Douglas Wiegmann, Ph.D.

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HFACS ANALYSIS OF GENERAL AVIATION ACCIDENT DATA
IMPLICATIONS FOR AERONAUTICAL DECISION-MAKING
Douglas Wiegmann, Ph.D. University of Illinois
Scott A. Shappell, Ph.D. Civil Aeromedical
Institute
FAA-99-G-006
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Method

• A comprehensive review of fatal 14 CFR Part 91
  accidents between January 1990 and December 1998
  was conducted using database records maintained
  by the NTSB and the FAA.
• A total of 2,391 accidents was identified for
  further analysis
• Only those accidents in which the accident
  investigation was completed and causal factors
  determined were included in this analysis.

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Method

• The 5,893 causal factors associated with these
  2,391 accidents were independently coded by five
  general aviation pilots
• This HFACS coding focused solely on the causal
  factors identified by the NTSB during the
  original accident investigation.
• No new cause factors were created during this
  error-coding process.

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Method

• Coders
• Five general aviation pilots/certified flight
  instructors
• Flight hours ranged from 1,250 to 11,000
• Training
• Four-hour workshop on HFACS
• Practice coding 20 accidents as a group
• Practice coding 50 accidents independently,
  followed by a review/consensus meeting

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Sample of the Types of Human Error Typically Found
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Procedure

• Each pilot was assigned 1/3 of the accidents for
  a given year.
• Independently coded NTSB cause factors (no new
  ones created)
• Randomly paired with a second pilot who coded the
  same set of accidents.
• Pilots met to compare codes and achieve consensus
• They were then assigned another 1/3 of the
  accidents for a particular year and randomly
  paired with another pilot.
• This process continued until all the accidents
  had been coded.

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Results

• Reliability of Coding Process
• On average, pilot agreed 79 of the time on how
  the causal factors should be coded using HFACS.
• These percentages varied only slight across the
  years of data analyzed in this study (range was
  77 to 83 agreement).
• When overall agreement was corrected for chance
  using Cohens Kappa, the resulting index was
  .722, which is considered good by conventional
  standards.

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Percentage of Accidents
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Results
Category of Cause-factors ()
Other (26.5)
Unsafe Acts (73.5)
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Results
Types of Unsafe Acts ()
N 2597
of Cause Factors
N 912
N 714
N 109
Type of Unsafe Act
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Percentage of Cause Factors
Skill-based Errors
Percentage of Cause Factors
Violations
Decision Errors
Perceptual Errors
Year
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Variety of Decision Codes (N 185)
inflight planning decision improper altitude
inadequate judgment poor inflight planning
decision poor inflight planning decision
inadequate planning decision improper refueling
not performed aborted take off not performed low
altitude flight maneuver performed remedial
action delayed aborted landing delayed wrong
runway selected all available runway not
used weather evaluation inadequate go around
delayed preflight briefing service
disregarded altitude improper aerobatics
performed unsuitable terrain for take off landing
selected preflight briefing service not
used hazardous weather advisory
disregarded procedure directives not followed vfr
into imc inadvertent lift off premature aborted
take off delayed
preflight briefing service not obtained weather
forecast disregarded go around not
performed altitude low inflight planning decision
delayed anti ice de ice system not used proper
altitude not selected flight to alternate
destination not performed weather evaluation
misjudged weather evaluation improper weather
evaluation poor missed approach not
performed emergency procedure simulated vfr into
imc attempted pull up excessive pull up
delayed planned approach improper planned
approach poor taxi speed excessive ostentatious
display fuel supply inadequate carburetor heat
improper use of flight into adverse weather
continued flight into adverse weather
inadvertent low altitude flight maneuver
intentional
flight into adverse weather initiated emergency
procedure delayed flight advisory
disregarded weather forecast not
obtained maneuver excessive remedial action
inadequate stall spin initiated missed approach
delayed gear retraction not performed low pass
performed planning decision poor carburetor heat
not used flight to alternate destination
delayed updating of recorded weather info not
obtained pull up performed go around
attempted emergency procedure not
performed altitude clearance inadequate improper
decision stall spin performed maneuver
performed remedial action not performed refueling
improper ac handling improper proper altitude not
maintained
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Top 10 Decision Errors
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Variety of Violation Codes (N 115)
ac control exceeded ac control not maintained ac
unattended engine running intentional ac weight
balance continued ac weight balance
disregarded ac weight balance exceeded ac weight
balance excessive ac weight balance improper act
clearance not complied aerobatics
attempted aerobatics improper aerobatics
initiated aerobatics intentional aerobatics
performed air speed exceeded altitude clearance
inadequate altitude disregarded altitude
inadequate altitude low ATC clearance not
followed attitude indicator not available buzzing
intentional buzzing performed certification
improper for flight decision height disregarded
decision height not complied with decision height
not maintained decision height not used descent
height disregarded design stress limits of ac
exceeded dispatch procedures not followed other
govt personnel external navigation lights not
used flight into adverse weather flight into
adverse weather attempted flight into adverse
weather continued flight into adverse weather
improper flight into adverse weather
inadvertent flight into adverse weather
initiated flight into adverse weather
intentional flight into adverse weather
performed flight into adverse weather
selected flight manuals disregarded flight
navigation instruments inadequate fuel supply
inadequate hazardous weather advisory
disregarded ice frost removal from ac
improper ifr procedure improper ifr procedure not
followed impairment alcohol
impairment drugs inflight briefing service not
used inflight planning decision
improper information insufficient designated
examiner information insufficient pic loading of
cargo improper low altitude flight maneuver
attempted low altitude flight maneuver
intentional low altitude flight maneuver
performed low pass intentional low pass
performed maintenance annual inspection not
complied with maintenance annual inspection not
performed maintenance design changes improper
pic maintenance installation improper maintenance
major alteration improper pic maintenance major
repair improper pic maintenance service bulletin
not complied with maneuver attempted minimum
descent altitude below minimum descent altitude
disregarded minimum descent altitude not complied
with minimum descent altitude not
maintained minimum descent altitude not
obtained/maintained
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Top 10 Violations
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Top 5 Decision Errors/Violations
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Conclusions

• Top factors involve weather-related issues
• These factors have been explore by the Wx JSAT
• FAA is sponsoring research to empirically explore
  these factors (go beyond expert opinion)
• When decision errors are addressed separately
  from violations
• Flight planning and management become the top
  issues
• This may be more inline with the traditional
  conceptualization of ADM