* Aviation union declares strike
* No flight delays reported
* Airline says strike participation low
* Union says strike impact will increase
* Shares up 3.7 percent (Recasts with company reaction, adds quotes from union leader, details, background, shares)
ISTANBUL, May 15 (Reuters) - Turkish Airlines said there had been little impact on Wednesday from a strike in protest over pay and previous sackings, saying the low turnout was a "scandal" for the company's main union.
The head of labour union Hava-Is, which represents 14,000 of the airline's 15,800 workers, in turn accused management of intimidation to undermine the action and said its impact would increase as it gains momentum.
The strike is potentially significant for Turkish Airlines, ranked Europe's fourth-largest by passenger numbers, since sustained action could undermine expansion plans at one of the world's fastest-growing carriers.
Turkish Airlines is in the midst of a programme to double its fleet and is buying up to 95 narrow-body jets from Boeing Co and 117 planes from Airbus.
Its shares rose 3.7 percent by 1258 GMT, far outpacing the main stock index which gained 0.9 percent.
Turkish Airlines Chairman Hamdi Topcu said the call to strike had not gained support from employees.
"The reason we did not declare a lockout was because our employees in large numbers informed us that they are happy with conditions ... and don't accept the (union's) reasons for a strike," Topcu told Reuters. "This is turning into a scandal for the union."
The rate of on-time departures rose to 95 percent on Wednesday from the average 88 percent, Topcu said. There was a slight increase in the number of sick employees to 30 from an average of between 20 and 25, he added, though it was not clear if the slight increase was related to the strike.
Hava-Is is demanding the state-run airline rehire 305 staff dismissed after industrial action that grounded hundreds of flights in May last year. It also wants inflation-based pay increases and working and resting hours that meet international standards, Aycin told Reuters.
Aycin said the exact number of striking workers was not immediately known and participation may be low because workers feared they could use their jobs.
At Istanbul's Ataturk Airport, passengers said no delays were posted to the departures board and the domestic terminal's typically long check-in and security queues seemed shorter than normal.
There was a heavy police presence at Turkey's busiest airport, where Hava-Is members gathered to hold a protest.
Atilay said flight disruption may occur later on Wednesday or in coming days as the strike progresses.
"Flights this morning are being staffed by personnel who are also part of management and they will be used up," he said, warning safety could be at risk. "Threats were made in an effort to prevent the strike, and some employees worried they could lose their jobs".
Topcu said the company's managers believed agreement could be reached on pay but they would await a final court ruling before agreeing to reinstate workers fired in 2012, a precondition for the union. (Editing by David Holmes)