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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.
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.
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.
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.
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 evidence of wrinkling, deformation, or bending from their inboard attach points, outboard to the wingtips.
The aileron and flaps remained connected at their respective attach points. Examination of the right aileron inboard hinge area revealed bending deformation and tearing of the upper, forward edge of the aileron hinge slot, adjacent to the hinge bracket. The bracket exhibited a horizontal impression and removal of paint corresponding to an aileron trailing edge up position. The left aileron exhibited similar bending deformation and paint removal from the hinge slot edge and the bracket.
The left aileron utilizes an adjustable trailing edge trim tab. Measurement of the trim tab free play revealed 0.23 inches of movement. According to the manufacturer, the maximum allowable trim tab free play is 0.10 inches. In addition, the following aileron measurements were obtained:
Left aileron control travel (UP): 19.5 degrees; allowable range, 21 to 25 degrees.
Left aileron control travel (DOWN): 15 degrees; allowable range, 13 to 17 degrees.
Right aileron control travel (UP): 24.5 degrees; allowable range, 21 to 25 degrees.
Right aileron control travel (DOWN): 11.5 degrees; allowable range, 13 to 17 degrees.
Aileron trim travel (UP): 31.5 degrees; allowable range, 14.5 to 19.5 degrees.
Aileron trim travel (DOWN): 11.5 degrees; allowable range, 14.5 to 19.5 degrees.
The left side of the horizontal stabilizer appeared undamaged. The left elevator remained attached to the stabilizer.
The right side of the horizontal stabilizer exhibited an upward and aft bend of about 180 degrees toward the inboard direction, about 4 feet outboard from the fuselage. The upper and lower surface of the stabilizer and elevator displayed wrinkling and distortion. The front spar at the point of the bend was fractured at the lower strap and through the center web structure. The upper strap was severely deformed and curled up, aft, and inward.
The center spar was deformed and fractured. The rear spar consists of a "C" shaped channel riveted to an upper and lower tapering strap where the elevator attaches. The rear spar was fractured at the outboard end of the upper and lower strap, 5 feet 9 inches outboard from the fuselage. The upper and lower skin of the stabilizer exhibited spanwise and chordwise tearing adjacent to the spar fractures with portions of skin pulled through the attaching rivets.
The right elevator remained attached to the stabilizer at the inboard and center hinges. The elevator was bent upwards about 30 degrees at the center hinge point and was separated from the stabilizer at the outboard hinge. All of the observed fracture surfaces exhibited numerous 45-degree angle planes.
The rudder and rudder trim tab assembly did not exhibit any signs of damage. The left side of the vertical stabilizer exhibited some evidence of buckling.
Examination of the elevator control assembly revealed that the control cables were attached to the elevator aft transfer tube assembly. The push-pull tubes, from the transfer tube to the left and right elevator torque tube fittings, were attached. The aft bulkhead (fuselage station 437), through which the push-pull tubes are routed, exhibited bending of the upper flange edge adjacent to the elevator pitch torque tube fittings. The edges of the elevator pitch torque tube fittings exhibited some bending and deformation. The deformed bulkhead and torque tube fitting impressions correspond to the elevators displaced in a trailing edge down position. The right elevator was displaced in the down position more so than the left. The difference in deflection between the elevators was 18 degrees.
Tests and Research
On October 28, 1993, the aircraft's flight control system parameters were measured by maintenance personnel at the Bishop Airport. The measurements are included in Item 14 of this report.
Metallurgical examinations of separated portions of the right elevator and stabilizer were conducted by Fowler Inc., Gardena, California. Their report indicated that all of the examined fractures were due to overload forces.
A National Transportation Safety Board aerospace engineer reviewed the accident sequence, aircraft rigging, and examined the airplane. According to the engineer, the damage to the airplane was consistent with an "aileron snatch" phenomenon. This phenomenon may produce an uncommanded roll excursion. The uncommanded roll can be precipitated by adverse aerolastic effects on the ailerons due to incorrect aileron travel and/or excessive free-play in the aileron trim tab.
Excessive free-play in the aileron trim tab and/or improper aileron primary and balance cable tensions can result in gross aerodynamic overbalance at high speeds and loss of lateral control. Chapter 12, Theory and Design of Control Surfaces, page 383 from "Airplane Aerodynamics" (Dommasch, Sherby, and Connolly) states in part:
"...very careful synchronization of the up-deflected and down- deflected ailerons is required to achieve the degree of balance desired. Stretch in the control-system linkages under load, particularly at high angles of attack, or distortion and flexing of the wings under load may destroy the normal static relation of the two ailerons. This usually results in a sudden abnormal overbalance of the ailerons as they are deflected. This phenomenon is called snatch and is dangerous. Frise balances are not recommended for large or high-performance airplanes, but have their particular utility for the small and moderate-sized airplanes of moderate performance characteristics."
The manufacturer utilized a consulting engineer to examine the airplane at Bishop. The engineer reported that upward loading on the horizontal stabilizer occurs when the trailing edge of the elevator is displaced downward. Normally, a pilot will not induce a rapid, full nose-down maneuver sufficient to fail the stabilizer in an upward direction. The failure of the right stabilizer, without damage to the left stabilizer, is evidence of non-symmetrical loading. The engineer indicated that this type of loading can occur due to a high vertical up-gust loading on only the right side of the tail, or a rapid recovery maneuver from an unusual aircraft attitude that probably included some spiraling motion of the aircraft.
The engineer reported that a high, upward gust on only the right side of the tail, sufficient to fail the stabilizer, would have rolled the aircraft to the left and would have produced vertical up loading sufficient to significantly deform the right wing. He stated that it is unlikely that the damage to the right horizontal stabilizer was directly caused by gust loading. He indicated that the damage was more likely caused by the loading induced by the pilot during recovery from an unusual attitude at a relatively high airspeed in excess of the airplane's design maneuvering airspeed, and possibly in excess of never-exceed airspeed limitations.
In addressing the elevator pitch torque tube damage, the engineer reported that excessive downward overtravel of the elevators was consistent with vertical up-loading on the right horizontal stabilizer. Due to elevator static balance, there would be no resulting inertia movement of the elevators due to gust loading. He indicated that the excessive overtravel of the elevators and resulting damage had to have been pilot induced through the control column and elevator control system.
The manufacturer's consulting engineer reported that the airplane's ailerons are slightly overbalanced, leading edge heavy. The engineer stated that the large overtravel of the right aileron was pilot induced and was probably done during the recovery sequence.
In addressing the topic of the left aileron trim tab free-play, the engineer reported that excessive free-play is undesirable and is usually a "nemesis" in the flutter characteristics of the wing-aileron-tab system. Excessive free-play can, in theory, allow tab flutter at any airspeed. Due to the combined linear behavior of the system, he concluded that this incident was not flutter related.
A second undesirable attribute of excessive tab free-play is a resulting ineffectiveness of the tab. The engineer concluded that excessive free-play of the aileron tab, although undesirable and unacceptable for an airworthy aircraft, had no significant part in this incident.
The FAA aerospace engineer who participated in the investigation attributed the loss of control to descent about maneuvering speed during turbulence in a mountainous area.
In addition to the persons indicated on page 5 of this report, the following participated in the investigation:
Roman Gabrys - FAA, Aircraft Certification, ANM-191D, Denver, Colorado. Paul Alexander - NTSB, AS-20, Washington, D.C.
The Safety Board released the airplane, located at Bishop, California, to the owner's representatives on October 15, 1993. No parts or components were retained by the NTSB.