T-910 on the Soberanes Fire south of Monterey, California in 2016. Photo by Wally Finck.
(Originally published at 9:50 a.m. MST March 5, 2018)
The National Transportation Safety Board published this article March 1, 2018 on their NTSB Safety Compass website. It provides details about how the U.S. Forest Service and the NTSB have worked together to attempt to mitigate some of the risks of flying old aircraft converted to air tankers low and slow close to the ground while experiencing high load factors.
By Jeff Marcus and Clint Crookshanks
One enduring image of the fight against forest fires, like those that devastated California last year, is of a large airplane flying low and dropping red fire retardant. These firefighting air tankers are invaluable, and they operate in extreme environments.
Over the years, we’ve investigated several accidents involving firefighting aircraft, identifying issues and making recommendations to ensure the safety of these important assets. For example, in 1994, we investigated an accident in which a retired Air Force Lockheed C-130A Hercules, which had been converted into a firefighting airplane and was under contract to the US Forest Service (USFS), crashed while battling a fire in the Tehachapi Mountains near Pearblossom, California, killing all three flight crewmembers. In June 2002, another retired Air Force Lockheed C-130A Hercules, also converted into a firefighting aircraft and under contract to the USFS, crashed while dropping fire retardant near Walker, California, killing the three flight crewmembers onboard. Just a month later, a retired Navy Consolidated Vultee P4Y-2 Privateer, again under contract to the USFS to fight forest fires, crashed while maneuvering to deliver fire retardant near Estes Park, Colorado, killing both flight crewmembers. We determined that the probable cause in each of these accidents was in‑flight structural failure due to fatigue cracking in the wings, and we concluded that maintenance procedures had been inadequate to detect the cracking.
Firefighting operations inherently involve frequent and high-magnitude low-level maneuvers with high acceleration loads and high levels of atmospheric turbulence. A 1974 NASA study found that, at that time, firefighting airplanes experienced maneuver load factors between 2.0 and 2.4—almost a thousand times more than those of aircraft flown as airliners. The NASA study concluded that, because the maneuver loading in firefighting airplanes was so severe relative to the design loads, the aircraft should be expected to have a shortened structural life. Repeated and high‑magnitude maneuvers and exposure to a turbulent environment are part of firefighting service, and these operational factors hasten fatigue cracking and increase the growth rate of cracking once it starts.
Aerial firefighting is an intrinsically high-risk operation; however, the risk of in‑flight structural failure is not an unavoidable hazard; rather, fatigue cracking and accelerated crack propagation should be addressed with thorough maintenance programs based on the missions flown. Aircraft maintenance programs, which are typically developed by airplane manufacturers, usually point out highly stressed parts that should be inspected for signs of fatigue cracking, and they give guidance on how often these parts should be inspected. When specifying a maintenance program, manufacturers typically consider the expected loads that an airplane will encounter; however, in the past, for many aircraft used in firefighting operations, very little, if any, ongoing technical and engineering support was available because the manufacturer no longer existed or did not support the airplane, or the military no longer operated that type of aircraft. The maintenance and inspection programs being used for the firefighting aircraft mentioned above did not account for the advanced age and the more severe stresses of the firefighting operating environment.
As a result of our investigations, we issued safety recommendations to the USFS to hire appropriate technical personnel to oversee their airtanker programs, improve maintenance programs for firefighting airplanes and to require its contractors to use these programs. The USFS responded promptly and effectively, substantially improving the safety of its firefighting operations. The USFS hired a team to build out its Airworthiness Branch, to lead their effort to comply with the NTSB recommendations, and with this staff of engineers and technicians made needed revisions to the contracting, oversight, and operations of the USFS program using airplanes to fight forest fires. The agency hired aircraft engineering companies that performed in‑depth stress analyses on the firefighting airplanes in operation. The results were used to improve maintenance programs by identifying parts of the aircraft structure in need of continuing inspections and proposed the time and use intervals needed between inspections to prevent fatigue cracks from developing into catastrophic structural failures. The USFS also outfitted firefighting aircraft (tankers as well as helicopters and lead aircraft) with equipment that measures and records the actual flight loads experienced while fighting forest fires, then used that data to further improve the inspection program for airplanes in use and to develop programs for new types of airplanes being introduced to fight forest fires.
Clint Crookshanks, an NTSB aviation structural engineer and aircraft accident investigator who worked on these airtanker accidents, helped the USFS review its contractors’ maintenance and inspection program documents and provided advice on how they could better address our recommendations. On November 5, 2010, the USFS issued its first iteration of a Special Mission Airworthiness Assurance Guide for Aerial Firefighting and Natural Resource Aircraft, which contained the method, schedule, and standards for ensuring the airworthiness of firefighting aircraft. The USFS has revised the guide twice since then, with the latest revision issued on November 6, 2015. The guide now includes standards for USFS aircraft contracts, which are required for all aircraft used in USFS firefighting missions, satisfying our recommendations. Since these improvements were implemented, no aircraft performing aerial firefighting missions for the USFS have experienced an in‑flight structural failure.
We continue to work with the staff at the USFS to improve the safety of firefighting flights. At the beginning of January 2018, Clint attended a meeting in Missoula, Montana, to discuss the current and future large airtankers on contract to the USFS. Our recommendations are still relevant to the USFS and its contract operators and were the basis for most of the discussion at the Missoula meeting. The current USFS contract requirements have ensured that all contractors have effective maintenance and inspection programs that account for the extreme operating environments seen in aerial firefighting. Aircraft providing aerial firefighting services contain equipment that records the loads on the aircraft and even provides an alarm in real-time when a flight’s loads may have overstressed the airplane. In addition, the data recorded is downloaded and supplied to Wichita State University for mission profile development. British Aerospace, which originally manufactured the jet powered BAe 146 and RJ-85 airplanes currently used for USFS firefighting operations, provides technical support for these airplanes’ operators. The US Air Force also provides firefighting service using C-130 airplanes equipped with a Mobile Airborne Firefighting System (MAFFS) to assist the USFS on an as needed basis. The manufacturer of the C-130, Lockheed-Martin, is working with the Air Force to continually monitor and analyze the loads on airplanes used in the firefighting mission.
The importance of keeping these unique aircraft and their crews safe and functional becomes even more evident during every forest fire season. The lessons we’ve learned from our accident investigations have been used to identify needed changes that have made it possible to more reliably and safely fight forest fires from the air and protect life and land.
Jeff Marcus is an Aviation Transportation Safety Specialist in the NTSB Office of Safety Recommendations and Communications. Clint Crookshanks is an aviation structural engineer and aircraft accident investigator in the NTSB Office of Aviation Safety.
Thanks and a tip of the hat go out to Isaac.
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