diff --git a/airport.h b/airport.h
--- a/airport.h
+++ b/airport.h
@@ -48,4 +48,20 @@ void InitializeAirports(void);
 void UnInitializeAirports(void);
 const AirportFTAClass* GetAirport(const byte airport_type);
 
+/** Get buildable airport bitmask.
+ * @return get all buildable airports at this given time, bitmasked.
+ * Bit 0 means the small airport is buildable, etc.
+ * @todo set availability of airports by year, instead of airplane
+ */
+static inline uint32 GetValidAirports(void)
+{
+	uint32 bytemask = _avail_aircraft; /// sets the first 3 bytes, 0 - 2, @see AdjustAvailAircraft()
+
+	// 1980-1-1 is --> 21915
+	// 1990-1-1 is --> 25568
+	if (_date >= 21915) {SETBIT(bytemask, 3);}	// metropilitan airport 1980
+	if (_date >= 25568) {SETBIT(bytemask, 4);}	// international airport 1990
+	return bytemask;
+}
+
 #endif /* AIRPORT_H */