Incident Overview
On September 29, 2008, at approximately 1125 eastern daylight time, a McDonnell-Douglas 369E helicopter, registration N801SB, performed a forced landing in a hay field near Allentown, Pennsylvania, following an engine failure. The helicopter was undamaged during the landing. The commercial pilot, the sole occupant, was uninjured.
Flight Details
The positioning flight departed Queen City Airport (XLL), Allentown, Pennsylvania, about 1115, en route to Stewart International Airport (SWF), Newburgh, New York. The flight was operated under 14 Code of Federal Regulations Part 91 in visual meteorological conditions; no flight plan was filed.
Earlier that day, the pilot had performed a preflight inspection and added fuel before departing Battlefield Heliport (PA14), Gettysburg, Pennsylvania, around 0915. The planned route included an intermediate stop at SWF before continuing to Laurence G Hanscom Field Airport (BED), Bedford, Massachusetts. During the initial leg, the left rear door became unlatched, prompting a diversion to XLL where the pilot secured the door and refueled.
Engine Failure and Forced Landing
After departing XLL, while cruising at 1,000 feet above ground level (agl) at a ground speed of 115 knots, the pilot heard a loud bang. The helicopter experienced a left yaw, activation of the "engine out" horn and light, and illumination of an engine chip detector light. The pilot noted a decline in rotor rpm and moderate vibration through the cyclic and anti-torque pedals. A forced landing was conducted in a field. The helicopter touched down level and slid approximately 65 feet before stopping.
Post-landing, the pilot shut off the start pump, generator, and battery switches, exited, and observed engine smoking and sounds similar to an engine cooling down. Anomalies included thermal damage to paint on the clam shell doors, engine oil throughout the engine compartment and tail section, unseating of the left exhaust stack, a crack around the mounting flange of the right exhaust, unseating of right discharge tube snap rings, loose igniters and fuel spray nozzles, and broken safety wiring.
Engine and Aircraft Examination
The helicopter was powered by a Rolls-Royce 250-C2B gas turbine engine. The helicopter and engine were transported to the operator's maintenance facility at PA14 and examined by an FAA inspector, Rolls-Royce representatives, and the operator's personnel. The engine manufacturer's representative reported minor internal impact damage to the exhaust stacks, consistent with high-velocity objects exiting the engine. The exhaust collector had a 2-inch crack along its forward face. The fourth stage turbine wheel outer ring was missing material along an approximate 30-degree arc and could not be rotated by hand.
FAA records showed the helicopter received its airworthiness certificate on October 28, 1986, and was added to the owner's Federal Aviation Regulations Part 133 certificate on September 19, 2008. The incident engine was installed in August 2008, with 2,158.8 hours of time in service at installation. Maintenance records indicated the engine had most recently been overhauled at a facility in Canada and was installed by the operator approximately 6 flight hours prior to the event.
Engine Disassembly and Laboratory Analysis
On October 21, 2008, the engine was examined at the Canadian overhaul facility with representatives from the Transportation Safety Board of Canada, Rolls-Royce, the helicopter owner, and the overhaul facility. The compressor section rotated freely, but the turbine section could not be rotated and showed thermal damage.
Internal examination revealed bearing and labyrinth seal damage, large carbon deposits, and a missing blade on the second stage turbine wheel (with no tip rub). The No. 6 bearing and associated stage exhibited thermal damage, while the No. 8 bearing had oil present and no heat damage indicative of oil starvation. Fuel nozzle spray pattern testing was within specifications, with no flame streaking across turbine components.
The NTSB Material Laboratory examined engine parts on November 27, 2009, and April 28, 2010. The combustion liner exterior showed dark gray regions with localized wrinkling around the two largest holes, consistent with localized hot spots. The interior had no hot spot or streaking indications.
Examination of the second stage turbine wheel revealed a fatigue fracture through an airfoil blade, initiating on the convex side at the transition radius between the wheel rim and blade, near a mold line from original manufacture. No metallurgical or mechanical discontinuities were at the surface. The other 23 blades on the same wheel had small cracks in similar locations. The second stage turbine wheel sectioning revealed cracks in the blade radius and wheel rim between blades, with transgranular propagation and oxidized surfaces consistent with thermal fatigue.
All 38 blades on the first stage turbine wheel were fractured near the tips. Six blades on the third stage turbine wheel were fractured outboard of mid-span, consistent with overstress separation, with no preexisting cracking found.
Weather Conditions
The 1051 recorded weather observation at Lehigh Valley International Airport (ABE), about 10 miles south-southwest of the landing site, reported winds from 290 degrees at 9 knots, a broken cloud layer at 2,900 feet agl, 10 miles visibility, temperature 20°C, dew point 14°C, and altimeter setting 29.99 inches of mercury.