SERA.15015 Construction of CPDLC messages

(a)The text of CPDLC messages shall be composed in standard message format, in plain language, or in abbreviations and codes. Plain language shall be avoided when the length of the text can be reduced by using appropriate abbreviations and codes. Non-essential words and phrases, such as expressions of politeness, shall not be used.

(b)The air traffic controller and the pilot shall construct CPDLC messages using standard message elements, free text message elements, or a combination of both. The use of free text message elements by air traffic controllers or pilots shall, be avoided.

(c)When the implemented CPDLC message set does not provide for specific circumstances, the competent authority may determine, in consultation with operators and other air traffic services providers, that it is acceptable to use free text message elements. In such cases, the competent authority concerned shall define the display format, intended use and attributes for each free text message element.

(d)The composition of a CPDLC message shall not exceed five message elements, only two of which may contain the route clearance variable.

(e)Construction of multi-element CPDLC messages:

(1)When a multi-element CPDLC message requires a response, the response shall apply to all message elements.

(2)When a single message element clearance or any part of a multi-element clearance message cannot be complied with, the pilot shall send an “UNABLE” response for the whole message.

(3)The controller shall respond with an “UNABLE” message that applies to all elements of the request when no element(s) of a single or multi-element clearance request can be approved. The current clearance(s) shall not be restated.

(4)When a multi-element clearance request can only be partially accommodated, the controller shall respond with an “UNABLE” message applying to all the message elements of the request and, if appropriate, include a reason and/or information on when clearance may be expected.

(5)When all elements of a single or multi-element clearance request can be accommodated, the controller shall respond with clearances corresponding to each element of the request. This response shall be a single uplink message.

(6)When a CPDLC message contains more than one message element and the response attribute for the message is “Y”, when utilised, the single response message shall contain the corresponding number of replies in the same order.