National airline carrier Swiss has been forced to cancel 112 flights over the next two weeks due to a temporary pilot shortage caused by illness and training.
Swiss has enlisted rival airline Helvetic to help plug the gap after 64 short-haul flights were struck off at the end of last week. Some observers partially blame a pilot pay dispute for the problem.
Swiss is now contacting passengers booked on the affected flights – which cover a number of European destinations – to make alternative arrangements. No routes have been cancelled, but some slots have been temporarily taken off the schedule.
Swiss put the unusually high incidence of pilots unfit to fly down to a number of non-work related injuries and the fact that several have not yet completed training for recently introduced aircraft.
But a spokesman dismissed as "speculation" suggestions that the increased absentee rate was a result of an ongoing dispute between former Crossair pilots and management that resulted in a one-day strike last month.
The Swiss Pilots Union denied that its members were conducting unofficial industrial action by declaring themselves unfit to fly. But spokesman Thomas Issler said the dispute, which has been running since 2002, and last month's strike may have indirectly played a role.
"We cannot say for sure why there is an unusually high level of unfitness to fly at the moment, but a number of factors might come in to play when a pilot determines whether they are fit to fly," he told swissinfo.
"These could include an increased workload leading to pressures at home, the bad environment within the company, frustration that their working conditions have not been improved and stress relating to management threats during the strike.
"But this is not unofficial industrial action. It is actually very professional behaviour for a pilot to admit that he is not fit to fly."
Issler added that he was confident that a resumption of talks would help to resolve the stress currently felt by pilots.
However, one Swiss pilot not involved in the dispute, who did not wish to be named, could not rule out that some colleagues are actually venting their anger.
"It could be that some people are taking matters into their own hands because they are disenchanted with both the management and their union," he told swissinfo.
"But difficult working conditions that do not allow them enough time off and the ongoing training programme are also big factors."
Aviation expert Sepp Moser dismissed the idea that this is an example of a "hidden strike". But he believes that the increasingly bitter dispute is taking its toll on pilots mentally.
"We have a phenomenan called corporate stress that is not the same as having the flu, but can still weaken health," he told swissinfo.
"If you don't know what is going to happen with your job and it causes you sleepless nights, then you could become unfit to fly. Flying an aircraft is not like doing an office job."
Moser also blamed Swiss for not having enough pilots to cover temporary shortfalls, which he believes could cause the company image problems with customers.
"This is yet another public relations blow. People do not trust this airline any more because it is in such bad shape," he said.
swissinfo, Matthew Allen
Up to 12% of short-haul pilots have reported unfit to fly in recent days. Last Sunday 22 flights were cancelled, the highest number in a single day since the problem surfaced.
The new flight cancellations have been announced until October 28, after which time the company hopes to restore its timetable.
By law, pilots have to declare themselves 100 per cent physically and mentally fit before they are allowed to fly.
The industrial dispute between former Crossair pilots and Swiss management started when the new airline was formed in 2002. These pilots have consistently complained that they have inferior pay and conditions to former Swissair pilots.
The Swiss Pilots Union called a 24-hour strike on September 26, which forced the cancellation of 142 flights. Swiss has threatened to sack anyone who takes part in a repeat strike.