Allegiant Air Flight 227

Allegiant Airlines flight 227 (AAY227), which originated from Asheville Regional Airport (AVL), Asheville, North Carolina, encountered convective-induced turbulence while descending to 13,000 ft during the approach into St. Petersburg-Clearwater International Airport (PIE), Clearwater, Florida. The captain was a check pilot, and the first officer was the pilot flying.

AAY227 established communication with air traffic control (ATC) and reported that the airplane was descending to cross the CAPOH intersection at flight level (FL) 270. The controller notified the flight crew of an area of moderate to heavy precipitation at the airplane’s 12:00 position which was 45 miles away and advised the crew to anticipate deviations to the right of the airplane’s course. The controller instructed AAY227 to descend and maintain FL 190, deviate to the right of course upon leaving FL 260, and proceed directly to the OLENE intersection when able. AAY227 was descending from FL 230 when the controller instructed the crew to cross OLENE at 13,000 ft and 250 knots.

The flight crewmembers reported that when they started the descent the airplane was in clear air and the ride was relatively smooth. The crewmembers also indicated that the initial crossing restriction for OLENE was given late by ATC and that the airplane was about 4,500 ft high on the descent profile, so they deployed the speed brakes and turned off the autopilot to meet the altitude and speed restrictions. The flight crew stated that no pilot reports (PIREPS) were conveyed during the flight and airborne weather radar did not show any significant precipitation returns. The cabin crewmembers stated that, although the seatbelt sign was on, they had not received the chimes that indicate final descent preparation should begin because the airplane had not yet descended through 10,000 ft.

As AAY227 approached its level-off altitude, the flight crewmembers observed a small cumulus buildup at an altitude of about 13,000 ft that was unavoidable. According to the first officer, the clouds were not dark and looked no different than the clouds that the airplane had previously flown through. As the airplane entered the clouds, it encountered some minor expected jolts, and just before exiting, a severe jolt of turbulence occurred that lasted about 1.5 seconds. After the turbulence encounter, the captain immediately called the cabin crew and was informed of multiple injuries. The captain declared an emergency and asked for priority handling direct to PIE. The airplane subsequently made an uneventful landing.

There were four flight attendants (FA) working the flight and they stated that just after the initial descent announcements, the lead FA (FA 1) was in the forward galley, FA 4 was in the aft galley, and FA 2 and FA 3 were walking through the cabin conducting seatbelt compliance checks. As the FAs were completing their checks, the airplane encountered turbulence, and all four flight attendants contacted the ceiling and then the cabin floor.

After landing, FA 2 and 4 were provided medical assistance for serious injuries. Four passengers received minor injuries. FA 1 and FA 3 were not injured. A postaccident review of the radar and satellite imagery indicated that several low-level gust fronts had merged near the accident site, resulting in convective buildup where the airplane had been operating. The National Weather Service issued a convective SIGMET (Significant Meteorological Information) for the area about 15 minutes prior to the encounter, of thunderstorms moving eastward at 10 knots, with implied potential for severe turbulence in and near convective cells. The Tampa WSR-88D detected a developing cell in the immediate vicinity of the turbulence event with echoes between 15 – 45 dBZ, however, cockpit weather radar does not typically pick up echoes less than 20 dBZ. Therefore, it is unlikely the flight crew were aware that they were entering into a larger area of convection that included higher intensity echoes located east of their flight path and encountered moderate or greater convectively induced turbulence (CIT) within the clouds.

Verbatim NTSB analysis and probable cause from the NTSB dataset