Fear of Flying: Hydraulic Loss Is One of Worst

The “complete hydraulic failure” reported by the pilot of doomed United Airlines Flight 232 is one of the most feared events in flying, rendering even a sophisticated three-engine jet like the DC-10 virtually uncontrollable.

The wide-bodied DC-10 has three independent hydraulic systems that operate all the plane’s control surfaces on its wings and tail, its landing gear and its brakes.

A failure of all three hydraulic systems is “rare, rare, rare,” said John Gallipault, director of the private Aviation Safety Institute in Worthington, Ohio. “Design engineers calculate that to happen something like once every 300 years.”

Gallipault, like several other aviation experts interviewed, cautioned that preliminary reports on air crashes are almost invariably incomplete or marred by errors and said that other factors likely would emerge as contributing to the disaster.

Manufacturer’s Warning

The aircraft’s manufacturer, McDonnell Douglas Corp., warns pilots that failure of even one hydraulic system can lead to disaster.

“Careful consideration should be given to continuation of a flight after loss of a single hydraulic system, keeping in mind that subsequent loss of a second system--through system or engine failure--may create a serious emergency situation,” the operating manual for the DC-10 says.

According to one unidentified survivor of the crash, the pilot told passengers that “one of our engines had blown and quit. He said when it blew it hurt the tail of the plane and the pilots couldn’t control the plane.”

Federal Aviation Administration spokesman John Leyden said the pilot reported “uncontained failure” of the jet’s No. 2 engine, in the plane’s tail, meaning that the engine blew apart and sent shards of metal through the tail section, where several critical hydraulic components are housed.

Reduces Control

According to the DC-10 manual, loss of the hydraulic system driven by the tail engine would render the lower half of the rudder inoperative, making steering difficult. It also would reduce control of the flaps at the rear surface of the wings, making it hard for the pilot to slow the big jet for a safe landing.

But backup systems operating off the two wing-mounted engines should have made the plane flyable, experts said.

A civil aviation expert who asked not to be named theorized that the No. 2 engine failed “catastrophically”--blowing apart and severing all the hydraulic lines in the tail of the plane. That would disable the entire steering rudder as well as the critical flaps in the horizontal stabilizer, which allow the pilot to raise or lower the jet’s nose.

The pilot could then use the two remaining wing engines to steer the plane and slow it with flaps and spoilers on the wings operated with hydraulic power provided by the two remaining engines, the expert, a former military transport pilot, said.

‘Incredibly Tricky’

But controlling the big airliner under those conditions is “incredibly tricky,” he said, and the pilot may not have been able to raise the nose as he attempted to land.

“He has a machine which is very difficult to get pointed to the right place,” the expert said after hearing descriptions of the crash. “He came pretty close to getting it landed. But the plane may have rolled slightly on him, and he dug in a wing tip.”

It is possible, also, that the main hydraulic fluid reservoir, also in the rear of the plane, was destroyed when the No. 2 engine blew, virtually assuring complete loss of all hydraulic power.

“Complete hydraulic failure was something he radioed and demonstrated, too,” Leyden, the FAA official, said. “But we can’t speculate on exactly how much control he had or what happened up there. All we know is he had a very, very difficult time controlling the aircraft.”

Power Steering Loss

An Eastern Airlines pilot, who asked not to be named, said that losing a DC-10’s hydraulic system would be similar to having the power steering go out in a car. A pilot in that situation, he said, would find it virtually impossible to control the direction of the aircraft and efforts to slow it for landing would be extremely sensitive.

“If you see your hydraulic pressure tanks go to zero and the lights and buzzers going off, you have to stabilize the aircraft,” said the pilot. “You don’t do anything to destabilize the aircraft, like pulling back on the power. You try to maintain the normal flight and get all the time you can and look for a big runway with crash crews.”

After past DC-10 crashes, critics have said that design features of the plane have made it particularly vulnerable to catastrophic hydraulic failures. But McDonnell Douglas has said that design changes have solved those problems.

Hydraulic system failures have been key elements in many of the worst air crashes of the past two decades. In most cases, the failure of some portion of the plane set off a series of events that burst hydraulic lines, causing the pilot to lose control of the plane.

For example, on May 25, 1979, an American Airlines DC-10, the same type of plane involved in Wednesday’s disaster, crashed on takeoff at Chicago’s O’Hare Airport, killing 271 persons on the plane and two more on the ground.

Investigators later determined that the crash was caused when an engine on the plane’s left wing ripped loose, damaging hydraulic lines that controlled the left wing flaps. The plane lunged to the left and slammed into the ground 31 seconds after leaving the runway.

Similarly, in August, 1985, in the worst single-plane accident in history, an aft bulkhead ruptured on a Japan Airlines 747 jumbo jet, releasing pressure that buckled other portions of the plane, severing hydraulic lines and causing the plane to go out of control and fly into a mountainside. All but four of 524 persons on board died.

Accident Over Paris

In an earlier DC-10 crash, in 1974, a Turkish Airlines DC-10 crashed killing 346 persons on board after it lost a cargo door over Paris. The explosive decompression of the plane buckled the floor, rupturing key hydraulic systems, investigators later found.

Hydraulic-system failures have also caused disastrous crashes for the military’s high-technology aircraft.

On Jan. 21, 1988, a $280-million Air Force B1-B bomber crashed after a large bird, probably a pelican, collided with the plane as it streaked through the air at 600 m.p.h. The collision ruptured a critical hydraulic line and set off a 3,000-degree fire, destroying the giant aircraft.

Later that same year, a Navy F-14A fighter jet crashed upside down near El Cajon after its crew had bailed out, seriously injuring three people on the ground. The two crewmen abandoned the plane after telling controllers that trouble with the jet’s hydraulic system had caused them to lose control of wing flaps.