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Software Flaw Zooms 777-200 To Near Stall at 41,000 Ft.
09/25/2005 07:44:05 PM
By Michael A. Dornheim
A WILD RIDE
The FAA has issued an emergency airworthiness directive covering Boeing 777 flight control software, following a recent incident in which an aircraft pitched up to stall speed at high altitude.
A Malaysian Airlines 777-200 was carrying passengers from Perth, Australia, to Kuala Lumpur, when it pitched up near the top of climb, gave conflicting signals to the crew and reached a dangerously low airspeed. Problems kept appearing down to the approach back to Perth, but the crew was able to make a safe landing.
The near-term fix, mandated by airworthiness directive 2005-18-51, involves reverting to an earlier version of software with known flaws, itself the subject of a prior airworthiness directive. The earlier flaws are not as severe as those discovered in the newer software, and the old version is seen as the acceptable lesser of two evils. The long-term fix is to eliminate all program bugs.
The problem is with "operational program software" (OPS) in the Honeywell air data inertial reference unit (Adiru), which supplies air data and attitude information that affects several systems, including the primary flight controls. Versions of the OPS after part no. 3470-HNC-100-03 have a flaw that enables them to use data from failed sensors.
The Aug. 1 Malaysian Airlines flight was climbing through 36,000 ft. on autopilot when it experienced a significant pitchup at 5:57 p.m. local time, 32 min. after takeoff and about 15 min. after sunset. The crew reported seeing a low airspeed warning on the engine indication and crew alerting system while the slip indicator swung to full right on the primary flight display (PFD), according to an account by the Australian Transportation Safety Board. Then the PFD airspeed tape indicated that the aircraft was approaching the overspeed and stall limits simultaneously. The pilot-in-command disconnected the autopilot and started to stabilize the aircraft, but during this time it zoomed to 41,000 ft. as indicated airspeed dropped from 270 kt. to 158 kt. Both the stall warning and stick- shaker activated.
The pilot lowered the nose, and the autothrottle increased thrust, but the pilot forced the levers back to idle. The aircraft pitched up again and climbed 2,000 ft. The pilot requested a descent and radar assistance from air traffic control and the crew verified airspeed and altitude with the service.
The pitch control returned to normal after 45 sec. as primary flight computer algorithms compensated for the erroneous Adiru inputs.
The PFD indications appeared normal while descending through 20,000 ft., according to the pilot. He turned the left autopilot on, but the aircraft banked to the right and the nose pitched down, and he turned it off. He then tried the right autopilot, but it gave a similar reaction, so he continued flying manually, and had no difficulty doing so. The autothrottle was left armed.
The aircraft was vectored for an instrument landing approach to runway 03 at Perth. When the crew was preparing for the approach at 3,000 ft., the PFD again indicated a low airspeed and the autothrottle responded by increasing thrust. During the approach the wind shear warning sounded, but the wind was gusty with moderate turbulence.
The accelerations recorded by the flight data recorder came from the Adiru and showed abrupt and persistent errors in all three axes. These values were used by the primary flight computer (PFC) during both manual and autopilot control. The PFC compares Adiru data with information from the standby air data and attitude reference unit (Saaru), and this reduced the severity of the initial pitching motion, the Australian Transportation Safety Board says.
Boeing issued a Multi Operators Message on Aug. 9 recommending that the 777 not be flown with an inoperative Saaru, and an Alert Service Bulletin on Aug. 26 describing how to install the acceptable older -03 model of the software in the Adiru, and how to cope with its known flaws. These are heading and drift indication errors. They were addressed with airworthiness directive (AD) 2005-10-03 issued on Apr. 29, which mandated replacing the -03 software with newer -06 or -07 versions, but these contained the flaw discovered in the Malaysian Airlines incident.
The recent FAA emergency AD 2005-18-51 was issued Aug. 29, supersedes the prior AD, and requires installing the Adiru -03 software within 72 hrs. of receipt. It notes that faulty Adiru data could cause anomalies in 777 primary flight controls, autopilot, autothrottle, pilot displays and autobrakes.
via aviation week
09/25/2005 07:44:05 PM
By Michael A. Dornheim
A WILD RIDE
The FAA has issued an emergency airworthiness directive covering Boeing 777 flight control software, following a recent incident in which an aircraft pitched up to stall speed at high altitude.
A Malaysian Airlines 777-200 was carrying passengers from Perth, Australia, to Kuala Lumpur, when it pitched up near the top of climb, gave conflicting signals to the crew and reached a dangerously low airspeed. Problems kept appearing down to the approach back to Perth, but the crew was able to make a safe landing.
The near-term fix, mandated by airworthiness directive 2005-18-51, involves reverting to an earlier version of software with known flaws, itself the subject of a prior airworthiness directive. The earlier flaws are not as severe as those discovered in the newer software, and the old version is seen as the acceptable lesser of two evils. The long-term fix is to eliminate all program bugs.
The problem is with "operational program software" (OPS) in the Honeywell air data inertial reference unit (Adiru), which supplies air data and attitude information that affects several systems, including the primary flight controls. Versions of the OPS after part no. 3470-HNC-100-03 have a flaw that enables them to use data from failed sensors.
The Aug. 1 Malaysian Airlines flight was climbing through 36,000 ft. on autopilot when it experienced a significant pitchup at 5:57 p.m. local time, 32 min. after takeoff and about 15 min. after sunset. The crew reported seeing a low airspeed warning on the engine indication and crew alerting system while the slip indicator swung to full right on the primary flight display (PFD), according to an account by the Australian Transportation Safety Board. Then the PFD airspeed tape indicated that the aircraft was approaching the overspeed and stall limits simultaneously. The pilot-in-command disconnected the autopilot and started to stabilize the aircraft, but during this time it zoomed to 41,000 ft. as indicated airspeed dropped from 270 kt. to 158 kt. Both the stall warning and stick- shaker activated.
The pilot lowered the nose, and the autothrottle increased thrust, but the pilot forced the levers back to idle. The aircraft pitched up again and climbed 2,000 ft. The pilot requested a descent and radar assistance from air traffic control and the crew verified airspeed and altitude with the service.
The pitch control returned to normal after 45 sec. as primary flight computer algorithms compensated for the erroneous Adiru inputs.
The PFD indications appeared normal while descending through 20,000 ft., according to the pilot. He turned the left autopilot on, but the aircraft banked to the right and the nose pitched down, and he turned it off. He then tried the right autopilot, but it gave a similar reaction, so he continued flying manually, and had no difficulty doing so. The autothrottle was left armed.
The aircraft was vectored for an instrument landing approach to runway 03 at Perth. When the crew was preparing for the approach at 3,000 ft., the PFD again indicated a low airspeed and the autothrottle responded by increasing thrust. During the approach the wind shear warning sounded, but the wind was gusty with moderate turbulence.
The accelerations recorded by the flight data recorder came from the Adiru and showed abrupt and persistent errors in all three axes. These values were used by the primary flight computer (PFC) during both manual and autopilot control. The PFC compares Adiru data with information from the standby air data and attitude reference unit (Saaru), and this reduced the severity of the initial pitching motion, the Australian Transportation Safety Board says.
Boeing issued a Multi Operators Message on Aug. 9 recommending that the 777 not be flown with an inoperative Saaru, and an Alert Service Bulletin on Aug. 26 describing how to install the acceptable older -03 model of the software in the Adiru, and how to cope with its known flaws. These are heading and drift indication errors. They were addressed with airworthiness directive (AD) 2005-10-03 issued on Apr. 29, which mandated replacing the -03 software with newer -06 or -07 versions, but these contained the flaw discovered in the Malaysian Airlines incident.
The recent FAA emergency AD 2005-18-51 was issued Aug. 29, supersedes the prior AD, and requires installing the Adiru -03 software within 72 hrs. of receipt. It notes that faulty Adiru data could cause anomalies in 777 primary flight controls, autopilot, autothrottle, pilot displays and autobrakes.
via aviation week
