Limited Edition Prints signed by veteran Pilots and aircrew are extemely popular with collectors. The 'golden age' of Aviation Art was probably in the 1990's where large Editions were selling out at the publishers in only a few weeks.
Care needs be taken with these litho prints - they are still printed in INK although the signatures are mostly in pencil. Some early Editions had the signatures in ink also.
There is absolutely nothing wrong with mounting INK signatures with prints or Drawings - the real culprit of fading is strong sunlight. I have seen many superb Prints totally RUINED not because the pencil signatures had faded but because the actual PRINT has. In all of these cases the Print was displayed on a wall or conservatory that had large amounts of direct sunlight through the day.
Some of the rarest autographs (Wartime especially) will be in Ink. Care still needs to be taken but the same rules apply. I have had mounted Ink signatures for over a decade on my walls and they have not faded at all. The reason? They are positioned on reverse walls where there is NO CHANCE of sunlight falling on them.
These will invariably be in Ink. There is a market for wartime signed documents, photos and letters - especially Luftwaffe. Caution should be applied as there are (regrettably) fakes being sold, especially on ebay. However, in the case of the Luftwaffe there ARE more wartime signatures available because of the 'culture' in Nazi Germany. High scoring pilots were used by the propaganda machine to be Heroes to the German public. Schoolboys used to send paper or cards through the feldpost for signatures which were then returned. Some would then cut out photos and stick these onto the signed paper. Care should still be exercised around buying such items and I will only purchase autographs which I am happy are genuine articles.