Alaska Airlines flight had cabin pressure problems Thursday.

For the sixth time since Christmas, an Alaska Airlines flight had cabin pressure problems Thursday.

A flight from Juneau to Anchorage was interrupted by a warning horn that indicates cabin pressure – the oxygen level inside the plane – was faulty, according to an Alaska Airlines spokeswoman.

Oxygen masks did not deploy, but the pilot took the plane down to between 10,000 and 12,000 feet – a level that provides more normal oxygen, the spokeswoman said. The flight continued without further incident to Anchorage, and no injuries were reported.

The cause of the warning alarm was still under investigation Thursday evening, Alaska said.

Earlier this week, the SeaTac-based airline had announced it was launching an investigation of the incidents for its entire 110-aircraft fleet.

The question the company is trying to answer:

Has misfortune frowned on Alaska Airlines, or does the SeaTac-based airline’s maintenance and operations program suffer from a systemic flaw?

“It is important for us to further satisfy ourselves and the flying public that we have no systemic issues and that we are taking all appropriate steps to prevent a recurrence of these incidents,” said Kevin Finan, Alaska’s executive vice president of operations.

Alaska says its initial investigations found no common threads linking the incidents. Here’s a quick review:

Cause: The causes of the incidents ranged from a collision between a baggage loader and an aircraft on the ground to an incorrectly latched door.

Models: The incidents happened on three models of airliners made by Boeing and McDonnell Douglas.

Locations: The incidents occurred on flights that originated at Sea-Tac Airport; Portland; Washington, D.C.; Ontario, Calif.; and Juneau.

What happened: In two of the incidents, the plane had reached high enough an altitude that oxygen masks deployed. In the other incidents, warning lights indicated a pressurization problems before the plane had reached an altitude at which the masks were necessary.

Injuries: No passengers were seriously injured in any of the incidents, but several were treated for minor ear problems.

Here are answers to common questions about aircraft pressurization problems compiled from the National Transportation Safety Board, Alaska Airlines and www.aviationow.com:

How dangerous are aircraft pressurization problems?

At lower altitudes, pressurization issues are not major problems because the air contains enough oxygen for passengers and crew members to breath without supplemental oxygen.

Above 14,000 feet cabin altitude – the lowest altitude at which oxygen masks deploy when pressurization fails – the situation becomes more serious. At 35,000 feet, some persons might lose consciousness in as little as 20 seconds without extra oxygen.

That’s why cockpit crew members have masks they can slap on with one hand in five seconds or less. It’s also why flight attendants instruct adults to put on their masks, which descend from the ceiling, before they attempt to help children with the masks.

What actions will the crew take when they learn of pressurization problems?

After quickly determining the structural integrity of the plane, the crew will quickly guide it to a lower altitude where supplemental oxygen is not necessary.

How long will the supplemental oxygen supply for passengers last?

About 10 minutes. That’s long enough for the plane to reach a lower altitude. The crew’s oxygen supply will last much longer.

Have major accidents happened because of aircraft depressurization?

In the U.S., one study found no loss of life aboard a commercial airliner since 1962, although there have been numerous pressurization failures. Crews are not required to file reports about minor incidents.

Crews train repeatedly how to handle pressurization problems.

But abroad as recently as last summer, a Helios Airways 737 crashed on a Greek mountaintop after loss of cabin pressure when crew members lost consciousness. One hundred twenty-one passengers and crew members died. Investigators suspect lack of training, crew communications problems and other issues led to the crew’s failure to handle the pressurization problems correctly.

Families of some passengers in that crash have sued The Boeing Co. claiming it didn’t publish a proper preflight checklist that could have detected the problem.

A private jet carrying professional golfer Payne Stewart crashed in South Dakota six years ago after everyone aboard lost consciousness after a pressurization failure.

What causes pressurization failures?

A number of causes are common. The failure of a door seal, a hole or crack in the airtight fuselage or failure of air pressurization equipment or valves can lead to pressurization problems.

Airliners are equipped with redundant pressurization equipment to minimize the possibility of a failure. In the case of the Alaska MD-80 on Dec. 26, an unreported collision between a piece of baggage handling equipment on the ground creased the plane’s metal skin. Under the stress of internal pressurization, the fuselage ripped open a foot-long gash at 26,000 feet that allowed pressurized air to escape.

Alaska AIRLINES CABIN PRESSURE problems

Date Alaska flight Origin Destination Aircraft type Cause

Dec. 26: 536 Sea-Tac Burbank MD-80 Ground handling collision

Feb. 14: 578 Sea-Tac Denver 737-400 Electrical issue

Feb. 18: 1 Washington Sea-Tac 737-700 Incorrectly latched door

Feb. 21: 100 Portland Denver 737-400 Still investigating

Feb. 22: 397 Ontario, Calif. Sea-Tac MD-80 No pressure problem found

Feb. 23: 65 Juneau, Alaska Anchorage 737-400 Still investigating

John Gillie: 253-597-8663 from this link