Separately, a five-day strike by Ryanair's cabin crew in Portugal got underway, but disruption to passengers appeared minimal, with no cancelled flights or major delays so far.
Earlier this month Ryanair's pilots in Ireland voted to join their UK-based colleagues in the first 48-hour strike if pay demands are not met. Ryanair is challenging the strike in court in Dublin, with a decision also expected on Wednesday.
Volunteers have come to the aid of the airline and travellers, Ryanair stated.
It means IALPA/FORSA members would be in breach of a court order if their strike goes ahead.
Ryanair has its main United Kingdom base at Stansted, although this is just one airport where strike action will take place.
He's acknowledged a lot of Ryanair passengers will be anxious about their travel plans this weekend - and is expected to make a decision on whether or not to grant the order later this morning.
Brian Strutton, BALPA General Secretary said: "Ryanair was foolish to bring this into the High Court rather than the negotiating room".
Other estimates have said pilots who fly in and out of the biggest hubs can earn as much as £185,000 per year.
Spain's STICPLA union also sent a letter to members, advising they were not obliged to "answer company calls" if they were in their rest time, on days off or on holiday.
"Instead Ryanair is relying on legal technicalities to try to persuade the High Court to block the strike".
Ryanair pilots based in the United Kingdom with the BALPA union have also been planning strike action for this week. It raised the issue of whether Europe's biggest budget airline would offer compensation to passengers if they ended up being unable to fly.