History of Flight
On July 20, 2018, about 1604 central daylight time, a De Havilland DH112 Venom airplane, registration N747J, was destroyed when it struck a farm structure near Sheboygan, Wisconsin. The private pilot sustained fatal injuries, and two people in the impacted building were seriously injured. The flight was conducted as a personal flight under Title 14 Code of Federal Regulations Part 91.
The pilot was performing formation training with another airplane, occupying the No. 2 position, staggered behind and to the right of the lead aircraft. During the preflight briefing, the pilot was instructed to depart 10 seconds after the lead airplane. Both airplanes took off from Sheboygan County Memorial Airport (SBM) in Sheboygan Falls, Wisconsin.
Witness video showed the accident airplane lifted off about 8 seconds after the lead. About 6 seconds after rotation, the left wing dipped downward then up, followed by the right wing rocking similarly before returning to wings level. The airplane climbed at an angle away from the runway. As it passed the witness's position on the ramp, the engine sound decreased then rapidly increased. The video, lasting 19 seconds after liftoff, showed the flaps extended and the landing gear retracting in the final seconds. The witness stopped recording, noting the airplane seemed sluggish but okay, then resumed after hearing that it was not climbing; he saw it sink behind trees.
Other witnesses observed the climb to about 200 ft above ground level before the airplane began to descend. One reported a quartering headwind from left to right. Another Venom pilot noted the gear retracted at a normal rate. No unusual sounds were heard; one witness could not hear well due to other engines. Three witnesses suggested the airplane encountered wake turbulence from the lead aircraft.
Cell phone video captured the final 5 seconds, showing the airplane at about 75 ft agl in a 25° left bank, then rolling wings-level while descending nose-up. The gear and flaps appeared retracted. The airplane impacted flat terrain, slid through a farm structure, and came to rest about 175 ft beyond.
Aircraft Information
The De Havilland DH112 Venom is a post-World War II-era fighter jet. A Venom-experienced witness described its split flap system, which provided more drag than lift. Flap positions were up, ½, and full; takeoffs were normally performed with flaps ½ or up. The landing gear was hydraulically powered by the engine, preventing retraction if the engine was not rotating. The Venom had a high-airflow, high-thrust engine that could experience flameouts or compressor stalls at high angles of attack and high power. The witness noted the airplane had excess power and that a low energy state would indicate flameout, failure, or shutdown. He added that leveling off at 200 ft would normally result in rapid acceleration.
Wreckage and Impact
The debris path extended 200 ft in length and 40 ft wide. A postimpact fire consumed much of the fuselage. The entire airplane was at the accident site, with the engine still attached. Compressor blades showed impact damage from about five to nine o'clock positions, but no foreign object damage or catastrophic failure evidence was noted.
The engine tailpipe area remained in place; the exterior skin was burned and blistered with a non-linear pattern. The compressor impeller and turbine disk could not rotate. The bifurcated inlet was thermally destroyed. The front bearing rotated with some resistance.
The compressor impeller was intact; 8 of 19 vanes had missing portions where the shroud burned away. Four consecutive vanes had circumferential rubbing on leading edges with metal displaced opposite rotation. Turbine vanes were undamaged with no thermal distress. The tailpipe and tailcone showed three areas of circumferential rubbing with heat discoloration; one had material transfer. A 90° arc of turbine blades had a 0.5-inch wide rub. No preimpact anomalies were found that would have prevented normal operation.
Additional Information
Compressor Stall: According to aviationglossary.com, a compressor stall occurs when one or more stages of rotor blades fail to pass air smoothly, caused by incompatible pressure ratio with engine rpm. Indications include exhaust temperature rise or rpm fluctuation; if sustained, it may lead to flameout and engine damage. Typical causes include abrupt acceleration, operation outside rpm design, turbulent inlet airflow, blade damage, or abrupt deceleration. Transient stalls may produce intermittent bangs; developed stalls cause strong vibration and loud roar.
Wake Turbulence: As described in the NTSB report on the USAir flight 427 accident, a wake vortex is a counterrotating airmass from wingtips. Its strength depends on weight, speed, and wing shape; greatest when the generating aircraft is heavy, clean-configured, and at slow airspeed.
Medical and Pathological Information
An autopsy by the Sheboygan County Medical Examiner's Office listed the pilot's cause of death as multiple injuries. Toxicology testing by the Wisconsin State Laboratory of Hygiene identified diazepam (10 ng/ml) and nordiazepam (11 ng/ml) in the pilot's blood. The FAA Forensic Sciences Laboratory detected carbon monoxide, no ethanol, and found loratadine, dextromethorphan, zolpidem in blood and urine, and oxazepam, temazepam, dextrorphan in urine. Diazepam (Valium) is a sedating benzodiazepine; the lowest psychoactive level is 100 ng/ml. Zolpidem is a sedating sleep aid; both carry warnings about operating machinery. Dextromethorphan and loratadine are not typically impairing at usual doses.