From the NTSB Safety Recommendation 4/23/04 (11 page pdf file)

-- Role of the Forest Service and Department of the Interior

As previously mentioned, public firefighting flights are not statutorily required to comply with most FAA regulations including those pertaining to airworthiness and maintenance) nor, accordingly, are they subject to FAA oversight in those areas. Therefore, the Forest Service and the DOI, as the operators of these flights, are primarily responsible for ensuring he safety of these operations. Although they attempt to compel safe operations through the use of contract requirements, the Board’s investigation revealed that their oversight and infrastructure simply are not adequate to assure safe operations. The Safety Board concludes that these agencies must ensure the continuing airworthiness of their firefighting aircraft, which necessarily includes monitoring the adequacy of their maintenance programs.

Therefore, the Safety Board believes that the Forest Service Service and the DOI should each develop maintenance and inspection programs for aircraft that are used in firefighting operations that take into account and are based on:

1) the airplane’s original design requirements and its intended mission and operational life; 
2) the amount of operational life that has been used before entering firefighting service; 
3) the magnitude of maneuver loading and the level of turbulence in the firefighting environment and the effect of these factors on remaining operational life; 
4) the impact of all previous flight hours (both public and civil) on the airplane’s remaining operational life; and 
5) a detailed engineering evaluation and analysis to predict and prevent fatigue separations such as those involved in the three accidents discussed above. In addition, the Safety Board believes that the Forest Service and the DOI should require that aircraft used in firefighting operations be maintained in accordance with these maintenance and inspection programs.

Further, the Safety Board believes that the Forest Service and the DOI should hire personnel with aviation engineering and maintenance expertise to conduct appropriate oversight to ensure these maintenance requirements are met.

The Safety Board is aware that the Forest Service has recently embarked on a multiyear plan to evaluate and improve the airworthiness of its air tanker fleet, including modification of its maintenance program so that it more closely reflects the firefighting mission. The Board supports this initiative and looks forward to learning more about the progress and results of this plan.

<snipped the part about the FAA>

Therefore, the National Transportation Safety Board makes the following recommendations:

Develop maintenance and inspection programs for aircraft that are used in firefighting operations that take into account and are based on:

1) the airplane’s original design requirements and its intended mission and operational life; 
2) the amount of operational life that has been used before entering firefighting service; 
3) the magnitude of maneuver loading and the level of turbulence in the firefighting environment and the effect of these factors on remaining operational life; 
4) the impact of all previous flight hours (both public and civil) on the airplane’s remaining operational life; and 
5) a detailed engineering evaluation and analysis to predict and prevent fatigue separations. (A-04-29) 

Require that aircraft used in firefighting operations be maintained in accordance with the maintenance and inspection programs developed in response to Safety Recommendation A-04-29 (A-04-30).

Hire personnel with aviation engineering and maintenance expertise to conduct appropriate oversight to ensure the Maintenance requirements specified in Safety Recommendation A-04-29 are met. (A-04-31)

<snipped the part about the FAA>