Plane1 Essay

This essay has a total of 4305 words and 24 pages.

plane1








Instructor: Greg Alston Abstract This paper examines the in-flight
separation of the number two pylon and engine
from a Boeing 747-121 shortly after takeoff from the Anchorage
International Airport on March 31, 1993. The
safety issues discussed focus on the inspection of Boeing 747 engine
pylons, meteorological hazards to aircraft, the
lateral load-carrying capability of engine pylon structures, and aircraft
departure routes at Anchorage International
Airport during turbulent weather conditions. Shortly after noon on March
31, 1993 the number two engine and
pylon separated from Japan Airlines Inc. flight 46E shortly after departure
from the Anchorage International Airport.
The aircraft, a Boeing 747-121, had been leased from Evergreen
International Airlines Inc. The flight was a
scheduled cargo flight from Anchorage to Chicago-O'Hare International
Airport. On board the airplane was the
flight crew and two nonrevenue company employees. The airplane was
substantialy damaged during the separation
of the engine but no one on board the airplane or on the ground was
injured. Flight 46E departed Anchorage about
1224 local time. The flight release and weather package provided to the
pilots by Evergreen operations contained a
forecast for severe turbulence. As fight 46E taxied onto the runway to
await its takeoff clearance, the local controller
informed the flight crew that the pilot of another Evergreen aircraft
reported severe turbulence at 2,500 feet while
climbing out from runway 6R. After takeoff, at an altitude of about 2,000
feet, the airplane experienced an
uncommanded left bank of approximately fifty degrees. Although the
desired air speed was 183 knots, the air speed
fluctuated from a high of 245 knots to a low of 170 knots. Shortly
thereafter the flight crew reported the number two
throttle slammed to its aft stop, the number two thrust reverse indication
showed thrust reverser deployment, and the
number two engine electrical bus failed. Several witnesses on the ground
reported that the airplane experienced
several severe pitch and roll oscillations before the engine separated.
Shortly after the engine separated from the
airplane, the flight crew declared an emergency, and the captain initiated a
large radius turn to the left to return and
land on runway 6R. The number one engine was maintained at maximum
power. While on the downwind portion of
the landing pattern bank angles momentarily exceeded forty degrees
alternating with wings level. About twenty
minutes after takeoff flight 46E advised the tower they were on the
runway. The aircraft was substantially damaged
as a result of the separation of the number two engine. Estimated repair
costs exceeded twelve million dollars. In
addition, several private dwellings, automobiles, and landscaping were
damaged by the impact of the number two
engine and various parts of the engine pylon and the wing leading edge
devices. The National Transportation Safety
Board (NTSB) determined the probable cause of this accident was the
lateral separation of the number two engine
pylon due to an encounter with severe or possibly extreme turbulence.
This resulted in dynamic lateral loadings
coming from many directions that exceeded the lateral load-carrying
capability of the pylon. It was later discovered
that the load-carrying capability of the pylon was already reduced by the
presence of the fatigue crack near the
forward end of the pylon's forward firewall web. As a result of this
investigation the NTSB made seven
recommendations to the Federal Aviation Administration (FAA),
including the inspection of Boeing 747 engine
pylons, the potential meteorological hazards to aircraft, an increase in the
lateral load capability of engine pylon
structures, and the modification of the aircraft departure routes at
Anchorage International Airport during periods of
moderate or severe turbulence. The NTSB also recommended that the
National Weather Service (NWS) use the
WSR-88D Doppler weather radar system to document
mountain-generated wind fields in the Anchorage area and
to develop detailed low altitude turbulence forecasts. In the course of the
investigation the NTSB explored virtually
every contributing factor contributing to the aircraft accident. These
included weather, mechanical failure, design
deficiencies, and human factors. The flight crew was properly trained and
qualified for this fight. None of the crew
members' Federal Aviation Administration (FAA) records contained any
history of accidents, incidents, or
violations. The flight crew and the mechanics who had worked on the
airplane before the flight volunteered to be
tested for the presence of alcohol and both lawful and illegal drugs. All of
the test results were negative. The
investigation revealed that the flight crew was in good health. The
airplane, registration N473EV, was a Boeing
model 747-121, serial number 19657. The airplane was manufactured in
June 1970, and was originally configured
to carry passengers. The airplane was acquired by Evergreen
International Airlines in December 1988, and was
subsequently reconfigured to carry cargo. The airplane had seating for the
three flight crew members and two
observers or passengers. The airplane was equipped with four Pratt &
Whitney JT9D-7 engines and appropriate
equipment for Instrument Flight Rules (IFR) operations. At the time of the
accident, the airplane had accumulated
83,906 flight hours and 18,387 cycles. The estimated economic design
life for the Boeing 747 is 20,000 flights,
60,000 hours, and 20 years. The number two engine, serial number
662812, had accumulated a total of 56,709.8
hours and 10,923 cycles since new and had accumulated 5,752.5 hours
and 1,200 cycles since overhaul two years
prior. The maintenance logs had no reports of severe e! ngine vibration on
the number two engine. The maintenance
records contained no deferred repair items regarding the number two
engine pylon structure. The airplane was
equipped with a Sundstrand Data Control Mark VI-J4 ground proximity
warning system (GPWS). In addition to
providing GPWS alerts, this system provides windshear caution,
windshear warning, and bank angle warning. The
system provides windshear warning and cautions between five feet and
1,500 feet during the initial takeoff and
between 1,500 feet and thirty feet during the final approach phases of
flight. The bank angle advisory indicates a roll
attitude that is excessive for the flight condition. Generally, above 1,500
feet, the callout occurs at forty degrees of
bank. The callout occurs again if roll attitude increases by twenty percent.
When roll attitude increases to forty
percent above the initial callout angle, the callout repeats continuously.
Below 1,500 feet, the callout angle is reduced
progressively. The windshear caution or windshear warning did not
activate because the turbulence was encountered
above 1,500 feet, well outside the warning envelope of the system.
However, the system did provide bank angle
warnings during the turbulence encounter. A significant meteorological
advisory (SIGMET) was issued at 1145 and
was valid until 1545. This SIGMET advised that moderate and frequent
severe turbulence could be encountered
from the surface to 12,000 feet. In addition, moderate and frequent
severe mountain wave turbulence could be
encountered from 12,000 feet to 39,000 feet within an area bounded by
Bethel, Johnstone Point, Sitkinak Island,
and Dillingham. The northern extent of the SIGMET area was about
thirty-six nautical miles south of Anchorage. A
correction to the SIGMET was made at 1342 adding the Anchorage area
to the list of locations within the advisory
area. According to an individual of the NWS forecast office at
Anchorage, the delay in issuing the correction (about
2 hours) was due to the workload. The delay caused the omission of
Anchorage from the SIGMET location points
to go unnoticed. The aviation weather forecaster also stated that
turbulence east of the airport was not an infrequent
event in the presence of a strong easterly flow near mountain top level. He
believed that in addition to the strong
easterly flow the turbulence was increased by an upper level trough
moving through the area, which, coupled with
heating, made the atmosphere unstable. He also stated that in the eighteen
years as a forecaster at Anchorage he did
not remember previously seeing as many severe turbulence pilot reports
as he saw that afternoon. Several other
pilots reported severe turbulence encounters about the time of the
accident. At 1210, a pilot of another Boeing 747
reported severe turbulence at 2,500 feet and moderate turbulence
between 3,000 feet and 10,000 feet during the
climbout to the north. The pilot of a U.S. Marshall Service Cessna 310
reported that he took off from runway 15 at
Merrill Field to perform a maintenance fight about noon. At 300 feet
above the ground, the airplane encountered a
downdraft and the airplane's air speed went from 120 knots to 90 knots
and lost about 200 feet of altitude. After he
emerged from the downdraft, the pilot turned the airplane to a heading of
120 and climbed to 900 feet. Shortly
thereafter, the airplane encountered an updraft. The vertical velocity
indicator pegged the needle at 4,000 feet per
minute upward and that despite reducing the throttles to idle the airspeed
would not fall below 160 knots. The pilot
stated that as he maneuvered the airplane back to the airport for landing,
the airplane encountered severe turbulence
with fifty-knot variations in air speed. The pilot concluded in his written
report that, in twenty years of flying, this was
the worst turbulence he has encountered. The NTSB also inspected the
navigation aids and communications within
in Anchorage area. No difficulties or problems were found. Damage to
the airplane consisted of the loss of the
number two engine and its pylon and the loss of most of the left wing
leading edge devices between engines number
one and two. During the investigation, the fuse pins holding the engine
pylons to the wings were removed from the
airplane. The two midspar fuse pins for the number two engine were
found to be deformed. The aft diagonal brace
fuse pin was fractured. The inboard midspar fuse pin for the number one
engine was found to be substantially
deformed. None of the other fuse pins on the airplane had any indications
of damage or deformation. Relatively
small areas of impact damage were also noted on the wings and trailing
edge flaps. The number two engine, all
portions of the number two engine pylon, and most of the leading edge
structure between the number one and
number two engines were recovered. There was no evidence of an
in-flight fire prior to the separation of the number
two engine. Several witnesses on the ground reported seeing a flash or
ball of fire as the engine separated from the
airplane. There were no reported fires on the ground as a result of falling
debris. Persons who first saw the engine
after it struck the ground reported steam rising from the engine.
Firefighters from the Anchorage Fire Department
sprayed water on the engine to prevent a possible fire. The pylon is
designed to carry the thrust and torque loads of
the engine as well as lateral, longitudinal, and vertical loads from
maneuvers and gusts. Lateral loads are ultimately
absorbed by the midspar fuse pins and side brace. According to Boeing,
the fuse pins can withstand an ultimate
lateral load of more than 2.8 G on the engine. Additionally, Boeing
reported that the portion of the structure of the
pylon that is critical under lateral loads is the firewall just aft of the
forward engine mount. The Boeing calculations
indicated that this firewall will fracture at a lateral load of between 2.35 G
and 2.88 G when it contains a fatigue
crack of the size found in this structure. The Boeing 747 airplane and its
pylon structure were designed in the
mid-1960's using the computer capabilities and analytical skills of the
time. Boeing's current computer modeling of
the pylon structure and the loads applied to it are considerably more
complicated and provide greater resolution of
the data than would have been possible with the techniques employed
when the airplane was designed. The use of
modern computer structural design programs allowed considerable
modeling of the pylon's response to various load
inputs with various structural failures. The number two engine pylon was
separated into four pieces as a result of
three principal fracture areas. These fractures were located just aft of the
forward engine mount bulkhead, among a
jagged vertical plane aft of the rear engine mount bulkhead, and around
the inboard midspar fuse pin fitting. The two
forward pieces of the pylon remained attached to the engine through the
forward and rear engine mounts.
Examination of the fractures around the perimeter of the break aft of the
forward engine mount bulkhead revealed
features typical of overstress separations, except for a small flat fracture
region in the firewall web. The flat fracture
area was approximately in the middle of the web on the outboard side of
the web centerline. The fracture was a
lateral fracture about two inches long through the thickness of the web
and was aft of the third transverse stiffener
behind the forward engine mount bulkhead. Investigators cut the flat
fracture area from the remainder o! f the firewall
and examined it in detail with a bench binocular microscope and a
scanning electron microscope. The mating
fracture faces had been heavily rubbed. Despite the rubbing, isolated
areas of contrasting color, indicative of
through-the-thickness propagation, was noted. Compression buckling of
the firewall web extended from the fatigue
crack area forward to the outboard side of the pylon at the second
transverse stiffener. Inspection of the other three
pylons on the airplane found no similar cracks. The fuse pin from the
underwing fitting for the diagonal brace was the
only one that was found broken. The outboard portion of the pin was
cocked within the underwing fitting. The
inboard piece of this fuse pin was recovered on the ground near the aft
portion of the pylon. The fractures on the
fuse pin and retainer bolt appeared typical of overstress separations. The
investigation found that all of the remaining
fractures and buckling of the structure were consistent with deformation
of the pylon structure in an outboard and
upward direction. Examination of the other fracture surfaces disclosed no
evidence of pre-accident damage or
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