HONOLULU, HI, USA
N801WP
de Havilland DHC-8
Hydraulic fluid was leaking from a deteriorated O-ring in the landing gear selector solenoid valve, rendering the normal landing gear extension system inoperative. The crew actuated the alternate gear extension system and was unable to extend the right main landing gear. The first officer pulled the main landing gear alternate extension cable until it stopped and the right main landing gear uplock actuator did not release. On advice from company maintenance, after completing the alternate gear extension procedure, the crew retracted the nose gear and the left main gear using the normal system and then attempted another normal extension. Only the left main landing gear extended. The airplane's number 2 hydraulic system was depleted with 1.5 quarts of fluid remaining. The crew attempted another retraction to raise the left main landing gear without success. The captain then landed the airplane with only the left main landing gear extended. Examination of the alternate gear system revealed the main gear extension cable would bind after the left main landing gear uplock actuator would release. Subsequent testing of the system while the airplane was on jack stands reduced the binding, but in all tests the left main landing gear uplock actuator would release before the right uplock actuator. Examination of the main landing gear alternate extension cable revealed the plastic coating was missing in two places adjacent to a pulley. The right main landing gear wheel well was excessively dirty with areas of exfoliated white paint, exposing the underlying primer.
History of the Flight On December 15,1996, at 2109 hours Hawaiian standard time, a DeHavilland DHC-8, N801WP, landed with the right main and nose landing gear retracted on runway 4L at the Honolulu International Airport, Honolulu, Hawaii. The airplane was operated by Aloha IslandAir, Inc., d.b.a. Island Air, as Princeville Flight 8633, a scheduled domestic passenger flight under 14 CFR Part 121. The airplane sustained substantial damage. The 3 flight crew members and 10 passengers were not injured. The flight originated in Lanai, Hawaii, about 2000. Night visual meteorological conditions prevailed at the time and an IFR flight plan was filed. The cockpit crew attempted to extend the landing gear while approaching Honolulu airport and failed to get a safe landing gear indication. They cycled the landing gear switch two more times and continued to get the unsafe indication. At 2024, the captain indicated to the Honolulu air traffic control tower (ATCT) local controller that they needed vectors from the final approach due to a landing gear problem. The local controller then instructed him to contact departure control. After contacting departure control, the crew performed the emergency checklist alternate gear extension to manually extend the landing gear. The first officer opened the alternate release/extension door located on the floor of the cockpit and noted only two of the landing gear down lock verification lights were illuminated. The right main landing gear had not extended. The crew completed the alternate gear extension emergency checklist, resulting in two green indicator lights illuminated with associated amber gear door indicator lights for the left main and nose gears, and one red indicator light for the right main landing gear. The captain then contacted Aloha IslandAir dispatch and requested assistance from company maintenance personnel. The captain informed maintenance personnel through the dispatcher that the right main landing gear would not extend after accomplishing the alternate landing gear extension procedure. Maintenance personnel suggested the captain attempt to shake the gear loose. The captain replied that they had already performed that operation. According to the captain, maintenance then suggested that the power transfer unit (PTU) be manually selected. The captain told maintenance personnel that the emergency checklist specifically prohibited the use of the PTU. The captain then informed the company maintenance personnel that the airplane was losing hydraulic fluid in the number 2 system. The hydraulic quantity indicator was showing 1.5 quarts remaining, which was below minimum dispatch level of 3 quarts. Company maintenance personnel requested the crew to retract the landing gear and attempt a wheels up landing. The captain reported in his written statement that the checklist did not prohibit raising the landing gear after a loss of hydraulic fluid; however, the checklist indicated it could not be physically done. The captain decided to attempt to raise the gear. The inhibit switch of the alternate gear extension system was returned to the normal position. The gear handle was placed in the up position and the left main landing gear and nose gear retracted. The landing gear handle was then placed in the down position and only the left main landing gear extended. The captain then attempted to retract the left main landing gear; however, it remained extended. At 2053, the captain declared an emergency with departure control. Departure control vectored the airplane for a visual approach to runway 4L. At 2104, the captain was cleared for the visual approach and instructed to contact the Honolulu ATCT. After receiving landing clearance, the aircraft landed on runway 4L with only the left main landing gear extended. The airplane settled with a right wing low attitude. The airplane veered off the right side of the runway, dragging the right wing tip. The airplane came to rest in the sod area east of runway 4L between taxiway E and runway 8L. Crew Information Captain The captain holds an airline transport pilot certificate, which was issued on July 23, 1987. He completed his type rating training in the Dehavilland DHC-8 on June 8, 1995, and received the rating on July 6, 1995. The captain's total aeronautical experience consists of about 11,801.3 hours, of which 736.5 were accrued in the DHC-8. In the preceding 90 and 30 days before the accident, respectively, the captain flew a total of 128.2 and 60.9 hours. The captain had accrued about 26 hours of night flight experience in the DHC-8, of which 17.6 hours were in the past 90 days. The most recent first class medical certificate was issued to the pilot on July 1, 1996, and contained no limitations. First Officer The first officer holds an airline transport pilot certificate, which was issued on November 12, 1992, with a Boeing 737 type rating and ratings for multiengine airplanes. The first officer also holds a commercial pilot rating for single engine airplanes. The most recent first class medical certificate was issued to the second pilot on January 10, 1996, and contained no limitations. The first officer's total aeronautical experience consists of about 4,517.4 hours, of which 262.4 were accrued in the DHC-8. In the preceding 90 and 30 days before the accident, the first officer flew a total of 150.4 and 46.5 hours respectively in the DHC-8. The first officer reported during an interview that he attempted to accomplish the alternate gear extension procedure. He stated he had performed the procedure before in a DHC-8 simulator in Seattle, Washington. He also indicated that the effort required to unlock the main landing gear in the simulator required less force than the effort he made during the emergency in the accident airplane and during the subsequent postaccident testing. Aircraft Information The airplane was maintained under an equalized maintenance program (EMP). Review of maintenance records revealed the last scheduled check of the airplane's alternate landing gear extension system was on January 19, 1996. An EMP maintenance procedure requiring overhaul of the left main and nose landing gear struts was accomplished on June 11, 1996, at a total airframe time of 21,728.8 flight hours. The operator removed the left main landing gear and replaced the strut with a spare. The old left main landing gear strut was sent to a repair station for overhaul, and then became the stock spare. At the completion of the maintenance, both the left-hand main landing gear and the nose gear were lubricated. The airplane's alternate gear extension system was also checked during this maintenance period. On July 30, 1996, an EMP procedure requiring the overhaul of the right main landing gear was accomplished. The airplane had accumulated 21,990.1 flight hours. The right main landing strut was removed and replaced with a spare strut, which was formally the accident airplane's left strut. Upon completion of the maintenance, the airplane's alternate gear extension system was checked. The was no written record of the right main landing gear strut being lubricated at the completion of the maintenance. Review of the airplane's maintenance records revealed the nose gear and both main landing gear were lubricated under the EMP in 1996 on August 3rd and 4th. According to the EMP, the lubrication of the main landing gear includes lubrication of the gear uplock actuators. Wreckage and Impact Information Structural damage was observed on the underside of the airplane's fuselage starting at the rear bulkhead of the nose gear wheel well. The damage progressed aft along the airplane's keel beam about 48 feet. The propeller ice shield and two windows were damaged on the right side of the airplane's fuselage. The right propeller was found in the feathered position and all four blades exhibited damage. The right propeller's composite tips were missing and the metal spar in each propeller blade was bent opposite the direction of rotation. The right wing was also damaged. Several stringers and ribs were found bowed on the bottom of the wing, between wing stations 225 and 297. The fiberglass tip was broken aft on the underside. The outboard tip of the right aileron was also damaged. Hydraulic fluid was leaking through the airframe from a compartment in the right wing root. The number 2 hydraulic system indicated 0.5 quarts remaining. The alternate gear extension system for the main landing gear was examined. The uplock cable release mechanism was traced from the cockpit through the cabin ceiling, and then outboard along the leading edge of the wings to each nacelle wheel uplock. The cable was found to be continuous and routed through the appropriate pulleys. The cable was coated with a clear plastic material in the area of the cable exposed in the cockpit when the cable is pulled to release the main landing gear doors and uplock actuators. There were two 3-inch segments found in the cable where the clear plastic was missing. Both areas of missing clear plastic corresponded to places where the cable was routed over a pulley in the ceiling of the cockpit above the first officer's head. Examination of the main landing gear wheel well revealed the right wheel well had accumulated more dirt debris than the left. The white paint in the right main landing gear well was exfoliating, revealing the underlying green primer. The exfoliated area was about 1 foot from the right main landing gear uplock actuator. A buildup of dirt was also observed on both of the main landing gear struts specifically on the uplock rollers. Tests and Research Alternate Gear Extension Examination of the airplane revealed the hydraulic fluid was leaking from the landing gear selector solenoid valve. A deteriorated O-ring seal was found in a fitting at the source of the leak. The leak was repaired by the operator's maintenance personnel. The airplane was placed on jack stands and a hydraulic mule was attached to the airplane's hydraulic system. The landing gear was retracted and extended with the hydraulic mule pressure. There were no further discrepancies noted with the airplane's normal hydraulic retraction and extension system. The airplane's alternate gear extension system was examined and tested. It was noted that the left main landing gear uplock actuator would release at the same time the main landing gear door uplocks would release when the T-handle was pulled. It was necessary to pull the T-handle further and with greater effort to release the right main landing gear uplock actuator. Several repetitive tests of the alternate gear extension system were performed. The first test was accomplished with the operator's director of maintenance in the first officer's seat position and an FAA operations inspector in the captain's position. Subsequent tests were performed by the first officer of flight 8633. In each test, the sequence of the main landing gear uplock actuator release remained the same, with the left main landing gear releasing first. According to the FAA inspector, the cable attached to the main landing gear alternate extension handle appeared to be binding after the left uplock actuator released. It was noted in all tests that a greater force was needed to release the right main landing gear uplock actuator; however, the force required became less with each subsequent test. During the tests, the first officer stated the force he applied to the manual release handle in flight during the emergency was greater than the force required to release the uplock actuators while the airplane was on jack stands. Additional Information Cockpit Voice Recorder And Digital Flight Data Recorder The Safety Board retained the airplane's cockpit voice recorder (CVR) and digital flight data recorder (DFDR) for readout. Both the CVR and DFDR were found to function properly, but failed to provide useful information during the investigation. The CVR had recorded over the event. Thirty minutes had passed since the problem with the landing gear system and the landing. Airplane Release The airplane was released to the operator for repair on December 19, 1996.
A failure of the normal landing gear extension system due to a hydraulic leak, and the failure of the airplane's emergency landing gear extension system due to inadequate servicing by company maintenance personnel. Factors were the emergency landing gear extension systems dirty and binding condition, a worn emergency landing gear extension cable, and the training simulator's lack of fidelity concerning the force required to operate the emergency gear extension T-handle.
Source: NTSB Aviation Accident Database
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