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N64KL accident description

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Tail numberN64KL
Accident dateJuly 18, 1998
Aircraft typeSikorsky CH-54A
LocationYucca Valley, CA
Additional details: None

NTSB description

HISTORY OF FLIGHT

On July 18, 1998, at 1625 hours Pacific daylight time, a Sikorsky CH-54A, N64KL, crashed near Yucca Valley, California, after experiencing a main rotor blade separation and a subsequent in-flight explosion and fire. The aircraft was destroyed and the three crewmembers received fatal injuries. The aircraft was being operated as a repositioning flight by Heavy Lift Helicopters under 14 CFR Part 91 when the accident occurred. The last leg of the flight to Apple Valley, California, originated in Blythe, California, about 1543 on the same day. Visual meteorological conditions prevailed at the cruising altitude and a company flight plan was filed.

The chief pilot and his copilot, who were in the second aircraft in a flight of two, reported that they were flying west along a heading of 285 degrees about 110 knots indicated airspeed (IAS). They were in loose trail, about .25 to .50 miles behind and above the lead aircraft, about 5,000 feet msl, when they saw a rectangular shaped object suddenly separate from the lead aircraft's 7 to 8 o'clock position. The lead air aircraft then reacted by abruptly spinning 180 degrees to the right. As the aircraft turned, the tailboom collapsed and folded downward and to the right, near the position of the aft end of the auxiliary fuel tank. This was accompanied by an explosion and fire that engulfed the aft portion of the aircraft. The aircraft's nose pitched up briefly about 10 to 15 degrees and then nosed over as the aircraft rolled to the right about 60 to 70 degrees. The aircraft continued to rotate about its longitudinal axis in an 80- to 90-degree nose down attitude until it collided with terrain.

The chief pilot reported that the accident aircraft's green blade had failed a blade indicator monitor (BIM) test check during a 3 flight hour inspection that the crew chief had performed the evening prior in Brownwood, Texas. The crew chief subsequently recharged the blade with nitrogen and the aircraft was allowed to remain parked overnight on the ramp. The next morning the crew chief checked the BIM indicator and found that the green blade had retained its charge. The crew of the accident aircraft made the decision to fly the aircraft to its destination.

Note: The BIM measures the pressure of the nitrogen filled main rotor spars. The purpose of monitoring the nitrogen pressure is to detect cracks in the spars that could lead to a catastrophic loss of the aircraft. The BIM indicator is pressure activated and requires a small release of nitrogen each time the spar pressure is checked. The indicator is read by observing the color being displayed. When fully charged, a series of yellow stripes cover a red indicator. As pressure is lost; however, the stripes recede until the red indicator is fully visible. A complete change in indication (yellow to red) occurs with a 2-pound change in absolute pressure.

During the next 7.6 hours of flight, the aircraft made four subsequent fuel stops. The crew, according to the both pilots, apparently checked the BIM on each stop and there was no mention of any further discrepancies.

The operator was scheduled to begin a new exclusive-use contract with the United States Forest Service (USFS) utilizing both aircraft on July 15, 1998.

Inspectors for the Riverside Flight Standards District Office (FSDO) subpoenaed cellular telephone records for the phone carried by the accident crew. On the evening prior to the accident, there were several calls made to and received from the operator on the crew's cellular phone. Both the chief pilot and the operator denied to inspectors that there was any discussion about the failed BIM check communicated during any of those calls.

An absolute pressure tester, which is a piece of required maintenance equipment for checking pressure in the main rotor spars, was not be found in the wreckage or in the accompanying service van. The van was also en route to the operator's home station; however, it was still about 1 or 2 days behind when the accident aircraft landed at Brownwood. Inspectors at the Riverside FSDO reported that another operator called their office a few days after the accident and told them that the operator of the accident aircraft had called asking to borrow an absolute pressure tester.

PILOT INFORMATION

According to flight and training records provided by the operator, the pilot and copilot were current and qualified in the accident aircraft. Both were also qualified to act as pilot-in-command. No training deficiencies were noted in their training records. Both had undergone Part 135 checks within the previous 90 days, and both were currently qualified by the USFS as part of the operator's contract requirements.

The duty day began at 0700 central daylight time in Brownwood with an initial takeoff time of 0740. The pilot and copilot routinely change positions in the aircraft at the end of each flying leg. They had flown a total of 7.6 hours on the day of the accident. The accident occurred on the last leg of a cross-country flight to their home station after a 3-month out-of-state deployment.

AIRCRAFT INFORMATION

The date and accumulated aircraft time at the last approved aircraft inspection program (AAIP) is not known. The AAIP that the operator was using at the time of the accident is based on a modified version of The Army Maintenance Management System (TAMMS). The CH-54A is no longer in the military inventory, and as such, receives no ongoing engineering support from the Department of Defense (DOD) or the manufacturer. The type certificate holder is A.I.R., Inc.

A BIM check is required to be performed by an airworthiness directive (AD) after every 3 hours of flight time, as well as every 6 months as directed by the maintenance program. According to Federal Aviation Administration (FAA) airworthiness inspectors and the AAIP, the failure of a BIM check renders the aircraft non-airworthy until the cause of the BIM check failure is positively identified and the problem has been corrected. If the problem cannot be positively identified, the aircraft is continued to be deemed non-airworthy and shall not be flown until the failing blade(s) is (are) replaced.

The operator contended that a "red" BIM indication can result from other than a crack in the spar. Repetitive checks can lower spar pressure as can changes in temperature (hot to cold), faulty O-rings in the indicator itself, or a leaking blade tip cap. Their maintenance manual, TM 55-1520-217-23-1, cautions that while testing the BIM indicator, the valve plunger under the manual lever must be pushed all the way down. This will shut off all spar pressure. It may be necessary to use both thumbs to do this. Also, press on ridged part of lever, not on the smooth tip. A partly pressed plunger may cause loss of spar pressure and a slow indication or no indication at all.

A review of the maintenance history of the green blade revealed a report of an unspecified "minor repair" 1.5-flight hours after the last reported overhaul. No maintenance work order (MWO) describing the nature of the repair was located. The blade is life-limited to 5,050 flight hours. This blade had 2148.5 flight hours at the time it was removed and reinstalled. The aircraft flight and maintenance logbook was destroyed in the accident. Safety Board investigators, using other available maintenance records, estimated that the blade had accumulated about 3,477.6 flight hours at the time of the failure.

The operator stated that it had, at no time, performed any maintenance or repair to the blade. The blade had been removed from another CH-54A aircraft the operator had obtained for parts cannibalization. After removal, the blade was next returned to service when it was installed on the accident aircraft. According to the operator, it had remained in continuous service until the accident.

Stop-drilling cracks in blade pockets to prevent crack propagation is an authorized military maintenance procedure for this aircraft. Drilling into the spar as part of the process is not authorized.

The aircraft had been modified with the addition of a supplemental type certificated (STC) belly-mounted tank/dispensing system that was designed to carry and spread fire suppressive chemicals.

COMMUNICATIONS

The crews of both aircraft were internally flight following with each other on 159.585 FM (primary), and 122.8 VHF. The chief pilot reported that the last air-to-air transmissions occurred as both aircraft were departing Blythe, California.

WRECKAGE AND IMPACT INFORMATION

The main aircraft wreckage was found in open, flat, dry desert terrain at an elevation of approximately 3,000 feet. The fuselage and both engines were located at 34 degrees 11.058 minutes north latitude and 116 degrees 17.123 minutes west longitude. The fuselage was destroyed during the impact, explosion and fire, therefore, no final heading was noted. The entire main wreckage site had been completely involved in an ensuing ground fire.

The outboard section of the green blade was the first piece of debris found along the approximate 1-mile long wreckage distribution. Sections of the remaining main rotor blades, tailboom structural components, Thomas couplings, Plexiglas fragments, and tail rotor gear boxes were found as far as 1 mile north of the main wreckage. The transmission, tail rotor drive shaft, and main rotor hub were located within 300 yards of the main wreckage.

A postaccident inspection of the aircraft conducted by Safety Board investigators failed to reveal any further discrepancies with the airframe, flight controls, engines, or related systems.

MEDICAL AND PATHOLOGICAL INFORMATION

Autopsies were conducted on July 22, 1998, by Dr. Frank Sheridan, M.D., a pathologist for the San Bernardino County Coroner's Office, San Bernardino, California. Specimens were retained for toxicological examination. The toxicological test results were negative for alcohol and all screened drug substances, with the exception of the postmortem production of ethanol.

FIRE

An immediate in-flight explosion and fire was observed by both of the pilots in the trail aircraft. They described a sudden burst of orange flame accompanied by black smoke as the tailboom collapsed. A secondary explosion, with orange flame and black smoke, was described about 10 seconds later as the aircraft impacted the ground.

TESTS AND RESEARCH

The green main rotor blade's (P/N 6415-20201-042) (S/N 64M-5424-5603) mating fracture surfaces were preserved and shipped by registered carrier to the Safety Board's Materials Laboratory in Washington, D.C. for further examination. The examination revealed that a hole had been drilled from the underside into the spar. The hole was about .093 inches in depth, terminating at a conical point which is consistent with that produced by a drill bit. At 5X, fatigue banding could be seen in the immediate vicinity of the hole. Under a scanning electron microscope (SEM), the surface was again examined, revealing striations located near the conical point that are typical of fatigue. The number of flight hours or cycles to failure could not be determined.

The drill hole had been located under a blade pocket and therefore, was not visible during a visual inspection of the blade. There is no prescribed inspection/maintenance procedure short of removing the blade pockets that would have indicated the presence of the drill hole. Blade pockets are condition items.

ADDITIONAL INFORMATION

The aircraft wreckage was released to Rob Cheek, Citrus Investigations, a representative of the registered owner, on March 17, 1999.

The emergency locator transmitter (ELT) was not located in the burned wreckage. No ELT signal was reported within geographic constraints during the time period of this accident.

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