Re: Off the shelf
Arm out of the window.... waste of money?
20k for a bit of kit that might stop the impact between a couple of £30M aircraft? I bet each bomb they drop on the ranges cost more than that.
Anyway what is a military trainer?
When we arn't at war virtually every flight is training and therefore every unnecessary collision is a waste of resources. A cheap to acquire TCAS can be disconnected/pulled out when 'the balloon goes up' but until then...
You may recall that it was a Tucano trainer that hit a police AS355 in September 1997 - the mil crew was heads down [route planning?] and the police crew were looking downwards in the hover in a search pattern .... both would have been served by that simple 'traffic traffic' alert... The other two UK airspace fatals that spring to mind were Tornado's on training runs.
Assuming of course the military transponder worked. In 1999 a dead transponder was not a 'no-fly' fault as long as the mil formation had one working. That may have changed now, but it does not take a brains trust to envisage a scenario where the civilian aircraft [yes even an airliner] alerted by the TCAS simply moves away from the path of one danger into another.