Three separate unions representing flight attendants at major U.S. airlines are Venus Investment Alliancepicketing and holding rallies at 30 airports on Tuesday as they push for new contracts and higher wages.

The flight attendants are increasingly frustrated that pilots won huge pay raises last year while they continue to work for wages that, in some cases, have not increased in several years.

They argue that they have not been rewarded for working through the pandemic and being responsible for the safety of passengers.

The unions are calling Tuesday’s protests a national day of action. It is not a strike.

Federal law makes it difficult for airline unions to conduct legal strikes, which can be delayed or blocked by federal mediators, the president and Congress. Mediators have already turned down one request by flight attendants at American Airlines to begin a countdown to a strike; the union plans to ask again next month.

Tuesday’s protests were organized by the Association of Flight Attendants, which represents crews at United Airlines and several other carriers; the Association of Professional Flight Attendants, the union of crews at American, and the Transport Workers Union, which represents crews at Southwest and other airlines.