Index: aircraft_cmd.c
===================================================================
--- aircraft_cmd.c	(revision 5369)
+++ aircraft_cmd.c	(working copy)
@@ -220,7 +220,7 @@
 	// prevent building of aircraft in helidepot/helistation
 	st2 = GetStationByTile(tile);
 	if ((avi->subtype != 0) && (GetAirport(st2->airport_type)->acc_planes == HELICOPTERS_ONLY)) {
-		return_cmd_error(STR_ENGINE_NOT_BUILDABLE);
+		return_cmd_error(STR_NO_SUITABLE_AIRPORT_TYPE);
 	}
 
 	unit_num = (HASBIT(p2, 0) == true) ? 0 : GetFreeUnitNumber(VEH_Aircraft);
Index: lang/english.txt
===================================================================
--- lang/english.txt	(revision 5369)
+++ lang/english.txt	(working copy)
@@ -2828,6 +2828,7 @@
 STR_REPLACE_ENGINE_WAGON_SELECT                                 :{BLACK}Replacing: {ORANGE}{SKIP}{SKIP}{STRING}
 STR_REPLACE_ENGINE_WAGON_SELECT_HELP                            :{BLACK} EXPERIMENTAL FEATURE {}Switch between engine and wagon replacement windows.{}Wagon replacement will only be done if the new wagon can be refitted into carrying the same type of cargo as the old one. This is checked for each wagon when the actual replacement takes place.
 STR_ENGINE_NOT_BUILDABLE                                        :{WHITE}Engine is not buildable
+STR_NO_SUITABLE_AIRPORT_TYPE                                    :{WHITE}No suitable airport type
 
 STR_ENGINES                                                     :Engines
 STR_WAGONS                                                      :Wagons