Tens of thousands of Ryanair passengers across Europe are suffering disruption as strikes cancel 250 flights.
Unions claim this is the biggest day of action yet against a series of complaints about conditions by Ryanair workers in a raft of countries.
Ryanair say 35,000 passengers have been affected.
But it added 90% of flights would operate as normal. Some 20 flights to and from Ryanair's biggest European hub, Stansted, have been cancelled.
The travel editor of the Independent, Simon Calder, said these were mostly to and from Germany.
He also said eight Manchester services - to and from Barcelona, Berlin, Faro and Malaga - have been grounded, as well as flights from Birmingham to Lanzarote and East Midlands to Malaga.
Ryanair said the action was taking place in six of its 37 markets and that most of its 400,000 passengers would not be affected.
It called the action "regrettable and unjustified" and apologised to customers.
Action had been planned across a range of countries by pilots and cabin crew earlier this week.
On Thursday, this was boosted by German pilots, who said they would join those in the Netherlands and Belgium by taking action.
Cabin crews in Belgium, Germany, Italy, the Netherlands, Portugal and Spain have also stopped work.
Unions want staff to be given contracts in the countries where they live, rather than under Irish law.
They say employing staff under Irish law inconveniences workers and affects their ability to access social security benefits.
Chief executive Michael O'Leary said the company had written to unions offering to move all staff to local contracts, which made the strike action "unnecessary".
However, the Dutch pilots union said it had only verbally offered its members local contracts and had refused to put the offer in writing.
Joost Van Doesburg, of the VNV union, said his members also wanted pensions in line with Dutch standards, and firmer guarantees on sick pay.
The German pilots' union said it could not rule out further strikes.
Tom Burridge, BBC transport correspondent
Meet with Michael O'Leary and beyond the bullish facade, this multi-faceted dispute is more complex and potentially damaging than he is willing to let on.
The roots of today's row stretch back to autumn last year when Ryanair 400,000 Ryanair passengers had their flights cancelled.
The airline did not have enough pilots to honour its schedule. It was in Mr O'Leary's words "a mess of our own making".
The subsequent decision to start recognising pilot and cabin crew unions around Europe was a multinational can-of-worms.
Some deals with some unions in some countries have been done.
But overall there is plenty to resolve and incendiary language, on both sides, is the flavour of the day.
Ryanair this week signed deals with cabin crew unions in Italy to provide employment contracts under Italian law and agreed to arbitration with the union representing its German pilots.
The European Commission said Ryanair employees should have contracts in the countries where they live rather than in Ireland, where its planes are registered.
EU social affairs commissioner Marianne Thyssen told Mr O'Leary at a meeting in Brussels on Wednesday that EU rules on employment of air crews were based on where workers left in the morning and returned in the evening - and not where aircraft were registered.
"Respecting EU law is not something over which workers should have to negotiate, nor is it something which can be done differently from country to country," Ms Thyssen said.
"The internal market is not a jungle - it has clear rules on fair labour mobility and worker protection. This is not an academic debate, but about concrete social rights of workers."
Passengers whose flights have been cancelled were contacted by email and text message on Tuesday to advise them of their options.
"We sincerely apologise to those customers affected by these unnecessary strikes on Friday which we have done our utmost to avoid," Ryanair said.
It has rejected calls by the UK's Civil Aviation Authority to compensate passengers whose flights have been cancelled, claiming they were caused by "competitor airline crew, unions and lobby groups" and were therefore "extraordinary circumstances".
However, Coby Benson, a lawyer specialising in flight delay compensation at Bott and Co, said Ryanair's arguments did not comply with the precedent set in April by a case in Germany.
Last month, Ryanair pilots across Europe staged a co-ordinated 24-hour strike to push their demands for better pay and conditions, plunging tens of thousands of passengers into transport chaos at the height of the summer holiday season.
In July, strikes by cockpit and cabin crew disrupted 600 flights in Belgium, Ireland, Italy, Portugal and Spain, affecting 100,000 travellers.
Another indication of the company's rethink on contracts came on Thursday when it announced two new bases in France. They will be the first in the country since it closed Marseille in early 2011 after being sued for employing French workers on Irish contracts.
It will also open another base at Bordeaux for summer 2019 and had another four under consideration.
Two aircraft will be based at both Marseille and Bordeaux and will offer a total of 64 routes and handle 3.5 million passengers a year.