Thursday, May 1, 2008
Avoiding CFIT Incidents
Maintaining proper altitude
About 12 minutes after takeoff, the flight was handed off to Los Angeles Center. The flight subsequently climbed to 8,400 feet MSL and then descended to 6,400 feet. At about 10 p.m., the airplane was flying between 6,300 and 6,500 feet when it disappeared from radar. The controller attempted to radio the pilot several times, without success.
Satellite images showed that a band of clouds north of Santa Barbara, extending from west to east, was obscuring higher terrain, including the accident site. Satellite data indicated that the cloud tops were near 10,000 feet. Pilot reports indicated a broken to overcast cloud layer, with tops from 9,000 feet to 11,000 feet, in the vicinity of the accident site.
The NTSB determined that the probable cause of this accident was the pilot’s failure to select and maintain an adequate terrain-avoidance cruise altitude. Contributing factors were the dark nighttime conditions, the rising mountainous terrain and the FAA controller’s failure to issue a terrain-related safety alert.
On February 6, 2007, a Beech King Air medical aircraft was being repositioned from Great Falls, Mont., to Belgrade, Mont. The airplane was on an IFR flight plan and carried an ATP-rated pilot, flight paramedic and a flight nurse. Weather observed at 8:56 p.m., at the Gallatin Field Airport in Belgrade, included an overcast ceiling at 11,000 feet AGL, visibility for 10 miles, wind at 260 degrees at four knots, a temperature of 4 degrees C and a 1 degree C dew point.
The aircraft was being handled by Salt Lake Center and, about 16 minutes after takeoff, the pilot advised that he had the airport at Belgrade in sight. At 8:57 p.m., the pilot was cleared to descend at his discretion from 15,000 feet to 13,000 feet. The pilot acknowledged and stated, “I’ve got the airport in sight at BZN requesting a visual approach.” The pilot was cleared for the visual approach, radar services were terminated and the pilot was told to contact the tower controller at Gallatin Field. At 9:01, the pilot radioed the tower, but most of the transmission was unintelligible. A follow-up transmission also was unintelligible. Investigators learned that it was common for radio transmissions from the area in which the aircraft was located to be garbled and unreadable.
Radar data indicated that the airplane began its descent at 8:57 p.m. from 15,000 feet. At the time, it was about 30 nm north of the accident site. The last radar target associated with the King Air was received just after 9:01, about 13 nm north of the accident site. It was at 11,300 feet MSL. The minimum obstruction clearance altitude (MOCA) for the accident area is 9,100 feet.
The wreckage was located on the north side of a ridge—the highest obstruction between the accident site and the airport. The elevation was 5,700 feet MSL. The airplane struck trees, then the terrain. All three occupants were killed. Investigators found no evidence of any preimpact problems with the aircraft’s engines, propellers or systems.
The airplane was equipped with an enhanced GPWS, but impact damage prevented testing of the unit. Investigators couldn’t determine how the unit was configured during the flight or whether it issued any warnings to the pilot before the impact with terrain. The NTSB noted that the airport is in a large valley and is surrounded by rising mountainous terrain. At night, clouds and the ground are difficult for pilots to detect, and a gradual loss of visual cues can occur as a flight continues toward darker terrain. Additionally, the horizon is less visible and distinct. According to the Safety Board, because the airplane was descending over terrain that provided few visual references, and because the overcast cloud layer would have prevented moonlight from illuminating the terrain, it’s likely that the pilot didn’t see the rising terrain as the airplane continued toward it.
The NTSB determined that the probable cause of this accident was the pilot’s failure to maintain an adequate altitude and descent rate during a night visual approach. Dark night conditions and mountainous terrain were factors in the accident.
Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.
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