Pilot Debrief

BEECH C99 near Norfolk, NE — 2024-10-08

Final reportCEN25FA006
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Date
2024-10-08
Location
Norfolk, NE, USA
Airport
OFK
Aircraft
BEECH C99
Registration
N130GP
Category
Airplane
Highest injury
Fatal
Fatalities
1

Probable cause

The pilot’s failure to maintain adequate airspeed following a partial loss of engine power during takeoff, which resulted in a loss of airplane control. Contributing to the accident was the fatigue failure of the compressor turbine fan disk blade.

Contributing factors

Contributing to the accident was the fatigue failure of the compressor turbine fan disk blade.

NTSB narrative

The pilot was departing on the final leg of a four-leg cargo flight. Airport surveillance video captured the airplane as it climbed away from the runway, entered a left bank, and began to descend. The airplane was in a nose-down, steep left bank as it descended out of view. Publicly available ADS-B flight track data indicated that the airplane’s ground speed decayed from 105 knots (kts) to about 84 kts as the airplane turned left of the runway’s extended centerline and climbed to a peak altitude around 160 ft above ground level (agl). The last recorded position showed the airplane about 88 ft agl at 85 kts ground speed. The airplane’s wings level, no-flap aerodynamic stall speed was 87 kts calibrated airspeed (KCAS), and its single-engine minimum controllable airspeed was 89 KCAS. Recorded weather information for the airport about the time of the accident included a 6-knot left quartering headwind for the takeoff runway. The airplane impacted terrain about 640 yards from the departure end of the runway. The distribution of the wreckage indicated that the airplane impacted in a near-wings-level attitude with relatively little forward motion, consistent with an aerodynamic stall or near-recovery from an aerodynamic stall. The observed damage to the propellers was consistent with the left engine producing low power, and the right engine operating at a high power setting. Examination of the engines revealed damage to multiple fan blades of the left engine compressor turbine (CT) disk. Most blades displayed damage signatures consistent with overstress; however, one blade displayed signatures consistent with fatigue. Metallurgic and microhardness examinations of the blade did not reveal any defects. The fracture surface bisected multiple grains and no plastic deformation was observed, all features consistent with intragranular fatigue; however, the root cause of the fatigue cracking could not be determined. It is likely that the first blade failed as a result of fatigue, and subsequently resulted in secondary overstress damage to other CT disk blades and further internal damage as the separated blades continued through the engine’s gas path. The observed engine damage would have resulted in a disruption of engine power, and likely noticeable unusual engine vibrations and possibly sparks from the engine’s exhaust. The airplane was equipped with an autofeather system. Since the left propeller was not found in the feathered position, it is unlikely that the left engine stopped producing power but, rather, continued to operate at a low power setting following the CT disk blade failure. Performance information contained in the airplane’s flight manual indicated that the airplane was capable of climbing with one engine inoperative if the inoperative engine’s propeller was feathered and the landing gear was retracted. The pilot completed training for the accident operator on the accident airplane about 1 month before the accident. Notes from his training flights indicated that he exhibited deficiencies during practice engine failure after takeoff scenarios on several occasions, including comments that his response to the simulated failure was “slow.” Available flight track data showed that the airplane’s ground speed decayed from takeoff speed to about its aerodynamic stall speed during the initial climb. It is likely that the pilot became distracted while attempting to manage and diagnose a partial loss of left engine power during the initial climb and did not maintain adequate airspeed, which resulted in a loss of control at an altitude too low for recovery. The CT disk blades were subject to recurring inspections every 2,000 hours of operation and replacement every 5,000 hours of operation. At the time of the accident, the blades had accumulated about 3,382 hours of operation and were most recently inspected about 20 months before the accident. Whether the fatigue cracking was present or would have been detectable during the most recent inspection could not be determined.

NTSB coding

Evidence available

  • Video
  • ADS-B / radar
  • Photos
  • 17 docket documents
View NTSB final reportView NTSB docket

Docket documents17

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