Of the many factors involved in the process of communication, phraseology is perhaps the most important because it enables us to communicate quickly and effectively despite differences in language and reduces the opportunity for misunderstanding.
Standard phraseology reduces the risk that a message will be misunderstood and aids the read-back/hear-back process so that any error is quickly detected. Ambiguous or non-standard phraseology is a frequent causal or contributory factor in aircraft accidents and incidents.
International standards of phraseology are laid down in ICAO Annex 10 Volume II Chapter 5, ICAO Doc 4444 PANS-ATM Chapter 12 and in ICAO Doc 9432 - Manual of Radiotelephony. Many national authorities also publish radiotelephony manuals which amplify ICAO provisions, and in some cases modify them to suit local conditions.
This article deals with non-standard phraseology, which is sometimes adopted unilaterally by national or local air traffic services in an attempt to alleviate problems; however, standard phraseology minimises the potential for misunderstanding.
Where non-standard phraseology is introduced after careful consideration to address a particular problem, it can make a positive contribution to flight safety; however, this must be balanced with the possibility of confusion for pilots or ATCOs not familiar with the phraseology used.
Non-standard phraseology in Europe
Regulation 2016/1185 introduces some deviations from the standard ICAO phraseology at EU level:
- Flight levels which are whole hundreds (e.g. FL 100, FL 200, FL 300, etc.) are to be pronounced as "Flight level (number) hundred".
- Altimeter setting of 1000 hPa is to be pronounced as "One thousand".
- Transponder codes containing whole thousands are to be pronounced as "(number) thousand".
- For transfers of communication within one ATS unit, the call sign of the ATS unit may be omitted, when so authorised by the competent authority.
The UK CAA has adopted certain non-standard phraseology designed to reduce the chance of mishearing or misunderstanding RTF communications. This phraseology is not in accordance with ICAO but is based on careful study of the breakdown of pilot/controller communications. Some other European countries have also adopted similar non-standard phraseology.
The following paragraphs taken from the UK Manual of Radiotelephony summarise the main differences.
- The word ‘to’ is to be omitted from messages relating to FLIGHT LEVELS.
- All messages relating to an aircraft’s climb or descent to a HEIGHT or ALTITUDE employ the word ‘to’ followed immediately by the word HEIGHT or ALTITUDE. Furthermore, the initial message in any such RTF exchange will also include the appropriate QFE or QNH.
- When transmitting messages containing flight levels each digit shall be transmitted separately. However, in an endeavour to reduce ‘level busts’ caused by the confusion between some levels (100/110, 200/220 etc.), levels which are whole hundreds e.g. FL 100, 200, 300 shall be spoken as “Flight level (number) HUNDRED”. The word hundred must not be used for headings.
- Examples of the above are:
- “RUSHAIR G-BC climb flight level wun too zero.”
- “RUSHAIR G-BC descend to altitude tree tousand feet QNH 1014.”
- “RUSHAIR G-BC climb flight level wun hundred.”
- “RUSHAIR G-BC turn right heading wun wun zero.”
Non-standard North American phraseology
A particular example of non-standard phraseology which is in regular use in North America is the instruction “taxi into position and hold”, (which has the same meaning as the ICAO standard phrase “line up and wait”).This can be confused with the old ICAO phraseology “taxi to holding position” (which means taxi to, and hold at, a point clear of the runway).
Use of this non-ICAO standard phraseology is fail-safe in North America, but in Europe can lead to an aircraft taxiing onto the runway when not cleared to do so. To overcome this problem ICAO has amended its phraseology to "taxi to holding POINT".
Non-standard Phraseology in Abnormal/Emergency Situations
It is often necessary for pilots and controllers to revert to non-standard phraseology in abnormal and emergency situations. The extent to which this occurs, and leads to effective communication, will depend upon the quality of the both speech delivery and language proficiency of those involved.
Neither Standard, Nor Approved
Sometimes controllers and pilots use phraseology that is neither standard, nor approved by a national civil aviation authority. The reasons for this may be various, e.g. poor knowledge or training, phrase that is rarely used, personal experience or preference, etc. The main difference between approved and non-approved phraseology is that the latter has not undergone any safety impact assessment.
There are several major risks associated with such phraseology:
- The other party may not hear the message correctly. When standard phraseology was developed, special attention was given to choosing words and phrases that sound distinctly different and therefore cannot be confused under any readability circumstances. When replacing standard phraseology with their own people do not perform thorough research as to whether their custom phrase may sound similarly to another one.
- The other party may not understand the message. This may be due to e.g. using phrasal verbs or other words that are not commonly known. The different levels of knowledge of the English language contributes to this as well.
- The message may be ambiguous, i.e. the transmitting person may mean one thing and the other one may understand something else, as was the case with the vehicle incursion at Perth in 2012 or in an incident at Toronto in 2016.
Examples of unofficial "phraseology" (the list is not exclusive):
- Ten/eleven thousand (instead of one zero thousand or one one thousand). This was considered by the investigation to be the cause of an incident in 2011
- Read you five by five (or any other X by Y combination) instead of Read you (number)
- ARL10 pronounced as Airline ten (instead of Airline one zero)
- Light chops, smooth ride, what's the ride, instead of phrases containing the word turbulence
- Affirmative instead of affirm (note that affirmative may, under certain low-readability circumstances, be confused with negative due to having the same ending)
- Double and triple (instead of pronouncing each digit separately)
- Keep heading, speed, etc. (instead of continue or maintain)
- Up and down instead of climb and descend
- Pronouncing 9 as nine instead of niner which may lead to confusion with 5
- Amending clearance staring with While we wait (which can be understood as line up and wait), which was considered as a contributor in a runway incursion event
- Using take-off instead of departure in situations where no take-off clearance is issued or cancelled. This has caused a number of occurrences, e.g. an accident in 1977 and an incident in 2008
- A description of an ACAS manoeuvre instead of the standard TCAS RA. Such a description may be lengthy, unstructured, incorrect or incomplete, and therefore the controller may request a repetition or clarification
Note that in all the above cases there is a standard alternative to the words and phrases used.
Accidents and Incidents
The following events include "Phraseology" as a contributing factor:
On 20 October 2021, a Bombardier CRJ1000 making a LNAV/VNAV approach at Nantes using Baro-VNAV minima read back a significantly incorrect QNH which was not noticed by the controller. The crew then flew the approach approximately 530 feet below the procedure vertical profile which led to the MSAW system being activated and advised to the flight. The crew response was delayed until the controller had twice repeated the correct QNH after which the error was recognised and the vertical profile corrected. The investigation noted that neither the operator’s procedures nor aircraft instruments allowed straightforward crew detection of their error.
On 24 October 2021, a Shorts SD360 intending to land at the international airport serving Ndola did so at the recently closed old international airport after visually navigating there in hazy conditions whilst unknowingly in contact with ATC at the very recently opened new airport which had taken the same name and radio frequencies as the old one. The Investigation found multiple aspects of the airport changeover and re-designation had been mismanaged, particularly but not only failure to publish new flight procedures for both airports and ensure that NOTAM communication of the changes internationally had been effective.
On 18 March 2016, at Cheongju in thick fog at night with visibility just above the minimum permitted for landing, an Airbus A319 began to enter the departure runway as a Boeing 737-800 was landing on it as cleared. Only when the 737 crew saw the other aircraft ahead when still at high speed were they able to initiate a lateral deviation and thereby avoid a collision by creating a 3 metre separation between their aircraft and the A319.The Investigation found that the A319 had exceeded the taxi clearance given by Ground Control but noted that this used poor phraseology.
On 23 May 2022, an Airbus A320 came extremely close to collision with terrain as the crew commenced a go around they did not obtain any visual reference during a RNP approach at Paris CDG for which they were using baro-VNAV reference to fly to VNAV/LNAV minima. The corresponding ILS was out of service. The Investigation has not yet completely established the context for the event but this has been confirmed to include the use of an incorrect QNH which resulted in the approach being continued significantly below the procedure MDA. Six Interim Safety Recommendations have been issued.
On 5 August 2019, a Cessa 560XLS touched down in runway undershot at Aarhus whilst making a night ILS approach there and damage sustained when it collided with parts of the ILS LOC antenna caused a fuel leak which after injury-free evacuation of the occupants then ignited destroying most of the aircraft. The Investigation attributed the accident to the Captain’s decision to intentionally fly below the ILS glideslope in order to touch down at the threshold and to the disabling of the EGWPS alerting function in the presence of a steep authority gradient, procedural non-compliance and poor CRM.
AGC Safety Letters:
EUROCONTROL Action Plan for Air-Ground Communications Safety: