This is just some idle speculation that might amuse some people.
Originally I thought the tales of plane sabotaging gremlins from WWII to be, in Ars Magica terms, a prime example of faeries developing a new role - to punish airmen who don't take sufficient care of their aircraft.
But recently I saw a New Zealand horror movie called "Shadows in the Cloud" where the primary antagonist is a gremlin. The portrayal of the gremlin as a sort of vicious rodent monkey-bat hybrid makes me imagine that for ArsMagica they are creatures of the Magic realm.
Living in hidden regionnes in clouds they are either very territorial and/or elemental in their dislike of manufactured items - eg metal extracted from the Earth.
WWII military aircraft are the first major incursion into their cloud regio territory, so the flying bat-monkeys fight back. Later aircraft fly too high and too fast for gremlins to catch.
Which suggests that they are up there in ME times, rarely if ever interacting with ground dwellers.
But if a mage ever traveled to these cloud lands regio, especially with a Companion knight in shining armour, I could see the knight being attacked by metal hating gremlins seeking to dis-assemble his armour while the knight is wearing it.
Just a note that WW2 planes d very little metal content aside from the engine and guns for the simple reason that metal was heavy compared to things like cloth and wood.
The classic reasoning is air pollution that would set them off, but that wouldn't apply to mages generally. Of course given where they live they may just not like company in general, or they may have thought the pilots rude for not having given them the proper (in their culture) salutations s they entered their airspace.
Since canvas and wood are still grown from the earth, the earth hating part would work while adding some complications since a large portion of cloth is either linen or cotton.
Needing to wear outfits that contain nothing grown or mined directly from the earth is something that most people don't do regularly, but can be arranged with the resources of a covenant.
If you really want to take weapons and armour, you could use a woolen gambeson and a bone cudgel.
From what I can tell, that was more true of the first world war, by ww2 most planes were starting to be made of metal, especially the fuselage with the wings staying wooden for longer.
The hurricane had metal structural elements and a variant with steel wings.
While the mosquito was considered outdated for its wooden construction, it proved to be a great plane.
To summarize before I start rambling, while there were mostly or almost entirely wooden planes in the period(some even quite successful), with advances in metallurgy and an increasing need for durability led to the rise in numbers of metal planes.
I am extrapolating backwards for gremlin behaviour in ME should troupe enter their cloud regio homelands.
Addenda - unless it is something the troupe does in their cloud regio that affects gremlins so strongly they take it out on WWII aircraft centuries later.
It could be a funny setup for what is basically a cutaway gag.
A magus has made a flying house, pilots it into the clouds, runs over what would be some gremlin analogue for a dog.
Smash cut to almost 700 years later, that gremlin whose dog you ran over is now taking her anger out on any plane ballsy enough to fly through her back garden.
another thing to keep in mind is that the gremlins might be easy to placate but have a hair trigger if you don't even try, so even having a magic lore of 1 would be enough to keep them off your back, but how many pilots in the 20th century had magic lore?
WWII gremlins went after complex and dangerous things; airplanes. (Their creator, Roald Dahl, had been a combat pilot.)
Medieval gremlins might go after the most complex devices available, ships, and be offended by poorly kept ropes and sails, disorganized holds, cluttered decks, and so on.
