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High-tech anxiety at 36,000 feet

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High-tech anxiety at 36,000 feet

Old 4th Dec 2003, 15:06
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High-tech anxiety at 36,000 feet

High-tech anxiety at 36,000 feet
Twice in two years, crews flying 747s inexplicably had their flight display screens go dark


It's the kind of airplane incident one might expect to find in stories about flights through the Bermuda Triangle.

But it happened on modern jetliners flying between Sydney and Singapore.

Two Boeing 747-400 passenger jets, operated by the same airline two years apart, suffered unheard of failures of all flight display screens in the cockpit -- high-tech anxiety at 36,000 feet.

Both planes landed safely.

The National Transportation Safety Board detailed the two incidents in a letter Tuesday to the Federal Aviation Administration, recommending the federal agency make sure pilots can quickly identify what measures to take if a similar event should occur.

The Boeing Co. said yesterday it does not yet understand how such a failure happened. The 747-400 has a modern glass cockpit, as do other newer Boeing and Airbus jets, in which flight data is displayed to the crew in a battery of large screens. Older jets used analog instruments.

A device called the electronic instrument unit collects data from numerous airplane systems and transmits it to the display screens. This includes engine and navigation data as well as such things as rate of climb or descent, airspeed, altitude and attitude.

This is what happened, based on investigations by the Civil Aviation Administration of Singapore and by the Australian Transportation Safety Board.

On Jan. 23, 2003, a Singapore Airlines 747-400 was at cruise altitude, en route from Singapore to Sydney, Australia, when it experienced a complete loss of information from all six integrated display units on the flight deck instrument panel.

Using standby instruments -- which provide limited data -- the pilots flew the plane for 45 minutes while trouble-shooting the problem with the airline's maintenance people on the ground. The crew was advised to pull out and then push back in the circuit breakers -- a process known as cycling -- for the electronic instrument units.

The crew followed this procedure and all six display units in the cockpit came back on. The plane continued to Sydney where it landed without further incident.

There are three identical electronic instrument units on the 747-400, providing triple redundancy.

But as this incident showed, the system can still fail.

Nearly two years earlier, on Nov. 6, 2001, the same thing had happened on a different Singapore Airlines 747-400 heading the other way -- from Sydney to Singapore.

That plane was approaching its cruise altitude of 36,000 feet when the crew received a cockpit warning about a decrease in cabin pressurization.

Unable to control the cabin "altitude," as that system is called, the crew began an emergency descent. That's when the integrated display units on the instrument panel began to go blank over the next few minutes -- just as would happen on the 747-400 flight last January.

Using standby instruments, the crew landed the plane safely. When maintenance people later cycled the electronic instrument unit circuit breakers, the six display units returned to normal function.

Standby instruments provide the crew only limited information to fly the plane -- altimeter, airspeed indicator and artificial horizon/attitude indicator.

They provide no data, however, about navigation or how the engines are performing.

"The flight crew has only the position of the engine thrust levers to monitor engine performance," the NTSB noted in its letter.

And the loss of data from the integrated display units left the crews with no information coming from the plane's traffic alert and collision avoidance system, the enhanced ground proximity warning system or from weather radar.

Also, as soon as the main display units went off, the autopilot was disengaged and the crews had to manually fly and navigate the planes.

The safety board asked the FAA to incorporate the procedures for handling this kind of situation in the quick reference handbook that pilots use.

"In the event that all six flight displays blank, flight crew workload would significantly increase and a crew would need to be able to rapidly identify and execute the appropriate sequence of actions to address the problem," wrote safety board chairman Ellen Engleman.

The two 747-400 incidents remain under investigation.

There have been many documented cases where electronic devices being operated by passengers have caused the cockpit instruments of modern jetliners to go haywire.

Also, powerful sunspot activity can interfere with some airplane electronics.

Liz Verdier, Boeing's safety spokeswoman, said there have been cases of cockpit display screens going blank on various jetliner models.

But she was not aware of other incidents in which all display screens went out.

"There are always backup systems," she noted. "So you are never out of it."

P-I aerospace reporter James Wallace can be reached at 206-448-8040 or [email protected]
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