NTSB and Forest Service work to reduce in‑flight structural failures on air tankers

Metal fatigue cracking was identified as an issue in several crashes

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.

Range Fire air tanker
Air tanker 12 on the Range Fire in Southern California, August 27, 2016. Photo by Kern County Fire Department.

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.

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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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More information about Croman’s S-61A crash

S-61A crash sikorsky helicopter
S-61A crash

It was initially described as a “hard landing”. However, information from the FAA and a photo we received indicate an incident that involved one of Croman’s S-61A Sikorsky helicopters on August 19, 2015 (that we wrote about on August 24) was more than that. We can’t verify with 100 percent certainty that the helicopter in the photo above is Croman’s S-61A, N1043T that crashed that day while working on the Eldorado Fire eight miles southeast of Unity, Oregon. But the person who sent us the photo said it is, and the paint job, the position of the helicopter, and the damage to the tail boom match the NTSB’s description of the crash.

Below is text from the NTSB Preliminary Report, ID# WPR15LA248, that was updated on September 3, 2015:

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“14 CFR Part 133: Rotorcraft Ext. Load
Accident occurred Wednesday, August 19, 2015 in Ironside, OR
Aircraft: SIKORSKY S 61A, registration: N1043T
Injuries: 1 Minor, 1 Uninjured.

This is preliminary information, subject to change, and may contain errors. Any errors in this report will be corrected when the final report has been completed. NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

On August 19, 2015, about 1930 Pacific daylight time, a Sikorsky S-61A, N1043T, landed on a mountainside after experiencing a partial loss of engine power about 7 miles west of Ironside, Oregon. The commercial pilot sustained no injuries and the air transport pilot sustained minor injuries. The helicopter sustained substantial damage to the tailboom. The helicopter was registered to, and operated by, Croman Corp under the provisions of 14 Code of Federal Regulations Part 133 as a firefighting flight. Visual meteorological conditions prevailed for the flight, which operated under a company flight plan. The flight originated from Baker City Municipal Airport (BKE), Baker City, Oregon at 1715.

The commercial pilot reported that shortly after picking up a bucket of water from a pond he gained airspeed and initiated a climbing left turn back towards the fire. As the helicopter started to climb, he heard a drop in RPM and the helicopter lost power. He attempted to continue the climb; however, the helicopter was too heavy. He released the water and landed the helicopter on a mountain side; subsequently, the helicopter rolled onto its right side.

The helicopter has been recovered to a secure location for further examination.”

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UPDATE, September 15, 2015:

Earlier this year a Croman S-61A helicopter’s main rotor hit a tree while dipping water on the Cabin Fire on the Sequoia National Forest in California. Below is an excerpt from the Rapid Lesson Sharing report dated August 4, 2015:

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“…As the pilots descended into the dipsite, the SIC communicated instructions to the PIC to “stay left” of the trees. While in the dip, the PIC heard what he suspected was a blade strike, called out the strike, jettisoned the water and immediately initiated a climb out to get clear of the area.

The pilots assessed the condition of the blades and saw no noticeable damage while in flight. On the climb out, the SIC noticed a smaller di-ameter tree (estimated to be about 8 ft. in height) that had been located at the helicopter’s 4 o’clock position, and missing its top. The Air Attack was notified about the potential blade strike and the pilots provided their intentions to land at the first opportunity. During the short flight to the first suitable landing site, the pilots noted no vibrations or abnormalities.

The crew performed a precautionary landing in a field located approximately 10 minutes away from the dip site. The Helicopter Manager was notified of the situation via cell phone. After shut down was complete, the pilots inspected the main rotor blade damage. Maintenance inspectors determined the main rotor blades, rotor-head, transmission and high speed shafts required replacement. The NTSB deter-mined the blade strike as an “Incident”, and it was further classified by the Forest Service as an “Incident with Potential”…”

NTSB: S-2T that crashed may have struck a tree

S-2T air tanker crash
Site of the S-2T air tanker crash. Photo by Ken Yager.

In a preliminary report that was released Tuesday night, the National Transportation Safety Board said the S-2T air tanker that crashed near Yosemite National Park in California on October 7 may have struck a tree which broke off a part of the aircraft’s wing.

Two other firefighting aircraft were in the area at the time. A lead plane preceded the air tanker into the drop area but that pilot did not see the crash. However the crew of an air attack ship overhead did, and they told the NTSB that the S-2T may have struck a tree, causing part of a wing to break off.

Both aircrews reported that there was smoke in the area, but visibility was good.

The air tanker was stationed at the air tanker base at Hollister, California, and had been dispatched to the Dog Rock fire. The airplane arrived on scene, made one drop on the fire, then proceeded to the Columbia Airport to be reloaded with fire retardant before it returned and made its final flight. Pilot Geoffrey “Craig” Hunt died in the accident.

A resident in the area of the crash site told us that locals took quite a few photos and a video that will help the NTSB’s investigation. They are unwilling to release the imagery to the public until after the investigation is complete.

It will be many more months before the NTSB releases their final report.

CAL FIRE Director Ken Pimlott issued the following statement regarding the NTSB’s preliminary information on the crash.

“Aerial firefighting is not simply flying from one airport to another. The wildland firefighting environment is a challenging one, both on the ground and in the air,” said Chief Ken Pimlott, director of CAL FIRE. “We look forward to the final NTSB report to see if we can use the findings to help mitigate the inherent dangers of the job. We owe that to Craig, who traded his life in an effort to protect the lives of others.”

NTSB report: EMS helicopter crashed after running out of fuel and failure to autorotate

(Updated at 1:51 p.m. MT, April 10, 2013)

The NTSB report mentions that the pilot was texting on his cell phone the day of the accident, including “during the accident flight”. An article at Bloomberg.com has more details about the texting, including:

…The NTSB documented at least 240 texts sent and received by the pilot during his shift the day of the accident, according to records cited by Bill Bramble, an NTSB investigator. There were 20 such texts with a coworker before and during the accident, the safety board found.

Freudenbert received four texts, three of them from a friend at work, and sent three others during the flight, according to NTSB records. He was planning to have dinner with the coworker, according to the records.

Another 13 texts were logged on his phone in the 71 minutes before the flight, including two during a previous flight, according to NTSB records.

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(Originally published April 9, 2013)

The National Transportation Safety Board has released the cause of the crash of an EMS helicopter August 26, 2011 near Mosby, Missouri. The agency’s report concludes the crash, which killed the pilot, flight nurse, flight paramedic and patient, occurred because the helicopter ran out of fuel and the failure of the pilot to execute a successful autorotation.

The finding about the possible reason for the autorotation failure after the engine failure at cruise speed may have implications for other pilots.

Below is the NTSB’s announcement:

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“April 9, 2013

NTSB DETERMINES FATAL MISSOURI HELICOPTER ACCIDENT WAS CAUSED BY FUEL EXHAUSTION, POOR DECISION MAKING AND INABILITY TO PERFORM CRITICAL FLIGHT MANEUVER

WASHINGTON — A pilot’s decision to depart on a mission despite a critically low fuel level as well as his inability to perform a crucial flight maneuver following the engine flameout from fuel exhaustion was the probable cause of an emergency medical services helicopter accident that killed four in Missouri, the National Transportation Safety Board said today.
Continue reading “NTSB report: EMS helicopter crashed after running out of fuel and failure to autorotate”