, this paper will focus on APM. Another ground-based safety net – APW (Area Proximity Warning) – is also included as an example.
2.1 The subject of phraseology on safety nets was addressed during an IFALPA ATS Committee Meeting in November 2016. The paper development of Phraseology for Safety Nets was then presented. It was noted that the ICAO ATMOPS Panel had also been tasked with developing this subject. It was suggested that the currently implemented safety nets, ground based and airborne, need to have an appropriate legal and operational framework in terms of:
1. Phraseology (“what” to say)
2. Procedures (“who” and “when”)
3. Legal liability (who’s responsible for terrain and obstacle clearance)
2.1.1 IFALPA made the proposal to focus on three safety nets: two ground-based being MSAW (Minimum Safe Altitude Warning) and APM (Approach Path Monitoring) and one airborne, E-GPWS (Terrain Avoidance and Warning System).
Note- as E-GPWS is an airborne safety net, this paper will only address the topic of ground based safety nets.
2.1.2 There was considerable discussion as to the need for phraseology but it was agreed to develop this further.
2.1.3 These safety nets generate alerts and it was suggested that the alerts should remain as consistent as possible to avoid any confusion either for pilots or for controllers. IFALPA decided to develop a position paper for their next meeting and it was envisaged that the paper would also assist the ICAO ATMOPS Panel in their development of phraseology for safety nets (IFALPA ATS Committee Oct 2016 Meeting Minutes).
2.1.4 At the time of writing, IFALPA’s position paper hasn’t been made available to the TOC.
The importance of safety nets for aviation safety
2.2 ICAO has developed the Global Air Navigation Plan (GANP), which spans a period of 15 years and aims to help Member States to upgrade current technologies with the newest ones. All the system’s upgrades are known as Aviation System Block Upgrades (ASBU). Gradually all ANSPs should upgrade their existing ATM systems with safety enhancing functionalities such as safety nets. The ASBUs were formulated with the goal of regional performance improvements, and the ASBU concept has four main performance improvement areas:
– airport operations;
– global interoperable systems and data
– optimum capacity and flexible flights
– and efficient flight paths
However, there is no standardization in place for what system functionalities should be implemented and all ANSPs will upgrade their current ATM systems to fit their own specific operational requirements.
2.2.1. Comprised within these upgrades are the safety nets: STCA, MSAW, APW and APM. These systems are part of the ASBU-process (ASBU 0 and ASBU 1)(ASBU Implementation Monitoring (ICAO, EUROCONTROL, edition 1.0, 8 Nov 2017)). In an operational environment, these functionalities trigger warnings that notify controllers and pilots about potentially dangerous situations such as possible Controlled Flight Into Terrain (CFIT).
2.2.2. CFIT is an accident in which there is in-flight collision with terrain, water, or obstacle without indication of loss of control. According to IATA from 2005 to 2014 there were 67 CFIT causing 1,346 fatalities and 99% hull loss rate (IATA Safety Report 2016, published 2017). It was observed that in circumstances where controllers need to pass information to pilots about system alerts they “rarely used […] phraseologies” and consequently, “were likely to fall back on plain language” (IFALPA ATS Com Nov 2016 Meeting Minutes)
2.2.3 The safety nets subject is being addressed by:
– IFATCA: through this Working Paper
– ICAO: by monitoring the ASBU implementation state for their Member States; also by currently developing the Ground Based Safety Nets Manual. (expected to be published in early 2018)
– ICAO (ATMOPS Panel): currently developing phraseologies on safety nets.
– IFALPA ATS Committee: writing a position paper and pushing for the establishment of standardized phraseology and work procedures.
– EUROCONTROL: through their guidance material on APM, MSAW and numerous briefing notes
2.3. Legal liability sets the boundaries for the responsibility for the safety of the flight. IFATCA’s policy related to the subject is found in IFATCA’s Technical and Professional Manual:
|“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.”|
(ATS 3.38 Terrain and obstacle clearance responsibilities – IFATCA TPM 2017 ed.)
2.4 ICAO Doc 4444 clearly states the responsibilities of air traffic controllers in an event of an MSAW:
|When a MSAW is generated in respect of a controlled flight, the following action shall be taken without delay:
a) if the aircraft is being vectored, the aircraft shall be instructed to climb immediately to the applicable safe level and, if necessary to avoid terrain, be assigned a new heading;
b) in other cases, the flight crew shall immediately be advised that a minimum safe altitude warning has been generated and be instructed to check the level of the aircraft.
(ICAO (November 2016) PANS-ATM (DOC 4444) 16th edition, chapter 15, 15.7.4 MSAW procedures)
2.5 As for Annex 11 the responsibilities of an air traffic controller are “to prevent collisions between aircraft” and “to provide advice and information useful for the safe and efficient conduct of flights” (ICAO Annex 11, 14th edition, 2016, chapter 2, 2.2 “Objectives of air traffic services”, page 2-2). However they “do not include prevention of collision with terrain” and “the procedures described […] do not relieve pilots of their responsibility to ensure that any clearance given by the air traffic control units are safe in this respect” (ICAO PANS-ATM, 16th edition, 2016, Foreword, 2. Scope and purpose, Note 2, page 10). Should a safety alert be triggered for a specific flight, the controller has the responsibility to inform the crew of the flight. The responsibility for the safety of the flight should rest with the crew.
2.6 Controllers will become responsible for the terrain clearance during vectoring or when giving a direct routing that would take the aircraft off an ATS route (ICAO PANS-ATM, 16th edition, 2016, Chapter 8, 18.104.22.168, page 8-13). In such circumstances the controller “shall issue clearances such that the prescribed obstacle clearance will exist at all times until aircraft will resume own navigation”. Therefore, upon receiving a warning from a safety net, the controller will provide information but will also issue instructions if deemed necessary.
2.7 Last but not least, both air traffic controllers and pilots are invited to exercise their “best judgement”(ICAO PANS-ATM, 16th edition, 2016, Chapter 15, 22.214.171.124, page 15-1) in the interest of aviation safety. It is of high importance to highlight the fact that safety nets are seen as ATM system “add-ons” so that they shouldn’t change how the operators (controllers and pilots) perform their duties. In this respect the use of safety nets should only support the controller’s work without interfering with the decision-making process. Ground based safety nets are to be used only to increase the controller’s situational awareness. Ground based safety nets shall not be relied upon in order to meet the target level of safety acceptable to the ANSP. (ICAO GBSN Manual, draft, 2017, 126.96.36.199)
Phraseology – standardised language for air-ground communications
2.8 Aviation phraseology was developed and implemented to insure a standardized vocabulary and might as well be considered an international language for the entire aeronautical community. Technically it is a manner of organizing words and phrases into elements with a specific meaning.
2.8.1 ICAO phraseologies were developed to provide efficient, clear, concise, and unambiguous communications. ICAO DOC 4444, ICAO DOC 9432 Manual of radiotelephony, ICAO Annex 11 and ICAO Annex 10, Vol II all contain the phraseology to be used in R/T. The aforementioned documents provide the framework for aeronautical communications and also describe the specific phrasing to be used in various traffic situations, from standard words and phrases to emergency phraseology.
2.9 Apart from the specifications for the MSAW function, the ICAO documents do not contain any other standard phraseology for the rest of the safety nets discussed in this paper, nor do they refer to the context in which they should be used. At the same time, ICAO acknowledges that “the importance of using correct and precise standardized phraseology cannot be overemphasized.”(ICAO Manual of Radiotelephony Doc 9432, 4th edition, 2007, chapter 2.1, page 2-1) However, the ICAO “phraseologies […] are not intended to be exhaustive, and when circumstances differ, pilots, ATS personnel and other ground personnel will be expected to use appropriate subsidiary phraseologies which should be as clear and concise as possible and designed to avoid possible confusion by those persons using a language other than one of their national languages” (ICAO’s Manual of Radiotelephony Doc 9432, 4th edition, 2007, chapter 3, .2, page 3-1).
2.9.1 ICAO has already established standards for language proficiency. Since 14th of July 2003, air traffic controllers, pilots and other aviation personnel should at least attain ICAO Level 4 English proficiency (ICAO Annex 1 “Personnel Licensing” 11th edition, 2011, Appendix 1, 1-1).
188.8.131.52 A proficient speaker of English holding an ICAO level 4 certificate would have demonstrated a “vocabulary range and accuracy are usually sufficient to communicate effectively on common, concrete, and work-related topics” and that his/her “comprehension is mostly accurate on common, concrete, and work- related topics when the accent or variety used is sufficiently intelligible for an international community of users”(ICAO Annex 1 “Personnel Licensing” 11th edition, 2011, Attachment A, ICAO descriptors for level 4, table). In this respect, the lack of phraseologies for safety nets and specific traffic situations might be overcome with the use of subsidiary phraseologies.
184.108.40.206 While a Level 4 English speaker can theoretically formulate a plain-English phrase in response to a safety net alert, it is known that the startle factor affects response time and effectiveness so simple standardized phraseology that avoids the need for controllers to formulate a unique response and gives pilots a readily understandable alert is preferred, given the safety-critical nature of situations that activate a safety net.
Safety nets – definition, functions
2.10 Safety nets are ground-based or airborne system functions that alert controllers or pilots to an increased risk to flight safety and help draw attention to potentially hazardous situations. They are an additional layer of safety functions designed to provide extra protection to the existing layers and they are designed to prevent imminent or actual hazardous situations from developing into major incidents or even accidents. In doing so they provide additional safety barriers in the overall ATM system (EUROCONTROL Safety Nets- A guide for ensuring effectiveness, 2017).
2.11 The ground-based safety nets Minimum Safe Altitude Warning (MSAW) and Approach Path Monitor (APM) help prevent CFIT accidents, and should work jointly with their airborne counterpart the E-GPWS.
According to ICAO Manual of GBSN (draft), 2017, Page 10:
|Ground-based safety net. A ground-based safety net is a function within the ATM system alerting air traffic controllers to an increased risk to flight safety requiring immediate attention.|
|220.127.116.11 GBSNs are designed, configured and used to make a significant positive contribution to safety including:
a) prevention of collisions;
b) avoidance of controlled flight into terrain (CFIT) accidents;
c) avoidance of controlled flight into terrain (CFIT) accidents during final approach; and
d) prevention of incidents and accidents arising from unauthorized penetration of an airspace volume.
(ICAO Manual of GBSN, draft, 2017, page 13)
2.11.1 Some of the precursors of CFIT are wrong altimeter reference settings, flight below minimum sector altitude and deviation below glideslope. These precursors now have an additional safety countermeasure in the currently presented safety nets.
Ground based safety nets (MSAW, APM)
2.12 Ground-based safety nets are part of the ATM system. Using primarily ATS surveillance data, they are designed to monitor the operational environment and to generate timely alerts if the safety of the flight is at risk; they provide warning times of up to two minutes (EUROCONTROL Guidelines for MSAW Part I – Concept and Requirements, 1st ed, 2017). Upon receiving an alert, air traffic controllers are expected to immediately assess the situation and take appropriate action.
2.13 Both MSAW and APM are implemented in airspace where ATS surveillance services are provided and where the risk of controlled flight into terrain exists (EUROCONTROL Specifications for APM Part I – Concept and Requirements, 1st ed, 2017). Both systems should work in concert with their airborne equivalent, E-GPWS (EUROCONTROL Safety Nets Ensuring Effectiveness Guide, 2017).
2.13.1 Nowadays the functionalities of the MSAW and APM are put to good use in Switzerland, where the terrain configuration represents a potential hazard to the flight operations. This will be detailed in later in this paper.
2.14 As mentioned before, a Manual on Ground Based Safety Nets (GBSN) is currently being developed by ICAO. The draft version provided to the TOC describes more the technicalities of these systems (planning, implementation, operation, optimization) rather than the actual working procedures. The manual does not include any phraseology to be used for the respective systems. Ideally the next steps should be followed by a controller when receiving an alert and should be complemented by the pilot’s ensuing actions:
2.14.1 The figure above represents the chain of actions that take place once a GBSN alert is generated at the controller working position. The following safety nets are presented in this manual: STCA (Short Term Conflict Alert), MSAW (Minimum Safe Altitude Warning), APM (Approach Path Monitoring) and APW (Area Proximity Warning). This paper will focus on MSAW and APM.
2.15 Minimum Safe Altitude Warning (MSAW) is a GBSN. Its sole purpose is to enhance safety and its presence is ignored when calculating sector capacity. The system uses ATS surveillance data and is designed, configured and used to help avoid the risk of controlled flight into terrain accidents by generating, in a timely manner, an alert of aircraft proximity to terrain or obstacles. In literature, MSAW is known as “the controller’s GPWS” (Ground Proximity Warning System).
2.15.1 Approach Path Monitor (APM) is also a GBSN its sole purpose being to enhance safety. APM too uses ATS surveillance data and is designed, configured and used to help avoid the risk of controlled flight into terrain accidents by generating, in a timely manner, an alert of aircraft proximity to terrain or obstacles during final approach (ICAO GBSN Manual, draft, 2017, chapter 3, 18.104.22.168, page 35). It is not uncommon to have MSAW and APM configured to work together, with different but complementary functions and also serving different parts of airspace volume (example MSAW may “cover” the initial approach part and APM, the final approach – see the Geneva example).
2.15.2 Some of the differences and similarities between MSAW and APM:
2.16 As for integrating GBSN into operational environments, the draft manual specifies that “comprehensive GBSN procedures should be developed, integrated into operations and reviewed periodically” (ICAO GBSN Manual, draft, 2017, chapter 1, 1.2.1, page 19). Therefore, the need for implementing phraseologies and procedures.
“The ICAO provisions for the final design and specification of the ground based safety nets must be able to support the procedures defined the DOC4444, sections 15.7.2 and 15.7.4.” (ICAO GBSN Manual, draft, 2017, chapter 3, 22.214.171.124, page 29)
Note – safety nets should not interfere with current work procedures for ATC and their contribution to ATM system should not be relied upon in order to meet the target level of safety acceptable to the ANSP.
Phraseology and procedures
2.17 According to the specifications of PANS-ATM, a controller is expected to respond to a MSAW in the following manner:
|126.96.36.199 In the event an MSAW is generated in respect of a controlled flight, the following action shall be taken without delay:
a) if the aircraft is being vectored, the aircraft shall be instructed to climb immediately to the applicable safe level and, if necessary to avoid terrain, be assigned a new heading;
b) in other cases, the flight crew shall immediately be advised that a minimum safe altitude warning has been generated and be instructed to check the level of the aircraft.”
(ICAO (November 2016) PANS-ATM (DOC 4444) 16th edition chapter 15, 15.7.4 Minimum safe altitude warning (MSAW) Procedures)
2.17.1 Crew response to an MSAW generated ATC action is included in the cited fragment. Basically, the crew will comply with the ATC’s instructions, advise if unable and also check and/or report required information.
2.71.2 Existing phraseology for MSAW is contained both in PANS-ATM and in DOC 9432:
|12.6.1 Alerting phraseologies
188.8.131.52 LOW ALTITUDE WARNING (aircraft call sign) LOW ALTITUDE WARNING, CHECK YOUR ALTITUDE IMMEDIATELY, QNH IS (number) [(units)]. [THE MINIMUM FLIGHT ALTITUDE IS (altitude)].
184.108.40.206 TERRAIN ALERT (aircraft call sign) TERRAIN ALERT, (suggested pilot action, if possible).”
(ICAO (November 2016) PANS-ATM (DOC 4444) 16th edition chapter 12)
2.18 Unlike for MSAW (PANS-ATM, 16th edition, 2016, chapter 15, 15.7.4, page 15-20), there are no controller actions nor phraseology prescribed by ICAO in case of an APM warning. In cooperation with IFALPA, TOC considers a few requirements recommendable.
2.18.1 In a work environment fitted with a safety net such as the APM, controllers should always inform the pilots about an APM warning. As time is limited, controllers should also require the crew to check their altitude and/or the altimeter setting, providing also the instant QNH value. In some instances, the controller, using his/her best judgement, might instruct the crew to climb or to go around in the interest of safety.
2.18.2 Pilots should respond in the manner described for MSAW alerts, by complying with the controller’s instructions, advising if unable and also checking and/or reporting required information.
2.18.3 If considered operationally plausible and taking into account both controller’s duty of care and the legal liability, TOC suggests that when adapting a new phraseology for APM, ICAO should consider making use of the existing MSAW phraseology.
2.18.4 As an aircraft is much closer to terrain and obstacles when an APM-alert occurs, consideration should be given to developing a “more directive” phraseology, compared to the one used for an MSAW alert.
Overview on the safety nets and associated phraseologies used in France
2.19 The use of safety nets and their associated phraseologies in France were presented in an information paper during ICAO’s ATMOPS Panel (WG5) in September 2017 in Montreal. The paper presented work procedures and phraseologies for STCA, APW (Area Proximity Warning), MSAW and also an airport safety net, RIMCAS.
2.19.1 For the given ANSP, the responsibilities of controllers are to issue a safety alert to aircraft in all classes of airspace when they become aware that an aircraft is in unsafe proximity to:
a. other aircraft (STCA)
b. terrain (MSAW)
c. runway incursion (RIMCAS) or
d. active restricted or prohibited areas (APW)
2.19.2 It is important to mention that MSAW is activated only in terminal areas and that phraseologies depend on whether the aircraft is being vectored or not:
1. Aircraft being vectored:
TERRAIN ALERT (vertical or lateral avoiding action).
2. Aircraft not being vectored:
TERRAIN ALERT, CHECK YOUR ALTITUDE IMMEDIATELY, Q_N_H IS (number of QNH).
2.19.3 For airports using APW as terrain alert, controllers use the same phraseologies as for MSAW. The APW function warns the controller about unauthorized penetration of an airspace by generating, in a timely manner, an alert of a potential or actual infringement of the required distance to that airspace volume.(ICAO GBSN Manual – draft, chapter 1, 1.1.2, page 13) Just like MSAW and APM, the APW is a GBSN, using surveillance data, with the sole purpose is to enhance safety and its presence is ignored when calculating sector capacity.
2.19.4 One of these airports is Paris Charles du Gaulle where due to the airport configuration, controllers make consistent use of APW. The landing runways are only 2 NM away from each other whereas the minimum radar separation is 3 NM so controllers use APW to monitor GP interception “from above” or “from below”. CDG airport provides published approach charts for this particular runway configuration.
Use of GBSN (MSAW, APM, APW) in Switzerland
2.20 Our ATC colleagues from Switzerland kindly provided some insight on the work procedures associated with the use of APM, APW and MSAW at their units, as follows: – Geneva airport (Mr. Verly Benoit, Skyguide, President of TWR and APP Association, Geneve): APM and MSAW are used combined as one set, generically called MSAW.
The system is configured to serve two different functions:
a. Generally the MSAW provides an alert in case an aircraft significantly deviates below the minimum vectoring altitude.
b. It also provides an alert when an aircraft on the Geneva final approach segment deviates below a user-defined approach profile and this is the effect of the APM function (EUROCONTROL Guidance Material for Approach Path Monitor, Appendix D-1: Enhancement for APM for Geneva, 1st edition, 19 May 2009, chapter 1, 1.1, page 3).
The phraseology for this functionality is the same as is used for MSAW: a safety alert shall be issued to the crew. The next actions depend on pilot’s decision and are usually corroborated with ATC information and instructions. The function can be deactivated as needed for visual approach, visual departure or visual descent.
Zurich (Mr. Christoph Gilgen, former ATC Geneva, Switzerland, former IFATCA Surveillance Panel ICAO, frequent member of TOC): APW function is used like a “wall” (its defined airspace volume incorporates the high terrain in the vicinity of the airport) so that it alerts the controller if an aircraft during the approach phase (normally on downwind for the two Northern runways) comes close to the defined volume of airspace with high obstacles situated behind. The next actions depend on pilot’s decision and are usually corroborated with ATC information and instructions.
3.1 In the ever-changing world of civil aviation, in a safety centred environment, IFATCA and IFALPA strongly agree that the use of standard phraseology by aviation professionals will improve communication and reduce confusion in R/T. Introducing new ATM System functions (ASBU) in the form of ground based safety nets requires adapting old phraseologies and developing new phraseologies and procedures that would ideally cover all traffic scenarios. Such phraseologies should leave no room for misunderstanding on either side of the controller-pilot communication loop.
3.2 In respect to the currently available safety nets, standard phraseology and specific procedures are established only for the MSAW function.
3.3 Ground based safety nets are to be used only to increase the controller’s situational awareness. Ground based safety nets shall not be relied upon in order to meet the target level of safety acceptable to the ANSP. (ICAO GBSN Manual, draft, 2017, 220.127.116.11, page 35)
3.4 For newly implemented ground based safety nets (APM, APW) standard phraseologies should be developed in the image of those already used for the MSAW function. Consideration should be given to provide an operational framework for the use of safety nets including phraseology on specific traffic situations. IFATCA deems the MSAW guidelines of the PANS-ATM (4444) as a good and valid example.
4.1 It is recommended that IFATCA policy is:
When implementing ground based safety nets, due consideration should be given to the common use of phraseology and procedures to be used in the operation of these safety nets.
And is included in the IFATCA Technical and Professional Manual.
ICAO Annex 1 “Personnel Licensing” 11th edition, 2011
ICAO Annex 6 Operation of Aircraft (Part I 10th edition, 2016)
ICAO Manual for GBSN (ground based safety nets), draft version, 2017
ICAO Phraseology Reference Guide All Clear AGC Safety Initiative (launched in 2004 by EUROCONTROL Safety Team, available on Skybrary site)
ICAO ATMOPSP/WG/5 Information Paper “Safety nets and associated phraseology in France” presented by Claire Navarro, September 2017
ICAO Annex 10 (Aeronautical Telecommunications Vol II- 7th edition, 2016)
ICAO Annex 11 (Air Traffic Services) 14th edition, 2016
ICAO PANS-ATM (Doc 4444), 16th edition, 2016
ICAO Manual of Radiotelephony Doc 9432, 4th edition, 2007
IFATCA’s Technical and Professional Manual, 2017 edition
EUROCONTROL specifications for APW (0.5 edition, May 2009) MSAW (1st edition, 22 November 2007), APM (0.5 edition, May 2009)
EUROCONTROL Guidance Material for Approach Path Monitor, Appendix D-1: Enhancement for APM for Geneva, 1st edition, May 2009
Guidance material EUROCONTROL Guidelines for APW (1st edition, 18 January 2017), MSAW (1st edition, 18 January 2017), APM (1st edition, 18 January 2017)
EUROCONTROL Safety Nets A guide for ensuring effectiveness, August 2017
EAM4/GUI6 EUROCONTROL Explanatory Material on Ground Based Safety Nets (1st edition, 15 April 2010)
Approach and Landing Accident Reduction (ALAR) Toolkit, Skybrary. EUROCONTROL (content control), 2016
NetAlert Newsletters (21st edition 2016)
CAP 789 CFIT Education and Training Aid 2nd edition 18 February 2011
IATA CONTROLLED Flight Into Terrain Accident Analysis Report (2010-2014)
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http://www.eurocontrol.int/articles/minimum-safe-altitude-warningmsawguidelines date accessed: 10 January 2018
http://www.eurocontrol.int/publications/eurocontrol-guidelines-minimumsafealtitude-warning-msaw-part-i-iii date accessed: 10 January 2018
http://www.eurocontrol.int/service/safety-nets-support date accessed: 10 January 2018
Last Update: October 1, 2020