Eurocopter AS 350 B2 accident near Oklahoma City, Oklahoma, February 22, 2013

The NTSB's narrative final · quoted from the NTSB record

The emergency medical services helicopter departed a hospital helipad in dark night visual flight rules conditions and proceeded on its mission. Satellite data showed that, after takeoff, the helicopter began a gradual climb toward its planned destination. The data stopped about 3 minutes and 30 seconds into the flight. No distress calls were heard from the pilot. Fixed video surveillance cameras located near the accident site showed the last few seconds of the helicopter descending toward the ground. The helicopter impacted a parking lot, and a postimpact fire occurred. Examination of the wreckage revealed that three of the engine's first-stage axial compressor blades exhibited deformation consistent with soft body foreign object damage. The remainder of the engine and airframe exhibited no evidence of malfunction that would have contributed to an in-flight loss of engine power. The helicopter's air intake design, which had been modified to accommodate a different engine than that originally supplied by the helicopter's manufacturer, incorporated a blanking plate attached to the top side of the engine cowling that covered a portion of the air inlet screen. A gap in the area where the blanking plate and the screen overlapped made it possible, in certain meteorological conditions, for water or snow to pass through the screen, accumulate on the blanking plate, and freeze into ice. Ice accumulation in this area, if left undetected, could result in the ice detaching from the blanking plate and entering the engine during operation, causing soft body foreign object damage and a loss of engine power. Precipitation and outside temperatures ranging from 35 to 19 degrees F occurred during the 12-hour period preceding the accident. The combination of these meteorological conditions was conducive to the formation and accumulation of ice in the area between the air inlet screen and the blanking plate. Although the helicopter's flight manual supplement for cold weather operations recommended installation of an air inlet cover after the last flight of the day, during the day and night before the flight, the helicopter was parked outside on the helipad without an air inlet cover installed. According to the helicopter's mechanic, he inspected the helicopter on the afternoon before the flight and noted that some snow had accumulated on it. It is likely that the lack of an engine air inlet cover allowed precipitation to accumulate in the vicinity of the engine air intake. The helicopter's flight manual cold weather operations supplement also contained instructions for the pilot to perform a visual and manual (tactile) inspection of the air intake duct up to the first-stage compressor for evidence of snow and ice. Furthermore, the manufacturer and the Federal Aviation Administration had previously released information notices regarding inflight loss of engine power due to snow or ice ingestion caused by inadequate inspection or removal of snow or ice from the engine air inlet. These notices recommended a thorough inspection in and around the engine inlet area in order to detect and remove any snow or ice accumulation before flight. The initial on-scene examination found no remnants of ice or snow on these components because exposure to the postcrash fire would have melted such evidence. Surveillance video of the helipad showed that most of the helipad lights were off at the time of the pilot's preflight inspection immediately before the flight, making it difficult for him to detect any ice or snow accumulation in the area of the engine air intake. Thus, the ice accumulation between the air inlet screen and the blanking plate remained undetected, and shortly after takeoff, the ice detached from the blanking plate, slid into the air inlet, and was subsequently ingested by the engine, resulting in an in-flight loss of engine power. .

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.