N840DSDiamond Aircraft Ind DA 40 · 2007

On December 10, 2012, at approximately 1950 eastern standard time, a Diamond Aircraft Industries DA 40; N840DS, was substantially damaged when it impacted trees and terrain after a loss of control during climb, after departure from Valdosta Regional Airport (VLD), Valdosta, Georgia. The certificated private pilot was fatally injured. Instrument meteorological conditions (IMC) prevailed, and no flight plan was filed for the Title 14 Code of Federal Regulations Part 91 business flight, destined for Jesup-Wayne County Airport (JES), Jesup, Georgia.

According to the VLD Air Traffic Control Tower (ATCT) supervisor, at approximately 1935, the pilot radioed VLD ATCT and advised that he was ready to taxi for departure. The pilot was then issued current weather and taxi instructions to the active runway. The pilot then advised the controller that he would be departing to the east to JES.

At 1939, the pilot advised ATCT that he was ready for departure and was issued current wind information and was cleared for takeoff.

At 1942, the pilot was advised to squawk a beacon code of "1200" and that he could receive visual flight rules (VFR) advisory service with Moody Air Force Base Radar Approach Control (RAPCON) on frequency 126.6. The pilot then advised that he was changing to frequency 126.6 for advisory services.

According to the RAPCON supervisor, after the pilot contacted the RAPCON for VFR flight following the pilot was advised to squawk a beacon code of "5576" but, at 1950, before the airplane was radar identified by the RAPCON, the pilot radioed "I'm in trouble." Moments later, both radar and radio contact was lost.

At 1953, downed airplane procedures were initiated and a search for the airplane by federal, state, and local authorities was initiated.

According to Federal Aviation Administration (FAA) and pilot records, the pilot held a private pilot certificate with a rating for airplane single-engine land.

His most recent FAA third-class medical certificate was issued on October 15, 2012. He had accrued approximately 208 total hours of flight experience, 123 hours of which was in the DA 40.

Further review of the pilot's flight records revealed that of his 208 total hours of flight experience he had accrued approximately 3 hours of night time and approximately 7 hours of simulated instrument time.

He did not possess an instrument rating and no record of any actual instrument time being logged was discovered.

The accident airplane was a low wing, T-tailed, single engine monoplane, manufactured primarily of fiberglass reinforced plastic (FRP). The ailerons, elevator and wing flaps were operated through control rods, while the rudder was controlled by cable. The wing flaps were electrically operated. Elevator forces could be balanced by a trim tab on the elevator, which was operated by a Bowden cable.

It was powered by a 180 horsepower, air-cooled, four-cylinder, horizontally-opposed, fuel injected, direct-drive engine, driving a 3-bladed constant speed propeller.

It was capable of flying in instrument meteorological conditions and was equipped with an electronic flight information system (EFIS) that integrated flight, engine, communication, navigation, and surveillance instrumentation systems to allow a pilot to operate the airplane without visual reference. The system consisted of a Primary Flight Display (PFD), Multi-Function Display (MFD), audio panel, Air Data Computer, Attitude and Heading Reference System, engine sensors, a processing unit, and integrated avionics containing Very High Frequency (VHF) communications, VHF navigation, and GPS (Global Positioning System).

The primary function of the PFD was to provide attitude, heading, air data, navigation, and alerting information to the pilot. The PFD could also be used for flight planning. The primary function of the MFD was to provide engine information, mapping, terrain information, and flight planning. The audio panel was used for selection of radios for transmitting and listening, intercom functions, and marker beacon functions.

In the event of a malfunction of the EFIS system's PFD or MFD, mounted directly above them were a set of standby analog instruments which consisted of an attitude indicator, airspeed indicator, altimeter, and magnetic compass.

According to FAA and maintenance records, the airplane was manufactured in 2007. The airplane's most recent annual inspection was completed on April 27, 2012. At the time of the inspection, the airplane had accrued 203.1 total hours of operation.

Meteorological information for this investigation was derived from numerous sources.

Surface Analysis Chart

The National Weather Service (NWS) Surface Analysis Chart for 1900 depicted a deep low pressure system at 995-hectopascals and associated occluded front impacting the northeastern United States with a cold front extending southward across northwestern Georgia and eastern Alabama into the Florida Panhandle, and then into the Gulf of Mexico. The station models ahead of the front over Florida and southern Georgia indicated warm-moist light southerly winds ahead of the front with broken to overcast clouds and scattered rain showers.

Soundings

The NWS Tallahassee 1900 sounding indicated a moist low level environment with the lifted condensation level (LCL) at 967 feet above ground level (agl), with the sounding relative humidity greater than 80 percent from the surface to 5,000 feet, with the sounding being conditionally unstable with a Lifted Index of 0.2 even with a low level temperature inversion. The freezing level was identified at 14,240 feet. The sounding wind profile indicated a surface winds from the south at 5 knots which veered to the west above the surface and increased in speed. The mean 0 to 6 kilometer (18,000 feet) wind was from 240° at 41 knots. The wind at 2,000 feet was from 220° at 14 knots.

Radar Imagery

The NWS regional radar mosaic for 1945 depicted several scattered areas of rain showers over Georgia and northern Florida to the southwest and east of the accident site.

The Moody Air Force Base WSR-88D radar composite reflectivity image for 1947 depicted very light intensity echoes associated with biological targets and ground clutter associated with a developing nocturnal inversion over the area and no meteorological echoes. No significant weather echoes were identified within 50 miles of the accident site.

Satellite Imagery

The Geostationary Operational Environmental Satellite (GOES-13) infrared image at 1945 depicted a layer of low stratus clouds over Valdosta area with a radiative cloud top temperature of 289° Kelvin or 15.84° C, which corresponded to cloud tops near 4,000 feet. A large area of enhanced clouds was identified to the south extending from the Gulf of Mexico across northern and central Florida associated with cumulonimbus clouds or thunderstorms. No cumulonimbus clouds were identified in the vicinity of the accident site.

Recorded Weather

The recorded weather at VLD, at 1953, approximately 3 minutes after the accident included: wind variable at 4 knots, visibility 10 miles, broken clouds at 1,400 feet, temperature 22 degrees C, dew point 19 degrees C, and an altimeter setting of 29.84 inches of mercury.

Weather Depiction Chart

The NWS Weather Depiction Chart for 2000 depicted an extensive area of Marginal Visual Flight Rules (MVFR) conditions along the front across Alabama and western and northern Georgia, as well as a small portion of southern Georgia and northern Florida. MVFR conditions were depicted over the Valdosta area.

Destination Weather

Conditions at the planned destination of JES located approximately 85 miles east-northeast of Valdosta indicated light southerly wind, with visibility unrestricted, and high scattered clouds, with lightning detected in the distant south at the time of the accident. Other airports in the immediate vicinity of the destination were reporting scattered to broken clouds at 400 feet agl. during the period.

Airman's Meteorological Information Advisory

The NWS had issued an Airman's Meteorological Information Advisory (AIRMET) update at 1545. AIRMET Sierra update 3 issued at 1545 EST warned of ceilings below 1,000 feet and visibilities less than 3 miles in mist and fog developing after 1900 through 2100, and continuing through 0400 on December 11, 2012 across northern and southern Georgia. This advisory extended over the departure, the destination, and the accident site.

Astronomical Conditions

United States Naval Observatory data indicated that sunset occurred at Valdosta at 1732, with the end of civil twilight at 1759. At the time of the accident, both the Sun and the Moon were more than 15 degrees below the horizon.

The accident airplane was a low wing, T-tailed, single engine monoplane, manufactured primarily of fiberglass reinforced plastic (FRP). The ailerons, elevator and wing flaps were operated through control rods, while the rudder was controlled by cable. The wing flaps were electrically operated. Elevator forces could be balanced by a trim tab on the elevator, which was operated by a Bowden cable.

It was powered by a 180 horsepower, air-cooled, four-cylinder, horizontally-opposed, fuel injected, direct-drive engine, driving a 3-bladed constant speed propeller.

It was capable of flying in instrument meteorological conditions and was equipped with an electronic flight information system (EFIS) that integrated flight, engine, communication, navigation, and surveillance instrumentation systems to allow a pilot to operate the airplane without visual reference. The system consisted of a Primary Flight Display (PFD), Multi-Function Display (MFD), audio panel, Air Data Computer, Attitude and Heading Reference System, engine sensors, a processing unit, and integrated avionics containing Very High Frequency (VHF) communications, VHF navigation, and GPS (Global Positioning System).

The primary function of the PFD was to provide attitude, heading, air data, navigation, and alerting information to the pilot. The PFD could also be used for flight planning. The primary function of the MFD was to provide engine information, mapping, terrain information, and flight planning. The audio panel was used for selection of radios for transmitting and listening, intercom functions, and marker beacon functions.

In the event of a malfunction of the EFIS system's PFD or MFD, mounted directly above them were a set of standby analog instruments which consisted of an attitude indicator, airspeed indicator, altimeter, and magnetic compass.

According to FAA and maintenance records, the airplane was manufactured in 2007. The airplane's most recent annual inspection was completed on April 27, 2012. At the time of the inspection, the airplane had accrued 203.1 total hours of operation.

Examination of the Accident Site

On December 11, 2012, at 1115, the wreckage of the airplane was discovered by the crew of Georgia State Patrol helicopter in a heavily wooded area, approximately 7 miles from VLD.

Examination of the accident site revealed that the airplane initially made contact with an approximately 56 foot high pine tree before striking two smaller trees and then the ground about 50 feet further on, from the initial impact point with the tree. The impact angle was measured at an approximate 45-degree nose down angle. The airplane came to rest on a 107 degree magnetic heading in a depression on the forest floor, at an approximate elevation of 200 feet above mean sea level (msl).

Examination of the Wreckage

Examination of the wreckage revealed that the airplane was heavily fragmented. Further examination revealed however, that all major components of the airplane were present and control continuity was able to be established for all of the primary flight controls, and for the wing flaps.

Continuity for the elevator trim system could not however be confirmed, as it was discovered that the Bowden cable was disconnected from the trim control wheel in the cockpit. Examination of the Bowden cable revealed that it had pulled out of a swaged rod end which displayed a crack on the outer surface of the swage which ran along its longitudinal axis. Further examination of the crack revealed, that the exposed fractured surfaces were not the same color as the rest of swaged rod end but instead, displayed a brown discoloration on the fracture surfaces. Further examination of the cable end revealed that it also displayed evidence of a powdery looking brown discoloration.

Examination of the Propeller and Engine

Examination of the propeller and engine did not reveal any evidence of any preimpact failures or malfunctions.

The engine was separated from the airframe and found lying inverted about 10 feet forward of the main wreckage. The oil sump and both crankcase halves were impact fractured. The propeller and crankshaft flange were separated from the remainder of the crankshaft just aft of the crankcase nose. Cylinders No.1, No. 2, and No. 4, were impact damaged, and wood fibers were observed to be embedded in the cooling fins of cylinder No. 2.

The propeller was discovered separated from the engine near the main wreckage. The spinner was fragmented. The propeller had remained attached the crankshaft flange but, the flange had separated from the engine and was found near the remains of the airplane's fuselage. One of its composite blades was broken off flush with the propeller blade cuff ,with the other two propeller blades broken off and splintered, about 7 inches outboard of their cuffs.

The crankshaft exhibited a radial fracture with a 45-degree shear lip just aft of where the propeller flange had separated which was indicative of crankshaft rotation during impact. Drive train continuity was established from the back of the engine forward to the fracture just aft of the crankcase nose, and from the fracture to the propeller hub.

The fuel injector servo was impact separated from the engine. The mixture and throttle controls were separated at the servo control arms. Wood fibers consistent with those of the trees impacted by the aircraft were embedded in the servo air inlet opening. The fuel inlet hose was separated from the servo. The fuel inlet screen was absent of debris. The servo regulator section was disassembled and no damage to the internal parts was noted. Liquid with an odor consistent with aviation gasoline was noted in the fuel injector servo and in the fuel flow divider. The engine driven fuel pump was impact separated from the engine and fragmented. The fuel flow divider was partially separated from the engine. The flow divider was disassembled and no internal damage noted. The No. 2 and No. 4 fuel injector nozzles were fractured. No obstruction was noted in any of the nozzles.

The oil sump was fractured. Oil however, was observed in the engine. The oil filter was crushed. No debris was noted in the oil suction screen or the propeller governor screen. The oil cooler and oil cooler hoses were impact damaged.

The magnetos were impact separated from the engine. The spark plugs were medium gray in color.

Spatial Disorientation

According to Advisory Circular (AC) 60-4A titled, "Pilot's Spatial Disorientation," surface references and the natural horizon may become obscured even though visibility may be above VFR minimums and that an inability to perceive the natural horizon or surface references is common during flights overwater, at night, in sparsely populated areas, and in low-visibility conditions.

According to the FAA Airplane Flying Handbook (FAA-H-8083-3), "Night flying is very different from day flying and demands more attention of the pilot. The most noticeable difference is the limited availability of outside visual references. Therefore, flight instruments should be used to a greater degree.… Generally, at night it is difficult to see clouds and restrictions to visibility, particularly on dark nights or under overcast. The pilot flying under VFR must exercise caution to avoid flying into clouds or a layer of fog." The handbook described some hazards associated with flying in airplanes under VFR when visual references, such as the ground or horizon, are obscured. "The vestibular sense (motion sensing by the inner ear) in particular tends to confuse the pilot. Because of inertia, the sensory areas of the inner ear cannot detect slight changes in the attitude of the airplane, nor can they accurately sense attitude changes that occur at a uniform rate over a period of time. On the other hand, false sensations are often generated; leading the pilot to believe the attitude of the airplane has changed when in fact, it has not. These false sensations result in the pilot experiencing spatial disorientation."

The FAA publication Medical Facts for Pilots (AM-400-03/1), described several vestibular illusions associated with the operation of aircraft in low visibility conditions. Somatogyral illusions, those involving the semicircular canals of the vestibular system, were generally placed into one of four categories, one of which was the "graveyard spiral." According to the text, the graveyard spiral, "…is associated with a return to level flight following an intentional or unintentional prolonged bank turn. For example, a pilot who enters a banking turn to the left will initially have a sensation of a turn in the same direction. If the left turn continues (~20 seconds or more), the pilot will experience the sensation that the airplane is no longer turning to the left. At this point, if the pilot attempts to level the wings this action will produce a sensation that the airplane is turning and banking in the opposite direction (to the right). If the pilot believes the illusion of a right turn (which can be very compelling), he/she will reenter the original left turn in an attempt to counteract the sensation of a right turn. Unfortunately, while this is happening, the airplane is still turning to the left and losing latitude.

Pulling the control yoke/stick and applying power while turning would not be a good idea–because it would only make the left turn tighter. If the pilot fails to recognize the illusion and does not level the wings, the airplane will continue turning left and losing altitude until it impacts the ground."

An Autopsy was performed on the pilot by the Georgia Bureau of Investigation. Cause of death was multiple blunt force trauma.

Toxicological testing of the pilot was conducted at the FAA Bioaeronautical Sciences Research Laboratory, Oklahoma City, Oklahoma.

The pilot's specimens were negative for carbon monoxide, cyanide, basic, acidic, and neutral drugs, with the exception of ethanol which was detected in Muscle, and was from a source other than ingestion, and Atorvastatin, which is a member of the drug class known as statins, and is used for lowering blood cholesterol.

The pilot had previously reported his use of Atorvastatin to his FAA airman medical examiner.

Weather Briefing

Review of the outlook weather briefing requested by the pilot from the Princeton Contracted Flight Service Station (FCFSS) revealed that prior to departure; he had been advised of AIRMET Sierra. Additionally, he had also been advised of the weather conditions that had been reported in the Valdosta area which included scattered clouds at 1,600 feet, broken clouds at 2,200 feet, and broken clouds at 4,300 feet. Furthermore, He was also given the temperature and dew point which at the time were within 4 degrees of each other, and was advised that it was marginal VFR, which is defined by the NWS as a ceiling between 1,000 and 3,000 feet or visibility in the three- to five-mile range.

Radar Data

Review of correlated radar data indicated that at 19:46:13.11, the airplane's climb rate was steady until reaching an altitude of approximately 2,100 feet msl. Approximately 7 seconds later the airplane's altitude dropped to 1,900 feet msl, and the airplane had begun to turn right.

At 19:46:22.927, the airplane was still turning to the right and had descending through 1,700 feet msl. Approximately 5 seconds later, it was still continuing to turn right, and was at 1,400 feet msl.

Further examination of the radar data, indicated that the last radar contact occurred at 19:46:32.507, when the airplane was still at 1,400 feet msl. The average rate of descent up to that point was approximately 3,420 feet per minute.

Comparison of the last radar contact to the location of the accident site, indicated that the last radar contact had occurred when the airplane was approximately 359 yards from its initial impact point with the trees.

Elevator Trim System

The DA 40 elevator trim system included a mechanically operated trim tab. This allowed the pilot to trim the airplane for different speeds and center-of-gravity positions.

The elevator trim system had three main parts:

- The handwheel assembly with trim indicator.

- The Bowden Cable (Elevator Trim Cable) which connected the handwheel to the trim tab.

- The trim tab actuator assembly.

The handwheel assembly on the center console controlled the elevator trim system. The assembly had a metal mounting frame. The frame attached to the rear of the engine control assembly and the top of the control bulkhead. A long bolt through the mounting frame carried the handwheel. The bolt also held friction disks, plain washers, and spring washers, which were mounted against the handwheel. Two jam-nuts could be used by the pilot to adjust the friction by applying compression to the friction disks and washers.

A small gear wheel attached to the handwheel. The small gear wheel engaged with a large gear segment with internal teeth. The gear segment had a pivot bolt at the bottom of the mounting frame. A ball-stud attached the eye-end of the Bowden cable to the gear segment. An extension to the mounting frame to the rear made the anchor point for the outer sheath of the cable.

The gear segment was also the trim indicator. The top face of the segment had a white line across it midway between the front, and back. The top face could be seen through a slot in the cover plate. The sides of the cover plate had markings to show the trim position.

The Bowden cable connected the trim handwheel assembly to the trim tab. The cable went through holes in the front and rear main bulkheads, the baggage frame and each of the ring frames. It then went up the front face of the front web of the vertical stabilizer and through a slot near the top, and then through a large hole at the top of the rear web of the vertical stabilizer to the trim tab actuator assembly. The cable had an inner core with threaded end fittings. Spherical end fittings at each end connected to the gear segment and trim tab actuator assembly. Clamp blocks held the outer core to the mounting frame at the front and a bracket from the horizontal stabilizer at the back.

The trim tab was a one-piece FRP molding. The tab had two integral levers. Two cranked actuating levers were attached to the integral levers. The left cranked actuating lever connected to the Bowden cable, and the right actuating lever connected to a friction damper. The friction damper had a clamp-block with a hole for a rod. The rod connected to the right actuating lever on the trim tab. The friction on the rod in the clamp block was adjustable.

When the top of the trim handwheel moved forward:

- The small gear wheel moved the top of the gear segment forward.

- The gear segment pulled the inner core of the flexible cable forward.

- The inner core of the flexible cable pulled the left cranked actuating lever forward.

- The left cranked actuating lever pulled the trim tab lever forward to move the tab up.

- The up movement of the trim tab would push the elevator down in flight giving nose-down trim.

When the top of the handwheel was moved aft, the gear segment moved aft, the cable moved aft and the trim tab would move down. This would push the elevator up and give nose-up trim.

In each case, the pilot could see the trim position from the white mark on the gear segment.

Laboratory Examination of the Elevator Trim Cable and Swaged Rod End

Examination of the elevator trim cable (Bowden cable) and swaged rod end from the accident airplane by the NTSB Materials Laboratory revealed that the core cable consisted of the cable strand with a plastic coating that was removed in the area where the bolt was swaged onto it.

Examination of the cable strand revealed twelve outer wires that exhibited bands of deformed material along their outermost surfaces. The strand contained nineteen wires in the configuration of 7 inner wires and 12 outer wires (e.g. 7 x 12, 19-wire strand). The average strand outside diameter was about 0.142 inch (3.6 mm). Measured by digital microscopy, the average wire diameter was about 0.027 inch (0.7 mm). The outer wires were wound in a right lay. Manufacturer's drawings indicated that the strand had a diameter of 3.5 mm (0.138 inch) and consisted of nineteen zinc-coated wires, each with a diameter of 0.7 mm (0.027 inch). The drawings however illustrated and specified a left lay.

The ball end and the nut were removed from the bolt and measurements revealed that the bolt satisfied the dimensional requirements in the manufacturer's drawings. The drawings specified the bolt material as DIN 1.4305 which is compositionally equivalent to UNS S30300, and specified the bolt material as stainless steel.

Examination revealed that the swaging tool during manufacture had pressed five flat regions equally spaced around the circumference of the swage. Ribs of extruded material were formed between the flat swaging jaws. Further examination revealed that the fracture had occurred along one of the ribs of material that was extruded between the swaging jaws during manufacture. Brown-colored deposits were present in the fracture.

Examination of the inner surface of the bolt swage area adjacent to the fracture revealed circumferential lines consistent with a drilling operation and helical impression marks consistent with contact with the cable wires.

The deposits in the crack were evaluated by scanning electron microscopy (SEM) and standardless semi-quantitative energy dispersive spectroscopy (EDS). The EDS spectra, revealed the presence of carbon, oxygen, iron, zinc, magnesium, aluminum, silicon, phosphorous, sulfur, chlorine, potassium, calcium, chromium, manganese, and nickel.

The fracture surface of the swage area was also examined by SEM and a typical scanning electron fractograph which revealed that the fracture micro-mode was microvoid coalescence (MVC) due to overstress. The longitudinal orientation of stringers within the fracture surface, were consistent with a resulfurized steel such as UNS S30300.

The inside surfaces of the bolt swage area exhibited helical impression marks (e.g. grooves) consistent with permanent deformation from contact with the cable wires In some of the helical impression grooves, flakes of zinc electroplating from the cable strands were present. EDS analysis revealed that the flakes were zinc, and in many regions of the inside surfaces of the swage, white-colored salts were present in the helical impression grooves. EDS analysis indicated that the white salts were composed largely of zinc and oxygen, consistent with zinc corrosion product.

Exemplar Swage Joint Tensile Tests

At the request of the NTSB, the trim cable manufacturer performed five tensile tests on cable and bolt samples that had been assembled in accordance with the production specification. According to the manufacturer, the cable assembly (including the swaged area) was rated for 2000 N (about 450 lbf.) In each instance, the swaged region of the bolt developed a longitudinal crack. In all instances the cracks developed along the extruded rib between the flat regions of the swage. Similar to the inner surface of the accident swage, the inner surface of the tensile test swages exhibited helical impression marks consistent with contact with the cable wires. SEM Analysis of each fracture revealed that the crack occurred in overstress with a microvoid coalescence fracture mechanism.

Criticality of Elevator Trim Tab Failure

According to the manufacturer, review of the elevator trim system of the DA 40 by the manufacturer and certification authority revealed that it was designed for low criticality of each single component. No failure of a single component would lead to an uncontrollable situation for the pilot and several failure modes had been looked at during the design and certification of the elevator trim system.

The elevator trim tab attachment to the elevator, and the trim tab drive was redundant, to prevent flutter (within design speeds) in case of disconnect of the Bowden cable or a component failure.

The most critical failure in the system would be disconnect of the Bowden cable at the actuating lever. The friction damper and friction clamp block were installed to prevent flutter for that failure case.

In case of disconnect of the Bowden cable at the throttle quadrant, the mass and the friction of the cable would be sufficient to prevent flutter, and the friction damper would act as an additional damper.

As a certification and flight test requirement, it was also demonstrated that an average pilot could handle a trim run-away or fully mis-trimmed condition.

A disconnect would lead initially to a floating of the trim tab with the elevator, leading to reduced elevator control forces, as the trim system was designed as an anti-servo. As such, a reduction of longitudinal stability would occur. In case the trim tab floated towards an extreme position, stops would limit that movement. The stop towards "Nose Up" was designed to lock the tab in that position. This was considered by the manufacturer to be the most convenient and safest position. The manufacturer also considered that the low speed characteristics for the DA 40 were gentle and that there were additional warnings for the pilot before approaching a limit (stall warning, and /or buffeting).

Additionally, a Bowden cable disconnect at the throttle quadrant or a failure of the forward swaging followed by the cable moving of the sleeve would have resulted in an additional length of the actuating mechanism driving the trim tab towards "Nose Up", which would result in the above described condition.