diff -r 641c137f4c2d -r 5d84c056a2b1 newgrf_engine.c
--- a/newgrf_engine.c	Wed Jun 14 09:52:50 2006 +0000
+++ b/newgrf_engine.c	Wed Jun 14 11:05:30 2006 +0000
@@ -850,7 +850,7 @@
 	assert(engine < AIRCRAFT_ENGINES_INDEX + NUM_AIRCRAFT_ENGINES);
 
 	/* Only valid for helicopters */
-	assert((AircraftVehInfo(engine)->subtype & 1) == 0);
+	assert(!(AircraftVehInfo(engine)->subtype & AIR_CTOL));
 
 	NewVehicleResolver(&object, v);