The proper use of this card can be unintuitive. Even though one of its abilities is to prevent three points of damage, when you receive three points of damage you do not spend this card unless you have to spend it to survive. You want to keep this card in play in order to use the Aerial ability; your goal, ideally, is to never spend the card. That might seem like it means that the ability to prevent damage then becomes useless, but that isn't the case. It basically means the card serves the same general purpose as gaining +3 hit points. Note that the only purpose of gaining +3 hit points is to prevent you from being killed, if you don't take so much damage that you would have been killed had you not had the extra hit points, those extra hit points technically never got used. However, it is so important for your character to survive that extra hit points are still valuable. Similarly, having a Cosmic Flight which just sits around waiting to prevent a hit from killing you is valuable and worthwhile.

Usually, the only thing that Aerial does for you is to turn your worthless Crisis Interdiction cards into useful cards. This isn't a huge benefit, but you do have three of them, and they are your only thwarting cards. It also gives a boost to your helmet, but this won't matter unless you are playing a special deck based around playing the helmet and defending, in which case it is pretty obvious you are playing Cosmic Flight as part of your combo.

Although the protection ability is useful, I generally think it isn't that amazing for the cost, especially since Captain Marvel isn't a hero who is especially short on hit points. And the ability to go Aerial also isn't that amazing. But together they make the card well worth playing. I wouldn't say it was a top priority must-play card, but I always make an extra effort to play the card because, not only is it effective, it is much more fun being able to use Crisis Interdiction at a critical point instead of having it be a dead card in your deck, and it is comforting having the protection from being killed if you make a miscalculation and take more damage than you can handle.

Once you have one copy in play, the second copy can be played and spent any time you like, exactly like a copy of Defensive Stance. I usually don't bother, though, unless I have spare resources, since between her high recovery, self-healing ability, and already having one copy in play, doing this usually seems like overkill.

A NOOB'S RANDOM THOUGHTS

I really do not like this card. If you don’t build your deck to accommodate it, it essentially results in 6 blank cards (Crisis Interdiction; Captain Marvel's Helmet; Cosmic Flight). But let’s try to review the card objectively.

• For 3 ER (Effective Resources) you get the Aerial trait, as well as the ability to discard the card to prevent 3 damage.
• Unfortunately the two effects are (largely) mutually exclusive. If you want Aerial you can’t use the Interrupt and if you do use the Interrupt, you lose Aerial.
• This in effect means you are either playing the card to constantly have Aerial (with the Interrupt ability as a nice-to-have safety net) or you are building your deck around Captain Marvel's Helmet; Protection Aspect and the Interrupt ability, with Aerial as an accidental bonus here and there.
• Paying 3 ER to get Aerial for only Crisis Interdiction and Captain Marvel's Helmet is too much of a tempo hit to make it worth it. As more cards are released that use the Aerial keyword, this card will undoubtedly become more valuable and a whole Aerial-Arche type will revolve around it.
• Paying 3 ER to avoid 3 damage is not great, however if combined with the Protection Aspect and Captain Marvel's Helmet it becomes a case of the whole is greater than the sum of its parts.
• Against Villains that have abilities/effects that trigger off successfully dealing damage (ex Stampede; Rage of Ultron) this card gains extra utility and value.

All said, I still don’t like that unless you go for a very specific build, 6 of Captain Marvel’s cards (Crisis Interdiction; Captain Marvel's Helmet; Cosmic Flight) are essentially blank resources.

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Most of these comments were written before cards like Honed Technique and Dive Bomb existed. Even Agile Flight makes it extremely worthwhile to have a passive Aerial in the deck. Aerial has become a much more important keyword since this card was introduced, and, yes Carol likes her energy resources, but for the low cost of a Quincarrier you can be wiping minions and villains off the board with a Dive Bomb every turn. It might’ve been a mediocre card when the game released, but it’s a beast now.