School of Aeronautics and Astronautics The Impact of Maintenance on Passenger Airline Safety Matthew Robichaud August 28 th, 2010
Outline 1. Motivation for the project. 2. Maintenance-related accident case studies. 3. Description of NTSB accident database. 4. Discussion of safety metrics. 5. Results of accident analysis. 2
Motivation A significant amount of research has been done on reducing mechanical failures and aircrew mistakes. The impact of maintenance on aviation safety has not attracted the same amount of attention. Project aims to provide insight and quantify the contribution of maintenance to aviation risk. Analyzed all Part 121 accidents investigated by the NTSB between 1962 and 2008. 3
Case Studies (1 of 3) Japanese Airlines Flight 123 (Boeing 747) Crashed on August 12 th, 1985. Aircraft suffered a tail strike in 1978. Inadequate repairs were made to the damaged area. Suffered an explosive decompression due to metal fatigue 7 years later. Failure or rear bulkhead caused the vertical stabilizer to separate from the aircraft. Deadliest single aircraft accident to date. 505/509 lost. Source: http://www.airdisaster.com/special/special-jal123.shtml 4
Case Studies (2 of 3) United Airlines Flight 232 (DC-10) Crashed on July 19 th, 1989. Maintenance failed to detect a fatigue crack on one of the engine s fan disks. Inspection and quality control procedures were inadequate. Uncontained destruction of the engine during flight. Self-destruction of engine cut all three hydraulic lines. 111/296 lost. Source: http://www.airdisaster.com/special/special-ua232.shtml 5
Case Studies (3 of 3) Alaska Airlines Flight 261 (MD-83) Crashed on January 31 st, 2000. Aircraft lost vertical pitch control and impacted the Pacific Ocean at high velocity. Lack of lubrication on the threads of the jackscrew assembly which controls the trim of the horizontal stabilizer. Source: Snapshot from: Mayday - Air Disasters. Episode: Cutting Corners 6
NTSB Accident Database (1 of 2) Analyzed all Part 121 accidents investigated by the NTSB between 1962 and 2008. Definition of Accident: an occurrence associated with the operation of an aircraft which takes place between the time any person boards the aircraft with the intention of flight and all such persons have disembarked, and in which any person suffers death or serious injury, or in which the aircraft receives substantial damage (NTSB, 2009b) 14 CFR Part 121: scheduled or non-scheduled passenger-carrying operations that adhere to regulations that limit operations to controlled airspace and controlled airports for which specific weather, navigational, operational, and maintenance support are available (NTSB, 2009a) 7
NTSB Accident Database (2 of 2) NTSB database summarized the causes and factors of each accident. NTSB Introduced a new classification scheme in 1982. Database does not provide the relative importance of each cause or factor. Maintenance-related accidents were defined as possessing at least 1 maintenance-related cause or factor. Table 1. Pre-1982 Maintenance Subject Codes Code Description 68D0 Improper Maintenance (Maintenance Personnel) 68D1 Improper Maintenance (Owner Personnel) 68D2 Improperly Serviced Aircraft (Ground Crew) 68D3 Improperly Serviced Aircraft (Owner-Pilot) 68D4 Inadequate Inspection of Aircraft (Maintenance Personnel) 68D5 Inadequate inspection of Aircraft (Owner-Pilot) 68D6 Inadequate Maintenance and Inspection 68D7 Other 68D8 Unknown/Not Reported *Table 2. Post-1982 Maintenance Subject Codes Code Description 24100 Maintenance 24101 Service of Aircraft/Equipment 24102 Inspection 24107 Adjustment 24111 Installation 24115 Replacement 24119 Service Bulletin/Letter * Table 2 only includes the most frequently reported maintenance codes. 8
Safety Metrics Metrics used: 1. Accidents per year 2. Fatalities per year 3. Fatal accidents per year 4. Average number of fatalities per accident per year 5. Passenger mortality risk Issues surrounding chosen metrics: Passenger mortality risk is the most sound metric but does not lend itself well to year to year comparisons. Metrics 1, 2 and 3 do not convey the severity of the accidents when considered by themselves. The number of fatal accidents does not accurately reflect passenger mortality risk since aviation accidents are often survivable. Average passenger survival rate: 35% (for all fatal accidents between 1982-2008) All metrics must be considered together to obtain a holistic view of the impact maintenance has on passenger airline safety. 9
Accident Analysis (Number of accidents per annum) Maintenance has contributed towards: 70 Total Accidents Maintenance-Related Accidents 7% of all accidents between 1962 and 2008. 5% of all accidents between 1998 and 2008. Number of Accidents 60 50 40 30 20 10 0 Change in Classification Scheme 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002 2006 Year Accidents Fatalities Fraction of Accidents and Fatalities Related to Maintenance 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 1962 1966 1970 1974 1978 1982 Year Change in Classification Scheme 1986 1990 1994 1998 2002 2006 10
Accident Analysis (Number of fatalities per annum) Maintenance has contributed towards: 700 Total Fatalities Maintenance-Related Fatalaties 15% of all fatalities between 1962 and 2008. 13% of all accidents between 1998 and 2008. Number of Fatalities 600 500 400 300 200 100 0 Change in Classification Scheme Maintenance-related accidents tend to have a slightly higher fatality rate than accidents overall. Fraction of Accidents and Fatalities Related to Maintenance 1962 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 1962 1966 1966 1970 1970 1974 1974 1978 1982 Year Accidents 1978 1982 Year 1986 1990 1994 1998 2002 Fatalities Change in Classification Scheme 1986 1990 1994 1998 2002 2006 2006 11
Accident Analysis (Average number of fatalities per accidents per annum) Peaks in the number of fatalities are the result of a few high fatality accidents. Total Maintenance Related Average Number of Fatalities per Accide ent 60 50 40 30 20 10 0 1962 1966 (1979, 273) Change in Classification Scheme 1970 1974 1978 1982 1986 1990 1994 1998 2002 2006 Year 12
Accident Analysis (Number of fatal accidents per annum) 13% (20%) of all fatal accidents between 1962 and 2008 (1998 and 2008) are maintenance-related. Peaks are due to a few severe accidents. Maintenance accidents, if fatal, tend to be severe. Maintenance accidents are almost twice as likely to be fatal compared to accidents overall. 13% of maintenance-related accidents are fatal. 7% of all accidents are fatal. Total Maintenance Related Number of Fatal Accide ents 8 7 6 5 4 3 2 1 0 Change in Classification Scheme 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002 2006 Year 13
Accident Analysis (Passenger Mortality Risk) Considers only passenger fatalities. Used data from US Bureau of Transportation Statistics Approximate mortality risk: 1 in 10 million (total) 0.3 in 10 million (maintenance-related) Maintenance Accounts for 30% of passenger mortality risk in commercial aviation. Average passenger survival rate: Fatal maintenance-related accidents: 1982-2008: 37% 1996-2008: 20% All fatal accidents: 1982-2008: 35% 1996-2008: 24% 14
Accident Analysis (Distribution of Maintenance Codes) Cause Factor Cause Factor N/A 68D8 - Unknown/Not Reported 24116 - Major Repair 24113 - Modification 24103 - Compliance with AD Mainte enance Code 68D2 - Improperly Serviced Aicraft (Ground Crew) 68D4 - Inadequate Inspection of Aicraft (Maintenance Personnel) 68D6 - Inadequate Maintenance and Inspection Main ntenance Code 24122 - Major Overhaul (engine) 24118 - Record Keeping 24112 - Lubrication 24106 - AAIP/Progressive Program 24121 - Overhaul 24115 - Replacement 24107 - Adjustment 24111 - Installation 24119 - Service Bulletin/Letter 68D0 - Improper Maintenance (Maintenance Personnel) 24101 - Service of Aircraft/Equipment 24100 - Maintenance 24102 - Inspection 0 20 40 60 Frequency 0 5 10 15 20 Frequency 15
Summary Improper maintenance can have devastating consequences. JAL 123, UA 232, ASA 261 7% (5%) of all aviation accidents, 15% (13%) of all aviation fatalities, and 13% (20%) of all fatal accidents between 1962 and 2008 (1998 and 2008) were maintenance related. Maintenance plays a role in up to 2 fatal accidents each year. Maintenance-related accidents are almost twice as likely to be fatal compared to accidents overall (13% vs. 7%). Maintenance accounts for 30% of the passenger mortality risk in commercial aviation. 16
References Barnett, A. (2007, November). Measure for Measure. Aerosafety World Magazine, pp. 48-52. NTSB. (2009a). Annual Review of Aircraft Accident Data -2005. Washington, DC: National Transportation Safety Board (NTSB). NTSB. (2009b). NTSB Aviation. Retrieved February 17, 2010, from Aviation Accident Database - General Aviation: http://www.ntsb.gov/aviationquery/aviationqueryhelp.htm 17