N173CWWilson Charlie H WILSON RV4 · 1997

On June 9, 2020, about 0846 mountain standard time, an experimental, amateur-built RV4, N173CW, was substantially damaged when it was involved in an accident near Safford, Arizona. The pilot was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

ADS-B data revealed that the airplane departed from runway 11R at Tucson International Airport (TUS), Tucson, Arizona, about 0810, and initiated a climbing left turn to the northeast. The airplane continued on the same track, reaching an altitude of about 9,800 ft mean sea level (msl) about 7 minutes later. For the next 25 minutes, the airplane maintained the same general altitude and heading over the mountain passes northeast of Tucson, between 4,000 ft and 7,000 ft above ground level (agl).

About 0836, 28 miles southwest of Safford Regional Airport (SAD), Safford, Arizona, the airplane began to descend at a rate of about 500 ft per minute (fpm). Seven minutes later, the airplane changed track to the north, toward SAD, descending at a rate of about 800 fpm, and an airspeed factored for wind and atmospheric conditions of about 150 kts. The airplane slowed to about 110 kts about 1.25 miles short of runway 30. Then, having reached an altitude of 4,200 ft msl (about 1,250 ft agl), the airplane began an almost 5,000-fpm, 15-second descent, while accelerating to a speed of 150 kts (see figure 1).

The first identified point of impact consisted of a 25-ft-long ground disruption located at an elevation of 3,090 ft msl, about 500 ft north of the last ADS-B target and 1/2 mile southwest of the SAD runway 30 threshold.

Figure 1. Flight track and airport location

The pilot had extensive flight experience as a captain in the US Navy, with about 7,000 hours total flight time. His logbooks indicated a total civilian flight experience of about 2,500 hours.

According to family members, the pilot had flown to SAD multiple times before, and it was one of his favorite airports.

Construction of the kit-built airplane was completed in 1997. The pilot purchased the airplane in September 2015, after which he flew it for about 280 hours. Family members stated that he typically flew the airplane every week. A new engine was installed at the time of the last condition inspection.

Flight documentation recovered from the airplane indicated that the best glide speed (VG) was 71 kts. The maximum flap-extended speed (VFE) was 96 kts with 20° of extension, and 87 kts at 40° extension. The design maneuvering speed (VA) was 115 kts, the maximum structural cruising speed (VNO) was 156 kts, and the never-exceed speed (VNE) was 185 kts.

A High-Resolution Rapid Refresh (HRRR) model sounding was created for the accident location at 1600 UTC. At an elevation of 4,127 ft msl, the HRRR sounding indicated wind from 149° at 3 knots, temperature 18.2°C, and dewpoint -14.4°C, with a relative humidity of 10%.

The model did not indicate any turbulence below 12,000 ft msl.

Construction of the kit-built airplane was completed in 1997. The pilot purchased the airplane in September 2015, after which he flew it for about 280 hours. Family members stated that he typically flew the airplane every week. A new engine was installed at the time of the last condition inspection.

Flight documentation recovered from the airplane indicated that the best glide speed (VG) was 71 kts. The maximum flap-extended speed (VFE) was 96 kts with 20° of extension, and 87 kts at 40° extension. The design maneuvering speed (VA) was 115 kts, the maximum structural cruising speed (VNO) was 156 kts, and the never-exceed speed (VNE) was 185 kts.

Due to the Covid-19 pandemic, neither the NTSB nor the FAA responded to the accident site, and onsite photographic documentation was accomplished by local law enforcement personnel. An airframe and engine examination were performed by representatives from the NTSB, FAA, and Lycoming Engines following recovery of the wreckage from the accident site.

The disruption identified at the impact point was located on a south-facing bluff, and projected uphill on a northerly heading toward the main wreckage. The ensuing 300-ft-long debris field included fragments of wing tip, main landing gear, a secondary ground disruption that contained fragments of the exhaust pipe assembly, and the propeller and the left aileron. The main wreckage came to rest 40 ft above the first point of impact, and 40 ft below runway elevation (see figures 2 and 3).

Figure 2. Initial impact point facing up hill with the airplane in the background (Photo Courtesy of the Safford Police Department)

Figure 3. Debris Field and Main Wreckage (Photo Courtesy of the Safford Police Department)

The entire cabin sustained crush damage through to the aft cabin bulkhead. The leading edges and forward undersides of both wings sustained crush damage along their entire length. The vertical and horizonal stabilizers remained attached to the aft fuselage, which was effectively undamaged. The pilot was in the forward seat and appeared to be wearing his lap belt and shoulder harness. There was no evidence of ballast.

All primary flight control and aircraft components were found in the immediate vicinity of the debris field. The canopy was separated and came to rest next to the main cabin; its lock pins were extended, and lock handle was set at the locked position. There was no indication of bird strike, and samples from the airframe sent for analysis did not reveal any evidence of wildlife DNA.

Flight control continuity was established from each control surface to the respective pilot controls. The airplane was retrofitted with an electrically-driven flap system. The flap control assemblies were either intact or exhibited damage signatures consistent with impact overload. The flap actuator jackscrew was fully retracted, which corresponded to a full flap deployment position at the time of impact. There was no evidence of preexisting actuator damage. Damage to the flap control switch and its electrical cables prevented an accurate assessment of its functionality.

The pitch trim tab was intact and connected to the elevator, and its control cable was continuous through to the aft end of the cabin; however, the trim controls in the forward fuselage sustained significant impact damage, preventing an accurate determination of both the trim position and trim system viability at the time of impact. Vans Aircraft Service Bulletin SB 06-9-20, which applied to the trim cable anchor, had been incorporated.

There was no indication that the airplane was fitted with an operational autopilot; maintenance records indicated that the autopilot was removed in 2016.

The rear control stick had been removed, and there was no boot around the 8-inch-wide opening at its base (see figure 4). A position light socket was located within the underfloor bay area of the aft control stick assembly (see figure 5). The socket was slightly crushed and wrapped with electrical insulating tape, which exhibited shredding and dark fretting marks. The conductor wires had been cut, and the metal case of the lamp was still in the socket. The clearance between the belly of the airplane and the control stick assembly was about 2 1/4 inches. The maintenance logbooks did not contain an entry indicating that the lamp socket had been replaced.

Figure 4. Aft control stick (left) with opening and no boot, and forward control stick with boot (right)

Figure 5. Position light socket (inset) and rear lower control assembly.

The engine remained attached to its mount, which had detached from the airframe. The bulk of the engine remained largely intact, and there was no evidence of catastrophic internal engine failure. The spark plug electrodes were mechanically undamaged and displayed coloration and deposits consistent with normal operation. There was no streaking along the airframe sides or any other evidence of a significant engine oil or fluid leak.

The propeller remained attached to the crankshaft flange, which had broken away from the engine at the forward crankshaft bearing. The crankshaft separation surface exhibited a radial 45° lip around its circumference. Both propeller blades exhibited leading edge gouging, torsional twisting, chordwise striations across the cambered surface, and trailing edge “S” bending.

Hydrodynamic damage to the fuel tanks and carburetor floats was consistent with fuel onboard at the time of impact.

An autopsy of the pilot was performed by the Pima County Medical Examiner’s Office at the request of the FAA. The cause of death was due to blunt force injuries of the head, and the manner of death was accident. The examination was limited by the severity of injury.

Toxicology testing performed by the FAA Forensic Sciences Laboratory did not identify the presence of any screened drug substances or ingested alcohol.

The configuration of the airplane was such that, with flaps deployed, the airplane would transition to a nose-down pitch attitude, requiring aft application of the control stick to maintain level flight; however, due to the speed of the flap actuator motor, the movement would have been progressive.

A flight test was performed by Vans Aircraft in calm wind conditions to confirm the flight characteristics and stick forces required with varying extreme combinations of flap deployment and trim configurations.

A similarly loaded RV-7 airplane was used for the test; the RV-7 had comparable physical dimensions to the RV-4, used the same wing and horizontal stabilizer airfoils, and similarly sized elevator mass and aerodynamic counterbalance. The tests were performed at an entry speed of 115 kts, with the engine speed set at 2,300 rpm.

Flight tests revealed that, when trimmed for an 800-fpm descent, as observed during the initial approach phase of the accident flight, full flap deployment resulted in a reduction of speed to 103 kts, with only light aft pressure required to maintain level flight. With the release of control pressure, the nose dropped and the descent angle increased, but the airplane settled to a speed of 115 kts, with about 6 lbs of aft control pressure force required to arrest the descent.

For the next test, pitch trim was set to full nose-down and the flaps were then deployed as before. About 17 lbs of aft control force was required to arrest the descent.

For the third test, flap and nose-down trim were deployed simultaneously, and the airplane’s speed was allowed to accelerate to 127 kts before a recovery was attempted. Slightly higher control forces were required to arrest the descent; however, the test pilot stated that at no point during any of the tests did the control forces become unmanageable, and that they were low enough to be easily overcome with control inputs.

Vans Aircraft reiterated that the flight test was conducted with flap deployment beyond the approved VFE in smooth air with no potential for gust loads, at or as close as possible to 1g, and warned that gust and applied g-loading can overstress and fail the flap mechanism.

On September 6, 2022, Vans Aircraft issued Service Letter 060, “Control Stick Opening Covers”. The letter addressed the possibility of foreign objects jamming flight control systems, and recommended that, when possible, all control stick openings where the stick passes through a floorboard or bulkhead should be covered by a stick boot.