Phil:
These are some of the attractions you can find in Barcelona (not actually Northern Spain, but I have no idea yet where you will be going). If you are going really North, you may try to look Vitoria (Alaba province) in the last link I provided; Vitoria is the capital of the Basque region of Spain.
Given that airplane nuts are many times also attracted to other mechanical devices, I have included this link to one of the best railroad museums in Europe:
Railroad Museum
MUSEO DEL FERROCARRIL MF
Plaza Eduard Maristany s/n. Tel.938 158 491.
http://www.ffe.es/vilanova/index.html
MUSEO CENTRO AEREO
Air Center Museum
Fundación Parque Aeronáutico de Catalunya.
Av. Aragai s/n. Tel. 938 154 481.
Schedule:
Open to the public Tuesday through Friday, 18:00 - 20:00 hrs.
A collection of helicopters and airplanes can be seen outside the Aeronautical School. Open daily for visitors.
This link works in English and Spanish, and has information on aeronautical activities in Spain, including museums:
http://www.aire.org/portal/index.php
Saludos,
Tulio