Delta Air Lines Flight 1889

The captain and first officer were operating Delta Air Lines flight 1889 as a regularly scheduled passenger flight that was originally scheduled to depart General Edward Lawrence Logan International Airport, Boston, Massachusetts, at 2130 coordinated universal time (all times in this report are in coordinated universal time unless otherwise noted), but air traffic control (ATC) delayed the departure until 2225. According to a review of automatic communication and recording system data, the flight release was generated and transmitted to the flight crew at 2103. According to the company flight dispatcher located at the Delta Operations Control Center, it included an ATC-preferred routing to Salt Lake City International Airport, Salt Lake City, Utah, because "there wasn't much weather enroute prior to getting out to the west." The dispatcher sent the initial weather briefing for the flight to the flight crew at 2106. At 2150, the dispatcher sent an updated weather briefing, which indicated that there were no weather updates since the previous transmission. The flight departed at 2244.

The National Weather Service (NWS) Storm Prediction Center day 1 convective outlook and area forecast that were issued and valid before the incident flight crew received their preflight weather information forecast areas of severe weather along the flight route and areas of thunderstorms, possibly severe, respectively, along the flight route. Although the preflight weather package discussed a chance of afternoon thunderstorms around the destination airport, it did not mention thunderstorm activity along the flight route.

A review of meteorological information indicated that the NWS also issued a severe thunderstorm watch for the incident site and surrounding area at 2105 with severe weather possible through 0500, which was along the originally planned flight route. Further, the NWS issued a severe thunderstorm warning for the area north of the airplane's flightpath as early as 0133, at which time the incident flight was about 230 miles east-northeast of the incident location. This information would have been available to the company dispatcher; however, the dispatcher did not pass either the severe thunderstorm watch or warning information to the flight crew before or during the flight.

At 0129, the Minneapolis Air Route Traffic Control Center (ZMP ARTCC) controller, who was handling the flight at that time, initially advised the flight crew about a large area of moderate-to-extreme precipitation on their current flight route in western Nebraska and that they would likely need to significantly deviate to get around it. The company dispatcher reported that he also observed the weather. At 0138, he sent a message to the flight crew indicating that there were thunderstorms ahead of the flight that extended up to 60,000 ft; suggesting that they fly direct to Thurman, Myton, and then continue on their planned flight route; this amended routing would take them further south of their original routing toward a gap in the developing thunderstorm activity. The dispatcher also indicated that he had sent the flight crew turbulence plot (TP) CN11, which was issued at 0049. Delta Air Lines had issued TPs for thunderstorm activity at 0113 (TP CN12) and at 0159 (TP CN13), both of which indicated potential thunderstorm activity along their flight route. However, the dispatcher only transmitted TP CN11, which did not include information about thunderstorm activity along the flight route. According to the captain, the revised routing put them directly toward a gap in the weather that corresponded with what they observed on their weather radar.

At 0141, the ZMP controller again informed the flight crew of the large area of moderate-to-extreme precipitation at their 12-o'clock position and 130 miles out and advised them of the decisions of other flight crews of flights headed in that direction to deviate well north or south around the area of precipitation. Two minutes later, the flight was transferred to the Denver (ZDV) ARTCC; at that time, the weather radar data available to both the ZDV controller and the company dispatcher indicated that the gap between the two areas of thunderstorms had reduced from 30 to 26 miles. The pilots queried ZDV about the line of thunderstorms ahead of them and were advised that there was a gap in the weather, which the pilots noted corresponded to the dispatcher's earlier suggested route. The flight crew chose to continue toward the gap in the weather, and both pilots believed they would be able to maintain at least 20 miles lateral separation between each individual thunderstorm cell, as required by Delta Air Lines procedures for thunderstorm avoidance. By 0154, with the flight still more than 50 miles from the thunderstorms, the gap in thunderstorm activity had closed to 12 miles. At the time that the flight entered the gap, the distance between the areas of thunderstorm activity had reduced to less than 10 miles and was continuing to decrease.

Also about 0154, the pilot of eastbound Southwest Airlines flight 3318, which had just passed through the hole in the weather in the opposite direction, provided a pilot weather report (PIREP) to the ZDV controller indicating that the hole was starting to close and that he did not think anybody should go through it behind them. However, the ZDV controller did not provide this PIREP to the incident flight crew. Also about 0154, as the incident flight was about 50 to 60 miles from the thunderstorms, the pilot of eastbound American Airlines flight 1155, which had passed the incident flight earlier in the opposite direction, reported a dramatic temperature increase at cruise altitude. Despite the ZDV controller's awareness of the weather conditions from the PIREPs, he did not relay this information to the Delta flight, which continued toward the thunderstorms.

According to the incident captain, he also noted a temperature increase at cruise altitude followed by a "rough ride" and "static discharge" and was briefly unable to hear the radio due to the static interference. The captain decided to turn away from the weather, and the ZDV controller provided him a heading to avoid the weather. As the airplane began to turn left, their radar display showed additional weather, and the flight crew chose to turn back to the right. The flight then began to encounter hail, which shattered the outside panes of the forward windows. The captain declared an emergency and diverted to Denver International Airport.

The ZMP controller did an excellent job of relaying known weather information to the flight crew. However, the ZDV controller did not. Specifically, the ZDV controller failed to issue the displayed weather to the flight as required by Federal Aviation Administration (FAA) Order 7110.65, "Air Traffic Control." The ZDV controller also failed to disseminate significant PIREP information to the incident flight crew as required by FAA Order 7110.65. Further, the company dispatcher who provided the flight crew with the initial and revised preflight weather briefings did not provide adequate weather information to the flight crew because it did not include information about the thunderstorm activity forecast along the flight route, and the dispatcher subsequently failed to provide the flight crew with complete and timely weather information, including weather radar data information, while the flight was en route to the destination airport.

Verbatim NTSB analysis and probable cause from the NTSB dataset