The key point here is IMHO "it's own". The weapon itself would be safe from the aura of the spirit in it, but not the guy handling it,as the effect says that the aura increase damages done by melee attacks made by the spirit or is inflicted on anyone attacking the spirit, but does not say it damages the spirit itself...

To take a parallel : imagine a dwarf with the spell "Elemental Aura" sustained on him. He is safe from the aura (rule-wise the wording of the spell is nearly word for word the same as the critter power).

Suppose a troll grab him by a feet to slam him repeteadly into a wall. Would you inflict damage on the troll just once, when he grabs him (and be able to wield his magically-enhanced metahuman club without further damage), or for as long as he keeps his hold on the dwarf sheated in destructive magical forces ?

Of course he will be damaged repeatedly as he is repeatedly attacking the dwarf via subduing. First he grabs and then he does STR S damage (which is described as slamming him into the wall). Would the troll perform a melee attack with the dwarf on a target, that target would by RAW not be damaged by the dwarf's aura, because the dwarf is definitely not attacking the secondary target.

Fringe cases can be a source of endless fun with RAW ! Here are a few more :
- if the troll blocks an incoming attack with his "club", would that count as an attack against the dwarf and expose the attacker to the aura or not ?

- if the dwarf is willing to be used as a club, does his wielder take no damage as he is now not attacking his "weapon" ?

- if the troll throws the dwarf at a target the dwarf wants to body-slam (for extra damage), does that count as a melee attack from the dwarf for the purpose of the spell ?

Ok i can imagine this. So that dwarf can burn my ass just spanking it, but can he scratch is nose ? Did he burn his face? Touching is a melee attack by the rule, if he touch himself, he will burn.
The rules are just stupid the way they are written. They just need interpretation. In my mind a fire spirit is immune to fire. But not by th rules.

If i ordered the fire spirit to posses my sword i burn myself with the sword?
ok right so my order is : posses de blade of my sword. Do i need a fire-resistant glove or is it ok now ?

You can't possess just part of a whole.

And why not?

It's not a monobloc object. If a spirit posses an emerald i can put that emerald on a jewel mount to make a pendant.

Once again it's just an interpretation of the rules, because their is no clear rules. Each GM have to decide how it works.

And a hand is only part of a body, made of individual bones and flesh and muscle, so the spirit can choose to possess just the bones of the hand, right?

and the bones are made of atoms, so can he possess just an atom?

That's the point, unclear rules lead to houserules. So the question is:
A spirit possessing a blade is it overpowered or not? If you thinck it is, so your rules say it's not possible.
I am thinking it's regular stuff, as possessed armor, possessed jewels, possessed cars, and so on.
In both case it would nedd Houserules.

It's been consistent, AFAIK, through all editions that magic cannot affect a part of something. It can only affect the whole object or organism.

I agree, especially with mana spells, and area spells as a whole. In the sword instance, yes, the spirit can possess the blade. As long as the blade is not attached to the rest of the weapon at the same time. If you rule that Element aura affects anything touching the object (as it probably should, but that argument has been done to death), good luck putting the sword back together.

In this case, I would disagree with the spirit possessing armour while it's on the body, anyway. That means the magician must plan that ahead, or undergo the process of an actual possession to himself. But I would allow the ITnW to affect the whole, either way. Of course, I also use the rules for wearing down armour (if the damage value is more then the armour value, reduce the relevant armour value by one), and will break items of armour when I feel it's appropriate.

Hmm. I wonder if 'Destroy Hand' would be a viable spell, or 'Steal Heart'.

No. As a basic principle, magic doesn't work on parts of things (especially living things). That's why Turn to Goo is such a weird error in this edition.

Reading the book, it mentioned that physical ratings get augmented. Weapon damage is a "gear rating", but my player wants to know if counts as a "physical rating".

"Physical rating" is about the intrinsic material characteristics of the object. "Gear rating" is about how well it can be used. I think the former is affected by Possession, the latter not.

You are correct on the description but Gear ratings can indeed be physical ratings like the Chemsuit or the insulation modification.

The former could possibly work but on the latter you will never get LOS to a valid target unless the thorax is open, at which point you won't need magic to kill the victim.

AFAIK, there are no definitions for 'physical ratings'. The GM must decide what effects the (mess of) possession has on anything not explicitly given as an example in the book. If you think a spirit would make the armor more chem-sealed, I guess go for it.

IMO, this is making an already verging-on-broken mechanic even stronger. If you want to make your armor a vessel, go ahead, but I'd rule that it won't protect you any better.

Not at all. Manifestation is a purely psychic ability. A summoned spirit arrives on the Astral and can manifest to its summoner. Manifestation is useful for appearing to the team at the exact spot where an invisible enemy is and keeping pace with him (Information Guided Indirect Fire (ar162)). It can also be an irritant (read the section on Watchers).

Materialisation (a totally different power) allows access to the material plane (a bit like possession but without needing a vessel, as you say).

It's also why Wreck Cyberware is such an oddity in SR3. It would make sense if the description explicitly stated it only applied to non-implanted 'ware. Otherwise, as soon as the cyber is "paid for with essence" it should not be targetable in isolation.

Still, the main problems here seem to be reliably summonning and commanding F10-14 Spirits (with Intelligence and Willpower double their summoner's); doing so and surviving the drain; and applying their Force bonus to damage/armour ratings. With F10 spirits there's no point asking if he has bothered to prepare the vessels first to make the possession test easier.