2Nd Decompression Reported Aboard 777 -- Faa Limits Test Flights To Lower Altitude Pending Investigation

Two separate Boeing 777 test flights were forced into emergency landings on consecutive days this week because of rapid decompression, prompting the Federal Aviation Administration to limit the 777's altitude in test flights to 25,000 feet.

The airplanes landed safely following tests Thursday in Seattle and Wednesday in Hawaii after a clamp and safety valve failed in the cabin-pressurization system.

Seven passengers were injured and four hospitalized after the test flight of WA002 in Seattle, which was flying at 43,000 feet when it lost air pressure. There were no injuries in the Hawaii incident and the altitude of that aircraft, WA003, was not available.

The mishaps complicate Boeing's effort to earn special FAA approval to fly the high-tech new twinjet on long overseas routes as soon as it enters commercial service. The 777 is scheduled to make its first commercial flight, bearing the colors of United Airlines, in June.

The incidents highlighted concern by pilots and safety experts about what might happen if the 777 lost cabin pressure in the middle of an ocean crossing.

The jetliner would have to descend from its typical cruising altitude of 40,000 or more feet to 10,000 feet or lower, the height at which passengers can breathe comfortably, said Mike Hynes, an Oklahoma City-based aviation-crash investigator.

But jet fuel burns more efficiently at high altitude. For example, four hours of fuel at 40,000 feet translates into just 30 minutes of fuel at 10,000 feet, Hynes said. The aircraft conceivably could run out of fuel over an ocean if it had to fly at low altitude for any length of time.

"Pressurization is the one system that can bring you the most grief on a long overwater flight," he said.

Yesterday Boeing replaced the failed clamp and valve on 777 No. WA002 and sent it back into test flights about 24 hours after the jet made an emergency landing at Boeing Field, and four crew members were sent to Virginia Mason Hospital for hyperbaric chamber treatment.

Back in test flights, too, is 777 No. WA003, which was involved in the Hawaii mishap on Wednesday.

The FAA imposed the 25,000-foot altitude limit on all 777 test flights until Boeing determines why the clamp and valve failed in the Seattle and Kona incidents, and comes up with an acceptable modification, said FAA spokesman Dick Meyer.

Boeing flight-test director Kenneth Higgins characterized the effort to fix the problem as routine. "The problem is going to be solved long before the 25,000-foot restriction has an impact on the testing program," he said.

The company is continuing a special series of 1,000 simulated airline flights the FAA has said it must execute flawlessly to win approval for the oceanic flights.

Here is what went wrong on the 777's pressurization system:

Super-hot, compressed air siphoned from the engines feeds into air conditioning "packs" housed in unpressurized wells on the plane's underbelly. The packs then channel cooled air into the interior of the fuselage via 11-inch-diameter ducts.

In the Seattle accident, a clamp on the left pack came loose where the duct feeds into the interior of the fuselage. A special "check" valve was supposed to seal off the leak. The check valve broke open explosively, letting pressurized air rush out of the duct opening.

Normally, the air-conditioning pack siphoning air from the right engine should have had plenty of power to keep the airplane pressurized, even with a breached duct and blown-out check valve on the left pack.

But Capt. Jim McRoberts had shut down the right pack to conduct certain tests, so it wasn't available to keep the cabin from decompressing rapidly.

In the Kona incident, one of the air-conditioning packs was shut off because it was not working properly. A duct clamp and check valve failed much the same way those parts were to fail in Seattle a day later.

Jetliners are permitted to fly passengers with just one pressurization pack in operation, although having two packs eases the workload on each and provides a margin of safety. In the Kona case, Boeing thinks repeated opening and closing of the check valve by engineers checking monitoring equipment may have been a contributing factor.