NTSB Factual Report

On February 18, 2012, about 1900 central standard time, a Boeing 737-832, N374DA, encountered turbulence while descending through 29,000 feet as it approached the Detroit Metropolitan Wayne County Airport (DTW), Detroit, Michigan. Delta Airlines operated flight 1806 under the provisions of 14 Code of Federal Regulations Part 121 as a scheduled domestic passenger flight. Two flight attendants and one passenger sustained minor injuries, and one flight attendant sustained serious injuries. The captain, first officer, another flight attendant, and 129 passengers were not injured. The airplane was not damaged. Visual meteorological conditions prevailed, and the flight was operating on an instrument flight rules (IFR) flight plan. The flight departed the Los Angeles International Airport (LAX), Los Angeles, California and was en route to Detroit, Michigan.

The flight crew reported that they received reports of turbulence near DTW and informed the cabin crew to plan on completing having cabin service prior to starting the descent into DTW. The flight attendants reported that they had completed service and were doing their final checks before the approach to DTW. The flight crew also stated that about 2 minutes prior to entering the area of expected turbulence, they made an announcement to the passengers and crew to remain seated and make sure the seatbelts were fastened. The flight attendants stated that the airplane hit turbulence before they were expecting it, and before they could get seated.

The first officer reported that as they descended from flight level 290, the airplane encountered a significant jolt of moderate turbulence. The flight encountered decreasing turbulence as they descended before reaching smooth air below flight level 190.

According to the flight data recorder data (FDR), during the event, the maximum vertical acceleration was + 1.568 G’s and the minimum was -0.30 G’s over a time period of 0.625 seconds. The FDR data indicated that vertical accelerations were severe, as defined by International Civil Aviation Organization (ICAO) standards.

A weather study was conducted by an NTSB meteorologist. According to the study, the High level Significant Weather Prognostic chart depicted the polar jet stream moving northeastward form Texas into Ohio then eastward into the ridge with the maximum wind of 180 knots at 35,000 feet, wind of 70 knots and greater associated with the jet stream extended from 17,000 to 51,000. A second subtropical jet stream was located south of the first jet stream. A large area of moderate turbulence was forecasted with the polar jet stream from Missouri,Iowa, Illinois, Wisconsin, Michigan, Indiana, and Ohio. The chart also expected another area of moderate to severe turbulence from 25,000 to 32,000 feet with the larger area over northern Missouri, Illinois, Iowa, Wisconsin, and Michigan. The accident site was located within the area of moderate to severe turbulence potential. The National Weather Service (NWS) also issued multiple in-flight weather advisories current for the area including a significant meteorological (SIGMET) advisory for occasional severe turbulence between 22,000 and 30,000 feet, a Center Weather Advisory (CWA) and airman meteorological (AIRMET) advisory for moderate turbulence between 18,000 and 41,000 feet.

The Delta Meteorology Department provided the flight crew with weather documents for the intended route; the document included several weather alerts for turbulence along the route of flight, the crew also received updates en route via ACARS (Aircraft Communications Addressing and Reporting System). The first update at 1547 CST warned of light to moderate, turbulence. At 1559 CST the advisory was updated to include parts of Ohio and Michigan and advised to avoid altitudes form flight level 200 to 250 for moderate to severe turbulence and moderate turbulence between flight level 260 to 300. Another ACARS message was sent at 1929 CST that indicated the area of moderate to severe turbulence had moved south and east of Detroit.

NTSB Probable Cause

An in-flight encounter with forecasted turbulence, which resulted in a serious injury to a flight attendant. Contributing to the accident was the flight attendant not being seated with her seatbelt fastened when the encounter occurred.