N922MACessna TU206F · 1976

On August 28, 2021, about 1316 mountain daylight time, a Cessna TU206F, N922MA, was substantially damaged when it was involved in an accident near Bernhard, Idaho. Both passengers were fatally injured, and the pilot sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 135 nonscheduled charter flight.

The accident flight was a scenic flight that one of the passengers had arranged for the purpose of scouting fishing locations.

The airplane was equipped with an Appareo Stratus 3i GPS device. Data obtained from the unit indicated that the airplane departed McCall, Idaho, at 1232 and made a climbing left turn to the northeast toward mountainous terrain. At 1240, the airplane turned to the east and continued its climb until it reached an altitude of about 8,800 ft mean sea level (msl). The airplane then began a slow descent into the canyons of the Frank Church Wilderness. About 8 minutes later, the airplane made a left turn to the north and started a climb from about 7,200 ft msl. The airplane followed a path between canyon walls and then descended again after it reached an altitude of about 8,000 ft msl. The data showed that the airplane began a right turn to the east at 1257 and descended to about 5,300 ft msl, where it completed a 180° turn inside a canyon and then continued to fly east. The airplane overflew Soldier Bar Airport at 1309 and continued east for about 3 nautical miles (nm). Three minutes later, the airplane made a right turn to the south.

The pilot was able to recount few of the final moments of the accident flight. He recalled that his passengers were ready to return home after they overflew Soldier Bar Airport, so he began to search for a drainage in the direction of their departure airport. The data showed that, once the airplane was established on its southerly heading, the pilot began a climb from an altitude of about 5,300 ft msl. At 1312, the pilot located a suitable drainage and made a right turn to the west toward McCall while he continued to climb. His goal was to overfly the mountain peaks at the end of the drainage.

As the airplane started to approach the top of the drainage, near the ridge line, the pilot, who had been flying near the right side of the drainage, crossed over to the left side of the drainage. About this time, he determined that the airplane’s rate of climb was insufficient to fly over the peak at the end of the drainage and he decided to make a 180° turn to climb to a higher altitude. At this point, the data showed the airplane about 1 nm from the top of the ridgeline and the airplane slowed to a groundspeed of about 62 knots while it continued to climb, at which time the pilot began a right turn. According to the pilot, he encountered a downdraft after starting the turn and was unable to maintain altitude despite the application of full power and full propeller rpm, and his vertical speed indicator showed a rapid descent. The pilot stated that he remembered extending the wing flaps but could not recall when and stated that he would not have extended the flaps fully. He recalled “riding the edge of stall into the trees because I had nowhere to go.” He added that the stall warning horn was triggered intermittently while the airplane descended into trees.

During the last approximate 2 minutes of recorded data, the airplane climbed from an altitude about 7,000 ft to about 8,300 ft. The airplane’s pitch attitude, which had not previously exceeded about 9° nose-up during the flight, exceeded 10° nose-up for much of the final minute of the flight, and increased to a maximum of 17° nose-up about 5 seconds before the end of the data. During the final minute of the flight, the speed remained below 80 knots, ultimately decreasing to below 65 knots during the final 15 seconds of recorded data. The data showed that the airplane’s groundspeed was about 55 kts at an altitude of 8,397 ft msl when the flight track data ended about 90 ft from the accident site.

A signal from the airplane’s 406 MHz emergency locator transmitter was received by the Air Force Rescue Coordination Center at 1318, and a search was initiated. The wreckage was located about 1646.

Figure 1: Accident airplane flight path from Soldier Bar Airstrip to Accident Site

The pilot was an experienced military fighter jet pilot with extensive low-level flight training. He reported that this was the first scenic flight he had flown. Before the accident flight, he had completed multiple charter flights that involved flying passengers between destinations.

The pilot reported that he was not familiar with the Cessna 206 before May 2021. He completed most of his training in the accident airplane make and model with the director of operations of McCall Aviation. These flights comprised “landing pattern stuff…just basic flight characteristics. We’d go out and do approach turn stalls, regular stalls…full stall characteristics of the airplane.”

During this time, they performed several local flights together to nearby airfields such as Donnelly and New Meadows, towns located about 10 nm south (at an elevation about 4,800 ft msl) and 9 nm northwest (at an elevation about 3,900 ft msl), respectively, of the operator’s base. The pilot stated that he felt the training was “pretty thorough for general aviation” and that each flight was accompanied by a “one hour long brief and debriefing afterwards.” During his training, they did not fly into any drainages; however, the director of operations would use nearby peaks to demonstrate how to properly fly within the mountain range while flying at nearby airports such as New Meadows. The pilot provided the example that he was taught to find the updraft on one side of a ridge between McCall and New Meadows for additional lift during a climb.

That summer, he flew the Cessna 206 into small airfields within the Frank Church Wilderness. Having recently started flying charter in the Cessna 206, he frequently followed the other airplanes during group flights so he could observe their movements. The pilot added that, by the time of the accident, he had flown in and out of drainages multiple times by himself in the accident airplane make and model.

According to the operator’s records, the pilot was signed off to fly the accident airplane on June 14, 2021. On the day of the accident, pilot departed on a charter flight to Salmon, Idaho, at 0640 and returned about 1025.

Another charter pilot who flew to an airfield located about 43 nm east of the accident site on the morning of the accident reported that the weather conditions were “beautiful” and the wind was light.

Weather observations captured by an unofficial station located about 8 miles north of the accident site at an elevation of 4,575 ft msl showed a temperature of 75°F, dewpoint 37°F, wind from 050° true, sustained wind magnitude of 2 mph and a wind gust magnitude of 7 mph.

A high resolution rapid refresh (HRRR) model for the accident location at 1300 indicated the following conditions:

Height (ft)

Temp (C)

Density Altitude

Pressure Altitude (ft) (computed)

8182

13

9483

7845

8218

11.5

9349

7880

8254

10.7

9303

7914

8435

10.2

9457

8088

8690

9.2

9644

8333

9058

8

9939

8686

8504

6.5

10295

9114

Table 1: HRRR model meteorological data

A review of Graphical Turbulence Guidance, which provides forecasts of information related to the expected intensity of clear-air or mountain wave turbulence, showed some light mountain wave activity in Colorado and southeastern Idaho between 7,000 and 11,000 feet at the time of the accident. The graphical forecast did not show any evidence of eddy dissipation in the Frank Church Wilderness at the time of the accident.

High-resolution rapid refresh (HRRR) model sounding data was analyzed by the RAwinsone Observation Program. The table below shows the vertical wind profile from the RAOB data.

Table 2: HRRR Wind profile interpolated by RAOB

A pilot with about 13 years of experience flying the Idaho backcountry and who was familiar with the accident area noted that he typically worried about downdrafts when the wind reaches about 25 kts at 9,000 ft but reported that mechanical downdrafts can also occur when the wind speed is lower than 25 kts.

The airplane was located on the east side of a mountain in the Payette National Forest at an elevation of about 8,370 ft msl, about 1,000 ft below the mountain’s peak. The initial point of impact was marked by an approximate 60-ft-tall tree that was severed at the top, located 120 ft from the main wreckage. A 50-ft-wide debris path was oriented on a northwesterly heading and identified by several broken trees between the initial impact point and the main wreckage that were consistent with an impact with terrain while in level flight. A portion of the right wing was located in the debris path a few feet from the main wreckage, which comprised the rest of the airplane. The airplane was oriented on an easterly heading. All major components of the airplane were accounted for at the accident site.

Figure 2: Accident site and surrounding terrain

Postaccident examination of the wreckage did not reveal any preimpact mechanical anomalies that would have precluded normal operation. Flight control continuity was traced from the cockpit controls to the aileron, elevator, and rudder control surfaces. Both the elevator control cables and the rudder cables were cut by recovery personnel. The right aileron direct cable exhibited signatures consistent with overload separation. The elevator trim tab had separated from the horizontal stabilizer; however, the actuator measured outside the normal operation range in the tab trailing end up deflection. The flap actuator position was consistent with the flaps in the near full down position, which was also consistent with the indications at the flap handle and flap position indicator.

Examination of the engine established mechanical continuity throughout the rotating group, valve train, and accessory section as the crankshaft was manually rotated by hand at the propeller. The magnetos produced spark at all six plug leads and most of the spark plugs displayed signatures consistent with normal wear, except for three spark plugs that had some evidence of oil. Examination of the interior components, including the cylinders, piston, and valves using a lighted borescope revealed no indications of catastrophic engine failure.

Performance Calculations

The airplane's rate of climb was calculated using performance charts derived from the pilot’s operating handbook (POH). Using the operator’s reported weight at the time of the accident of about 3,200 lbs, the climb figures from 6,000 ft (the starting altitude at the drainage) to 9,000 ft (the lowest peak in the drainage where the accident occurred) were based on a nearby reported temperature of 18ºC at 6,000 ft and 8°C at 9,000 ft. The calculations indicated that, under normal operating limitations (at a maximum weight of 3,300 lbs per the POH), and an airspeed of about 95 kts, the airplane would have required about 5 minutes to climb from 6,000 ft msl to 9,000 ft msl. Factoring in an additional 15% to adjust for temperature, which was about 11° warmer than standard, the airplane would have required about 5 minutes 45 seconds to make the climb. According to the flight data, the accident occurred about 4 minutes and 5 seconds after the pilot began climbing from 6,000 ft msl.

The terrain elevation at the bottom of the drainage increased from about 4,000 ft msl when the airplane turned into the drainage to about 8,000 ft msl, when the airplane began to descend over the course of about 6 nm. From the point of descent to the top of the lowest peak increased about 1,000 ft in elevation over the course of about 0.6 nm.

Mountain Flying Training

In addition to the practical training the pilot received, he also attended company training in May 2021, which included a presentation on mountain flying. Some of the subjects that were covered included “Clearing Mountains,” which comprised excerpts from a book on mountain flying, and an additional topic on “Course Reversal.”

The “Clearing Mountains” section contained some basic guidance on having an “escape route” and selecting a canyon (drainage):

? Always remain in a position where you can turn toward lowering terrain.

o This axiom also encompasses the idea that you will not enter or fly in a canyon where there is not sufficient room to turn around. Another way of stating this truth is to have an escape route in mind and be in a position to exercise this option.

The pilot stated that his escape route was “simply to turn around” and that he did not remember having selected a point to abort when he was flying up the drainage.

The “Clearing Mountains” section also incorporated instructions for turning around in rising terrain:

? Do not fly beyond the point of no return.

o This is the position when flying upslope terrain where, if you reduce the throttle to idle and begin a normal glide, you will have sufficient altitude to turn around without impacting the terrain.”

o As you near the ridge, when arriving at a position where the power can be reduced to idle and the airplane will glide to the top of the ridgeline, a commitment to cross the ridge can be made.

The “Course Reversal” section comprised 1 page and included the following guidance:

? Everyone flying in the mountains will encounter situations when it becomes necessary to make a 180° turn.

o To turn around, slow down. This will decrease the radius of turn.

o Pull back on the control wheel to trade airspeed for altitude if you have extra speed.

o Then make the steepest turn you can comfortably make, up to 60°.

Airplane Stall Characteristics and Indicators

According to the pilot’s operating handbook, “the stall characteristics are conventional and aural warning is provided by a stall warning horn which sounds between 5 and 10 knots above the stall in all configurations.”

On August 22, 2013, about 1050 mountain daylight time, a Cessna TU206F, N922MA, was substantially damaged during landing on runway 10L at Boise Air Terminal/Gowen Field (BOI), Boise, Idaho. The commercial pilot and the four passengers were uninjured. The backcountry charter flight was operated by McCall Air, and was conducted under the provisions of Title 14 Code of Federal Regulations Part 135. Visual meteorological conditions prevailed, and no Federal Aviation Administration (FAA) flight plan was filed for the flight.

According to the pilot, he had flown to Wilson Bar USFS Airport (C48) Dixie, Idaho, to pick up the backcountry passengers and transport them to BOI. The landing at, and departure from, C48 were uneventful, as was the flight to BOI. The approach to, and initial touchdown on, runway 10L at BOI were uneventful. However, when the pilot lowered the nose landing gear (NLG) to the runway, the airplane began to vibrate severely, and the pilot was unable to maintain directional control. The airplane veered left, and came to a stop near the edge of the runway. The pilot secured the airplane and all persons exited safely. Examination of the airplane revealed that the NLG had collapsed, and the cowling, propeller, and the right wing had contacted the runway. The damaged NLG components were retained by the NTSB for detailed examination.

FAA records indicated that the pilot held a commercial pilot certificate that included airplane single- and multi-engine land ratings. According to information provided by the pilot, he had approximately 6,837 total hours of flight experience, including about 4,568 hours in the accident airplane make and model. His most recent flight review was completed in July 2013, and his most recent FAA second-class medical certificate was issued in May 2013.

FAA information indicated that the airplane was manufactured in 1976, and was equipped with a Continental Motors TSIO-520 series engine. Operator-provided information indicated that the airplane had a total time (TT) in service of about 6,706 hours. The most recent completed element of the continuous inspection program was completed on August 6, 2013, and the airplane had accumulated about 52 hours in the period between that inspection and the accident.

The 1053 BOI automated weather observation included winds from 160 degrees at 10 knots, visibility 10 miles, clear skies, temperature 31 degrees C, dew point 8 degrees C, and an altimeter setting of 29.99 inches of mercury.

FAA information indicated that the airplane was manufactured in 1976, and was equipped with a Continental Motors TSIO-520 series engine. Operator-provided information indicated that the airplane had a total time (TT) in service of about 6,706 hours. The most recent completed element of the continuous inspection program was completed on August 6, 2013, and the airplane had accumulated about 52 hours in the period between that inspection and the accident.

The airplane came to rest upright, with its NLG collapsed, near the north (left) edge of the runway, and about 1,400 feet beyond the landing threshold. The outboard end of the right wing incurred scraping and buckling damage as a result of contact with the runway.

The NLG strut was folded aft under the fuselage, and the upper link of the torque assembly was fracture-separated from the strut. The upper end of the upper link of the torque link was fracture-separated from the landing gear, and the diagonal braces were fractured and/or and gouged. No non-impact damage of the nose wheel steering mechanism was noted. A detailed damage description is included in a subsequent section of this report.

Nose Landing Gear Design

The NLG can be considered as consisting of two primary sections; the trunnion assembly, and the shock strut assembly. The trunnion assembly affixes to the airframe, and includes the shock strut cylinder, two diagonal braces, the steering actuator, and the shimmy dampener.

The shock strut assembly mates with the trunnion assembly, and consists of the shock strut piston, and the fork, wheel, and tire assembly. The shock strut assembly pivots about an axis approximately parallel to the airplane vertical axis, to enable rotation for nose wheel steering. A torque link assembly enables control of the rotation of the shock strut assembly. The upper torque link attaches to the steering collar on the trunnion assembly, while the lower link attaches at the junction of the strut piston and fork assemblies. The attach bolt holes in the torque links are bushed with NAS77 bushings

Nose Landing Gear Inspections and Maintenance

The operator reported that the airplane was maintained using a manufacturer's/continuous airworthiness inspection program.

According to the operator, most of the NLG components were the original components from when the airplane was manufactured. Review of the recent maintenance records indicated that the wheel bearings were replaced about 2 years before the accident, and had accumulated about 812 hours TT.

The most recent NLG maintenance was completed on August 22, 2012, when the airplane had a TT of about 6,354 hours. At that time, the upper torque link and its hardware were replaced. At the time of the accident, that component had accumulated about 352 hours in service.

In addition to the standard inspections, in 2011 Cessna published a "Supplemental Inspection (SI) 32-20-01" (Title: "Nose Landing Gear Inspection") applicable to the 206 series airplanes from model years 1969 to 1976. This SI was applicable to the accident airplane. The "Initial" and "Repeat" inspection intervals were 3,000 hours or 5 years. The stated purpose of the SI was to "ensure structural integrity of the nose gear torque links, drag link, bolts, nose gear fork and collar." Specific inspection items included dimensional checks, checks for wear, and inspecting for "bent bolts" and cracks in the collar assembly. Neither deformed bolts nor cracks of any size were permitted.

According to the operator's maintenance manager, although not explicitly cited in the maintenance records entries, the SI was accomplished at least twice, on September 8, 2010, and most recently on August 22, 2012. Based on the maintenance records, that SI was most recently accomplished about 1 year, and 352 hours in service, before the accident, which was well within the specified inspection intervals.

Nose Landing Gear Laboratory Examination

The NLG components were sent to the Cessna Material and Process Engineering laboratory in July 2015 for examination, with federal oversight provided by FAA personnel. The intent was to characterize the damage and, if possible, determine the cause of the NLG collapse. An initial exam was conducted in late July 2015, and the detailed exam was completed in late September 2015. During that interim period, the NLG assembly was stored inside, first at the recovery facility, then at another facility, and then from July 22, 2015 on, at the Textron Aviation Materials and Process laboratory..

All observed components were the proper parts/part numbers. All required components and hardware were present and accounted for. The shimmy dampener was intact, filled with fluid, and functioned normally under manual manipulation. The examination noted the absence of any S-1450-6B14-010 shims at the attach point of the upper torque link to the steering collar, but the shim quantity is variable as a function of the fit between the two components. Therefore, a secure fit with no shims is an allowable configuration. Damage to the components precluded a determination of their pre-accident fit.

The two lugs on the steering collar had fractured, which liberated the upper end of the upper torque link from the collar. The lug failure modes were determined to be ductile overload. No pre-existing damage or other material abnormalities of the lugs were observed. However, on the right outboard lug, a "shadow" in the topcoat paint was consistent with the presence of an approximately 0.5967 inch diameter object, located approximately 0.1 inch off-center from the 0.377 inch diameter machined hole in the lug, at the time of the topcoat application. The shadow off-center direction was approximately the same as the lug failure direction. For reference purposes, the specified AN960-416 washer has an outside diameter (OD) of 0.500 inches, and the (absent) S-1450 shim have an OD of approximately 0.874 inches. The reason for this shadow could not be determined.

The attach bolt, nut, and cotter key for that joint remained installed in the upper torque link. The fact that this attach hardware was undisturbed, combined with the absence of the two S-1450 shims, enabled the determination that those shims had not been installed at the time of the accident. The 0543047-2 spacer in the upper torque link was immobilized in place in the link, which was not in accordance with the design. Neither the timing of, nor the reason for the immobilization could be determined; it could have been a result of impact damage, or exposure to a corrosive environment or substance, either in service or subsequent to the accident. The paint on the upper link was noticeably cleaner and brighter than that of the lower link; this was consistent with its recent replacement.

The upper torque link remained attached to the lower link, which remained attached to the nose gear fork. The 0543047-1 spacer in the lower torque link moved freely in the link, per design.

The upper lugs of the right diagonal brace had fracture-separated from the trunnion; the attach hardware remained in the trunnion, and the separated lug segments remained captive by the hardware. The lower end of the left diagonal brace and its mating lug on the trunnion were deformed. The head of that attach bolt was fracture-separated. The bolt fracture face did not contain any evidence of pre-existing abnormalities or deficiencies; the fracture was consistent with ductile overload. The upper attach bolt was bent but otherwise intact.

Both diagonal braces contained gouges at their lower ends. The gouges were consistent with damage that occurred either during the deformation of the assembly as a result of the NLG collapse, or during the subsequent recovery efforts.

Once the steering collar was cleaned, a crack was observed on its upper circumference. The crack measured approximately 0.35 inches long, and was oriented approximately vertically with respect to the airplane, and radially with respect to the collar. Microscopic examination revealed that the crack was high-cycle fatigue, but no geometric discontinuities, corrosion, or other features were present at the initiation site of the crack. The metallurgy of the collar was per the design specifications.

The investigation was unable to determine the age of the crack, or whether the crack had been present and undetected during the operator's most recent NLG inspection. In addition, because the age and service time of the crack was unknown, the investigation was unable to determine whether the manufacturer's NLG inspection intervals were adequate.

The collar normally mounted on the upper strut, and a roller bearing assembly between the two enabled the collar to rotate on that strut. During the examination, the roller bearing did not rotate freely in its installed position. The roller bearing, as well as the races on the strut and collar, exhibited light corrosion, consistent with insufficient lubrication and protection from the elements. The timing of the roller bearing immobilization and corrosion could not be determined; it could have occurred either in service, or subsequent to the accident.

Nose Gear Shimmy

The FAA Aviation Maintenance Technician Handbook (H-8083-31) defines shimmy as "Abnormal, and often violent, vibration of the nose wheel of an airplane, and states that shimmying "is usually caused by looseness of the nose wheel support mechanism or an unbalanced wheel."

Shimmy, particularly when left uncorrected, can damage components, with the potential to result in a component or system failure. Operations on unprepared runways or rough pavement can impose different and/or higher loads than operations on smooth runways, which can accelerate wear and thus susceptibility to shimmy. Operations on unprepared runways or rough pavement can also mask shimmy.

In 1984, Cessna issued Service Information Letter (SIL) SE84-21, which provided maintenance-related guidance for detecting and correcting the causes of shimmy. The SIL stressed the importance of eliminating improper system freeplay (looseness), as well as other abnormalities. The accident-induced damage precluded any determination of the accident airplane's overall NLG condition with regard to shimmy susceptibility. The pilot did not report any previous shimmy problems with the airplane. He reported that the previous landing and takeoff at C48 were normal, and that the accident landing was normal until the vibration and loss of control occurred.