34TH ANNUAL CONFERENCE, Jerusalem, Israel, 27-31 March 1995
WP No. 101
The Use of Voice Communications in ADS Operations
An ADS service will not use voice communications for routine message transmission of reports between the controller and the flight deck crew for en-route and oceanic ATC. All routine reports that are to be sent are done so automatically by the system itself, thus negating the need for voice position reports. Any other communications between ground and air can be sent via the datalink service. The cost of providing an air-ground-air network in addition to the ADS communications links will be costly, and this calls into question the operational need to retain voice communications. This issue must therefore be addressed.
Is there a requirement for voice communications?
One of the benefits of an ADS-ATC service is that controller routine workload is reduced by automating some tasks or carrying them out in a more efficient way. The problems of operating with HF are well known and documented. As a communications link it is at best barely acceptable as a communications medium. For this reason, separation standards and the capacity of procedural ATC where HF is used are restrictive.
To some extent, in an ADS-ATC service, the communications workload may be replaced with a reduced controller communications workload. This will typically involve the construction and inputting of messages etc. into the message transmission system. There should be less of this type of activity – until the traffic increases to take up the workload freed! There will also be a change of workload on the flight deck. The precise level of workload on the flight deck and at the controller workstation is as yet to be determined.
The paper presented to the Ottawa conference on the application of air ground datalink, discussed a number of problems at the operational level of datalink communications. In brief the typical transaction times which have been achieved using Mode ‘S’ and VHF datalink, are in general slower than voice communications. Experience in Canada has shown a much better performance in the case of oceanic clearance delivery using datalink, a significant improvement over the equivalent HF service. There are a number of human factors reasons which also lead to potential delays in transaction times – low arousal rates in cockpits etc.
Routine messages, it can be envisaged, may be despatched in standard message formats that can be easily constructed from skeleton messages contained within the workstation communications system; suitably amended to reflect the specific request. However, non- routine messages or free text messages will take considerably longer by virtue of the fact that they must be typed into the message entry panel, checked and then transmitted. An increase in workload can reasonably be expected in these circumstances. This may potentially be unacceptable at times of high workload.
There are occasions when operationally it is necessary for an immediate response to be given to an instruction, or when the communications delay inherent in present day datalink message transmission will be unacceptable. Such occasions may be in the event of an emergency ( it should be noted that on such occasions datalink will be a powerful tool, and may be used in conjunction with voice communications). It may be needed in the event of an untoward event requiring avoiding action to be taken to maintain or regain separation, or perhaps when the datalink medium has failed for whatever reason.
Much criticism is levelled, justly, at the limitations that voice RT communications pose in ATC operations today. One major failing, is that when the communications medium itself fails, it has a catastrophic effect upon the conduct of the ATC service provided. By removing voice communications in ADS airspace, then operational flexibility and the performance of the ATC system, is lessened to the same degree that voice comms is. In effect the removal of voice communications leaves the system susceptible to common mode failures. (A common mode failure is one where the same root cause affects each part of a system). The use of satellites for communications and navigation can potentially make common mode failures possible, thereby reducing the resilience of the whole system to failure.
Therefore for the foreseeable future it can be envisaged that there is a demonstrable operational need to retain the flexibility and urgency that voice communications provide.
Pilot controller communication awareness
In today’s operating environment, in en-route and oceanic airspace, the use of VHF or HF RTF provides a common communications channel for all those involved. This is a open channel, that provides a level of awareness to all those engaged in communications on a particular frequency. This notion is known as ‘ the party-line effect’. There is much concern by both controllers and aircrew at the loss of this attribute of voice communications. To the pilot there will in effect be a silent cockpit, and this may substantially detract from the pilots awareness of proximate traffic. In the latter case it has been suggested that ACAS be used to compensate for the loss of information to the pilot. This is not in keeping with the designed use for ACAS, and therefore is unacceptable. Due regard for the benefits that accrue from the open channel nature of voice communications, and its withdrawal, must be given by designers and planners of an ADS-ATC service.
The introduction of ADS into operational service will greatly reduce the need for controllers and pilots to communicate by voice. However, there will be occasions when the performance of the datalink will not be quick enough to meet the operational need to communicate with pilots on the flight deck. It is desirable to retain as much flexibility in carrying out the control task as possible to enable a responsive ATS service to aircraft operators.
ADS system design should enable the control task to be maintained when communications links other than voice fail for whatever reason. The dependent nature of ADS operations, where communications and navigation use the same medium make the system vulnerable to common mode failures. The retention of voice communications provides some defence in this event.
It is recommended that:
In all circumstances where an ADS-ATC service is provided there shall be an independent voice communications capability fully operational.
When entering ADS airspace existing HF and/or VHF voice communications shall be established to confirm their serviceability.
Last Update: September 28, 2020