diff --git a/aircraft_gui.c b/aircraft_gui.c
--- a/aircraft_gui.c
+++ b/aircraft_gui.c
@@ -129,7 +129,7 @@ static void NewAircraftWndProc(Window *w
 			int sel = WP(w,buildtrain_d).sel_index;
 			int pos = w->vscroll.pos;
 			EngineID engine_id = AIRCRAFT_ENGINES_INDEX;
-			int selected_id = -1;
+			EngineID selected_id = INVALID_ENGINE;
 
 			do {
 				if (HASBIT(e->player_avail, _local_player)) {
@@ -145,7 +145,7 @@ static void NewAircraftWndProc(Window *w
 
 			WP(w,buildtrain_d).sel_engine = selected_id;
 
-			if (selected_id != -1) {
+			if (selected_id != INVALID_ENGINE) {
 				DrawAircraftPurchaseInfo(2, w->widget[4].top + 1, selected_id);
 			}
 		}
@@ -162,14 +162,14 @@ static void NewAircraftWndProc(Window *w
 		} break;
 
 		case 5: { /* build */
-			int sel_eng = WP(w,buildtrain_d).sel_engine;
-			if (sel_eng != -1)
+			EngineID sel_eng = WP(w,buildtrain_d).sel_engine;
+			if (sel_eng != INVALID_ENGINE)
 				DoCommandP(w->window_number, sel_eng, 0, CcBuildAircraft, CMD_BUILD_AIRCRAFT | CMD_MSG(STR_A008_CAN_T_BUILD_AIRCRAFT));
 		} break;
 
 		case 6:	{ /* rename */
-			int sel_eng = WP(w,buildtrain_d).sel_engine;
-			if (sel_eng != -1) {
+			EngineID sel_eng = WP(w,buildtrain_d).sel_engine;
+			if (sel_eng != INVALID_ENGINE) {
 				WP(w,buildtrain_d).rename_engine = sel_eng;
 				ShowQueryString(GetCustomEngineName(sel_eng),
 					STR_A039_RENAME_AIRCRAFT_TYPE, 31, 160, w->window_class, w->window_number);