Hundreds of British Airways (BA) cabin crew have walked out in a row over pay.
Picket lines were set up around Heathrow Airport as the 48-hour walkout got under way on Tuesday (January 10) morning.
"Mixed-fleet" cabin crew members are striking over what the union has called 'poverty wages'.
Staff say earnings are much lower than the £21,000 plus per year advertised.
But BA argues that full time mixed-fleet crew do in fact earn this.
The airline played down the impact of the strike and said most flights will not be affected.
The strike was announced last week after the airline refused talks to resolve an ongoing dispute, union Unite said.
Pay rises of 2% and 2.5% were rejected by union members shortly after Christmas.
But the airline has refused to increase its offer.
BA's Heathrow contingency plans included merging a small number of flights to and from the airport while all flights to and from London Gatwick and London City were to operate as normal.
BA told getwestlondon: "Despite the industrial action, all our customers will be able to travel to their destinations as planned."
A spokesperson for BA said on the first day of the walkout that flights are departing "as planned today".
On day two (January 11) of the industrial action cabin crew marched over to Marks & Spencer HQ to hand over a letter calling on the retailer to demand BA stops paying "poverty wages" to sell M&S food.
It coincided with the roll out of the new M&S food range on short haul flights across BA.
However, M&S said this is not something they will comment on as it is an issue for BA.
A planned strike on Christmas Day and Boxing Day was called off following a revised offer from the airline.
But the offer was rejected by 93% of Unite members and Unite said the airline has refused to return for talks.
Unite said it has had a surge in memberships, taking its numbers to more than 2,900.
Unite national officer, Oliver Richardson, said: "This is a low paid workforce struggling to make ends meet on wages which are among the lowest in the airline industry.
"It is to the shame of British Airways, a company which prides itself as a 'premium brand', that members of its loyal workforce are forced to take second jobs to make ends meet or turn up to work unfit to fly because they can't afford to take the day off sick.
"Refusing to meaningfully address the concerns of 'mixed fleet' cabin crew and instead seeking to poison industrial relations through confrontation does passengers a disservice and will lead to plummeting morale.
"We would urge British Airways to listen to our members and address their legitimate concerns over poverty pay."
Unite estimates that on average "mixed fleet" cabin crew earn £16,000, including allowances, a year.