Guidance on simplifying the following diagram
No, you shouldn't merge the 3 tables.


For a given flight, aircraft may change
For a given aircraft, flight may change


However, I'd suggest that your model isn't complex enough


FlightSeats depends on Aircraft model (seat layout etc)
Aircraft departs Airport too...
.. but Flights leave and departs Airports. Or is it FlightAirport?
Flight is arguably 2 entities: the base flight (number, schedule etc) and the actual FlightInstance (date etc)


Have you written down all your facts first? Say in tabular format like this MSDN example (actually for ORM which I use because ERDs don't capture all the information you need)

Edit, more...


As per @ypercube and @Simon Righart comments below
From @Simon Righart: A given aircraft might have three different classes of seats (economy, business + first) with different numbers of each each flight number might be flown by multiple aircraft