Songbird 29 :
The problems are numerous and complex and would vary depending on the medium used to down link the RA.
But as many believe that Mode S is the the only cost effective way to downlink RAs,. so let's take Mode S :
A recent QinetiQ study show that the average delay for controllers to be aware of an RA via Mode S is about 9 seconds and to get a verbal report from a pilot after an RA is currently about 29 seconds.
The main problem is what happens during those 20 seconds.
According to the current ICAO documentation the controller is still responsible and could/should issue instructions as long as the aircraft /pilot do not manoeuvre. The chances to have controllers intervene in those 20 seconds is higher than without RA downlink.
Next is the problem of intermittent short false RAs transmitted on mode S ( and that are down linked today ) but that are not resulting in an RA in the cockpit.
Lastly , should we decide to downlink one day, there is a strong debate about displaying or not to controllers the sense of the RA. Seen the mode S delay to display the information, a risk exits that the displayed information will be different of the actual RA ( especially in case of sense reversal ) This could induce again more unwanted and potentially dangerous controller intervention, especially in the time frame when he is still responsible,( i.e. before he gets a pilot R/T confirmation that the aircrfat is following the RA.)
RA Down link will show that the system has issued an RA at one point of time but not that the pilot is following it.
According the latest SOFREAVIA study on the survey of 4 major European airlines , still 10% of the RAs are ignored by pilots , or are acted against .
.