Human Factors in General Aviation CFIT Accidents

Introduction

Why pilots fly perfectly serviceable aircraft into the ground has been a puzzling question since the dawn of aviation. Such controlled flight into terrain (CFIT) occurrences have been identified as a significant contributor to accident statistics in the General Aviation (GA) sector and although much effort has gone into the analysis of CFIT in the commercial and military environments, until recently little effort was put into researching CFIT in GA.

Expanding on the United States Federal Aviation Administration’s Joint Safety Analysis Team (JSAT) study into CFIT in this sector, Shappell and Wiegmann (20011, 20032) looked at applying the Human Factors Analysis and Classification System (HFACS) to this data as a tool to understanding why pilots continue to commit these “seemingly purest of human error accidents” (Shappell & Wiegmann, 2003, p. 1). As background to understanding their findings, it is useful to examine traditional explanations for CFIT and the strategies used to combat these events.

Traditional Explanations and Mitigation Strategies for CFIT

Common believe is that CFIT is usually the result of one, or a combination, of the following factors:

  • Loss of visual reference in conditions of low light or poor visibility
  • Spatial disorientation
  • Visual illusions
  • Inattention
  • Distraction
  • Poor piloting skills

With this in mind, strategies to mitigate the risk of CFIT have included:

  • Encouraged use of ground proximity warning systems (GPWS)
  • Training to improve pilot awareness of factors contributing to CFIT
  • More restrictive organisation-imposed altitude restrictions
  • Strict enforcement of rules

Although effective in the civil and military environment, most of these tools are difficult to implement in the general aviation sector. GPWS, for example, is an excellent tool for enhancing safety, but too expensive for all, but a few, involved in GA. Similarly, strict enforcement of rules becomes a logistical nightmare for the enormous numbers of GA operators, who often fly from remote locations in uncontrolled airspace and any safety added by company SOPs are not relevant to private owners, who need only meet the minimum legal requirements.

Practical intervention strategies for reducing CFIT in the particular sector of general aviation are required to properly address this problem.

HFACS Application to General Aviation CFIT Accidents

Shappell and Wiegmann initially studied 164 fixed-wing general aviation CFIT accidents over a two year period, 1993-1994, and then subsequently 1407 similar events over the period 1990-1998. Causal factors as identified in NTSB reports were examined by independent experts and categorised for each accident using the HFACS framework. Ultra-light aircraft and helicopters, as well as agricultural and commercial ops were not included in the study and as GA accidents had been previously identified as mostly populating the lower two HFACS levels, only these have been focussed on.

Results from HFACS Application to General Aviation CFIT Accidents

The percentage of general aviation accidents found to have causal factors in individual HFACS categories
HFACS Category 1993-1994 Statistics 1990-1998 Statistics
CFIT Accidents CFIT Accidents Non-CFIT Accidents Odds Ratio1
% % %
Unsafe Acts
Skill-based errors 48.8 76.3 73.2 1.178
Decision errors 44.5 33.5 35.3 0.923
Perceptual errors 17.1 12.5 7.2 1.847
Violations 30.5 31.6 12.4 3.264
Preconditions for Unsafe Acts
Adverse Mental States 7.3 12.2 4.5 2.907
Adverse Physiological States 5.5 3.7 2.5 1.497
Physical/Mental Limitations 12.8 12.9 18.9 0.639
Crew Resource Mismanagement 14.0 7.2 11.0 0.631
Personal Readiness 0.0 6.3 1.6 4.089
Reproduced using data from Shappell and Wiegmann (2001, 2003).

Significant findings from the HFACS analysis include:

  • Skill-based and decision errors are the predominant unsafe acts contributing to general aviation CFIT accidents although their prominence is equally strong in non-CFIT accidents.
  • Violations are identified as causal factors in 30% of CFIT accidents, but more significantly the difference between this and the non-CFIT case indicates that in CFIT accidents, violations are more than three times more likely to have been a causal factor than in other accidents.
  • Surprisingly, perceptual errors and adverse physiological states are of minor significance, featuring in only 15% and 5% of CFIT cases respectively. This conflicts strongly with the traditional view that disorientation, visual illusions and poor visual reference are major factors in CFIT and to the contrary, over 50% of those CFIT accidents classified occurred in visual meteorological conditions (VMC).

Strategies to Prevent General Aviation CFIT Accidents

From the HFACS study, various strategies to prevent CFIT in general aviation can be suggested and include those shown below.

HFACS Category Strategy to Prevent General Aviation CFIT Accidents
Unsafe Acts
Skill-based errors Enhanced and more regular training to improve basic handling skills and instrument scans as well as comprehensive Biennial Flight Reviews (BFRs) to ensure pilot proficiency will minimise skill-base errors.
Decision errors Aeronautical Decision Making (ADM) and CFIT awareness training will reduce the likelihood of decision errors being made.
Perceptual errors Training to improve pilot visual scans and the use and standardisation of high visibility paint and obstacle lighting for masts, towers, wires and other obstacles above ground level will minimize perceptual errors.
Violations Exceptional violations are difficult to predict or control, but routine violations often have their roots in organisational culture. Through CFIT awareness training, strict enforcement of regulations and fostering a strong safety culture, pilot attitudes can be changed and violations can be minimized.
Preconditions for Unsafe Acts Enhanced training in CFIT causes, awareness and prevention, along with traditional Human Factors training will mitigate the risk of preconditions for unsafe acts in general.

Conclusion

It can be seen that CFIT accidents in general aviation are a result of not one, but many different unsafe acts and preconditions for those acts and although less prominent in the GA sector, organisational influences and unsafe supervision will likely contribute in some form. Any practical strategy to prevent CFIT accidents will have to address each of the factors. The HFACS framework is an excellent tool to identify and categorise human factors in these accidents, but can also be used to track the effect of any strategies used to prevent them.

References
1. Shappell, S.A. & Wiegmann, D. A. (2001). Unravelling the Mystery of General Aviation Controlled Flight into Terrain Accidents Using HFACS . Retrieved from http://www.humanfactors.illinois.edu/Reports&PapersPDFs/isap01/shappellwiegavpsy01.pdf on 16 August 2010.
2. Shappell, S.A. & Wiegmann, D. A. (2003). A Human Error Analysis of General Aviation Controlled Flight into Terrain Accidents Occurring Between 1990-1998. Retrieved from http://www.hf.faa.gov/docs/508/docs/cami/0304.pdf on 16 August 2010.


Want to know more?

AviationKnowledge - Human Factors Analysis and Classification Systems (HFACS)
Visit here to find out more about HFACS.
AviationKnowledge - Controlled Flight Into Terrain (CFIT)
Visit here to find out more about CFIT.
General Aviation CFIT Joint Safety Implementation Team: Final Report
This document details the strategies implemented to combat CFIT in GA following the FAA's JSAT study.
Controlled Flight into Terrain - Education and Training Aid
This ICAO training aid was produced by an international CFIT task force and contains useful information for policy makers through to pilots.
Flight Safety Foundation - CFIT Checklist
This checklist is a practical tool for pilots and aicraft operators to assess the risk of CFIT for various flights.

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