De Havilland DH112 VENOM N747J accident near Sheboygan, Wisconsin

Probable cause

The airplane's encounter with wake turbulence from the lead airplane in the formation flight, which resulted in a compressor stall and the airplane's subsequent inability to climb.

NTSB analysis

The pilot was conducting a formation training flight consisting of two airplanes; the accident airplane was in the No. 2 position. The airplane took off about 8 seconds behind the lead (No. 1 position) airplane. Witnesses described the airplane's takeoff as sluggish and indicated that the airplane's wings rocked back and forth shortly after lifting off. The witnesses' testimony was consistent with one witness' video recording, which showed the airplane climbing away from the runway and making a shallow left turn. As the airplane passed the witness' position, the engine sound decreased and rapidly increased. The airplane reached an altitude of about 200 ft above the ground before descending and impacting a farm structure. Another video captured the airplane descending in a slightly nose-up and wings-level attitude before impact. The flaps and landing gear both appeared retracted in that video.

No preimpact anomalies with the airframe and engine were found that would have precluded normal operation of the airplane. Signatures consistent with circumferential rub marks within the turbine were identified. Although the rate of rotation at the time of impact could not be determined, the retraction of the landing gear at a normal rate showed that the engine was likely producing some power at that time.

One of the witnesses to the accident was a pilot with experience in the accident make and model. He described the engine powering the airplane as "high airflow and thrust" and indicated that the engine could experience a compressor stall when operating at a high angle of attack and a high-power setting. He also stated that the airplane could get into a low energy state if an engine failure occurred and that the airplane would normally accelerate "very quickly" if it leveled off at 200 ft.

The witness accounts of the accident airplane's wings rocking back and forth were consistent with the airplane encountering the wake turbulence of the lead airplane. Wake turbulence can disrupt airflow to the engine of a trailing airplane. The final seconds of the witness' video showed the pilot controlling the airplane during the climb to 200 ft, after which the airplane began descending. Thus, the wake turbulence from the lead airplane likely disrupted airflow to the engine and caused a compressor stall and a partial loss of engine power, which resulted in the airplane's inability to continue to climb.

Toxicology testing identified sedating medications in the pilot's specimens, but the levels of the medications at the time of the crash were well below the levels that would have caused significant effects. Therefore, it is unlikely that impairing effects of these medications contributed to this accident.

Text from the NTSB. Open the NTSB docket for WPR18FA200 for the full record.