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N7UT accident description

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Tail numberN7UT
Accident dateFebruary 11, 2000
Aircraft typeBell OH-58A+
LocationWoods Cross, UT
Additional details: None

NTSB description

HISTORY OF FLIGHT

On February 11, 2000, at 1404 mountain standard time, a military surplus Bell OH-58A+ helicopter, N7UT, was destroyed when it impacted terrain while performing an autorotation during a maintenance flight at Skypark Airport, Woods Cross, Utah. The commercial pilot and his mechanic passenger were both fatally injured. The public use helicopter was being operated by the Utah Department of Public Safety under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the maintenance test flight that originated 13 minutes before the accident. The pilot had not filed a flight plan.

According to maintenance personnel, the helicopter was being test flown to evaluate the fuel control for engine RPM (revolutions per minute) droop and to conduct a functional check of the free-wheeling unit. The ground crew reported that the pilot started the engine at approximately 1320, and the helicopter became airborne at approximately 1350. Radar data indicates that the helicopter departed runway 16 and made a left turn to an approximate 360 degree ground track. The pilot reported to Salt Lake City approach control that he was "over Bountiful northbound to Lgoon and back." Lgoon is the name of a step-down fix on the Salt Lake City ILS approach to runway 16L (an estimated 6 nautical miles [nm] north of Skypark Airport).

At 1357:27, radar data indicates that the helicopter reversed course southbound. At 1401, the helicopter turned westbound (at approximately 750 feet above ground level [agl]) on a 1 nm base-leg for runway 16. Radar contact with the helicopter was last recorded at 1403:18, when it was at approximately 700 to 800 feet agl.

A witness sitting in his airplane at the approach end of runway 16 observed the helicopter on final approach. He heard the pilot transmit that he was going to practice an autorotation. He said that 2 to 5 seconds later, the helicopter was "100 to 150 feet agl, the main blades of the Jet Ranger [OH-58] stopped, and the craft started a slow spiral to the left, nose down." He said that the helicopter was "shaking violently, and the skids were flexing in and out." The main rotor was flexing up and down in an "extreme" manner; the tail rotor separated from the helicopter at approximately 50 to 60 feet agl. The witness said that the "Jet Ranger [OH-58] continued a slow left spiral until impacting on its left side." He said that he heard no transmissions from the pilot after "control was lost."

Another witness said, "I heard a helicopter and so I looked up and then the blades curled up weird and then made a weird left turn and plunged to the ground." Another witness reported that during the helicopter's fall, the main "rotor almost stop[ped] turning."

PERSONNEL INFORMATION

The pilot began flying in 1971, and flew only fixed winged aircraft until October 1991 when he received his commercial pilot certificate for helicopters. He began flying for the state of Utah in May 1978, and over the next 22 years flew for Wildlife Resources, Department of Transportation, and Department of Public Safety (DPS). At the time of the accident, DPS records indicate that the pilot had a total of 9,655 hours of flight experience. Approximately 1,313 hours of this time was in helicopters, of which 937 hours was in turbine powered helicopters. The pilot's last flight prior to the accident flight was in the accident helicopter, 23 days before the accident.

According to a DPS pilot, DPS helicopter pilots went to a flight training recurrency school once a year, and that was the only time that they practiced autorotations to the ground. With their own helicopters, they occasionally practiced autorotations with power recovery. He said that a consultant flight instructor came to Salt Lake City on November 2-5, 1999, and provided training to the pilot and himself in the use of night vision goggles. During this training, both DPS pilots flew three autorotations (during the day) to power recoveries. On one of these practice autorotations, the accident pilot over sped the main rotor (110% plus was observed by the flight instructor), and a maintenance inspection was performed.

The consulting flight instructor said that the pilot had "good air sense." He further stated that the pilot "had a tendency to concentrate on one thing, and leave other items out of his scan." When he left Salt Lake City, he made the recommendation that the two DPS helicopter pilots practice autorotations together.

On the morning of the accident, while the helicopter was being prepared to fly, the pilot said to one of the maintenance men that he was "not really very good at doing autorotations." He further said to the maintenance man that "I'll kind of do it; I'll let Steve do it later."

AIRCRAFT INFORMATION

The helicopter was a single engine, two-bladed main rotor, four seat aircraft, which was manufactured by Bell Helicopter in 1971. It was designed and manufactured for military use, and in May 1998 it was recertificated for government use (public use). The helicopter was powered by a T63-A720 (Allison 250-C20C) turboshaft, reverse flow engine that had a maximum rating of 420 horsepower, but was derated to 317 horsepower. The engine was last overhauled by the US Army on February 16, 1995, and had accumulated approximately 823 hours of operation by the time of the accident. The aircraft records indicate that the airframe (S/N 41703, Army No.71-20842) had accumulated approximately 5,015 hours, at the time of the accident.

The model designation of the helicopter was OH-58A+, which is the OH-58A with a bigger engine. The OH-58A+ and the OH-58C helicopters have the same engine and maximum gross weight of 3,200 pounds; the differences are in the instrument panel and avionics. The accident helicopter had several modifications: long range extender fuel tank (71 to 89 gallons usable), Night Bright light, external hook with mirror, power analyzer and recorder (PAR), and Bear Paws on the landing skids for snow landings.

The engine manufacturer's representative estimated that the engine's average fuel consumption was between 24 to 25 gallons per hour. The airframe manufacturer's representative said that the main rotor normal operating speed was from 93% to 110% RPM.

The pilot flew the helicopter from Salt Lake City International Airport to Skypark Airport, on January 19, 2000, for maintenance. The normal 25 hour inspection was scheduled; additionally, the fuel control rigging was to be adjusted, and a leaking freewheeling unit was to be repaired.

According to a Utah DPS pilot, the procedures for performing an autorotation are as follows: collective full down, smoothly roll throttle off [out of the fuel governor range], cyclic adjust for airspeed (too fast or too slow and the rate of decent can get excessive), confirm rotor in the green to ensure that the free wheeling unit disengaged, and confirm N1 stabilized showing that the engine was still making power. For the power recovery, smoothly roll throttle back on, confirm rotor and N2 (power turbine) needles are joined and in normal operating range, and simultaneously pull up collective while applying left pedal. Proceed with the climb out. He further stated that when they practiced autorotations, they entered them at approximately 1,200 feet agl.

The DPS pilot said the helicopter's main rotor rotates counter-clockwise (from the perspective of the pilot's seat), and that left anti torque pedal was required whenever torque was being generated by use of the collective. The left anti torque pedal counteracts the fuselage right turning tendencies whenever torque is being generated. He said that if torque is not being generated, and left anti torque pedal is applied, the helicopter would rotate to the left.

METEOROLOGICAL INFORMATION

At 1356, the weather conditions at the Salt Lake City International Airport (elevation 4,227 feet), Salt Lake City, Utah, (220 degrees 5 nm from the accident site) were as follows: wind 160 degrees at 8 knots; visibility 10 statute miles; scattered clouds at 4,200 feet, overcast at 12,000 feet; temperature 50 degrees Fahrenheit; dew point 37 degrees Fahrenheit; altimeter setting 29.86 inches of mercury.

AERODROME INFORMATION

The Skypark Airport (Bountiful/Woods Cross, Utah; elevation 4,234 feet) is not serviced by a control tower. The Salt Lake City International Airport supplies weather information and radar support to Skypark.

WRECKAGE AND IMPACT INFORMATION

The helicopter was found on a muddy earthen berm (elevation 4,239 feet, N40 degrees 52.51 minutes, W111 degrees 55.57 minutes) approximately 45 feet north of the street, W. 1500 St. S., in Woods Cross, Utah. It was lying on its left side (slightly inverted), and was longitudinally oriented approximately 060 degrees. There were no ground scars. The paved overrun to the approach end of runway 16 was located approximately 300 feet to the west and 75 feet to the south of the helicopter's impact location.

The fuselage sustained extensive vertical crushing, compromising approximately half of the viable living space (left side of the cabin area). The forward left seat was significantly displaced to the right side of the aircraft; the forward right seat (the pilot's seat) was separated from the helicopter, and it displayed downward and sidewards deformation. The fuselage was broken at several locations, and the aft portion of the tail boom was segmented and separated from the aircraft. The right skid was found separated from the fuselage and displaced forward approximately 10 feet; the left skid was broken from the fuselage, but in place. The left horizontal stabilizer was compressed in an accordion fashion towards the tail boom; the right horizontal stabilizer was minimally damaged.

The engine was found securely attached to the mounts, but had been slightly displaced to the right. The left side of the engine exhibited extensive impact damage; neither the compressor section nor the turbine section could be rotated. On scene and before recovery was initiated, the fuel control pointer was observed indicating slightly above flight idle position. The engine was removed from the airframe and sent to Rolls-Royce Allison, Indianapolis, Indiana, for disassembly and inspection.

The fuel bladder, with approximately 45 gallons of fuel in it, was intact without any detectable fuel leakage. The fuel selector arm was found in the ON position. The throttle linkage was intact and displayed continuity. Both left and right side throttles were observed positioned in slightly above the flight idle position. The N1 gauge (engine compressor section) was indicating 56%.

All of the helicopter's major components were accounted for at the accident site. The flight control system was found primarily intact, and all control tubes were in place and attached. Control continuity was established for all flight controls; all drive shafting, the main transmission, the tail rotor gear box, and engine throttle and governor controls. The main rotor blades remained attached to the main rotor hub, and they exhibited minimal damage. Similarly, the tail rotor blades were found securely attached to their hub assembly, and the pitch control system exhibited no abnormalities. One tail rotor blade was observed to be fractured approximately 6 inches from the root end of the blade, and the outer portion had separated.

The torque gauge, the warning and caution panel, the freewheeling unit, and the main rotor hub static stops were sent to Bell Helicopter, Fort Worth, Texas, for further examination. The Power Analyzer and Recorder (PAR computer) unit was sent to Avionics Specialties, Inc., Charlottesville, Virginia, for data recovery.

No preimpact engine or airframe anomalies, which might have affected the helicopter's performance, were identified. There was no evidence of pre or postimpact fire.

MEDICAL AND PATHOLOGICAL INFORMATION

An autopsy was performed on the pilot by the State of Utah, Department of Health, Salt Lake City, Utah, on February 12, 2000.

Toxicology tests were performed on the pilot by the FAA's Civil Aeromedical Institute (CAMI) in Oklahoma City, Oklahoma. According to CAMI's report (#200000036001), the pilot's blood was tested for carbon monoxide, cyanide, and drugs, with negative results. The pilot's vitreous was tested for ethanol with negative results.

TESTS AND RESEARCH

The National Transportation Safety Board's (NTSB) Recorded Radar Study group chairman stated that radar information was recorded approximately every 4.6 seconds. He said that the recorded data provided helicopter position resolution of +/- 95 feet, and altitude resolution of +/- 50 feet. The stored radar data recorded the flight from 1351:41 (first detection) to 1403:18 (last detection), an elapsed time of 11 minutes 37 seconds. The group chairman said, "the ground impact location was almost directly below the last recorded radar target position."

On March 8, 2000, at the facilities of Bell Helicopter in Fort Worth, Texas, under the supervision on an NTSB investigator from the Arlington, Texas office, the following four items were examined:

1. The engine torque meter, with the front glass broken, and the indicator needle fractured off of the stem. Examination of the face of the engine torque meter under black light did not reveal evidence of needle slap related to ground impact. An attempt to reposition the separated needle to the stem, for gauge indication at the time of impact, was determined to be inconclusive. The needle attachment cap, on the end of the torque gauge stem, was found to be movable.

2. The freewheeling unit. The freewheeling unit was manipulated by hand, and it operated appropriately. It was disassembled, and it was determined that it had been assembled properly. No abnormalities, which could have prevented normal operation, were noted, and only normal wear was noted.

3. The cockpit warning lights (Master Caution, Engine Out, Transmission Oil Pressure, and Transmission Oil Hot) were examined for filament stretching and distortion. Examination of these bulbs under a stereo-zoom microscope revealed that two of the warning lights, the Master Caution and the Transmission Oil Pressure, had bulb filaments which were stretched to the left (suggesting that they were illuminated at the time of ground impact). According to a manufacturer's representative, the Transmission Oil Pressure light will illuminate when the main rotor RPM (NR) drops below 65%.

4. The static stops, near the main rotor mast, were found deformed and fractured. Examination of the static stops indicated overload forces.

The PAR computer (SLZ7648, Serial Number DR0004) unit was read out by the manufacturer, Avionics Specialties, Inc., Charlottesville, Virginia. The PAR computer normally samples eight input parameters approximately 30 times per second, and only stores a log record upon the conclusion of an event (such as a start). The unit records performance exceedences, and non-exceedence events of Starts, Shutdowns, Trend Records, and Power Fails. The RAM (random access memory) of the PAR computer stored the following data at the time the PAR computer's power failed due to the helicopter's ground impact: Total Operating Temperature (TOT) 557C Torque (TRQ) 13.5% Compressor speed (N1) 61.9% Turbine speed (N2) 65.8% Main rotor speed (NR) 65.8% Pressure altitude (PALT) 4,036 Feet Indicated airspeed (IAS) 34 Kts Outside air temperature (OAT) 4C.

The internal time of the PAR computer was set, on December 1997, by the manufacturer, before it left the facility for installation. According to a manufacturer's representative, Eastern Standard Time was used. He said that when they began to examine the PAR computer, they determined that its internal clock was 33 minutes slow.

The engine examination and disassembly was accomplished on May 5, 2000. The engine had incurred major impact damage and the turbine would not rotate (see attached report). The compressor was intact, and it had ingested

(c) 2009-2011 Lee C. Baker. For informational purposes only.