Friday, September 1, 2017

Helio H-250 Courier, N6324V: Accident occurred September 26, 2016 at Lake Hood Seaplane Base (PALH), Anchorage, Alaska

Aviation Accident Final Report - National Transportation Safety Board: https://app.ntsb.gov/pdf

NTSB Identification: ANC16LA072
14 CFR Part 91: General Aviation
Accident occurred Monday, September 26, 2016 in Anchorage, AK
Probable Cause Approval Date: 10/17/2017
Aircraft: HELIO H-250, registration: N6324V
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The commercial pilot reported that, after an uneventful touchdown on the dry, gravel-surface runway, he felt a medium vibration as he applied the brakes. As the airplane continued the landing roll, both main landing gear (MLG) legs sheared off. The propeller impacted the runway, and the airplane came to rest on the bottom of the fuselage, which sustained substantial damage. 

A postaccident examination of the airplane revealed that both MLG legs failed due to fatigue cracking. The fatigue cracks were extended by intermittent overstress until the legs separated. The overstress extensions likely occurred during the accident landing or immediately before it because there was no corrosion of the crack surfaces. 

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
The fatigue fracture failure of both main landing gear (MLG) legs during the landing roll, which resulted in the collapse and separation of both MLG legs. 

The National Transportation Safety Board did not travel to the scene of this accident.

Additional Participating Entity:
Federal Aviation Administration / Flight Standards District Office; Anchorage, Alaska

Aviation Accident Factual Report - National Transportation Safety Board: https://app.ntsb.gov/pdf

Investigation Docket - National Transportation Safety Board: https://dms.ntsb.gov/pubdms

http://registry.faa.gov/N6324V

NTSB Identification: ANC16LA072
14 CFR Part 91: General Aviation
Accident occurred Monday, September 26, 2016 in Anchorage, AK
Aircraft: HELIO H-250, registration: N6324V
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

On September 26, 2016, about 1420 Alaska daylight time, a tailwheel-equipped Helio Courier H-250 airplane, N6324V, sustained substantial damage during the landing roll at the Lake Hood Seaplane Base, Anchorage, Alaska. The certificated commercial pilot sustained no injury. The airplane was registered to, and operated by, a private individual as a visual flight rules (VFR) flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91. Visual meteorological conditions prevailed at the time of the accident, and a VFR flight plan had been filed. The flight originated from a private airstrip near Igiugig, Alaska, about 1200.

The pilot reported in a written statement on September 29 that after an uneventful touchdown on the dry and gravel surface of runway 32, a medium vibration was felt as the pilot applied the brakes. As the airplane continued the landing roll, both main landing gear legs sheared off. The propeller impacted the runway, and the airplane came to rest on the bottom of the fuselage without further incident. 

The airplane sustained substantial damage to the fuselage. A postaccident inspection revealed that the main landing gear legs on both sides sheared off below a support bracket located at the intersection of the tubular steel structure of the fuselage attachment points. The main landing gear legs (left side, part number 250-040-451-0 and right side, part number 250-040-451-1) are constructed of 1/8-inch steel, which is molded and welded to form a 2-inch-square steel tube. The main landing gear legs are covered by a contoured fairing assembly. The pilot reported that the main landing gear legs were manufactured in 1966. He reported that the main landing gear legs had about 3,500 hours total time, about 15,000 cycles, and 15 hours since the last inspection. 

In a written statement on October 14, the pilot reported that he could not determine when the main landing gear was inspected as they were changed in 2013 for ski-wheel use and he was unsure if a dye penetrant process was performed or not. He reported that the main landing gear legs installed on the accident airplane came off another airplane that had about 3,500 hours total time.

The main landing gear legs were secured and transported to the NTSB Materials Laboratory in Washington, D.C., for further examination. 

In the recommendation section of the National Transportation Safety Board (NTSB) Accident/Incident Reporting Form 6120.1, the pilot reported that the main landing gear legs may previously have experienced hard landings, along with previous cracks, and that these cracks are difficult to detect during inspection. He further reported that it is advisable to have the main landing gear checked by specialists at annual inspections, especially if the main landing gear is being converted from ski-wheels to wheels, or vice versa.

METEOROLOGICAL INFORMATION

The closest weather reporting facility was the Lake Hood Seaplane Base. At 1353, an Aviation Routine Weather Report (METAR) was reporting in part: wind from 310 degrees at 5 knots; visibility 10 statute miles; sky condition, overcast 5,500 feet; temperature 46 degrees F; dew point 36 degrees F; altimeter 29.89 inHg.

TESTS AND RESEARCH

An examination by the NTSB Materials Laboratory revealed that both main landing gear legs were fractured through the square tube section between 29 and 30 inches above the axle centerlines. Several fatigue regions were present in the vertically oriented, aft walls of both main landing gear legs particularly near the upper and lower corners. All fatigue regions initiated at multiple sites on the outer (aft) surfaces of the tube structure and propagated inward (forward) through the wall of the main landing gear legs.

The NTSB Materials Laboratory examination report is in the public docket for this accident. 

ADDITIONAL INFORMATION

Previous Main Landing Gear Leg Failure

ANC05LA097 identified a fracture failure of the right side main landing gear leg (part number 250-040-451-1, with about 7,006 hours in service) with a Helio H-250 during the landing roll. 

Landing Gear Inspection and Maintenance 

The Federal Aviation Administration has published Advisory Circular 43.13-1B Acceptable Methods, Techniques, and Practices – Aircraft Inspection and Repair (2001). This document discusses inspection and maintenance of landing gear and states in part:

The entire structure of the landing gear should be closely examined for cracks, nicks, cuts, corrosion damage, or any other condition that can cause stress concentrations and eventual failure. Small nicks or cuts can be filed and burnished to a smooth contour, eliminating the point of stress concentration. If a crack is found in a landing gear member, the part must be replaced.

NTSB Identification: ANC16LA072
14 CFR Part 91: General Aviation
Accident occurred Monday, September 26, 2016 in Anchorage, AK
Aircraft: HELIO H-250, registration: N6324V
Injuries: 1 Uninjured.

This is preliminary information, subject to change, and may contain errors. Any errors in this report will be corrected when the final report has been completed. NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

On September 26, 2016, about 1420 Alaska daylight time, a tailwheel-equipped Helio Courier H-250 airplane, N6324V, sustained substantial damage during the landing roll at the Lake Hood Seaplane Base, Anchorage, Alaska. The certificated commercial pilot sustained no injury. The airplane was registered to, and operated by, a private individual as a visual flight rules (VFR) flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91. Visual meteorological conditions prevailed at the time of the accident, and a VFR flight plan had been filed. The flight originated from a private airstrip near Igiugig, Alaska, about 1200. 

During a telephone interview with the National Transportation Safety Board (NTSB) investigator-in-charge on September 26, the pilot stated that after an uneventful touchdown on the dry and gravel surface of runway 32, a medium vibration was felt as the pilot applied the brakes. As the airplane continued the landing roll, he released the brakes and both of the main landing gear leg assemblies sheared off. The propeller impacted the runway, and the airplane came to rest on the bottom of the fuselage without further incident. 

The airplane sustained substantial damage to the fuselage. A postaccident inspection revealed that the main landing gear leg assemblies on both sides sheared off below a support bracket located at the intersection of the tubular steel structure of the fuselage attachment points. The main landing gear leg assemblies (left side, part number 250-040-451-50 and right side, part number 250-040-451-1) are constructed of 1/8-inch steel, which is molded and welded to form a 2-inch-square steel tube. The main landing gear legs are covered by a contoured fairing assembly. The two main landing gear leg assemblies were secured and transported to the NTSB Materials Laboratory in Washington, D.C., for further examination. 

The closest weather reporting facility was the Lake Hood Seaplane Base. At 1353, an Aviation Routine Weather Report (METAR) was reporting in part: wind from 310 degrees at 5 knots; visibility 10 statute miles; sky condition, overcast 5,500 feet; temperature 46 degrees F; dew point 36 degrees F; altimeter 29.89 inHg.

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