Ryanair pilots from the union Balpa resumed their recent industrial action with a three-day walkout that started on Monday.
It comes as their peers at British Airways get ready to strike the following week.
Following on from this week's strike, members of the British Airline Pilots Association (Balpa) announced a 48-hour walkout on September 18-19 and 24-hour stoppages on September 21, 23, 25, 27 and 29.
Here's all you need to know about the likely impact on travel and where passengers stand.
When does the latest Ryanair walkout start and end?
The first round of action by members of Balpa (British Airline Pilots Association) this month began at a minute past midnight on Monday 2 and runs until 11:59pm on Wednesday 4.
There will also be a 48-hour walkout covering September 18-19 and 24-hour strikes on September 21, 23, 25, 27 and 29.
What has Ryanair said about the potential for disruption?
The airline reassured passengers that all its UK flights will run as planned with 95% of UK pilots on roster.
A Ryanair statement to booked flyers said: “We do not expect any pilot strike disruptions to our schedule.”
But planned action abroad by Ryanair staff abroad has disrupted the future flight plans of some UK flyers.
What's the impact?
Ryanair faces industrial action by cabin crews in Spain throughout September following the announcement 900 jobs are threatened.
As a result, the airline has cancelled 14 flights to and from Spain in the first week of the month. It said all affected passengers have been notified and made aware of their rights to rebook or be refunded.
The airline said “the closure of loss-making winter bases in the Canary Islands will not be reversed by these pointless strikes”.
How was Ryanair's service affected by the recent strikes?
The UK-based pilots left their cockpits for two days before the recent August Bank Holiday weekend after the airline failed to get the High Court in London to stop them.
However, Ryanair said its 892 daily flights flew as normal.
How could BA flyers be affected by strike action?
Some already have been. The airline sent out notifications of confirmed cancellations, only to u-turn and apologise the emails were issued in error.
Yet not before some passengers had already rebooked their travel.
British Airways pilots are still scheduled to strike on September 9, 10 and 27 after the airline lost its Court of Appeal bid to block them from walking out.
What is the likelihood of it being averted?
It rests on a resumption of negotiations, though there's no indication of a breakthrough yet.
BA said it had “continued to urge Balpa to return to talks” with its negotiating team “open to discussion”.
Brian Strutton, Balpa general secretary, said the union would only re-enter talks when BA “indicate” they will improve their wages offer.
On RyanAir, he said the union wants to settle the dispute and wants pilots to enjoy the same policies that exist in other airlines.
Issues range from pensions to maternity benefits and allowances.
“While this action has considerably disrupted Ryanair, forcing them to engage contractors and bring in foreign crews to run its operation, it has had limited impact on the public’s travel plans," he said.
“Ryanair should stop dragging its feet and get back to the negotiating table.”
What can passengers do if there is disruption or cancellations?
Airlines normally issue two to three days' notice of cancelled flights but if in doubt check with them.
Airlines are required under the European Air Passenger Regulations to reimburse passengers or find alternative travel solutions at no extra cost.
Will passengers be due compensation?
Potentially. Airlines are bound by EU law to pay passengers if a flight is cancelled less than a fortnight before departure - with the exception of "extraordinary circumstance" - or delayed by more than three hours (up to £530).
Strike action doesn't fall into the category of "extraordinary circumstance", so passengers can apply to airlines for compensation unless they accept a refund.
Travel insurance does not normally cover strike action by airline staff.
Why are Ryanair pilots leaving their cockpits?
The continued action at Ryanair is being held after a vote in favour of industrial action in a dispute over pay and conditions.
Members of the British Airline Pilots Association (Balpa) voted by four to one to back a campaign of action on a 72% turnout.
A Balpa statement said the pilots' claim includes "many issues including pensions; loss of license insurance; maternity benefits; allowances; and a fair, transparent, and consistent pay structure".
A Ryanair spokesperson said the late summer action is "ill-judged and ill-timed".
The airline said the support for it amounted to "less than 30 per cent" of Ryanair's UK pilots as not all are members of Balpa and "just 57 per cent voted in favour of industrial action".
Why are BA pilots walking out?
BA pilots in the Balpa union have failed to reach an agreement after an offer to rise pay by 11.5% over three years.
The union said pilots should be given a greater share of the “massive profits”, in part for their sacrifices and efforts to help the airline recover from recent struggles.
BA defended the offer as “fair” and said it had already been endorsed by the Unite and GMB trade unions to the almost 90% of BA staff they represent.