Raising The Bar: A Sikorsky Crash

By N.T.S.B. — February 01, 2013

On August 5, 2008, a Sikorsky S-61N helicopter impacted trees and terrain during the initial climb after takeoff at an elevation of about 6,000 feet in mountainous terrain near Weaverville, California. The pilot-in-command (PIC), the safety crewmember, and seven firefighters were fatally injured while the co-pilot and three firefighters were seriously injured. Impact forces and a post-crash fire destroyed the helicopter, which was being operated by the U.S. Forest Service (USFS) as a public flight to transport firefighters.

Crash During Take-off of Firefighting Helicopter

History of the Flight:

The mission was to reposition a number of hand crew firefighters working on a forest fire in the area. When they arrived at the pick-up point, a brownout required them to abort the landing and then touch down about 100 feet south of the original spot on a comparatively dust-free rock outcrop.

The first load of 10 firefighters boarded. According to the CVR transcript, while on the ground at H-44, the co-pilot asked the PIC if the helicopter would have “enough power” to depart vertically out of the LZ. The PIC responded, “Absolutely, yes.”

The CVR transcript indicated that, during the initial departure, the co-pilot announced, “seventy five percent torque,” referring to the engine torque gauge indication, followed by, “everything looks good.” As the take-off continued, the co-pilot announced, “eight seven,” again referring to the engine torque gauge, and then “one hundred and two percent power’s good,” referring to the main rotor speed (NR).

According to the sound spectrum analysis of the CVR recordings, the engines reached topping about 30 seconds after the power began to increase and
remained at topping for about 14 seconds, with the gas generator speeds (NG) steady at 102 percent and 101.4 percent on the individual engines. As the NGs increased and topped out, the NR gradually decayed, or “drooped,” over about 51 seconds from 108.6 to 101.5 percent. NR then began to increase and stabilized about 103.2 percent. Within two seconds of the increase in NR, both NGs decreased below their topping speeds. The CVR did not record any discussion by the pilots regarding the fact that the engines had reached topping.

One of the firefighters on board the helicopter during this departure reported that “the helicopter felt heavy, slow and sluggish” and that his eye level was “approximately five to eight feet below the tree tops.” Another firefighter on board reported that, as the helicopter lifted off, “it seemed very slow” and “took a little bit to get up above the tree line.”

The pilots calculated that darkness would require them to shut down in about 2 hours 20 minutes. They decided to go to Trinity Helibase for fuel before transporting the remaining three loads of firefighters. After refueling, they had an allowable payload of 2,552 lbs., which was more than the 2,400 lbs. that the helitack crewmembers had been loading that day.

Also, they would have an additional margin after burning 400 lbs. enroute to the pickup point.

While the helicopter was being refueled, two mechanics performed a routine visual inspection. They both noted a layer of ash on the leading edges of the main rotor blades and around the engine inlets. One mechanic stated that both engine intakes were covered in ash, but the compressors’ first stage stators were clean. He further stated that the amount of ash on the blades was more than he had seen previously during this particular fire but was equivalent to what he had seen when the helicopter was working on other fires. He began wiping the blades with a rag, which easily removed the ash, leaving the wiped area of the blades free of debris. The mechanic stated that, while he was wiping the blades, he asked the PIC how the helicopter was running, and the PIC replied that it was operating well.

The other mechanic reported that he had asked the co-pilot if any problems existed with the helicopter, and the co-pilot replied, “she is flying great.” The mechanic stated that, as he began to wipe the ash from the engine inlets, the PIC asked the two mechanics to finish their work so that the helicopter could depart since the required shutdown time was nearing. They stopped wiping the ash off the blades and inlets and finished preparing the helicopter for flight.

During the approach, the co-pilot told the PIC that the outside air temperature (OAT) gauge was reading 20°C, and the PIC replied, “So it’s gotten cooler.” The co-pilot stated that the helicopter would have “quite a bit of performance with the drop in temperature.” After the helicopter landed, a helitack crewmember asked the pilots if he should get another helicopter to aid in the transportation, as dark was nearing. The co-pilot responded that they should be able to complete the mission. The crewmember then informed the pilots that the manifested weight of the firefighters and cargo being boarded was 2,355 lbs.

After the pilots were notified of the manifested weight, the co-pilot stated that the performance load calculation indicated a maximum payload of 2,552 lbs at 32°C. He added that the temperature was 12 to 13 degrees cooler and their payload 200 lbs. less than calculated. Both pilots restated that they were indeed 200 lbs. lighter than the previously calculated maximum payload, and the co-pilot confirmed that the helicopter was “good to go.”

At 1940:46, the PIC began to increase the power for take-off. At 1940:47, the co-pilot stated, “okay, just nice and smooth here.” At 1941:03, he stated, “okay there’s seventy five–there’s eighty,” and then, at 1941:06, “there’s eighty five,” all of which were engine torque readings. About 4 seconds later, he stated, “there’s ninety showin’ ah hundred and three percent,” referring to an engine torque reading of 90 percent and an NR reading of 103 percent. About 9 seconds later, he informed the PIC that NR had decreased to 100 percent and was drooping. The CVR recording ended 20 seconds later at 1941:39.

Sound spectrum analysis of the CVR recording indicated that the engines reached topping 22 seconds after power was applied and remained at topping until the end of the recording, with NGs steady at 102.1 percent and 101.5 percent on the individual engines. Between 1940:46 and 1941:31, the NR drooped from 106.9 to 95.0 percent and remained there for about 3 seconds. At 1941:34, about 5 seconds before the end of the recording, the NR started to droop again, reaching a final value of 93.5 percent.

Ground witnesses stated that, as the helicopter began to lift off, the rate of climb appeared very slow, and the helicopter’s movement was labored. One witness stated that the take-off was at “extremely slow speed and low altitude,” while another witness stated that the helicopter was “moving extremely slow, inconsistent with the last two departures.” The witnesses reported that the helicopter began to move forward in a nose-low configuration and drift sideways to the right. The helicopter continued to move forward and then began losing altitude as it continued down slope. One witness described the take-off as follows:

After a vertical ascension of about 20 feet, the helicopter began to move forward and about 40 feet to the right. As the helicopter continued forward toward a section of lower trees, the belly of the fuselage contacted trees; it appeared as though the helicopter would fit between the trees, though the main rotor blades were not high enough to clear the trees. Debris began to fly from the surrounding trees and the helicopter settled into the vegetation.

The helicopter collided with the trees and subsequently impacted the down sloping terrain, coming to rest on its left side. Almost immediately after the impact, witnesses saw smoke and fire coming from the wreckage. One witness reported that both engines continued to run for about 30 seconds after the helicopter impacted the ground.

Probable Cause

The National Transportation Safety Board determines that the probable causes of this accident were the following actions by helicopter operator:

1) the intentional understatement of the helicopter’s empty weight,
2) the alteration of the power available chart to exaggerate the helicopter’s lift capability, and
3) the practice of using unapproved above-minimum specification torque in performance calculations that, collectively, resulted in the pilots relying on performance calculations that significantly overestimated the helicopter’s load-carrying capacity and did not provide an adequate performance margin for a successful takeoff; and insufficient oversight by the U.S. Forest Service and the Federal Aviation Administration.

Contributing to the accident was the failure of the flight crew members to address the fact that the helicopter had approached its maximum performance capability on their two prior departures from the accident site because they were accustomed to operating at the limit of the helicopter’s performance.

Contributing to the fatalities were the immediate, intense fire that resulted from the spillage of fuel upon impact from the fuel tanks that were not crash resistant, the separation from the floor of the cabin seats that were not crash resistant, and the use of an inappropriate release mechanism on the cabin seat restraints.

 

 

About The Author

N.T.S.B.

For more information, in the U.S.A. (N.T.S.B.) The NTSB is an independent Federal agency charged by Congress with investigating every civil aviation accident in the U.S. and significant accidents in other modes of transportation-railroad, highway, marine and pipeline. WEBSITE = http://www.ntsb.gov/ For more information, in CANADA (T.S.B.) The Transportation Safety Board of Canada (TSB) is an independent agency that advances transportation safety by investigating occurrences in the marine, pipeline, rail and air modes of transportation. WEBSITE = http://www.tsb.gc.ca/eng/

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