Originally Posted by
iwillflytheworld
As far as I know UA and AY interline, so you should be able to check in through HEL at SAN (i.e. get boarding passes for the whole itinerary and your baggage tagged to be delivered at HEL).
Err, interlining goes only as far as baggage. I'm almost certain that UA and AY systems are not compatible so that UA could issue an AY boarding pass at SAN (or even at the transatlantic departure point, be it SFO, ORD or IAD).
Baggage through-checkin will be OK.
The AY e-checkin opens 36 hours before the scheduled departure time, thus it should be possible for the OP to do that at SAN.
Originally Posted by
iwillflytheworld
In fact you must do that, because otherwise you'd need to claim your luggage in FRA and clear security again, and you'd almost surely miss your FRA-HEL flight, even if you already have the boarding pass.
Security must be cleared anyhow, as this is a non-Schengen to Schengen connection, and requires a Skytrain transfer T1->T2 in FRA.