N2571G

On September 26, 2018, at 1907 Alaska daylight time, a Cessna 182 airplane, N2571G, sustained substantial damage during a wake turbulence encounter, about 3.5 miles northwest of Anchorage, Alaska. The airplane was registered to and operated by the pilot as a visual flight rules (VFR) cross-country flight under the provisions of Title 14 Code of Federal Regulations Part 91, when the accident occurred. The private pilot and passenger sustained minor injuries. Visual meteorological conditions prevailed, and no flight plan had been filed.

The pilot stated that while southbound, enroute to Lake Hood Airstrip (LHD), he received a traffic advisory and wake turbulence cautionary advisory from air traffic control (ATC) regarding a Boeing C-17 (TREK324) that was on approach for runway 6 at Elmendorf Air Force Base (EDF). In an effort to avoid wake turbulence, the pilot executed a left 360° turn. Upon completion of the turn, he passed behind the C-17, and encountered "severe turbulence" which he described as "one violent instantaneous motion." After the event, the airplane continued for landing at LHD. During a postflight inspection of the airplane, he found both wings and the horizontal stabilizer exhibiting signs of buckling. Furthermore, it appeared the left wing had shifted aft, reducing the gap between the trailing edge of the flap and the fuselage by 3/8 of an inch. After removing an inspection panel and using a borescope to examine the left-wing spar, a crack was discovered.

A review of archived Federal Aviation Administration (FAA) radar and voice data revealed that the Cessna was VFR from Point MacKensie, inbound to LHD when issued a traffic and wake turbulence advisory. The Cessna pilot informed ATC that he had the C-17 in sight and executed a left 360° turn. Following the turn, the Cessna passed 3.66 miles behind, and 500 ft below, the C-17. On the inbound leg of the turn, the pilot descended from 900 ft GPS altitude to about 600 ft GPS altitude and which was below the altitude of the C-17, which was about 1,200ft GPS altitude.

Section 7 of FAA Advisory Circular AC90-23G, dated February 10, 2014 states in part: "Flight tests have shown that at higher altitude the vortices from large aircraft sink at a rate of several hundred feet per minute (fpm), slowing their descent and diminishing in strength with time and distance behind the wake-generating aircraft (see Figure 5, Descent of Vortices from Large Aircraft). Atmospheric turbulence hastens decay. Pilots should fly at or above the preceding aircraft's flightpath, altering course as necessary, to avoid the area behind and below the generating aircraft…The worst case atmospheric conditions are light winds, low atmospheric turbulence, and low stratification (stable atmosphere). In these atmospheric conditions, primarily in en route operations, vortices from Heavy and especially Super aircraft can descend more than 1,000 feet."

Section 8 of the above mentioned AC90-23G states in part: "Air traffic controllers apply procedures for separating instrument flight rules (IFR) aircraft that include required wake turbulence separations. However, if a pilot accepts a clearance to visually follow a preceding aircraft, the pilot accepts responsibility for both separation and wake turbulence avoidance. The controllers will also provide a Wake Turbulence Cautionary Advisory to pilots of visual flight rules (VFR) aircraft, with whom they are in communication and on whom, in the controller's opinion, wake turbulence may have an adverse effect. This advisory includes the position, altitude and direction of flight of larger aircraft followed by the phrase "CAUTION–WAKE TURBULENCE." After issuing the caution for wake turbulence, the air traffic controllers generally do not provide additional information to the following aircraft." Also listed are two notes, one of which that states "Whether or not a warning or information has been given, the pilot is expected to adjust aircraft operations and flightpath as necessary to preclude wake encounters." The second note states "When any doubt exists about maintaining safe separation distances between aircraft to avoid wake turbulence, pilots should ask ATC for updates on separation distances and groundspeed." No such queries were made by the Cessna pilot. Furthermore, 8e. states that pilots should avoid flight below and behind a larger aircraft's flightpath and if a larger aircraft is observed above on the same track, airplane position should be adjusted, preferable upwind.