This would be a strike at an airline NOT in bankruptcy.
Again, it's a pretty open-and-shut breach of contract case.
Actually, it's both. If, say, the USAirways contracts are voided, the flight attendants for that airline would be striking when the contract had been voided.
Your point is also well taken, the flight attendants for all other airlines would be striking while under a valid contract. This would be the equivalent of what the PATCO Air Traffic Controllers did under my Freeper namesake, President Reagan. Striking under that set of facts is a well-established replacement worker situation.
Reagan80
-as to the airline in bankruptcy, the court could not selectively abrogate the contract (ie by ordering wage concessions) but still hold the union to the no-strike clause; it's kind of all or nothing. If there were a strike in such a situation (after complying with Railway Labor Act mediation, strike notice, and "cooling off" requirements), the strike would not be in breach of a contract that didn't exist anymore and would not be in contempt of court, but permanent replacements could be hired (unless it was found to be an "unfair labor practice strike", in which case only temporary replacements could be hired);
-as to employees of other airlines, even those in bankruptcy but whose contracts had not yet been set aside, a strike would be in violation of any contractual no-strike clause and the employees could not only be permanently replaced but could be discharged, unless there were some provision in their contract for a "sympathy strike" to be OK and if this situation mest such a definition.
-not intended for legal advice (in other words, don't try this at home).