More than 400 Singapore Airlines (SIA) cabin crew have applied for no-pay leave, with durations ranging from five days to a month, The Straits Times (ST) has learnt.

This is after the airline had asked in a staff circular earlier this week for volunteers to take no-pay leave to address a temporary manpower surplus, which is expected from September to November this year.

A month ago, SIA had told its 8,200 cabin crew that training would be stepped up for a few months to ensure that they were occupied because an expected expansion in flights did not happen.

ST understands that the current surplus is partly because plans to operate a Singapore-Jakarta-Sydney service failed to get approval from the Indonesian authorities.

SIA did not comment on the extent or reason for the surplus.

Mr Alan Tan, president of the Singapore Airlines Staff Union, which represents cabin crew, said of the recent no-pay leave offer: "We have been assured by management that the surplus is a temporary situation, and there are also no plans to scale back on future recruitment.

"For crew who would like to take some time off, this is a good opportunity to do so. We expect that those who apply will be looking at taking a few days or weeks off."

A stewardess, who has applied for two weeks off in November, said: "I decided to take a short break... It's usually quite difficult to get leave approved during the year-end period so this was a good opportunity for me."

The last time the airline asked its cabin crew to take no-pay leave was in 2009, following a global financial crisis which crippled the air travel industry.

SIA and other full-service premium airlines that rely heavily on corporate travellers to fill their first- and business-class cabins were badly hit when companies slashed travel budgets, either by cutting the number of trips for staff or downgrading them to economy class.

Some firms also switched from full-service to budget airlines.

Since then, SIA has also had to grapple with high fuel prices and an overcapacity in seat supply that has forced carriers to cut fares - which has affected the bottom line - to woo travellers.

The challenges have forced SIA to restructure its business, which has seen its low-cost arm Scoot and regional carrier SilkAir grow faster than the parent premium airline.

This is reflected in the number of pilots at SIA, which has fallen from 2,331 in March 2011 to 2,072 at the end of March this year. During this time, some pilots left voluntarily while others were asked to go.

Pilots who ST spoke to said they have not heard of plans to also offer cockpit crew voluntary no-pay leave.

Said a senior pilot: "These things can happen quite quickly, though, so you never really know what's coming next."