N407NDCirrus Design SR20 · 2005

On December 5, 2014, about 1428 mountain standard time, a Cirrus SR20 airplane, N407ND, impacted terrain during approach at the Fort Collins-Loveland Municipal Airport (FNL), near Fort Collins, Colorado. The solo student pilot was seriously injured and the aircraft was substantially damaged. The aircraft was registered to and operated by Cirrus LLC under the provisions of 14 Code of Federal Regulations Part 91 as an instructional flight. Day visual meteorological conditions prevailed for the local flight, which departed without a flight plan.

The student pilot stated that he entered the traffic pattern at FNL 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, as he was concerned with wake turbulence and wanted to land beyond the helicopter's touchdown point. Just prior to landing, he encountered turbulent air, entered into an uncommanded steep left bank, and attempted to go around. The pilot was unable to maintain control and the airplane subsequently impacted terrain and cartwheeled, which resulted in damage to the fuselage and both wings.

An airport surveillance camera at FNL captured the accident airplane approaching the runway about 30 seconds in trail of the departing UH-60 helicopter.

At 1435 the weather observation station at FNL reported the following conditions: wind 110 degrees at 3 knots, visibility 10 miles, clear sky, temperature 14 degrees C, dew point 4 degrees C, altimeter setting 30.22 inches of mercury.

FAA Flight Test Report

In February 1996, the Federal Aviation Administration (FAA) Technical Center released a flight test report (DOT/FAA/CT-94/117) on the hazards of rotorcraft wake vortices in forward flight. The flight test utilized a laser Doppler velocimeter (LDV) to measure helicopter wake vortices. Four helicopters, with weights ranging from 7,600 to 70,000 pounds, were utilized as the wake vortex generating aircraft. The maximum duration for vortex life, as measured by the LDV, was 75 seconds for the UH-60. The FAA flight test report made the following conclusions:

--Medium weight helicopters, such as the S-76A and UH-1….can leave active, potentially hazardous vortices for up to 90 seconds. Separations for small aircraft behind these rotorcraft should therefore be in the 90-second range.

--Larger helicopters, such as the CH-47D and CH-53E ….were observed to have longer hazard times. A 120-second separation should be adequate for operations behind these rotorcraft.

--Information on the wake vortex hazard behind these rotorcraft, including delineation by class, should be included in the Airman's Information Manual and the Wake Vortex Advisory Circular.

The FAA flight test report is included in the public docket for this investigation.

FAA Pilot guidance

Current FAA airman information manual (AIM) and advisory circular (AC) 90-23G on aircraft wake turbulence do not recommend an in-trail distance or timing separation for an airplane following a helicopter. The AC contains a general wake turbulence statement: "pilots should avoid helicopter vortices since helicopter forward flight airspeeds are often very low, which generate strong wake turbulence."

AmSafe airbag seatbelt assemblies were mounted in the two front crew seats of the accident aircraft. Neither of the two airbag seatbelts deployed as the airplane cartwheeled. On January 6, 2015, a wreckage review was conducted, which included examination of the crew seats and airbag seatbelt assemblies.

The AmSafe inflatable restraint system is self-contained and not connected to aircraft electrical power. Each restraint system includes an inflatable restraint harness (which includes the harness/seatbelt, an airbag inserted in the seatbelt webbing and a Kevlar inflator hose), an inflator assembly (which includes a helium gas canister) and an inflator interface cable. The two front crew seat AmSafe inflatable restraints are controlled by an electronics module assembly (EMA).

The EMA's dual crash sensors evaluate the acceleration and impact energy or crash pulse. The EMA's crash sensors only sense a longitudinal G input and require a continuous pulse of a minimum of 6+G's, which must load both sensors at the same time for 40-50 millseconds before the system will send a crash pulse signal to the inflator. A review of data downloaded from the Primary Flight Display (PFD) revealed that longitudinal Gs necessary to trigger the EMA crash sensors did not occur during the accident sequence.

Examination of the right front crew seat revealed that the right airbag seatbelt's Kevlar inflator hose assembly was chafed. The inflator hose was chafed in a location about 6 to 7 inches from the hose fitting. The chafed area created a hole approximately 2 inches in length, 0.5 inches in width and completely through the Kevlar and rubber layers of the hose assembly. The chafed area was immediately forward of a storage pocket attached to the back of the right seat. The examination did not determine the source of the chafing.

According to the AmSafe's Supplemental Instructions for Continued Airworthiness, a detailed visual inspection should be performed at least annually. The inspection requires a check of exposed hoses and cables for fraying and excessive wear of the hose and cable protective sleeving. Hose wear is acceptable as long as no holes are present. If any exposed hoses or cables show significant signs of fraying or excessive wear, such as holes, the assembly should be replaced.

Examination also revealed that the electrical diagnostic check failed. The inflator interface cables had either loose and/or broken wires on the connectors for each of the two crew seats. These cables require inspection and testing annually.