History of Flight
On December 3, 2011, about 0830 mountain standard time, the pilot of a Beechcraft 1900D airplane, N247GL, discovered a hydraulic leak on the nose landing gear (NLG) during a post flight inspection. The airplane was registered to Great Lakes Aviation and operated by Great Lakes Airlines as a 14 Code of Federal Regulations, Part 121 scheduled passenger flight. The airplane had just completed a flight from Montrose, Colorado, to Denver.
Damage to Airplane
The pilot’s inspection of the NLG revealed a crack in the NLG actuator end cap, part number (P/N) 25703-4. The end cap, serial number (S/N) 540, was removed from the actuator and shipped to the NTSB.
Aircraft Information
The NLG actuator was delivered new with the airplane and had accumulated 31,828.2 hours and 38,831 cycles since new. Great Lakes Airlines performed an ultrasonic inspection in accordance with the instructions in the Model 1900D Airliner Maintenance Manual 485 cycles prior to the incident. According to the operator, the NLG actuator had never been overhauled.
Tests and Research
The NLG end cap was examined in the NTSB Materials Laboratory. A visible crack extended circumferentially from the primary extend port on one side of the end cap, around the diameter of the end cap, to the secondary extend port on the other side. The intact portion of the end cap between the ports was cut toward the center line of the shuttle valve bore, and the two pieces were separated to examine the fracture faces. Examination revealed a relatively flat, smooth zone and ratchet marks originating at the intersection of a radius with an inner diameter of the end cap, consistent with fatigue. The fatigue zone extended outwards almost uniformly through the wall thickness of the end cap, with a preference to the shuttle valve bore, and had propagated entirely through the thickness near the aft side of the end cap. The fracture face outside of the fatigue zone displayed a rough, grainy surface consistent with an overload event. Samples were removed and metallurgically mounted, polished, and etched. The etchant revealed a longitudinal grain structure oriented parallel to the shuttle valve bore. The surface finish and machined radius were found to satisfy the drawing requirements.
The fatigue zone was examined in the scanning electron microscope. A location near the shuttle valve bore with the longest fatigue propagation was chosen for striation counting. Near the beginning of the crack, there were 27 individual striations in a measured crack length of 11.25 micrometers. Near the end, 28 striations in a measured length of 24.25 micrometers. It was then calculated that there were approximately 14,898 striations along the crack propagation at this location. Since the full crack length could not be determined due to the fatigue zone propagating into the shuttle valve bore, the striation count is an approximate and minimum value.
According to the Metals Handbook Desk Edition, 2nd Edition, 1998, wrought aluminum products generally exhibit differences in tensile properties and resistance to fatigue stresses in the three orthogonal grain directions, with the longitudinal direction being superior. Typically, fatigue loading in the longitudinal grain direction can provide a 5 to 10 percent improvement in fatigue strength. The principal tensile stresses in the end cap are in the longitudinal direction and result from hydraulic pressure.
Additional Information
The fatigue cracking evident on the NLG end cap is similar to cracking found on the end caps of three different Beechcraft 1900D airplanes operated by Great Lakes Airlines, documented in NTSB investigations CEN11IA341, ENG12IA014, and ENG12IA024.
As a result of some previous NLG end cap fractures and preliminary findings from CEN11IA341, Hawker Beechcraft instituted changes to their recommended maintenance program for the NLG actuator. On August 1, 2010, the Model 1900D Airliner Maintenance Manual (AMM) was revised to add a recommended overhaul of the NLG assembly (including actuator) every 10,000 cycles or 5 years, whichever occurs first, to be complied with within 15 months. On May 1, 2011, instructions were added for inspecting the NLG actuator end cap for cracks using ultrasonic inspection, and a repetitive ultrasonic inspection every 1,200 cycles was recommended once the actuator had accumulated 8,000 cycles since new or newly overhauled. On November 1, 2011, a Temporary Revision allowed the actuator to remain in operation if ultrasonic inspection had been performed within the last 1,200 cycles, with recommended repeat inspections every 600 cycles.
The end cap is produced per Airight, Inc. (now APPH, Inc.) drawing 25703, currently revision P. Revision N, approved February 2010, added a specified longitudinal grain direction along the longitudinal axis; prior to this, no grain direction requirement existed. According to APPH, the change was initiated by the raw material supplier and unrelated to fatigue failures. APPH also stated that stamping the actuator serial number in the recessed area between ports on the end cap was standard practice for every overhauled actuator since at least 2004.
On February 13, 2013, the NTSB issued recommendations A-13-04, 05, and 06 to Hawker Beechcraft Corporation to address fatigue cracking of NLG end caps on Beechcraft 1900D airplanes.