LAX93FA333

A twin engine turboprop airplane was descending through 16,500 feet at 205 knots when the airplane began an uncommanded 360- degree right roll. The pilot regained control of the airplane and continued his descent and landed. After landing, the pilot found that the outboard half of the right horizontal stabilizer had broken, folding up and aft 180 degrees over the upper surface of the stabilizer. The failure of the stabilizer was attributed to overload. Meteorological conditions were described as a light-to-moderate chop. Examination of the airplane revealed overtravel of the elevators and ailerons. No other structural damage was found. The ailerons and aileron trim tab were out of adjustment.

History of the Flight

On August 26, 1993, about 1430 hours Pacific daylight time, an Aero Commander 690A, N706KC, experienced an in-flight loss of control about 39 miles northwest of Bishop, California. The airplane was being operated as a visual flight rules (VFR) cross country positioning flight under Title 14 CFR Part 91 when the accident occurred. The airplane, operated by Medic Air Inc., Reno, Nevada, sustained substantial damage. The certificated airline transport pilot and a medical technician crewmember were not injured. Visual meteorological conditions prevailed. The flight originated at Reno Cannon International Airport, Reno, Nevada, at 1400 hours.

The pilot reported that he was descending into Bishop from 17,500 feet mean sea level (msl) at 205 knots indicated airspeed (IAS) between 1,000 to 1,500 feet per minute rate of descent. The maximum airspeed indicator (VNE) was at 225 knots. At 16,900 feet, the airplane encountered light turbulence. The pilot had disarmed the autopilot system and, at 16,500 feet msl, the airplane suddenly began an uncommanded roll to the right. The aircraft continued to roll 360 degrees before the pilot regained control.

The pilot reported regaining control of the airplane in a slight nose-high attitude between 190 to 200 knots IAS. Both the pilot and crewmember reported experiencing only positive "G" loading during the upset and recovery. The pilot continued the approach to Bishop. When engine power was reduced for landing, the airplane pitched down sharply. The pilot landed without further damage by holding full aft elevator control and applying engine power. The pilot did not report any requirement to hold any gross rudder or aileron deflection to maintain directional control of the airplane during the landing.

After landing at Bishop, the pilot discovered that the outboard half of the right horizontal stabilizer and elevator were bent up and aft 180 degrees onto the upper surface of the stabilizer. No other aircraft damage was visible.

The accident occurred during the hours of daylight about latitude 38 degrees, 03 minutes north and longitude 118 degrees, 48 minutes west.

Crew Information

The pilot holds an airline transport pilot certificate with airplane multi-engine land and instrument-airplane ratings. The certificate is endorsed for commercial pilot privileges with an airplane single engine land rating. In addition, the pilot holds a flight instructor certificate with airplane single engine and instrument-airplane ratings. The most recent first class medical certificate was issued to the pilot on June 6, 1993, and contained no limitations. The medical technician crewmember holds a private pilot certificate with a glider rating.

According to the pilot/operator report submitted by the operator, the pilot's total aeronautical experience consists of about 8,595 hours, of which 2,364 were accrued in the accident aircraft make and model. In the preceding 90 and 30 days prior to the accident, the report lists a total of 150 and 50 hours, respectively, flown.

Aircraft Information

The airplane had accumulated a total time in service of 4,036.6 flight hours. Examination of the maintenance records revealed that the airplane is maintained according to the manufacturer's inspection program utilizing a phased inspection schedule. Each phase of the aircraft inspection is accomplished every 50 hours. The most recent inspection, a Phase C Inspection, was accomplished on August 25, 1993, 3.2 flight hours before the accident. The most recent inspection of the empennage was accomplished during the same inspection.

The most recent Phase A Inspection was accomplished on July 23, 1993, 78.5 hours before the accident. Phase B and D inspections primarily concentrate on the engines and landing gear. Examination of the maintenance and flight department records revealed no unresolved maintenance discrepancies against the aircraft prior to departure.

The flight control system from the pilot control column to the primary control surfaces is connected by push-pull tubes, bell cranks, and cables. The manufacturer's Phase A Inspection covers, among others, inspection of all flight control surfaces including the aileron skin, hinge points, aileron trim tab hinge points, bell cranks, and trim tab free play. A Phase C Inspection covers, among others, inspections of the flight controls and flight control system including trim tab free play on the aileron, rudder, and elevator.

The airplane's pilot operating handbook (POH) indicates that the maximum airspeed for flight in turbulence (maneuvering speed) is 137 knots indicated airspeed at an aircraft weight of 8,750 pounds. The accident airplane's weight at the time of the accident was about 8,749 pounds.

The FAA defines maneuvering speed as the maximum speed at which application of maximum available aerodynamic control will not overstress the airplane. Abrupt control movements above this speed can result in dangerous structural damage.

For pilot reporting purposes, the Federal Aviation Administration (FAA) defines turbulence in terms as light, moderate, severe, and extreme. The light and moderate categories can also have lesser degrees of turbulence that are reported as light or moderate chop.

The airplane is configured with non-symmetrical, Frise-type ailerons which deflect 25 degrees up and 17 degrees down. A significant portion of the leading edge of the Frise aileron extends forward of the hinge line and this forward area is subjected to the force of the slipstream when the aileron is deflected trailing edge up. The mechanical stops for the ailerons are located on the cable drum assembly located under the cockpit floor between the control columns.

Meteorological Information

The closest official weather observation station, Bishop, California, is located 39 nautical miles south of the airplane upset area and is located in the Owens Valley on the east side of the Sierra Nevada Mountains. At 1350 hours, a surface observation was reporting in part:

Sky condition and ceiling, clear; visibility, 50 miles; temperature, 89 degrees F; dew point, 40 degrees F; wind, 180 degrees at 10 knots; altimeter, 30.10 inHg.

The pilot reported that the encountered turbulence was best described as a light to moderate chop. He did not observe any meteorological signs of lenticular cloud formations or evidence of wind gusts on the ground.

A meteorological study of the area weather conditions was conducted by the National Transportation Safety Board, Operational Factors Division. The study revealed that turbulence can be expected in mountainous areas when wind conditions of 25 to 50 knots is perpendicular to and near the level of the mountain ridgeline.

In the area of the accident, the 700 millibar (about 10,000 feet) winds were less than 10 knots. The 500 millibar (about 18,000 feet) winds were less than 15 knots. Satellite photos of the area did not show any indicators of turbulence. Weather radar data from the National Weather Service did not show any echoes (thunderstorms) in the area.

Communications

Radar data of the accident airplane's flight was examined utilizing data from the Oakland, California, Air Route Traffic Control Center (ARTCC), National Track and Analysis Program (NTAP). The airplane's 1200 code transponder signal was tracked from 1430:13 hours at 17,600 feet msl to 1434:50 hours at 13,900 feet msl, a distance of 19.7 nautical miles. Calculations of the airplane's ground speed and rate of descent revealed that the aircraft averaged a ground speed of 256 knots. The rate of descent averaged 801 feet per minute. The data is based on linear calculations.

Wreckage and Impact Information

Safety Board investigators examined the airplane at the Bishop Airport on September 28, 1993. All of the airplane's major components were attached to the airplane. The wings were straight with no