The aircraft operator reported that:
- quick release fittings for the universal couplings on the ball
screw assemblies have been ordered, - in the interim, a hammer and chisel have been included in each
of the L382 aircraft tool kits to enable the crew, if required, to
hammer off the bolts that secure the ball screw universal couplings
in the event of a landing gear emergency, - the L-382 crew training now includes practical training in
emergency landing gear procedures using aircraft that are
undergoing hangar checks, - a notice has been issued to L-382 crews that provides further
guidance on actions required following landing gear malfunctions,
and - the L-382 aircraft have been fitted with satellite capable
telephones to allow easier communication with the maintenance
organisation.
Engineering Aspects
Although the aircraft was about 50 flying hours beyond the 600
hourly block check, it was operating within the approved ten
percent servicing extension. It was due to undergo the block check
in Singapore immediately after the Dili to Darwin flight. The block
check would have included a more in depth examination of the ball
screw assemblies and, in this case, the planned replacement of the
subsequently discovered damaged assembly. The preliminary
inspection conducted after the accidentin Singapore did not detect
the damage to the ball screw assembly. The damage was discovered
only after the removal and dismantling of the assembly. It was
therefore questionable that any developing damage would have been
noticed during prior maintenance checks or during the inspection
conducted by the flight engineer in Dili.
Both the operator and aircraft manufacturer reported that this
was the first reported failure of this type of ball screw assembly.
In the absence of other reports or evidence of a developing problem
with the ball screw assembly fitted to the aircraft, the
probability of recurrence of this type of failure was considered
low.
Crew Actions
After the aircraft was shutdown in Dili, the flight engineer
performed an on-ground check during which he found no fault with
the landing gear. As a result, the pilot in command decided to
continue operating the aircraft for the flight to Darwin the next
day. The aircraft manufacturer reported that the failure of the
main landing gear to lower normally at Dili should have been
subjected to a maintenance investigation, and this may have
occurred if the operator's maintenance organisation was aware of
the problem. There was no published guidance, however, by either
the manufacturer or the operator that was readily available to the
crew that provided assessment criteria about the continued
serviceability of the landing gear. Additionally, the operator had
experienced spurious landing gear indication problems that cleared
with the reselection of the landing gear position. The crew were
aware of these occurrences and without any landing gear system
faults being evident, their actions in recycling the gear position
several times and decisions about the potential cause of the
landing gear fault were probably influenced by the operator's
previous experiences.
The operator's prior experience with spurious indications,
associated with the air trafic controller's observation made during
the fly past in Dili, the failure to find a fault with the landing
gear after landing, and the lack of published guidance probably
contributed to the crew believing that the problem was a result of
an indication fault. The lack of readily accessible communications
for the crew to talk to the company's maintenance organisation may
have further contributed to the crew's decision to continue
operating the aircraft.
When the crew attempted to lower the landing gear at Darwin, the
damage within the left main rear landing gear ball screw assembly
caused the ball nut to jam on the screw jack. Consequently, due to
the interconnection of the gearboxes by the torque shafts, the
complete left main gear was jammed. It could not be lowered
normally or using any emergency method except by disconnection of
the universal joint mounted on top of the left forward main gear
ball screw. By the time the flight engineer and loadmaster had
managed to undo some of the nuts on universal joint bolts of the
forward left landing gear ball screw assembly, the aircraft only
had about 20 minutes endurance. The pilot in command assessed that
the crew probably did not have enough time to release the remaining
two bolts before the aircraft ran out of fuel. Landing the L-382
with just the right main landing gear and nose gear down would have
probably made directional control of the aircraft after touchdown
difficult. The consequences of the aircraft running out of fuel
while airborne were far greater than those associated with landing
the aircraft with the landing gear up.
The pilot in command's actions including directing the copilot
to fly while he worked through the problem with the flight engineer
probably assisted him in remaining situationally aware and making
valid assessments about the options available. The cockpit voice
recording revealed that the crew used appropriate crew management
principles, extensively discussing their options and helping the
pilot in command make informed and appropriate decisions.
The pilot in command's request for the laying of a foam path was
based on a common misconception that such a path would be effective
in reducing the chances of a fire during landing. The nature of the
foam agent used in Australia, however, meant that the laying a foam
path would have been ineffective in reducing the chance of fire
after landing.
CONCLUSION
The aircraft's left rear main landing gear ball screw assembly
had failed internally. The resultant damage required the crew to
reselect the landing gear during the approach to Dili and prevented
the crew lowering the landing gear normally during the later
approach to Darwin. The left main forward landing gear universal
joint bolts and nuts were damaged, preventing the crew from
lowering the landing gear using either normal or emergency lowering
methods within the time available. The pilot in command assessed
that the left landing gear could not be lowered before the
aircraft's fuel was exhausted.
History of Flight
The day before the accident, the Lockheed L-382G Hercules was
being used to conduct a United Nations charter flight from Darwin
in the Northern Territory to Dili in East Timor. During the
approach for landing, when the landing gear was selected down, the
main gear indication showed that the left main gear had not fully
lowered. The crew checked the electrical and hydraulic systems but
no fault was found. They also reported that after a fly-past of the
control tower, the air traffic controller advised that the gear
appeared down and locked. The crew then cycled the gear up then
down and it lowered normally with the indication showing the gear
down and locked. The landing at Dili was made without further
incident. The flight engineer reported that he inspected the
landing gear after landing at Dili and found no faults in the
landing gear system. He said that he suspected that a micro switch
might have been the cause of the indication. The aircraft operator
did not have any maintenance personnel stationed at Dili and the
crew did not report the problem to the operator's maintenance
organisation.
The aircraft returned to Darwin the next day. At about 1000
(CST), while the aircraft was on approach to Darwin airport, the
crew lowered the landing gear. The nose and right main gear
indicators showed that the respective gear was down and locked but
the left main gear position indicator showed unsafe. Still
suspecting an indication problem, the crew raised and lowered the
landing gear several times, but the left main gear indicator
continued to show an unsafe condition. The crew conducted a
fly-past of the control tower and the controller confirmed that the
left main gear was not down. Having confirmed that the nose and
right main landing gear operated correctly and that the left main
landing gear would not move, the pilot in command allocated flying
duties to the copilot. The pilot in command and flight engineer
conducted the checklist actions and attempted to lower the gear
using the emergency procedures. The attempt to lower the gear
hydraulically by using the landing gear override selector valve was
unsuccessful. An attempt to lower the gear using the manual drive
failed because the emergency engaging handle could not be moved.
The flight engineer unsuccessfuly attempted to manually move the
shift lever on the forward gearbox of the left landing gear from
"power" to "manual" and the loadmaster then attempted to lower the
gear by disconnecting the universal joints on the vertical torque
shafts of the left landing gear. However, the castellated nuts on
the bolts of both wheel vertical torque shaft universal joints
could not be undone without using a spanner. Even using a spanner,
only two of the four nuts had been undone after about 30
minutes.
At about 1020, the crew of a C5 Galaxy military cargo aircraft
also inbound to Darwin advised air traffic control that their
aircraft was experiencing a hydraulic problem. Twenty minutes
later, the crew of the L-382 informed air traffic control that the
aircraft would be making a gear-up landing. The pilot in command
requested that the airport's Rescue and Fire Fighting Service
(RFFS) lay a foam path along the last two thirds of Runway 36. The
air traffic controller informed the crew that a landing on Runway
29 was preferred and that the L-382 was number two in the
"emergency landing sequence". Air traffic control intended that the
Galaxy land first followed by the L-382.
The air traffic controller later informed the L-382 crew that
the airport RFFS advised that laying foam was not standard
procedure. The controller also advised that if foam was laid, no
foam would be available to attend the aircraft after it landed. The
pilot in command was concerned about the potential for fire caused
by sparks during the landing and he requested a clearance from air
traffic control to perform the landing on the grass alongside the
runway.
By the time two of the nuts on each of the universal joints had
been undone, the fuel state of the L-382 was approaching 1,500 lbs
or about 20 minutes endurance. The pilot in command decided that
due to the low fuel state, there was insufficient time to undo the
remaining nuts before a landing was required and he advised the
controller of the aircraft's low fuel state. Concerned at the
chances of the aircraft slewing off the runway after touchdown, the
pilot in command also decided that the nose and right main landing
gear would be raised for the landing. The L-382 was cleared to
track for final approach Runway 29. The Galaxy diverted to the
Royal Australian Air Force Base at Tindal.
The pilot in command subsequently decided that it would be more
prudent to land on the runway because of possible obstructions on
the grass area. After briefing the passengers, the crew conducted
their own emergency briefing, including actions after touchdown,
shutdown actions, and evacuation routes. The pilot in command
assumed control of the aircraft during final approach and conducted
the gear up landing.
At 1104, the L-382 landed on Runway 29. Touchdown was made at
approximately 90 kts and the aircraft slid about 300 metres before
stopping adjacent to Taxiway D. The aircraft remained straight on
the runway and none of the crew or passengers were injured. They
evacuated the aircraft soon after it came to rest. Although a flash
fire erupted at the rear lower fuselage area while the aircraft
slid along the runway, the fire did not spread. The RFFS applied
foam to the area around the aircraft after it came to a halt.
Damage to the Aircraft
The aircraft sustained extensive lower fuselage structural
damage due to the scraping along the runway. All rib lower end-caps
aft of the nose-wheel bay were damaged and there was evidence of a
flash fire in the rear section of the lower fuselage. Some damage
to the electrical wiring located in the lower fuselage area had
also been sustained.
Weight and Balance
The aircraft remained within the published centre of gravity and
weight envelopes throughout the flight.
Fuel
The aircraft taxied at Dili with about 17,600 lbs of fuel, for
the flight to Darwin. The departure fuel load was estimated to be
800 lbs more than the amount needed for the flight and required
reserves. The aircraft arrived at Darwin with over 8,000 lbs of
fuel and landed with about 1,500 lbs of fuel remaining.
Personnel Information
The pilot in command had accumulated 9611 hours of flying
experience and had about 4428 hours on the Hercules series of
aircraft including the military C-130 and civilian L-382. The
copilot had about 2,300 hours flying experience of which nearly 600
had been gained on the L-382. The flight engineer had 1,224 hours
operating experience of which about 484 hours were in the L-382.
The flight engineer reported that he had not conducted, nor had he
witnessed, a practice emergency lowering of the landing gear. There
was no regulatory requirement for him to conduct or witness a
practice emergency lowering of the landing gear.
Operator Information
The operator conducted freight and passenger flights in several
countries including remote areas of Africa, South America and
Antarctica.
The operator reported that because many of its operations were
in remote areas, crews were expected to assess aircraft
serviceability according to the company operations manuals,
Aircraft Flight Manuals (AFMs) and Minimum Equipment Lists. There
was no published guidance available to the crew regarding the
maintenance requirements following landing gear incidents such as
the one they encountered during the approach to Dili.
Meteorological Information
The weather conditions were clear with a 10 kt breeze from the
north-west.
The Main Landing Gear
The main landing gear of the L-382 comprises a main assembly on
each side of the aircraft. Each assembly has two shock struts each
equipped with a brake assembly, wheel and tyre assembly, torque
strut, and ball screw retracting mechanism. The brake, wheel, and
tyre assemblies are mounted on the bottom of each shock strut. The
two shock struts are mounted in tandem and connected to each other
by a torque strut.
Each main landing gear retracting mechanism has horizontal and
vertical torque shafts, three gearboxes, a hydraulic motor, ball
screws, and strut vertical guide tracks. An emergency manual drive
is provided for extension and retraction of the main landing gear
following a hydraulic system failure.
The left main landing gear gearboxes are mounted on the left
wheel well structure over the forward and aft shock struts, and
connected to each other by a horizontal torque shaft. A hydraulic
motor and a manual gearbox are mounted forward of the drive
assembly. A vertical torque shaft extends down from each of the two
gearboxes to a universal joint mounted on top of each of the ball
screws. Each ball screw is anchored through a ball bearing pillow
block at the upper end, and through a trunnion in the shelf bracket
at the lower end. A ball nut on the ball screw is connected to the
strut lower flange. The vertical ball screw assembly is installed
on each of the two main landing gear struts. Each screw is centred
between the strut guide tracks, and mounted to the inboard wheel
well wall through its upper pillow block and trunnion assembly at
the lower end of the ball screw shaft. Consequently, rotation of
the screw by the vertical torque shaft causes the ball nut attached
to the landing gear strut lower mounting flange to travel up or
down, directly raising, or lowering the landing gear.
Main Landing Gear Operation
When the landing gear selector in the cockpit is positioned
either up or down, an electrical signal commands the selector valve
to direct hydraulic pressure to the hydraulic motor mounted on the
front of each forward gearbox. Each of the forward gearboxes rotate
the attached vertical torque shaft and screw assembly while also
driving the horizontal torque shaft to drive the associated rear
gearbox. The rear gearbox turns the rear vertical torque shaft and
attached ball screw assembly.
Lowering of the landing gear can also be achieved by several
alternative methods, depending on the type of failure encountered.
Following a failure of the selector valve, over-ride buttons on the
landing gear selector valve can be used to manually direct
hydraulic power to the hydraulic motors of the main landing gear.
The main landing gear can also be lowered using the manual shift on
the forward gearbox. Pulling the emergency engaging handle moves
the shift lever on the forward gearbox from power to manual thereby
engaging the manual gearbox and releasing a spring-loaded brake.
The main gear should then free-fall. If the gear fails to
free-fall, a hand-crank is available to wind the gear down into
position. A malfunction that locks any component of the system
could prevent the main gear from moving. Consequently, the AFM
advises that the universal joints mounted on top of each of the
ball screws be disconnected. The gear should then free-fall under
its own weight. A wrench is provided to wind down the landing gear
should it not free-fall once the universal joints were
disconnected. The universal joints are connected to the torque
shaft by bolts and secured with castellated nuts. The nuts and
their associated bolts should be tightened during installation to a
torque of 25 to 30 inch pounds.
The aircraft manufacturer reported that it was possible, though
undesirable, to land the L-382 model Hercules with one landing gear
leg down on one side and both gear legs down on the other.
Engineering Investigation
A field structural repair was carried out in Darwin before the
aircraft was flown to Singapore for final rectification of the
damage. The landing gear was secured in the down position for the
flight. The engineers conducting the repairs visually inspected the
landing gear and found no abnormalities. The aircraft was then
jacked clear of the ground, and functionally tested the landing
gear. They found that the left main gear failed to extend normally.
When the engineers attempted to hand crank the gear down, they
noted that there was a high resistance within the system and the
left main gear would not lower. When the universal joints were
disconnected on the left main landing gear assembly, the forward
strut lowered freely but the rear strut remained up. On closer
inspection and disassembly, the left rear main gear ball screw
assembly was found to have excessive backlash and the grease on the
ball screw was found contaminated with accumulated debris. The
engineers also reported finding several defects within the
disassembled ball screw assembly including; excessively worn ball
inserts and numerous chipped and distorted bearing balls in the
ball nut assembly. Three circular scores with deep gouges were
found on the internal surface of the ball nut assembly return
sleeve and the scores coincided with the positions of the bearing
balls. The ball screw was also bowed. The engineering organisation
concluded that the damage was consistent with the bearing balls not
riding normally or freely along the sleeve, with the greatest
resistance probably occurring when the bearing balls rode across
the gouges. The examination found no faults in the left landing
gear hydraulic motor or associated gearboxes. The operator and
aircraft manufacturer reported that there had been no previous
failures of the type of ballscrew fitted to the accident
aircraft.
An inspection of the universal joint castellated nuts and
associated bolts found that none of the nuts could be fully unwound
without the use of a spanner. A subsequent materials analysis of
one of the castellated nut and bolt units revealed that the thread
of both the nut and bolt had been deformed by the imposition of a
load or loads along the axis of the bolt. Excessive tensile loads
being applied during the tightening of the bolts or a load caused,
for example, by abnormal operation of the landing gear could cause
the damage. The reason why the threads of the nuts and bolts
deformed could not be determined.
The aircraft manufacturer reported that no records could be
found specifically stating that difficulty was experienced in
removing the nuts from the bolts in the flanged connection.
The manufacturer had, however, introduced a torque shaft with a
quick release feature that replaced the nuts and bolts. The
manufacturer reported that the feature was introduced as a product
improvement to make it easier and safer to disconnect the torque
shaft from the ball screw.
Maintenance
A review of the aircraft maintenance documentation revealed no
precursory event or events that may have indicated to the operator
or the flight crew that there was an impending problem with the
left main gear. No landing gear system faults were recorded during
the previous block check. The manufacturer advised that the
inspection requirements of the L-382 included a daily, a "B" check
(the earlier of every 6 months or 600 flying hours), and a "C"
check (the earlier of 2 years or every 2,400 flying hours). The
daily and "B" checks required the checking of ball screws for
general condition, cleanliness and lubrication. The "C" Check
required a more detailed examination. The aircraft operator
reported that the approved company maintenance schedule for the
L-382 aircraft utilised block checks that occurred every 600 hours
flying time. The block checks incorporated the "B" and one quarter
of the "C" checks. Consequently, the aircraft would complete the
manufacturer maintenance requirements every four block checks. The
aircraft underwent a block check 647 hours before the incident at
Dili. The aircraft was operating on an approved ten percent
extension to the servicing schedule and was due to be flown to
Singapore after the return flight to Darwin to undergo the block
check.
The damage found within the ball screw assembly could be
identified only when the unit was dismantled. There was no
maintenance requirement to disassemble the unit for an in-service
inspection. The ball screw assembly was installed as a new
component and the aircraft operator approved documentation
indicated that life of the component was 6 years or 6,300 flying
hours. The ball screw assembly involved in the accident had been in
service for 5,506 hours and 65 months and was due for replacement
during the next block check. The ball screws were cleaned every 50
flying hours and were due for cleaning within six flying hours
after the flight to Dili.
The aircraft manufacturer reported that the failure of the main
landing gear to lower properly during the approach to Dili should
have been subject to a maintenance investigation using the
trouble-shooting procedures detailed in the aircraft maintenance
manual. According to the manufacturer, the main landing gear system
should have undergone an extensive maintenance inspection including
extension and retraction testing before the aircraft was released
for continued operation.
Operating Procedures
The AFM, in part, stated, "If the main and nose landing gears
fail to extend after normal actuation of the landing gear lever,
attempt to identify the malfunction before making further attempts
to lower the gear". The operator's standard operating procedures
provided the procedure for emergency lowering the landing gear but
did not provide any further elaboration on the AFM
requirements.
The aircraft operator reported that landing gear indication
problems had occurred on other occasions; mainly due to spurious
electrical signals that cleared with the reselection of the landing
gear position.
The crew had been provided with a mobile telephone but the
mobile service in Dili at the time was reported as being
unreliable.
Passenger Evacuation
One of the loadmasters in the crew gave a pre-flight briefing to
the passengers before the departure from Dili. Once the decision
was made to make an emergency landing at Darwin, the senior
loadmaster briefed the passengers on emergency procedures for the
landing. The loadmasters reported that procedures were conducted in
accordance with the operator's loadmaster training manual and drill
cards carried by each loadmaster.
Before landing, the passengers were briefed that the front
(crew) door and left emergency exit number two would be used. They
were told to move towards the nose of the aircraft after exiting
and to remain clear of the aircraft propellers. The passenger next
to the exit was briefed about operating the emergency door and only
to exit after the propellers had stopped. The passengers complied
with the evacuation briefings and no injuries were reported.
Rescue Fire-fighting Services
The Airport Services Manual provided a discussion and guidance
on the technique of foaming runways for an aircraft emergency
including gear-up landings. It noted that, "Flouroprotein foam,
film forming flouroprotein foam and aqueous film forming foam are
not considered suitable for runway foaming operations due to their
short drainage time". The Civil Aviation Safety Authority (CASA)
reported that all the Airport Fire Services in Australia audited by
CASA use Aqueous Film Forming Foam. Consequently, Airservices
Australia and the Civil Aviation Safety Authority had an agreed
policy that foam paths would not be laid at Australian airports.
This decision was due, in part, to the poor persistence qualities
of the foam agent (the foam path would only exist for a short
period) and because aircraft frequently missed the foam path during
landing. Both factors reduced the effectiveness of laying foam
paths. The investigation was also advised that an attempt to lay a
foam path for the aircraft probably would have exhausted the total
stocks of foam agent held by Darwin RFFS, leaving none to use on
the aircraft after it had landed.