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.