Using custom aircraft in campaigns

Anyone know how to have some of the custom, dloaded aircraft appear during campaigns?
I've modded the 'fortess europa.xml' file to start from a later date - no prob.
I tried to add the custom aircraft under techtree in the same format as the existing aircraft tags but no dice. Starting the campaign gives the error 'can't find file ...'
Any ideas?
I've modded the 'fortess europa.xml' file to start from a later date - no prob.
I tried to add the custom aircraft under techtree in the same format as the existing aircraft tags but no dice. Starting the campaign gives the error 'can't find file ...'
Any ideas?