diff --git a/src/newgrf_airport.cpp b/src/newgrf_airport.cpp
--- a/src/newgrf_airport.cpp
+++ b/src/newgrf_airport.cpp
@@ -175,7 +175,7 @@ void AirportOverrideManager::SetEntitySp
 {
 	byte airport_id = this->AddEntityID(as->grf_prop.local_id, as->grf_prop.grffile->grfid, as->grf_prop.subst_id);
 
-	if (airport_id == invalid_ID) {
+	if (airport_id == this->invalid_ID) {
 		grfmsg(1, "Airport.SetEntitySpec: Too many airports allocated. Ignoring.");
 		return;
 	}
@@ -183,15 +183,15 @@ void AirportOverrideManager::SetEntitySp
 	memcpy(AirportSpec::GetWithoutOverride(airport_id), as, sizeof(*as));
 
 	/* Now add the overrides. */
-	for (int i = 0; i < max_offset; i++) {
+	for (int i = 0; i < this->max_offset; i++) {
 		AirportSpec *overridden_as = AirportSpec::GetWithoutOverride(i);
 
-		if (entity_overrides[i] != as->grf_prop.local_id || grfid_overrides[i] != as->grf_prop.grffile->grfid) continue;
+		if (this->entity_overrides[i] != as->grf_prop.local_id || this->grfid_overrides[i] != as->grf_prop.grffile->grfid) continue;
 
 		overridden_as->grf_prop.override = airport_id;
 		overridden_as->enabled = false;
-		entity_overrides[i] = invalid_ID;
-		grfid_overrides[i] = 0;
+		this->entity_overrides[i] = this->invalid_ID;
+		this->grfid_overrides[i] = 0;
 	}
 }