The entry for Operate is a little confusing, as the various specialisations are not laid out the same way for all skills.

Is a single specialisation in Operate meant to only be for a single type of vehicle (like a bike or a jump pack) or a class of vehicles (like Surface or Voidship)?

AFAIK (from what's in Black Crusade), it's the latter. There are only three Operate types generally seen. It's possible that there might be som 'exotic' vehicles that require their own Operate skills (I would have thought that Titans would have required something beyond the typical Sentinel, but no).

Huh.  Reading the NPC entries in Only War, both Chaos and Imperial Troops get Operate (Surface) skills, while Dark Eldar and Orks get Operate (Surface [Hover]) and Operate (Surface [Rokkit Pack]) respectively.  Also, according to the Operate skill entry, jump packs should be an Aeronautica operate, not a Surface.

This looks like there is definately an error here- either its meant to be more general, as in (Surface) is all you need, or you are supposed to sub-specialize.  Look like this will have to be errata'd.

Alternately, those dirty xenos just can't learn to fly/drive everything with a single skill.  Proof of their inferiority to glorious Humans!

Armour said:

Actually up untill now jump packs have required Pilot[Personal].

I think the issue is that once again FFG have lazily copied and pasted from earlier books without properly checking that it makes sense.

Considering how powerful it would be to allow characters to pilot ALL types of vehicles with a mere 3 skill purchases (Surface,Aeronautica, Starship) i would rather it be split up into its constituent types of vehicles (Wheeled, Tracked, Skimmer, Personal, Aeronautica etc).

Looking back in my Rogue Trader book they use to split up all the vehicles skill between six different skills.  You had Drive (Ground Vehicle, Skimmer/Hover, Wakler) and Pilot (Personal, Flyers, Space Craft).  So back in those rules jump packs would be covered in personal.

For Only War using a jump back should be Operate (Aeronutica) as stated in the skill discription on page 128, its also the skill a PC gets from a Drop Troop Regiment.

You can also grab the Talent Hotshot Pilot as long as you have one of the Operate skills and then you will count the others all at Rank 1 (Known) and add one degree of success to any successful Operate Test.  A really good talent for an Operator to have for sure, any nice and cheep xp wise as they will have both the aptitudes.

Giving the Ork Stormboy Operate(Surface[Rokkit Pack]) might not be a mistake. Jump packs are usually treated as a Flyer-type skill because they're capable of full flight, even if only for a minute or so. Even the grav-chute can stay airborne for a while (depending on altitude). The Rokkit pack, on the other hand, states that it's basically incapable of flight and can only make powered 'hops'  - "he must be on the ground by the end of his Turn."