LAX94IA203

While descending into the downwind leg of the traffic pattern at the los angeles intl airport, the commuter airplane encountered wake turbulence from a preceding Boeing 747-400, and the airplane rolled inverted. The pilot recovered the airplane, and the flight continued and landed without further incident. The airplane sustained minor damage. Radar data showed that the cummuter was approx 5 mi from the B-747 up to and at the time it encountered the turbulence, which met current separation criteria.

On April 23, 1994, at 0909:06 hours Pacific daylight time, a Fairchild SA-227, N27220, operating as Skywest Airlines flight 5490 (SW 5490), a scheduled domestic commuter flight to Los Angeles International Airport (LAX), encountered wake turbulence from a preceding Boeing 747-400, United Airlines (UAL) flight 2. The pilot momentarily lost control of the airplane, but landed at LAX without further incident. Skywest Airlines maintenance personnel determined that the airplane sustained minor damage. Neither the 2 flight crewmembers nor 17 passengers were injured; 1 passenger reported receiving minor injuries. Visual meteorological conditions prevailed. The flight originated at Santa Maria Airport, Santa Maria, California, at 0830 hours.

Both SW 5490 pilots reported they encountered the wake turbulence while in a descent about 5 miles behind UAL flight 2. The first officer was flying the airplane. After encountering the wake turbulence, the airplane rolled to the left. As the first officer applied full right aileron, the airplane continued to roll to the left until it was inverted.

During the recovery from the inverted position, the airplane's nose pitched down, the airplane rolled to the right, and the pilot recovered the airplane to level flight. The flight continued to the airport without further incident.

The Federal Aviation Administration (FAA) Air Traffic Control Handbook 7110.65 requires 3 nautical miles between aircraft. This requirement increases to 5 nautical miles when a small or large aircraft is operating directly behind a heavy jet aircraft (i.e., a Boeing 747 type airplane). The radar data shows that the closest separation between SW 5490 and UAL flight 2 was 4.47 miles. This occurred north of the Santa Monica [California] very high frequency omni range [(VOR) - a navigational facility] before an "in trail" situation began and 2 minutes 47 seconds before the incident.

The 5 nautical miles criteria, in this instance, went into effect when SW 5490 turned onto the Los Angeles International Airport downwind leg from over the Santa Monica VOR. According to the FAA's Los Angeles Air Route Traffic Control National Track Analysis Program (NTAP), at 0908:06 hours, SW 5490 was 0.69 miles east of the Santa Monica VOR. Skywest 5490 was descending between 7,200 feet msl and 7,000 feet msl, and was 4.76 nautical miles behind UAL flight 2.

At 0908:18 hours, SW 5490 was 1.31 miles east of the Santa Monica VOR at 7,000 feet msl. The flight was 4.88 nautical miles behind UAL flight 2. At 0908:30 hours, the lateral separation between SW 5490 and UAL flight 2 increased to 5.03 nautical miles; a minimum of 5 nautical miles separation was maintained until the incident. When SW 5490 encountered the wake turbulence at 0909:06 hours, it was 5.01 nautical miles behind UAL flight 2 at 7,100 feet msl.