CPDLC Air Initiated Continuity

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The CPDLC Air Initiated Continuity is a part of the performance statistics gathered for air initiated transactions. It is the probability that an air initiated CPDLC transaction completes before the expiration timer expires (270 seconds). Note downlink messages which are responded to automatically (e.g. Current Data Authority exchanges) are not considered in the calculation of continuity; they complete almost immediately and so would skew the results and do not really reflect the continuity of the service as perceived by the pilot.

All air initiated requests and associated closing responses are analysed. For transactions that do not complete within the permitted delay (270s) an expired event is counted.

CPDLC Air Initiated Transactions Continuity = 1 – (number of air initiated CPDLC expired events / total number of air initiated CPDLC requests made)


Expired events can occur for one of two reasons: i) the transactions are completed by a uplinked error message indicating that the transaction could not be completed due to some temporary problem with the system (see below for the list of such error messages), or ii) because no closing response is received within 270 seconds (the expiration timer).

Note transactions that do not complete as the result of Provider Abort are not considered as expired events, since in the definition of Continuity the communication system has to be available when the transaction is initiated and in the case of a PA the communication system is not available.

The error messages that are considered as expired events are as follows:

  • UPLINK DELAYED IN NETWORK AND REJECTED
  • RESPONSE TIMER EXPIRED
  • AIR SYSTEM TIMEOUT