North Canton, OH, USA
N49NS
Cessna 560
The airplane was on approach to the destination airport when the low hydraulic level and flow cockpit warnings annunciated in the cockpit. The flight crew was unable to extend the right main landing gear after conducting the normal and emergency landing gear extension procedures and by performing g-loading maneuvers. The airplane landed with the right wing main landing gear retracted causing substantial damage to the right wing and fuselage structures. The total loss of hydraulic power to the airplane flaps, spoilers, thrust reversers, and landing gear was caused by a ruptured hydraulic pressure hose assembly. The examinations of the accident and exemplar hoses revealed internal wear between the fiberglass within the hose's fire sleeve and the stainless steel braid. The accident hose and two of the three exemplar hoses did not show any markings or damage to the exterior of the sleeve to indicate the existence of the internal wear abrading the stainless steel braiding. Functional testing of the landing gear system revealed that the extension sequence varied during eight actuations, and, in one actuation, the left main landing gear remained retracted. Following the accident, the airplane manufacturer was in the process of amending the airplane maintenance manual rigging procedures for the landing gear system, and placing service time limits on hydraulic hoses.
HISTORY OF FLIGHT On December 17, 2006, at 1907 eastern standard time, a Cessna 560, N49NS, operated by Mid-Ohio Aviation Inc., received substantial damage on impact with terrain during a partial gear-up landing on runway at Akron-Canton Regional Airport (CAK), North Canton, Ohio. The flight crew reported that they were unable to extend the right main landing gear. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 135 passenger flight was operating on an instrument rules flight plan. The pilot, copilot, and three passengers were uninjured. The flight departed about 1745 from Teterboro Airport, Teterboro, New Jersey, and was en route to CAK. The flight crew reported that the hydraulic level low and the hydraulic flow low annunciator panel lights illuminated while en route to CAK. They reportedly followed checklist procedures to extend the landing gear and upon pulling the auxiliary gear control handle only the nose and left main landing gears locked down. They did not see the right main landing gear extended when they looked through the airplane windows. They then attempted to extend the right main landing gear by yawing and turning the airplane and performing "several G-loading maneuvers." The flight crew performed two flybys at the CAK Air Traffic Control Tower in order to have air traffic controllers confirm that the right main landing gear was still retracted. The flight crew then actuated the pneumatic landing gear extension handle to ensure that the left main and nose landing gears were locked down. The airplane landed on runway 01 (7,601 feet by 150 feet, grooved asphalt) with the flaps retracted, right main landing gear retracted, spoilers, and thrust reversers inoperative. AIRCRAFT INFORMATION The Cessna 560, serial number 560-0116, airplane was inspected on April 19, 2006, when Phase B, 1-5, 11, 18, 22, and 49 inspections were performed at: aircraft total time 8,082.5 hours, total aircraft landings 7,789, total number one engine hours/cycles: 7,928.4/7,605 and total number two engine hours/cycles: 7,839.7/7,548. The airplane landing gear system was last tested during the phase 1-4 portion of the April 19, 2006, inspection. The airplane received its last inspection on September 12, 2006, at an aircraft total time of 8,236.5 hours. The aircraft total time at the time of the accident was 8,356.9 hours. The airplane hour meter at the time of the accident was 5,546.2 hours. WRECKAGE AND IMPACT INFORMATION Damage to the airplane included abrasion on the bottom right wing panels, main spar, and rear spar, which occurred during the landing rollout. Also, there was wrinkling to both sides of the fuselage near the rear bulkhead. Examination of the airplane revealed a ruptured left hydraulic pressure hose assembly. The installation position of the hose was such that it did contact an adjacent structure/surface and was not restrained along its intermediate length. Examination of the landing gear system revealed that there were no separations within the mechanical control system. The right main landing gear uplock roller bearing was noted to possess scratches/marks.. The left main landing gear had a larger gap between the uplock and the uplock roller bearing than the right main landing gear uplock and roller bearing. The left main landing gear bearing did not possess similar features as did the right bearing. The hose assembly and both uplock bearings were sent to the National Transportation Safety Board (NTSB) Materials Laboratory for further examination. Post accident operation of the landing gear was performed by repeated extension and retraction of the landing gear with the airplane on jacks and powered by a hydraulic cart and airplane battery power. The left and right main landing gears were lifted up into the retracted position. With the landing gear hydraulically stowed, the emergency landing gear handle was pulled "slowly." The nose landing gear was the first to extend at each actuation of the emergency landing gear handle. The right main landing gear extended before the left main landing gear during the first 3 or 4 actuations. Upon the sixth actuation, the left main landing gear extended before the right main landing gear extended. Upon the seventh extension, the right main landing gear extended and the left main landing gear remained retracted. Upon the eighth actuation, all of the landing gear extended with the left landing gear extending last. The T-handle overtravel beyond the main landing gear release point measured 0.32 inch (the maintenance manual specification is 0.50 inch). TESTS AND RESEARCH Accident Hose Examination The left hand hydraulic pressure hose assembly, identified by the Cessna 560 Parts Manual as part number AE2463531G0460, underwent examination at the NTSB Materials Laboratory. The hose had a metal identification tag stamped "TSO-C75 TYPE I-A P-F," "CM 3574D460B000S," "10 25/90." According to Stratoflex, the tag identification indicated a Stratoflex 124F003-6CR-0460 hose, which has a layered construction consisting of a polytetradfluorine inner tube, a single layer of corrosion-resistant steel (CRES) braid and a fire sleeve consisting of an internal glass fiber fabric and external silicon sleeve. The hose had been manufactured in 1990 and accumulated a total time in service of 8,356.9 hours and 8,077 cycles. There is no life limit in place relating to the hose. The hose was 46 inches long with a straight fitting at one end and a 45-degree elbow fitting at the other end. The silicone outer layer of the fire sleeve for the hose ruptured between 13.5 - 15.5 inches from the straight fitting end of the hose. Adjacent to the clamp at the elbow fitting end of the fire sleeve, the silicone layer of the fire sleeve was cracked, and the crack appeared to be filled with an orange filler material. Once the fire sleeve was removed, wear was observed on the CRES braid primarily in three areas. These areas were located at 8.5 - 9.25 inches (area A), 9.75 - 16 inches (area B), and 16.5 - 18 inches (area C) from the straight fitting end. In each of these areas, wear was characterized by thinned wires; and in areas of greatest wear, wires were thinned to a chisel-like point with wire material missing between the chisel-like ends. The area of the CRES braid in the vicinity of the wear also appeared darker than the rest of the hose braid. Wear was also observed on the CRES braid opposite of the hose circumference at the three areas. There was less wear on the opposite side. The exterior surface of the fire sleeve at each of the worn areas was intact. Some of the surface markings in area A were missing and the surface appeared disturbed. However, the fire sleeve outer surface in the area of wear had an overall appearance similar to that of the remainder of the fire sleeve. The thickness of the fire sleeve silicone layer was 0.044 inches. A close view of wear in Area A showed, in the area of greatest wear, wire thinned to a chisel point and missing material from between the chisel-like ends. Worn surfaces of the wires had linear scratch marks, and the width of the scratch marks was similar to the diameter of the glass fibers from the fire sleeve. The polytetrafluorine inner tube was ruptured with a longitudinal crack near the middle of area B and the wire braid in the area was spayed outward. The wire ends in the ruptured area displayed a chisel-like appearance of the worn wire ends. Individual glass fibers were observed in the worn and ruptured area. The worn ends of the braid wires had linear markings, and the width of the linear markings appeared similar to that of the glass fiber diameter. Similar thinning and linear marks on the thinned surfaced were observed in area B. The thickness of the fire sleeve silicone layer from the exemplar hoses was approximately 0.044 inches. Scanning electron microscopic (SEM) examination of the fire sleeve showed individual fiber diameter of approximately 6 micrometers. According to the airplane manufacturer, the hydraulic hose does not have an hourly or calendar time life limit. Uplock Roller Bearings The internal sleeves in both bearings rotated by hand within the outer sleeves without binding, and no excessive play was noted during hand manipulation. Exemplar Hose Examinations Three exemplar hoses from three different Cessna Model 560 airplanes were removed during their regular maintenance schedules so that they may undergo examination at the NTSB Materials Laboratory. The three hoses were labeled for purposes of examination as A, B, and C. All of the hoses were manufactured in 1995 and their hours and flight cycle time were noted. Hose A, had 8,215 hours and 8,496 cycles, hose B had 8,865 hours and 7,678 cycles, and hose C had 11,991 hours and 9,682 cycles. There is no life limit in place relating to the hose. The three hoses were Stratoflex part number 124F003-6CR-0460 and the following annotated on metal tags attached to each hose. The tags on hose A and C read, "71379 ASSY CM3574D460B000S," PT/A/55813 SIZE 6 1500 PSI," "MFR-98441-124F003-6CR-0460," and "TSO-C75 Type I-A-S-F 12/15/95." The tag on hose B read, "71379 ASSY CM3674D460B0000S," PT/A/55813 SIZE 6 1500 PSI," MFR-98441-124F003-6CR-0460," and "TSO-C75 TYPE I-A-S-F 12/15/95." The Stratoflex trademark "S" was on all three tags. The exemplar hose were examined visually and found the have a somewhat dimpled surface appearance. Observations of the hoses' external appearance is provided in Table 2 of Material Report No. 08-066. The exterior features included burn marks, cracks and a hole in one of these hoses. The fire sleeves were then removed. Hose A's fire sleeve had an interior surface that possessed a mostly white to light brown appearance, but had areas that appeared darker, including a burn area 4.5 inches from the straight fitting to the hose. An area of wear was observed on the CRES braid at 18.25 inches from the straight fitting end. Several adjacent wires of the CRES braid were also worn through approximately 50 percent of their thickness. Bits of silicone material were also present in the wear area. Fibers of the fiberglass were fractured in this area, leaving a hole in the fabric. This area of wear corresponded to a filled crack in the silicone observed on the fire sleeve exterior, and the fiberglass in this area was somewhat darker. Hose B had areas of CRES wire braid that appeared slightly darker at 13 inches, several locations between 22 - 24 inches, and at 31.5 inches from the straight fitting end. The darker area at 13 inches had no marks or damage on the fire sleeve exterior. However, wear was observed on the CRES wire braid. Some wires of the braid were worn approximately half way through the thickness in this area. Between 22-24 inches from the straight fitting, several dark spots were observed on the CRES wire braid, and several adjacent wires were worn to approximately 50 percent of their thickness at two of these spots. These spots corresponded to cracks observed in the silicone and a black spot on the fiberglass interior surface. At 31.5 inches from the straight end fitting, an area of the CRES area braid approximately 1/4 inch diameter had a blue to gold tint. Several adjacent wires were worn in this area with wear between approximately 25 - 50 percent of their thickness. This spot also corresponded to a crack in the silicone and to a black spot on the fiberglass interior surface. Hose C had areas of the CRES braid that contained dark deposits intermixed with fibers at 4.25 inches and 12 inches from the straight end and the braid was slightly darkened at 19.5 inches. Wear of the braid was observed at all three locations. At 4.25 and 12 inches, wear of several adjacent wires was between 25 and 50 percent of their thickness. This spot also corresponded to a crack in the silicone and to black spot on the fiber glass interior surface. The area of fiberglass at 19.5 inches in the fire sleeve also appeared darker. No marks or damage were observed on the fire sleeve exterior at this location. A closer view of the CRES braid at this location had wear of the individual wires. Some of the wires were worn nearly through the thickness. The thickness of the fire sleeve silicone layer from the exemplar hoses was approximately 0.033 inches. SEM examination of the fire sleeve showed individual fiber diameter of approximately 8 micrometers. Hose Installation/Inspection Details Inspection of another Cessna 500 series airplane was examined by a Cessna representative. The Cessna representative indicated that in the exemplar engine, tie straps attached the hydraulic hose to a tube and a suctions hose. These tie straps were approximately 6 inches, 12 inches, and 18 inches from the straight eng fitting of the hose. The Cessna 560 maintenance manual (revision 24), 20-10-06 (revision 15), page 2, Tubing and Hose Assemblies Removal/Installation states: NOTE: If new hose is to be installed, ensure that hose is manufactured of the correct material and length. B. (4) check that hose is free to expand, contract and clear of all structure. Where inadequate clearance exists between hose and structure, protection must be provided for hose to prevent damage from chafing. The Cessna 560 maintenance manual (revision 24), 20-10-06 (revision 15), page 5, Hydraulic, Pneumatic and Bleed Air Line, Fuel Line, Duct and Hose Inspection Criteria states: A. (1) Inspect for proper routing and clamping. Lines and ducts must be adequately supported to prevent fatigue cracks at fittings. (2) Inspect for any evidence of chafing or other damage. (3)(c) Replace pressure or return hydraulic or fuel tubing which has dents deeper than 5% of outside diameter. (4) Inspect hoses for twisting or weather checking or splitting (4)(a) Ensure hose assemblies have the proper freedom of movement necessary for their function and they do not chafe other items during their normal range of movement. (5) Inspect for evidence of fluid leakage. Emergency Landing Gear Extension Incidents Two additional incidents involving a partial emergency landing gear extension on Cessna 550, serial number 550-684 and Cessna 560, serial number 435 airplane were reported. The Cessna 550, serial number 684, emergency down lock procedure illuminated only one landing gear instrument light after the emergency uplock release handle and blow down knob were pulled. The standard hydraulic procedure was then used to extend the landing gear. The sequencing valve had been overhauled and damaged O-ring within the sequencing valve was reported. The Cessna 560, serial number 560-435, right main landing gear did not deploy after the landing gear handle could not moved. The emergency uplock release handle and blow down knob were then pulled. The right main landing gear uplock emergency cable was slack, requiring 1.5 inches of lateral movement before moving the uplock hook. The airplane landed with the right main landing gear extended. The crew landed with the flaps extended and during the landing rollout, the airplane skidded on the right flap thus precluding structural damage to the airplane. Cessna Aircraft is amending the airplane maintenance manual rigging procedures for the emergency landing gear system. Service Difficulty Reports A query of the FAA's Service Difficulty Report database using the search term "hydraulic system" yielded 77 voluntary reports. These reports included two reports involving hydraulic hose number "CM3574D460B0" on Cessna 550 airplanes. One of these reports indicates January 31, 2006, a Cessna 560, registration number and serial number unreported, experienced a burst hydraulic hose with a time in service of 6,000 hours. The second report indicates on May 12, 1998, a hydraulic hose was found leaking from an unreported location. The time in service of the hose was unreported.
The rupture of the hydraulic pressure hose assembly, which was caused by internal wear between the hose's fire sleeve and stainless steel braid, and the failure of the emergency landing gear assembly to deploy the right main landing gear during approach. Contributing to the accident were inadequate hydraulic hose assembly maintenance inspection procedures, the lack of a hydraulic hose life limit by the airplane manufacturer, and the inadequate landing gear rigging procedures.
Source: NTSB Aviation Accident Database
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