No Injuries When Landing Skid Collapsed, Mechanic Cited As
Partially At Fault
It was likely pretty surprising for the pilot and four
passengers aboard an MD369E getting set for a sightseeing tour in
Hawaii when the aircraft lurched left and the tail rotor
contacted the ground. An NTSB probable cause report indicates both
fatigue failure of the landing gear strut, and maintenance
personnel’s failure to perform an adequate inspection of the
landing gear, were the probable causes of the accident.
Fortunately, no one was injured.
NTSB Identification: WPR10LA060
Nonscheduled 14 CFR Part 135: Air Taxi & Commuter
Accident occurred Friday, November 20, 2009 in Kahuku, HI
Probable Cause Approval Date: 05/19/2011
Aircraft: MCDONNELL DOUGLAS HELI CO 369E, registration:
N551CR
Injuries: 5 Uninjured.
On November 20, 2009, at 1020 Hawaiian standard time, a
McDonnell Douglas 369E helicopter, N551CR, sustained substantial
damage when the left rear landing skid strut collapsed while the
helicopter was standing with its engine operating at the Turtle Bay
Resort Heliport, Kahuku, Hawaii. K & S Helicopters, Inc.,
d.b.a. Paradise Helicopters, operated the helicopter under the
provisions of Title 14 Code of Federal Regulations Part 135. The
commercial pilot and four passengers were not injured. Visual
meteorological conditions prevailed, and a company flight plan had
been filed.
The helicopter was positioned on a movable landing platform.
Four passengers had boarded the helicopter for a sightseeing tour
of Oahu. The pilot brought the engine up to flight idle, preparing
for takeoff, when the left rear landing gear collapsed. The
helicopter tilted to the left rear and the tail stinger and
vertical stabilizer contacted the ground. The tail rotor blades
also struck the ground. The pilot shut down the engine and stopped
the rotor using the rotor brake. Examination of the left rear
landing gear strut revealed that the strut fractured at the bolt
hole where the strut was attached to the fuselage drag brace. The
tail stinger and the vertical stabilizer were bent, and the tail
rotor blades were damaged.
Personnel Information
The pilot, age 36, held a commercial pilot certificate with a
rating for rotorcraft-helicopter issued on January 4, 2006, and a
second-class medical certificate issued June 26, 2009, with no
limitations. The pilot reported that he had 6,200 flight hours in
helicopters.
Aircraft Information
The five seat single engine helicopter, serial number (S/N)
0534E, was manufactured in 1997. It was powered by a Rolls Royce
250-C20B, 450-hp engine. The operator reported that the airframe
had 11,393 hours on it at the time of the accident, and the engine
had 15,997 hours. The last engine inspection was performed 30 hours
prior to the accident. The most recent airframe inspection was a
100-hour inspection performed on November 7, 2009, at a helicopter
total time of 11,361.8 hours, 30.2 hours prior to the accident. As
part of the 100-hour inspection, the landing gear was inspected in
accordance with the manufacturer’s maintenance inspection
program.
Tests & Research
The rear left and right landing gear struts were sent to the
National Transportation Safety Board’s Material Laboratory
for examination.
Of the two struts submitted for examination, one was fractured
and consisted of two pieces (left strut). The fractured skid strut
tube had broken in the area of the bolt holes used to attach the
strut to the drag brace. A section containing one end of the
fracture was cut off the smaller portion of the strut to facilitate
examination under the stereo microscope and in the scanning
electron microscope (SEM). Examination of the fracture surface
under the stereo microscope revealed an area adjacent to one of the
bolt holes which had a planar, thumbnail like appearance. This area
was further examined with the SEM, which revealed a series of
striations consistent with the growth of a crack due to fatigue.
Additional areas exhibiting striations consistent with a fatigue
crack were also observed on the opposite side of the same bolt hole
and spreading downwards along the fracture surface. The remainder
of the fracture surface had a distinct shear lip, consistent with
an over stress fracture.
On the exterior surface of the fractured strut, in the areas
surrounding the bolt holes, evidence was noted of grinding
consistent with an abrasive process to remove paint. On the
exterior surface of the intact skid strut tube there was also
evidence noted of grinding around the area of the bolt holes.
Measurements of the diameter of the bolt holes were made on the
intact (right) skid strut. The smaller bolt hole was found to have
an average diameter of 0.5057 inch (the maximum diameter allowed
per the maintenance manual is 0.500 inch). The larger bolt hole was
found to have an average diameter of 0.5336 inch (the maximum
diameter allowed per the maintenance manual is 0.5313 inch). In
addition to being oversize, the smaller of the two bolt holes had
evidence of cold working on the interior surface of the bolt hole.
This cold working was consistent with the bushing, which would have
been installed through the bolt hole, repeatedly pressing against
the bore of the hole due to relative movement between the two. On
the exterior surface of the intact strut, adjacent to the area of
cold working in the bore of the bolt hole, a crack is visible. This
crack was approximately 2 mm long and could be seen with the
unaided eye.
Following examination by the NTSB Materials Laboratory, the
struts were brought to the MD Helicopter facility in Mesa, Arizona,
under supervision of the Safety Board IIC, for a functional
installation check on a MD 369E airframe, in accordance with MD
Helicopters, Inc. Maintenance Manual (CSP-HMI-2, Chapter 32-10-00,
page 602, Landing Gear Strut Inspection and Chapter 32-10-00, page
401, Landing Gear Replacement). The landing gear strut inspection
is incorporated into the 100-hour or annual inspection. The aft
left side of the exemplar airframe was fitted with a new strut, the
aft right side of the airframe was fitted with the right strut from
the accident helicopter. According to the Maintenance Manual, the
inspection procedure is to jack the helicopter until the landing
gear dampers are fully extended, shake the landing gear assemblies,
and note any play or looseness at pivot bearings, inboard strut
assembly fittings, and drag brace holes. If any play or looseness
is noted the landing gear is to be removed and the strut requires
further inspection. The results of the inspection revealed that the
right strut exhibited some fore/aft and vertical play of the strut,
while the new strut on the left side was completely rigid,
exhibiting no play under the test conditions.
The MD Helicopters Maintenance Manual describes the installation
of the drag brace with the attaching hardware as
follows.”Attach brace to strut with attaching hardware.
Torque nut to 5 inch-pounds (0.56 Nm) maximum.” A caution
note precedes the installation step. The caution states, “Do
not over-tighten brace-to-strut hardware. Maximum permissible nut
torque is finger-tight; then backed off to nearest castellation for
installation of cotter pin.” The actual torque on the
brace-to-strut hardware on the accident helicopter was not
measured.