DL_Mech From United States of America, joined Feb 2000, 2239 posts, RR: 8
Reply 3, posted (15 years 3 weeks 6 days 14 hours ago) and read 1742 times:
On the 777, there is a PFC (Primary Flight Computer) disconnect switch above the Captains' head. This switch is a two position switch (norm. and disc.) that selects the mode of operation that the Primary Flight Control System (PFCS) is operating in.
The PFCS has three modes of operation: Normal, Secondary and Direct. The normal mode occurs when all information is available to the FBW computers. The secondary mode is a downgraded mode that is automatically selected when certain air and attitude data is lost. When the switch is moved to disc., the control surfaces are moved in direct response to analog signals generated by position transducers.
In addition, the 777 has three cable-controlled hydraulically-operated contol surfaces. Spoilers 4 & 11 ( one on each wing) receive control inputs to their respective Power Control Units (PCU) directly from cables attached to the control columns. Mechanical pitch control is obtained by operating the alternate stabilizer pitch trim handles (located between the Captains' right leg and throttles). These handles are directly connected to two stabilizer trim control modules that are located near the ceiling of the stabilizer compartment.
This plane is built to withstand anything... except a bad pilot.
WorldTraveller From Germany, joined Jun 1999, 624 posts, RR: 5
Reply 4, posted (15 years 3 weeks 4 days 9 hours ago) and read 1742 times:
There is a VERY good article on this topic which compares both Boeing's and Airbus' FBW philosophy/concepts.
Here it goes (it's rather long....but enjoy!):
"Unlike Airbus, Boeing lets aviator override fly-by-wire technology
Monday, March 20, 2000
By JAMES WALLACE
SEATTLE POST-INTELLIGENCER REPORTER
MIAMI -- As the Airbus A320 jetliner descended through 3,000 feet on its final approach to the airport, co-pilot Rudy Canto glanced off to the right and spotted a regional jetliner bearing down on the Airbus plane.
The regional jet also was descending, and the two planes were seconds away from a midair collision.
Canto barked a warning and the pilot immediately pushed the side-stick controller hard left and pulled back as far as it would go and held it there -- an abrupt maneuver that should have resulted in the plane's losing lift and stalling.
But one of seven flight-control computers on the A320 took over and the Airbus plane climbed sharply and turned away from the regional jet just in time to avoid a collision.
No report was filed on the near midair collision. The flight was actually a demonstration for a group of journalists earlier this month in one of the $12 million A320 flight simulators at the new Airbus Industrie training center in Miami.
What happened in the simulator, though, was no different from what would have occurred in the sky. And it is at the heart of an ongoing debate in aviation today:
Should pilots or a computer have the ultimate control authority over a commercial jetliner as the plane approaches its design limits in an emergency?
Airline passengers can't see it, but this is the most significant difference between Boeing and Airbus planes.
Dramatic advancements in technology have made it possible for planes built by either manufacturer to be flown by computers from shortly after takeoff through the landing.
But Airbus has taken a much different philosophical approach to using computers than its rival. The European airplane maker designed its new fly-by-wire jets such as the A320 with built-in hard limits, or "protections."
The Boeing Co., on the other hand, believes pilots should have the ultimate say. On Boeing jets, the pilot can override onboard computers and their built-in soft limits.
"It's not a lack of trust in technology," said John Cashman, director of flight-crew operations for Boeing. "We certainly don't have the feeling that we do not want to rely on technology. But the pilot in control of the aircraft should have the ultimate authority."
On all Airbus planes other than the older A300 and A310, computers prevent the pilot from putting the plane into a climb of more than 30 degrees where it might lose lift and stall. The maximum bank or roll allowed is 67 degrees. The plane's nose-down pitch is limited to 15 degrees. There are protections against overspeed.
And the computer won't allow the plane to make any extreme maneuvers that would exceed 2.5 times the force of gravity.
(As a plane maneuvers, for example turning from side to side or up and down, the forces against it can be measured in units of gravity called "g" forces. One "g" is equal to the force of gravity on earth.)
Consider the near-miss incident in the Miami flight simulator.
When the pilot yanked back hard on the control stick to avoid the simulated regional jet, one of the computers on the A320 applied full thrust to the plane's two engines. At the same time, the computer retracted the speed brakes on the wings, which had been deployed to slow the plane for landing. But the computer did not retract the plane's wing flaps, which had also been extended for landing. The flaps provided added lift during the steep climb.
Most important of all, the computer limited the angle of the plane's climb to 30 degrees to keep it from stalling and falling out of the sky.
"If we had been in a conventional plane, we probably would have stalled and crashed," Canto said.
But Cashman said such limits keep a plane from performing at its absolute capability.
"When you fully automate and protect the system, you have to take away some of the capability," he said.
"It makes no sense to us to limit the pull up capability, say to miss another airplane or the ground. . . . We feel the pilot should have that capability and should be able to achieve it by use of normal controls, providing cues that he is getting close to those limits but letting him exceed them if necessary."
These so-called "cues" tell the pilot the plane is approaching certain speed, load or attitude limits. As the jet nears its stall speed, for example, much more force is needed to pull back on the control column. The same is true as the "g" forces on the plane increase.
Planes are generally designed structurally to have more capability than what the book says, Cashman noted.
If the pilot can pull an extra "g" during the initial part of an upset (loss of control) when the airspeed is low, Cashman said, the pilot may avoid getting into a more serious high-speed recovery.
"By limiting the amount of 'g' you pull, you may prolong the recovery," Cashman said.
He recalled the case of a China Air 747 that tumbled out of control over the Pacific in 1985. The pilots were able to recover by subjecting the jumbo jet to upward of four times the force of gravity.
John Lauber, former board member of the National Transportation Safety Board and now vice president of safety and technical affairs for Airbus, said good arguments can be made for either the Airbus or Boeing cockpit philosophy on soft or hard limits.
"But hard protections are a better way to address loss of control," he said.
Though fly-by-wire was used on jet fighters and on the supersonic Concorde, the first Airbus plane with the technology was the A320, which entered service in 1988. The same system is also used on the Airbus A330 and A340 widebody jets.
Fly by wire simply means that computers on the plane transmit the pilot inputs into electrical signals that are sent through wires to actuators that move the control surfaces.
On conventional planes, the flight-control surfaces are moved by hydraulic devices controlled by cables that run through the airplane.
Airbus also eliminated the wheel-and-control column, or yoke, that is used on all Boeing jets. Instead, Airbus pilots control the plane by moving a small, hand-held joystick off to the side.
"The benefits are numerous," Canto said of the Airbus system. The computers, for example, automatically trim the plane even when it is being flown by the pilot. The trim system on other planes must be operated by the pilots when the plane is not on autopilot.
"It's like having power windows in your car," said Larry Rockliff, vice president of the Airbus training center in Miami who has served as an instructor for every model of Airbus plane. "It reduces the pilot workload."
The only Boeing plane with fly-by-wire technology is the 777.
Cashman, who was chief pilot for the 777 program, said Boeing could have designed the 777 with the same hard limits as those in the Airbus planes.
"They could have been absolutely identical, but we chose to go a different path," he said. "It was based on what our customers wanted, and what we believed was the safest way to operate jet transports."
The Airline Pilots Association, the largest pilot union, recently published a series of technical papers on the issue.
"The pilots had strong opinions -- both ways," said Capt. Paul McCarthy, executive air-safety chairman for the union.
"There are good arguments on both sides," he said. 'Both make legitimate points. And both sides are correct. . . . It's a good, healthy debate that will continue for the next five to 10 years until everyone is confident which way we should go."
The study by the pilots union recommended that Boeing and Airbus incorporate the best of both cockpit designs in future planes.
Statistically, the accident rates for Boeing and Airbus planes are about the same.
"Both types of planes are extremely safe," said Bob Vandel, executive vice president of the non-profit Flight Safety Foundation in Arlington, Va.
He said the pilots with whom he talks are pretty much divided about whether they prefer the Boeing or Airbus way.
'The envelope protection Airbus has is very good," he said, "but some pilots want total control. They don't ever want to be completely out of the loop."
Vandell said he cannot recall a particular crash of a Boeing or Airbus plane in which a case could be made that, had the plane used the other manufacturer's system on limits, the crash could have been prevented.
But there has been much discussion in aviation circles, he said, about the Boeing 757 that crashed into a mountain ridge while trying to land at Cali, Colombia, in 1995. All 159 people on the jet were killed.
The plane's ground-warning system alerted the crew they were about to hit the mountain, but the crew did not retract the plane's speed brakes as they tried to climb. They hit the mountain ridge about 250 feet from the top.
The speed brakes on an A320 would have been retracted automatically, Airbus points out.
But Cashman said Boeing believes the jet would have hit the ridge even if the speed brakes had been retracted.
Airbus planes with their fly-by-wire technology and hard limits have also crashed. Six of the A320s have so far been lost.
One of the very first A320 jets crashed shortly after the jet entered service in 1988, raising many questions about the Airbus philosophy.
The pilots were making a low and slow-speed pass at an air show in Habsheim, France. The subsequent crash was captured on video. It shows the Airbus plane disappearing into trees, as if it were making a very slow landing. Then a huge cloud of smoke billows from the forest.
Though the A320 was full of non-paying passengers, all but a few survived.
Lauber said the pilots were supposed to fly by with the gear down at about 100 feet. Instead, they came in at less than 30 feet off the ground. When the plane gets below 50 feet, the computer assumes the pilots are trying to land, Lauber said.
"The fact is, the plane did exactly what it was supposed to do," he said. Only it landed in the tress.
Airbus learned much from that incident, Lauber said.
Until the crash, he said, there was a "genuine psychology" around Airbus that it had designed a crash-proof airplane because of the hard protections.
"The repercussions from that accident continue to reverberate," Lauber acknowledged.
He and others at Airbus know that should one of the Airbus fly-by-wire planes crash in the United States, television will be showing endless video of the Habsheim incident, questioning whether Airbus has placed too much faith in computers."
As you can see, both philosophies have their advantages...