--- a/aircraft_cmd.c	Mon Jan 16 15:23:06 2006 +0000
+++ b/aircraft_cmd.c	Wed Jan 18 14:12:26 2006 +0000
@@ -141,7 +141,7 @@
 	const AircraftVehicleInfo *avi;
 	Engine *e;
 
-	if (!IsEngineBuildable(p1, VEH_Aircraft)) return CMD_ERROR;
+	if (!IsEngineBuildable(p1, VEH_Aircraft)) return_cmd_error(STR_ENGINE_NOT_BUILDABLE);
 
 	value = EstimateAircraftCost(p1);