Airline safety investments and operating conditions: determinants of aircraft damage severity.

• Author: Phillips, Richard A.

1. Introduction

   This study investigates determinants of the severity of damage to aircraft involved in commercial airline accidents and incidents.(1) Airline safety (or the risk of airline travel to the consumer) may be defined as the probability of sustaining injury or death. There are two aspects of this probability: (1) the likelihood of the occurrence of "safety related events" such as airline accidents or incidents (event probability) and (2) the severity of the event given that it has occurred (severity conditional probability). The research heretofore has focused on the former and not the latter aspect of airline safety.(2) However, Moses and Savage |11, 171~ note a limitation of the event probability approach by stating that "even if we were to define safety as the probability that a trip would end in an accident, there still would be the problem that accidents vary in severity from minor damage-only incidents to major tragedies with loss of life." An airline accident, for instance, may involve no deaths or injuries at one extreme to all persons killed and the aircraft destroyed at the other extreme.

   This paper contributes to the air safety literature by focusing on the severity of accidents and incidents as an alternative measure of safety and analyzing determinants of severity levels. A micro-data set (from the National Transportation Safety Board) which includes detailed information on individual aircraft accidents and incidents is utilized. Since the severity of an accident is conditioned upon the occurrence of the accident, determinants of the former will be correlated with determinants of the latter. Therefore, analyzing factors that affect severity will yield insight on determinants of the overall level of air safety.

   The results of the study suggest that airline safety investments such as more experienced pilots and aircraft safety equipment will significantly reduce event (accident and incident) severity as measured by damage to the aircraft. However, the results indicate that safety investments in newer and larger aircraft and more frequent mechanical and safety inspections will have little effect on aircraft damage severity. The results also indicate that visual flying conditions, the provision of certificated rather than commuter airline services(3) and a reduction in pilot utilization (measured by the number of aircraft landings in a specified time period) are operating conditions that are associated with reductions in the severity of aircraft damage (given that an accident or incident has occurred).

   The study is structured as follows. Section II presents a model of aircraft damage severity. Data and estimation results are detailed in section III. The conclusion (section IV) discusses the implications of the findings.

2. A Model of Aircraft Damage Severity

   The severity of the aircraft damage of an airline safety related event is a function of airline safety investments and operating conditions. Safety investments consist of actions by an airline to prove the safety of its service; operating conditions describe the environment in which an airline operates.(4) Examples of safety investments by an airline include hiring more experienced pilots, utilizing larger and newer aircraft, utilizing specific aircraft safety equipment and increasing the frequency of aircraft mechanical and safety inspections. Such investments are hypothesized to reduce the severity of a given aircraft accident or incident.

   Two measures of pilot experience are included in the analysis: PEXP, the cumulative flight time (in hours) of the first pilot for all commercial aircraft and PAGE, the age of the first pilot in years. More experienced pilots are less likely to make judgmental errors. In addition to lowering the probability of event occurrence, a more experienced pilot is also expected to lessen the severity of an event that does occur. For example, a more experienced pilot may be more adept in recognizing the safety implications of a weather system and the handling of a disabled aircraft. A negative relationship is expected between PEXP and the severity of aircraft damage.

   The age of the first pilot (PAGE) is included as an additional measure of pilot experience. The predicted sign of the PAGE coefficient is indeterminant. It is possible that commercial flight time may not include certain types of prior aviation experience. For example, military pilots frequently enter civilian aviation upon retirement at relatively early ages. If so, PEXP will not include this additional aviation experience and hence the relationship between aircraft damage severity and PAGE is expected to be negative. On the other hand, it is possible that advancing age negatively affects pilot performance. This might occur, for example, if age delays the response time of the pilot to in-flight hazards. This would tend to raise the degree of damage for a given event. If, for given levels of PEXP, increasing PAGE reduces pilot performance, the relationship between damage and PAGE is expected to be positive.

   The size of an aircraft (AIRWT) is measured by weight in pounds and age of aircraft (AIRAGE) is measured by the cumulative flight time (in hours) on the airframe. As the size of an aircraft (AIRWT) increases, the severity of aircraft damage is expected to decrease, other factors remaining the same. Aircraft size is correlated with airworthiness--larger aircraft are less susceptible to hazardous weather, wind, and other environmental conditions. Therefore, there is a higher probability of involvement by smaller aircraft in a safety related event. Further, a...