it is not possibly in 1979 to build air depots.
Thanks!
The retirement date of 1977 for hangars has already been fixed on my Github branch - thank you for reporting, however!
How do I download the update? It doesn't seem to help downloading this (https://github.com/jamespetts/simutrans-pak128.britain). What should I do?
you need to wait for the update to be made into a downloadable version - that could take a while. In the meantime you could try to compile the pakset yourself, that's a rather technical operation though.
The update is not available yet except in source format: the sources need to be compiled using Makeobj. Makeobj comes with the Simutrans-Experimental binaries in Windows. Have a look through the forum for general instructions for using Makeobj.
Thanks, a bit complicated, but I got it fixed now :)