£30, same as online contactless
The £100 is £30 lower that what we’re allowed to auth without SCA.
So if you consumed the entire £100 limit plus the extra £30 offline limit you would just be touching the max amount that can be authed via contactless without SCA.
Most airlines just ignore what the card says, and send us presentments later (i.e. they just take the money later, when we can’t prevent it).
In these cases Mastercard rules require us to make a best effort attempt to recover the money, but ultimately we can claim back any money we loose via a chargeback.
The airlines have decided to take on the risk of getting a chargeback, presumably because it allows them to sell more stuff. Also older banks don’t seem to bother with chargeback, we do, and this has caused some strife with airlines like EasyJet.
If you were to contest such a payment because SCA wasn’t applied, I’m not 100% sure what would happen. PSD2 gives us the legal right to reclaim the money from the airline, because they didn’t allow us to apply SCA properly (you might enter your PIN, but the card would refuse to authorise which would mean that technically SCA wasn’t completed). However we also have no obligation to refund you if we believe you were acting fraudulently i.e. getting a refund for something you actually bought on the plane.