implementation of the survey reached approximately 7,000 air carrier pilots each year.7 A total of 29,882 pilots were surveyed for the NAOMS study over the period from April 2001 to December 2004.8 Of these pilots, 25,105 participated in the AC survey. The GA survey was conducted for a much shorter period (August 2002 through April 2003) and involved 4,777 pilots. Groups of pilots for both surveys were selected monthly using a simple random sampling from the public ACD.

The sampled pilots were contacted first by mail with a pre-notification letter from the NAOMS team. This letter was followed by a telephone call during which the survey was administered. If the respondent was not available, a callback time was arranged. The survey questionnaire included a computer screen to allow checking for qualifying activity during the recall period, which consisted of the last n days before the survey, with the number n varying initially from 30 to 90 days but fixed at 60 days after March 2002. The survey was conducted by professionally trained interviewers using a computer-assisted telephone interview system.

Each pilot who responded to the survey was asked a set of questions—described in detail in Chapter 5—about his or her background, the number of hours and flights flown, the number of numerous possible safety-related events observed, some topic-specific questions, and feedback about the survey. The information in the responses was restricted to the recall period.

4.2
TARGET POPULATION AND SAMPLING METHOD

For the NAOMS surveys, the two target populations were all flight legs meeting the criteria for AC and GA during the recall period. The NAOMS questionnaires9 indicate that the qualifying AC flight legs were intended to be those conducted under Federal Aviation Regulations (FAR) Part 121 (under which the major passenger and large cargo airlines such as FedEx fly).10 Considering air carrier operations as those operating under Part 121 is consistent with the practices of the U.S. Department of Transportation.11 The flights of interest in the GA questionnaire were those conducted under FAR Parts 9112 and 135. However, because FAR Part 135 governs the operation of scheduled commuter air carriers and on-demand, for-hire air taxi and charter providers,13 the inclusion of flights operated under Part 135 in the general aviation survey extended the notion of general aviation well beyond normal usage of the term. In its general aviation safety statistics, the U.S. Department of Transportation specifically excludes Part 135 operations and considers Part 135 scheduled operations to be a segment of aviation separate from both general aviation and also from Part 135 on-demand operations.14 The GA survey did not collect the information that would have enabled events from these disparate segments to be disaggregated.

The ideal sampling frame for this population would be the list of all flight legs that occurred in the appropriate flight regimes, that is, Part 121 flights in the AC survey and Part 91 and Part 135 (given the NAOMS definition of general aviation) in the GA survey during the recall period. However, collecting data for a simple random sample of flight legs would not have been economical or even feasible. The NAOMS team decided to draw samples of pilots and to ask them about all events that occurred during the recall period. This strategy results in a cluster sampling of flight legs: pilots are the primary sampling units, and all the flights flown by the sampled pilots during the recall period are then included in the sample.

Such a cluster sample of flights differs from a simple random sample in several ways. In particular, the flight

7

Battelle, NAOMS Reference Report, 2007, p. 34.

8

Ibid., p. 13. Other sources provide slightly different numbers, in part because of reclassifications, different data releases, and so on.

9

Ibid., Appendixes 11 and 12.

10

FAR Part 121 refers to a section of the FAA Federal Aviation Regulations that prescribes safety rules governing the operation of air carriers and commercial operators of large aircraft. The term Part 121 carriers refers to carriers operating under these regulations; see Air Transport Association, The Learning Center: Glossary, 2009, available at http://learningcenter.airlines.org/Pages/Default.aspx?Filter=p, accessed July 15, 2009.

11

See, for example, Bureau of Transportation Statistics, National Transportation Statistics 2009, Research and Innovative Technology Administration, U.S. Department of Transportation, Washington, D.C., 2009, Table 2-9: U.S. Air Carrier Safety Data.

12

FAR Part 91 refers to a section of the FAA Federal Aviation Regulations that includes principally general aviation.

13

Air Transport Association, The Learning Center: Glossary, 2009.

14

See, for example, Bureau of Transportation Statistics, National Transportation Statistics 2009, Table 2-9: U.S. Air Carrier Safety Data.



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