History of Flight
On December 5, 2014, at about 1428 mountain standard time, a Cirrus SR20 airplane, N407ND, impacted terrain during approach at Fort Collins-Loveland Municipal Airport (FNL), near Fort Collins, Colorado. The solo student pilot was seriously injured, and the aircraft sustained substantial damage. The aircraft was registered to and operated by Cirrus LLC under 14 Code of Federal Regulations Part 91 as an instructional flight. Day visual meteorological conditions prevailed, and the local flight departed without a flight plan.
The student pilot stated that he entered the traffic pattern for a full stop landing on runway 33. He observed a Sikorsky UH-60 helicopter on downwind and delayed his turn to base until the helicopter was on final, abeam his position. While on final, the student pilot adjusted his aim point to land long to avoid wake turbulence from the helicopter. Just prior to landing, he encountered turbulent air, entered an uncommanded steep left bank, and attempted a go-around. The pilot was unable to maintain control, and the airplane impacted terrain and cartwheeled, damaging the fuselage and both wings.
An airport surveillance camera captured the accident airplane approaching the runway about 30 seconds in trail of the departing helicopter.
Meteorological Information
At 1435, the weather observation station at FNL reported wind from 110 degrees at 3 knots, visibility 10 miles, clear sky, temperature 14°C, dew point 4°C, and an altimeter setting of 30.22 inches of mercury.
Additional Information
The FAA Technical Center released a flight test report (DOT/FAA/CT-94/117) in February 1996 on rotorcraft wake vortices. The test used a laser Doppler velocimeter to measure wake vortices from four helicopters weighing 7,600 to 70,000 pounds. For the UH-60, the maximum vortex life was 75 seconds. The report concluded that medium-weight helicopters (e.g., S-76A, UH-1) can leave hazardous vortices for up to 90 seconds, recommending 90-second separations for small aircraft. Larger helicopters (e.g., CH-47D, CH-53E) had longer hazard times, with 120-second separations recommended. The report suggested including this information in the Airman's Information Manual and Wake Vortex Advisory Circular.
Current FAA guidance (AIM and AC 90-23G) does not recommend in-trail distance or timing separation for airplanes following helicopters. The AC states: "pilots should avoid helicopter vortices since helicopter forward flight airspeeds are often very low, which generate strong wake turbulence."
Survival Aspects
The aircraft was equipped with AmSafe airbag seatbelt assemblies in the two front crew seats. Neither airbag deployed during the cartwheel. A wreckage review on January 6, 2015, examined the seats and assemblies.
The AmSafe system is self-contained with an inflatable restraint harness, inflator assembly (helium gas canister), and inflator interface cable, controlled by an electronics module assembly (EMA). The EMA's dual crash sensors require a longitudinal G pulse of at least 6 G's for 40-50 milliseconds to trigger. Data from the Primary Flight Display showed that necessary longitudinal Gs did not occur.
Examination of the right front seat revealed the airbag's Kevlar inflator hose was chafed (2 inches long, 0.5 inches wide hole through Kevlar and rubber) about 6-7 inches from the hose fitting, near a storage pocket. The chafing source was not determined. According to AmSafe's Supplemental Instructions for Continued Airworthiness, annual visual inspection should check for fraying and wear; holes require replacement.
The electrical diagnostic check failed. The inflator interface cables had loose or broken wires on connectors for both seats. These cables require annual inspection and testing.