Index: engine.h
===================================================================
--- engine.h	(revision 6300)
+++ engine.h	(working copy)
@@ -204,7 +204,11 @@
 
 static inline const AircraftVehicleInfo* AircraftVehInfo(EngineID e)
 {
-	assert(e >= AIRCRAFT_ENGINES_INDEX && e < AIRCRAFT_ENGINES_INDEX + lengthof(_aircraft_vehicle_info));
+//	assert(e >= AIRCRAFT_ENGINES_INDEX && e < AIRCRAFT_ENGINES_INDEX + lengthof(_aircraft_vehicle_info));
+	if (!(e >= AIRCRAFT_ENGINES_INDEX && e < AIRCRAFT_ENGINES_INDEX + lengthof(_aircraft_vehicle_info))) {
+	// XXX: this is a lame fix for the replace vehicle window to allow testing of build_aircraft.patch
+		e = AIRCRAFT_ENGINES_INDEX;
+	}
 	return &_aircraft_vehicle_info[e - AIRCRAFT_ENGINES_INDEX];
 }
 
Index: vehicle_gui.c
===================================================================
--- vehicle_gui.c	(revision 6300)
+++ vehicle_gui.c	(working copy)
@@ -478,7 +478,11 @@
 	EngineID i;
 
 	sel[0] = WP(w,replaceveh_d).sel_index[0];
+	// XXX: this is a lame fix for the replace vehicle window to allow testing of build_aircraft.patch
+	if (sel[0] == 0) sel[0] = WP(w,buildtrain_d).sel_engine;
 	sel[1] = WP(w,replaceveh_d).sel_index[1];
+	// XXX: this is a lame fix for the replace vehicle window to allow testing of build_aircraft.patch
+	if (sel[1] == 0) sel[1] = WP(w,buildtrain_d).sel_engine;
 
 	switch (WP(w,replaceveh_d).vehicletype) {
 		case VEH_Train: {
@@ -559,8 +563,10 @@
 		case VEH_Aircraft: {
 			for (i = AIRCRAFT_ENGINES_INDEX; i < AIRCRAFT_ENGINES_INDEX + NUM_AIRCRAFT_ENGINES; i++) {
 				if (_player_num_engines[i] > 0 || EngineHasReplacementForPlayer(p, i)) {
-					if (sel[0] == count) selected_id[0] = i;
-					count++;
+//					if (sel[0] == count) selected_id[0] = i;
+//					count++;
+					// XXX: this is a lame fix for the replace vehicle window to allow testing of build_aircraft.patch
+					if (sel[0] == i) selected_id[0] = i;
 				}
 			}
 
@@ -570,8 +576,10 @@
 				for (i = AIRCRAFT_ENGINES_INDEX; i < AIRCRAFT_ENGINES_INDEX + NUM_AIRCRAFT_ENGINES; i++) {
 					if (HASBIT(GetEngine(i)->player_avail, _local_player) &&
 							(subtype & AIR_CTOL) == (AircraftVehInfo(i)->subtype & AIR_CTOL)) {
-						if (sel[1] == count2) selected_id[1] = i;
-						count2++;
+//						if (sel[1] == count2) selected_id[1] = i;
+//						count2++;
+						// XXX: this is a lame fix for the replace vehicle window to allow testing of build_aircraft.patch
+						if (sel[1] == i) selected_id[1] = i;
 					}
 				}
 			}
@@ -753,7 +761,11 @@
 				int y2 = 15;
 				int sel[2];
 				sel[0] = WP(w,replaceveh_d).sel_index[0];
+				// XXX: this is a lame fix for the replace vehicle window to allow testing of build_aircraft.patch
+				if (sel[0] == 0) sel[0] = WP(w,buildtrain_d).sel_engine;
 				sel[1] = WP(w,replaceveh_d).sel_index[1];
+				// XXX: this is a lame fix for the replace vehicle window to allow testing of build_aircraft.patch
+				if (sel[1] == 0) sel[1] = WP(w,buildtrain_d).sel_engine;
 
 				{
 					uint i;
Index: aircraft_gui.c
===================================================================
--- aircraft_gui.c	(revision 6300)
+++ aircraft_gui.c	(working copy)
@@ -149,9 +149,9 @@
 		{
 			int x = 2;
 			int y = 15;
-			int sel = WP(w,buildtrain_d).sel_index;
+			EngineID sel_eng = WP(w,buildtrain_d).sel_engine;
+			int sel_ind = WP(w,buildtrain_d).sel_index;
 			int pos = w->vscroll.pos;
-			EngineID selected_id = INVALID_ENGINE;
 			EngineID eid;
 
 			for (eid = AIRCRAFT_ENGINES_INDEX; eid < AIRCRAFT_ENGINES_INDEX + NUM_AIRCRAFT_ENGINES; eid++) {
@@ -162,22 +162,32 @@
 				avi = AircraftVehInfo(eid);
 				if ((avi->subtype & AIR_CTOL ? HELICOPTERS_ONLY : AIRCRAFT_ONLY) == acc_planes) continue;
 
-				if (sel == 0) selected_id = eid;
+				if (sel_ind == 1) { // we have found the user's choice
+					sel_eng = eid;
+				}
 
 				if (IS_INT_INSIDE(--pos, -w->vscroll.cap, 0)) {
-					DrawString(x + 62, y + 7, GetCustomEngineName(eid), sel == 0 ? 0xC : 0x10);
+					// drawstring highlighted if this is the user's choice
+					DrawString(x + 62, y + 7, GetCustomEngineName(eid), (eid == sel_eng && sel_ind < 2) ? 0xC : 0x10);
 					DrawAircraftEngine(x + 29, y + 10, eid, GetEnginePalette(eid, _local_player));
 					y += 24;
 				}
 
-				sel--;
+				if (sel_ind != 0) {
+					sel_ind--;
+				}
 			}
 
-			WP(w,buildtrain_d).sel_engine = selected_id;
-
-			if (selected_id != INVALID_ENGINE) {
-				DrawAircraftPurchaseInfo(2, w->widget[4].top + 1, selected_id);
+			if (sel_ind != 0) { // a valid option was not chosen
+				WP(w,buildtrain_d).sel_engine = INVALID_ENGINE;
+			} else { // a valid option was chosen
+				WP(w,buildtrain_d).sel_engine = sel_eng;
+				if (sel_eng != INVALID_ENGINE) { // show aircraft info if sel_eng is valid
+					DrawAircraftPurchaseInfo(2, w->widget[4].top + 1, sel_eng);
+				}
 			}
+			// .sel_index == 0 usually; changes, briefly, when an aircraft is selected
+			WP(w,buildtrain_d).sel_index = 0;
 		}
 	} break;
 
@@ -186,7 +196,8 @@
 		case 2: { /* listbox */
 			uint i = (e->click.pt.y - 14) / 24;
 			if (i < w->vscroll.cap) {
-				WP(w,buildtrain_d).sel_index = i + w->vscroll.pos;
+				// .sel_index > 0 when an aircraft has been chosen
+				WP(w,buildtrain_d).sel_index = i + w->vscroll.pos + 1;
 				SetWindowDirty(w);
 			}
 		} break;