KOREAN AIR FLIGHT 801 CRASH: A CASE STUDY Vidyadhar Rangojoo EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

EE 585: Fault tolearnt Computing On August 6, 1997 1:42 AM. Korean Airlines Flight 801 crashed into terrain on approach to Won Pat Int’l airport, Guam. The impact site lies at 658ft on Nimitz Hill. Death toll: 237. Survivals: 28. Classified as a Controlled-flight-into-terrain (CFIT) accident. On August 6, 1997 at about 1:42am Guam local time, Korean Air flight 801, a Boeing 747-300, crashed into Nimitz Hill, Guam while attempting a no precision approach to runway 6L at A.B. Won Guam International Airport. During its investigation, the NTSB found that a ground-based minimum safe altitude warning system (MSAW), designed to alert air traffic controllers of aircraft flying too low, had been inhibited. The Commercial Aviation Safety Team (CAST) defines CIFT accident as one in which “a fully qualified and certified crew flies a properly working airplane into the ground, water, or obstacles with no apparent awareness by the pilots”. EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

EE 585: Fault tolearnt Computing Wreckage of Flight 801… Ariel view of the location of crash Crash site Some of the available pictures of the location of crash. EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

Wreckage of Flight 801…CONTD KAL 801 after the crash Pictures of the wrecked air flight 801 . Ariel view EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

Reasons for the crash… Complex combination of events: Glide slope not operational. Pilot error. Inhibition of Minimum Safe Altitude Warning System: Ground based controller. Alerts controllers visually/aurally when tracked flight descend below a predetermined min safe altitude. The horizontal needle represents the glideslope. The horizontal needle represents the glide slope. EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

EE 585: Fault tolearnt Computing MSAW Alert EE 585: Fault tolearnt Computing

MSAW General Monitoring Minimum Safe Altitude (MSA) 500´ Threshold Terrain Clearance Altitude The MSAW system is a ground-based system that alerts controllers visually and aurally when an IFR-tracked flight descends below, or is predicted to descend below, a predetermined minimum safe altitude (MSA). The system itself is entirely software, relying on existing radar hardware to assist controllers in detecting low-flying aircraft. The system identifies low-flying aircraft, and if it has violated its MSA, the system raises an alert to air traffic controllers. EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

EE 585: Fault tolearnt Computing MSAW System Overview… 1990: System configured to monitor area within 55-nm radius of the Guam radar. 1993: New system installed; citing frequent nuisance warnings issued by system. This change reduced MSAW processing to a 1-nm ring b/w 54-nm and 55-nm from radar facility. New MSAW software installed at Guam to update terrain database; includes the inhibition zone. Facility Inspection of this software failed to note MSAW inhibition entirely. EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

The Guam MSAW Inhibition zone… MSAW Service Area Boundary (55 nm) Inhibit Zone (54 nm) Guam The MSAW’s path monitoring tracks aircraft operating within specially designated rectangular regions called capture boxes where aircraft typically performs final approach maneuvers. Inside each capture box, the MSAW system simulates a glide slope descent path to determine whether an aircraft on final approach has descended, or is projected to descend, below the desired path. In order to reduce the frequency of nuisance warnings generated by the system, inhibition zones were added to the configuration. Due to this the originally configured monitored area of 55-nm was modified to 54-nm inhibition zone , effectively reducing the MSAW processing to a 1-nm ring as shown above. Pacific Ocean EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

EE 585: Fault tolearnt Computing Analysis… The glideslope for runway 6L was out of service, captain lost awareness of flight’s position on final approach. Altitude callouts by onboard GPWS ignored by flight crew; mistaken for nuisance messages over Guam. Poor illumination surrounding Nimitz Hill. EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

Effectiveness of MSAW…. NTSB/FAA simulation indicated that, without the inhibition, MSAW would have generated an alert 64 seconds before impact. NTSB: This would have been sufficient for the controller to advise KA 801. NTSB concluded that FAA’s quality assurance of MSAW was inadequate. EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

EE 585: Fault tolearnt Computing Lessons learnt…. Configuration management. Human error. EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

EE 585: Fault tolearnt Computing Related Incidents… Dulles International Airport,1994. Houston Intercontinental Airport,1998. EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

EE 585: Fault tolearnt Computing References.. Paper titled “What should aviation safety incidents teach us?” by John C. Knight. Paper titled “Framing analysis of software failure with safety cases “.William S. Greenwell, John C. Knight . http://www.cnn.com/WORLD/9708/10/guam.crash/ http://ns.gov.gu/guam/indexmain.html EE 585: Fault tolearnt Computing EE 585: Fault Tolerant Computing.

EE 585: Fault tolearnt Computing QUESTIONS……??? EE 585: Fault tolearnt Computing