N747J

HISTORY OF FLIGHTOn July 20, 2018, about 1604 central daylight time, a De Havilland DH112 Venom airplane, N747J, was destroyed when it was involved in an accident near Sheboygan, Wisconsin. The private pilot was fatally injured, and two people in the impacted structure were seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot was conducting a formation training flight that consisted of two airplanes; the accident airplane was in the No. 2 position and was to be staggered behind and to the right of the lead (No. 1 position) airplane. During the flight briefing, the pilot was instructed to take off 10 seconds after the lead airplane. The airplanes departed from Sheboygan County Memorial Airport (SBM), Sheboygan Falls, Wisconsin.

Review of video provided by a witness showed that the accident airplane departed about 8 seconds after the lead airplane. About 6 seconds after the accident airplane lifted off, the left wing rocked downward then up, followed by the right wing rocking downward then up as the airplane returned to wings level. The accident airplane continued to climb at an angle away from the runway and the witness' position on an airport ramp. As the airplane passed the witness' position, the engine sound decreased and then rapidly increased. The video ended about 19 seconds after the airplane lifted off from the runway. The video showed that the accident airplane's flaps were extended throughout the 19-second video and that the landing gear was retracting during the last few seconds of the video. The witness stated that he stopped recording because the airplane "was a little sluggish as it climbed out, but it seemed to be ok" and that he started recording again after he heard someone state that the airplane was not climbing; the witness then saw that the airplane "sank out of sight behind some trees."

Other witnesses observed the takeoff and departure. According to one of these witnesses, the airplane climbed to about 200 ft above ground level (agl) and then started to descend. This witness reported a quartering headwind at takeoff from the left to right side of the runway. Another witness, who was a Venom airplane pilot, indicated that the gear was retracting at a normal rate. None of the witnesses recalled hearing any unusual sounds from the airplane during takeoff, but one witness commented that he could not hear the accident airplane well because of noise from running airplane engines on the airport ramp. Three of these witnesses stated that the accident airplane appeared to have encountered the wake turbulence of the lead airplane.

An additional witness traveling east on a road near the departure end of the runway captured cell phone video of the final 5 seconds of flight. The airplane is first seen heading away from the witness' position; at that time, the airplane was descending from an altitude of about 75 ft agl and was in a bank of about 25° to the left. The airplane then rolled to a wings-level attitude and continued to descend in a slightly nose-up attitude before impact. The landing gear and flaps appeared retracted in this video. The airplane impacted flat vegetated terrain, slid through a farm structure, and continued about 175 ft before coming to a stop. AIRCRAFT INFORMATIONThe DH112 Venom airplane was designed as a post-World War II-era fighter jet. The witness who had experience flying Venom airplanes stated that the airplane was equipped with a split flap system and that the flaps were used more to provide drag than lift. The flap selector had three positions: up, ½, and full, and takeoffs were normally made with flaps ½ or up. The landing gear was powered by a hydraulic pump driven by the engine, so the landing gear could not retract if the engine were not rotating. The Venom airplane pilot described the engine powering the airplane as "high airflow and thrust" and indicated that Venom airplanes could experience flameouts or compressor stalls when operating at high angles of attack and high-power settings. He stated that the Venom airplane had "excess power" and that the only ways for the airplane to be in a low energy state would be an engine flameout, engine failure, or engine shutdown. He added that the airplane would normally accelerate "very quickly" after takeoff if it leveled off at 200 ft. AIRPORT INFORMATIONThe DH112 Venom airplane was designed as a post-World War II-era fighter jet. The witness who had experience flying Venom airplanes stated that the airplane was equipped with a split flap system and that the flaps were used more to provide drag than lift. The flap selector had three positions: up, ½, and full, and takeoffs were normally made with flaps ½ or up. The landing gear was powered by a hydraulic pump driven by the engine, so the landing gear could not retract if the engine were not rotating. The Venom airplane pilot described the engine powering the airplane as "high airflow and thrust" and indicated that Venom airplanes could experience flameouts or compressor stalls when operating at high angles of attack and high-power settings. He stated that the Venom airplane had "excess power" and that the only ways for the airplane to be in a low energy state would be an engine flameout, engine failure, or engine shutdown. He added that the airplane would normally accelerate "very quickly" after takeoff if it leveled off at 200 ft. WRECKAGE AND IMPACT INFORMATIONThe airplane came to rest amongst several farm buildings; the debris path was 200 ft in length and 40 ft wide. Postimpact fire consumed a large portion of the fuselage. The entire airplane was located at the accident site.

The engine remained attached to the fuselage. The compressor blades displayed some impact damage from about the five to the nine o'clock positions. No signs of foreign object damage were observed, and no evidence of catastrophic engine failure was noted.

Further examination of the engine showed that the part of the airplane around the tailpipe was still in place. The exterior of the airplane skin was burned and blistered, but the burn pattern was not linear, and there was no burn damage to the interior of the skin. The compressor impeller and turbine disk could not be rotated. The bifurcated inlet at the front of the engine was thermally destroyed. The front bearing could be rotated, but there was some resistance during rotation.

The compressor impeller was intact. Of the 19 impeller vanes, 8 consecutive vanes were missing portions coincident with where the compressor shroud was burned away. Four consecutive impeller vanes had circumferential rubbing on the leading edges with metal displaced opposite the direction of rotation.

The turbine vanes were in place and displayed no damage or thermal distress, and no metal spray was on the airfoils. The tailpipe was in place on the rear of the turbine case with no evidence of thermal distress. The tailcone was in place in the tailpipe, and the forward edge of the tailcone had three areas of circumferential rubbing with heat discoloration. One of these areas also had material transfer. The diameter of the forward edge of the tailcone corresponded to about a 90° arc of turbine blades that had about a 0.5-inch wide circumferential rub on the trailing edge.

No preimpact anomalies with the airframe and engine were found that would have precluded normal operation of the airplane. ADDITIONAL INFORMATIONCompressor Stall

According to aviationglossary.com

A Compressor Stall in a gas turbine engine is a condition in an axial-flow compressor in which one or more stages of rotor blades fail to pass air smoothly to the succeeding stages. A stall condition is caused by a pressure ratio that is incompatible with the engine rpm. Compressor stall will be indicated by a rise in exhaust temperature or rpm fluctuation, and if allowed to continue, may result in flameout and physical damage to the engine.

The resource indicates that the typical causes of compressor stalls are the following:

Excessive fuel flow caused by abrupt engine acceleration when the axial velocity (speed) is reduced by increased pressure in the combustion chamber due to the added combustion and the resultant rotational speed increase

Operation of the engine outside of its RPM design parameters affecting the rotational speed of the compressor blade

Turbulent or distorted / disrupted airflow to the engine inlet reducing the axial velocity

Damage to either the compressor or turbine blades

Low fuel flow due to abrupt engine deceleration causing a decreased back pressure

The resource also indicates that compressor stalls can be transient and intermittent and provides the following information:

Indications of a transient/intermittent stall are usually an intermittent "bang" as backfire and flow reversal take place. If the stall develops and becomes steady, strong vibration and a loud roar may develop from the continuous flow reversal. Often, the flight deck gauges do not show a mild or transient stall, but they do indicate a developed stall. Typical instrument indications include fluctuations in rpm and an increase in exhaust gas temperature.

Wake Turbulence

The National Transportation Safety Board's report on the September 8, 1994, accident involving USAir flight 427 near Pittsburgh, Pennsylvania (NTSB/AAR-99/01), provided the following description of a wake vortex, which is also known as wake turbulence:

A counterrotating airmass trailing from an airplane's wingtips. The strength of the vortex is governed by the weight, speed, and shape of the wing of the generating aircraft; the greatest strength occurs when the wings of the generating aircraft are producing the most lift; that is, when the aircraft is heavy, in a clean configuration, and at a slow airspeed. MEDICAL AND PATHOLOGICAL INFORMATIONThe Sheboygan County Medical Examiner's Office,...