United Continental Holdings Inc., which is embroiled in contentious contract negotiations with pilots employed by its United and Continental subsidiaries, said six more Continental flights were cancelled Thursday following "abnormally high" pilot sick calls. On Wednesday, the parent company scrubbed 24 Continental flights for the same reason, a spokeswoman said.

Continental operates about 1,000 mainline flights a day. United Continental said the majority of the flights cancelled in the past two days due to sick calls were mainline flights out of Continental's big hub in Newark, N.J.

The Air Line Pilots Association union represents Continental's 4,300 pilots under one contract and United's 5,600 aviators under another. Capt. Jay Pierce, chairman of the ALPA branch at Continental, on Thursday sent an email to all his members warning them not to take any actions to disrupt the carrier's operations.

Capt. Pierce, who received a letter of concern on Wednesday from a United Continental labor executive, told the Continental pilots in the email that he doesn't know if the company is correct in its claim that the sick calls were part of an effort by some pilots to disrupt operations and pressure the company in contract negotiations. But he said the union leadership strongly opposes such actions, which put the union and individual pilots at legal risk and make contract negotiations more difficult. In the email, he reminded the pilots to make every reasonable effort to maintain reliable operations.

In a separate letter to Doug McKeen, United Continental's senior vice president of labor relations, the pilot chief said ALPA has repeatedly raised the issue of staffing shortages at Continental to company management. In response to Mr. McKeen's Wednesday letter, Capt. Pierce said he doesn't know and cannot say whether the recent increase in sick-leave usage is related to staffing issues or is the result of other factors. He denied that ALPA has encouraged pilots to call in sick, and said placing public blame on Continental pilots for flight cancellations "simply inflames the situation."

Under federal law, wildcat efforts such as work-to-rule campaigns and sick-ins are illegal. In 2008, United Airlines sued ALPA and some individual pilots, alleging they conducted an illegal campaign of sick-leave abuse and pilot intimidation that resulted in hundreds of flight cancellations. A federal court enjoined the union and pilots from such behavior, a court order that remains in effect.

United Continental on Thursday declined to say how many sick calls are normal among Continental aviators or disclose how many pilots actually called in sick this week. The company maintains there are 50 more Continental pilots available to fly this summer than a year ago, as those on furlough have been recalled. The Continental pilots' June schedule saw a 1.9% increase in hours actually flown over the year-ago month, the company said. July numbers aren't yet available.

In peak times, such as the summer travel period, airlines sometimes run out of pilots near the end of the month, as aviators hit their contractual limits on the number of monthly hours they can fly. Thunderstorms and other weather events also eat into pilot time and can result in staffing shortfalls, although carriers build their pilots' work schedules based on historical data and build in cushions. Airlines also keep a certain number of junior pilots "on reserve" to fill in when scheduled pilots are unable to work.

ALPA has been trying for nearly a year to negotiate a combined contract for the two pilot groups. The two carriers closed their legal merger in October 2010 but the employees of each unit continue to operate separately. Once the parent company achieves common labor contracts with the various work groups and receives from federal regulators a single operating certificate, United Continental will be able to blend the workers and the fleets.

Both of the current pilot contracts, which remain in effect until a new deal is reached, contain concessions. In United's case, the labor accord was agreed when the company was in bankruptcy-court protection early in the past decade, and came open for renewal in early 2010. The Continental pilots also agreed to givebacks in their current pact, which opened for renewal at the end of 2008.

Jeff Smisek, chief executive of United Continental, said on an earnings conference call last week that he no longer believes the company will reach joint contracts with all unions by year-end, which was his "admittedly aggressive goal." He said "the complexities of merging mature, long-standing contracts, as well as factors outside our control," are slowing the process.

His remarks rankled. "We are certainly doing our part at the bargaining table," Capt. Pierce said at the time. "We see no reason to delay getting a deal that will enable the benefits of this merger to be shared with our pilots and other airline employees..." In Thursday's email to Continental pilots, Capt. Pierce said the union expects management to do its part to settle contract issues in a timely fashion.