[Carts]

Haven't played a campaign for ages,got through the first mission O.K,second didnt load,it would appear that a couple of my I-16s have gone A.W.O.L.I got this message on my loading screen

INTERNAL ERROR/HierMeshObj cant load
HIM3DO/PLANE/I-1624/hier.him

this only applies to the I-1624s,any ideas what i need to do?