Final Report: Stall in icing downed King Air on approach

 - December 14, 2007, 6:18 AM

RAYTHEON KING AIR E90, RENO, NEV., MARCH 13, 2002–The NTSB determined the probable cause of the accident was the pilot’s inadequate approach airspeed for the existing adverse meteorological conditions, followed by his delayed action to avert stalling and subsequent loss of control of the airplane. Contributing factors were reduced visibility due to the inclement weather and icing conditions.

On final approach to Reno/Tahoe International Airport (RNO), King Air N948CC stalled, rolled left and, in an uncontrolled descent, hit a commercial building about one nautical mile from the runway’s displaced threshold. The ATP-rated pilot and four passengers suffered minor injuries and a fifth passenger was seriously injured. The airplane was destroyed. At the time of the accident, 7:40 p.m., RNO was reporting 2,500 overcast, 700 broken and 1.5 miles visibility with snow showers and freezing fog. By the time the twin turboprop reached the final approach fix, the visibility had decreased to a half mile, but the pilot was not informed of this deterioration. The pilot maintained the recommended 140-knot approach speed for icing conditions until about 3.5 miles from the runway. Thereafter, the airplane’s speed gradually decreased until reaching about 75 knots. When the airplane started vibrating, the pilot increased engine power.

The airplane was equipped with de-ice boots on its wings, vertical and horizontal stabilizers. Unlike the wing boots, the tail boots are not visible from the cockpit and there is no tail-boot-condition annunciator. On previous occasions, overheat conditions had occurred in which the environmental ducting melted and heat was conducted to the adjacent pneumatic tube that provides de-ice air to the empennage boots. The pneumatic de-ice system had undergone maintenance in January 1999, at which time the tubing was replaced. In December 2000, new boots were installed. In 2001, holes in the vertical stabilizer were repaired, the cabin temperature controller was replaced and the static-air pneumatic tubing located near the aft lavatory, which was found melted, was repaired. In September 2001, melted EVA tubing near the aft pressure bulkhead was reported. During the accident investigation, the de-ice tube was found completely melted closed, thus rendering the empennage de-ice boots useless (this was not listed as a contributing factor by the NTSB).

The flight was operated by Regent Air Service of Truckee, Calif., as a Part 135 on-demand air-taxi flight, en route to Truckee-Tahoe Airport (TRK), from Durango, Colo. On the approach to TRK, the pilot activated the airplane’s anti-ice systems and, due to inadequate visibility, executed a missed approach and flew to RNO. The de-icing boots were again activated as he received vectors to intercept the approach to RNO Runway 34L. He did not observe appreciable icing on the leading edge of the wing, but activated the boots, at which time “a little (ice) came off.” The pilot reported ice on the side of the wing’s stall strip, and thin ice on the boot. Control of the airplane became difficult inside the DME fix. The airplane’s approach was initially stabilized at the MDA, when the pilot increased engine power to 800 foot-pounds per engine to maintain altitude. Some ice, “way less than [a quarter of an] inch,” was visible on the wing’s leading-edge stall strip, but did not extend beyond the aft portion of the de-icing boot.

During the final seconds of flight, the controls started vibrating and the pilot felt a yawing moment. Indicated airspeed was between 111 and 115 knots. At full power, the airplane still shook and yawed. The pilot tried to stop the airplane’s descent by maintaining a level attitude. The engine power was full, but the airplane was not climbing. Thinking he was going to hit the building and aware of the need to reduce the impact, he pulled the yoke full back. The left wing stalled, and the airplane banked left. He said there might have been ice on the tail because the tail “felt really weird.”