Military helicopter wake turbulence destroys small plane

On December 5, 2014, about 1428 MST, a Cirrus SR20, registration N407ND, impacted terrain during approach at the Fort Collins-Loveland Municipal Airport, near Fort Collins, Colorado. The solo student pilot was seriously injured* and the aircraft was substantially damaged. The aircraft was being operated as an instructional flight.

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.

At 1435 the weather observation station at FL 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.

In February 1996, the FAA Technical Center released a flight test report 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.

Current FAA airman information manual 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, as the minimum threshold for triggering — a longitudinal G input of at least 6 G's, simultaneously loading both sensors for 40-50ms — did not occur during the accident sequence.

The NTSB determined the probable cause of this accident to be the student pilot's failure to comprehend the significance of the wake turbulence that a preceding helicopter would generate during departure, which resulted in a loss of airplane control during landing. Contributing to the accident was the lack of FAA wake turbulence separation criteria for a small airplane following a helicopter.

*Note: "seriously injured" is defined by statute. Serious injury means any injury which: (1) Requires hospitalization for more than 48 hours, commencing within 7 days from the date of the injury was received; (2) results in a fracture of any bone (except simple fractures of fingers, toes, or nose); (3) causes severe hemorrhages, nerve, muscle, or tendon damage; (4) involves any internal organ; or (5) involves second- or third-degree burns, or any burns affecting more than 5 percent of the body surface. Based on the accident pilot's description of his injuries, he broke his hand.

00:00 Accident video
01:24 Photos
01:33 Pilot accident report
02:43 Pilot's supplement
04:34 FAA Advisory Circular on wake turbulence
10:40 Vortex avoidance

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