In our Eberron campaign, airships are more common than usual. The bad guy in my adventure hides on an already flying airship, kills the pilot and crashes the airship against a castle next to the airship. A few seconds before it crashes, he jumps from the ship with a feather token.
Does my bad guy need a Mark of Storm for this? Or can anyone slightly change the direction of an airship by using his muscles for 30 seconds and crash it against a very large target? I understand that you need a Mark of Storm to communicate with the elemental in the core, but I am more thinking about a mechanical direction change which only uses the existing thrust and can only change the direction of the ship for a short time and for a short amount.
I don't want to give the bad guy a Dragonmark, since it would make it too easy to reduce the number of suspects to 12 half-elves in the town, where 11 will work for House Lyrandar and will very likely have an alibi.
Yes, you can control an airship without a dragonmark.
The rules for this are found in Eberron: Rising From the Last War on page 234, in the "Elemental Vessels" section under the header "Controlling the Elemental":
It goes on to talk about using spells as another way to take direct control; a dominate spell can take over the ship automatically, even wresting control away from an active pilot, while charm bypasses the Charisma check but can't override the pilot.
The referenced "magic item at the vehicle's helm" is the wheel of wind and water for an airship; other kinds of elemental vessels may have different helm items. (Lightning rails in 3rd Edition had "lightning reins" that the pilot used to control the bound elemental, but for some reason all reference to that item has been removed in the current edition; it simply says the pilot controls the elemental without describing the helm item.) If not using the helm, the required direct contact with the khyber crystal is more difficult than it sounds, as the crystal is typically sealed in a metal casing in a containment chamber accessible only though locked and trapped doors (described in the "Freeing the Elemental" section immediately after the quote above).
So yes, in extremis, with a strong will, anyone can temporarily take command of an airship or other elemental vessel, provided the pilot is out of the picture; but it takes powerful dominate magic to take control against the wishes of a conscious and active pilot. Assuming that isn't an issue and this bad guy can make that quite difficult check (or you, as DM, just decide they can for the purpose of the story), they can take control for one minute, which should be plenty of time to do something terrible, if they picked the right moment to act.