OPS15IA020

The flight crew's continued flight into a closing gap between areas of thunderstorm activity and their failure to maintain the required lateral separation from the thunderstorms, which resulted in the airplane's encounter with hail and subsequent airplane damage. Contributing to the incident were the company flight dispatcher's failure to provide complete and timely weather information to the flight crew and the Denver air route traffic controller's failure to provide significant pilot weather report information and alert the pilots of existing and worsening hazardous weather along their flight route, as required by Federal Aviation directives, both of which led to the airplane's encounter with hail.

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.

Neither the captain or first officer received, nor were they required to receive, drug and alcohol screening as a result of this incident. The National Weather Service (NWS) issued a severe thunderstorm watch for the incident site and surrounding area at 2105 with severe weather possible through 0500. 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 aircraft was still in ZMP airspace about 230 miles east-northeast of the developing thunderstorms they later encountered. The NWS Storm Prediction Center (SPC) day 1 convective outlook that was issued and valid before the incident flight crew received their preflight weather information forecast areas of severe weather along the flight route. The NWS area forecast issued before the incident flight received their preflight weather information also mentioned thunderstorms, possibly severe, along the flight route. Neither the NWS SPC day 1 convective outlook nor the area forecast were provided to the incident flight crew. As noted, the preflight weather briefing sent to the incident flight crew by the company dispatcher noted that there was a chance of afternoon thunderstorms around the destination airport, but it did not mention thunderstorm activity along the flight route until near the destination. Title 14 CFR 121.601(c) requires dispatchers to provide the pilot-in-command (PIC) any additional available information of meteorological conditions (including adverse weather phenomena, such as clear air turbulence, thunderstorms, and low altitude wind shear) during the flight. Delta Air Lines Flight Control Operations Manual (FCOM) reiterates the content of the CFR and further outlines the criticality of the dispatchers constant vigilance of significant meteorological events and requirements for use of TPs. For more detailed information, see the "Relevant Procedures" section of this report. When asked about NWS products, and why he had not provided that information in the preflight weather information, he said that he generally did not use NWS products because the products from his company meteorology department were more accurate and reliable. The NWS surface analysis chart for 0300 depicted a stationary front just south of the incident site located in northwestern Kansas with thunderstorms and rain reported in the two surface stations near the incident site. Cheyenne County Municipal Airport (SYF), located 24 miles southwest of the incident location, was the closest official surface weather station to the incident site. The 0155 SYF surface observation reported wind from 180° at 10 knots with gusts to 19 knots, 10 miles visibility, clear skies below 12,000 ft above ground level, temperature of 31° C, dew point temperature of 11° C, and an altimeter setting of 29.75 inches of mercury. Remarks: automated station with precipitation discriminator, temperature 30.5° C, dew point temperature 10.9° C. Imperial Municipal Airport (IML), located 29 miles north-northwest of the incident location. Although SYF was located physically closer to the incident location, the thunderstorm line was not located above SYF at 0155; however, at the time of the incident, the thunderstorm line was over IML; therefore, the 0201 weather observation from IML would have been more representative of the conditions the incident flight crew encountered during the incident. The IML surface observation at 0201 reported wind from 230° at 14 knots, thunderstorms, temperature of 22° C, dew point temperature of 19° C, altimeter setting of 29.90 inches of mercury. Remarks: automated station with precipitation discriminator, lightning distant all quadrants, temperature 21.7° C, dew point temperature 18.9° C, present weather identifier sensor is not operating, maintenance is needed on the system. SIGMETs 1C and 5C were valid for the area surrounding the incident site at the time of the incident. SIGMET 1C was issued at 0055 (valid through 0255) and SIGMET 5C was issued at 0155 (valid through 0355) and both warned of thunderstorm activity along the route of flight. SIGMET 1C warned of areas of severe thunderstorms moving from 250° at 30 knots with thunderstorms tops above FL450. Tornadoes, hail to 2 inches, and wind gusts to 60 knots were all possible in the severe thunderstorms discussed in SIGMET 1C. SIGMET 5C warned of areas of severe thunderstorms moving from 250° at 30 knots with thunderstorm tops above FL450. Tornadoes, hail to 2 inches, and wind gusts up to 60 knots were all possible in the severe thunderstorms discussed in SIGMET 5C. In addition, Delta Air Lines Meteorology issued TPs for thunderstorm activity (including along the incident aircraft's route of flight) at 0113 (TP CN12) and 0159 (TP CN13); however, neither TP CN12 or CN13 were sent to the incident flight crew. The dispatcher did send TP CN11, which was issued at 0049, to the incident flight at 0138, but TP CN11 did not contain information about thunderstorm activity along the flight route. As mentioned earlier, 14 CFR Part 121.601 would also apply here with regards to the requirement of the flight dispatcher to provide the PIC with additional meteorological information. Composite reflectivity is the weather radar data that goes into the ATC weather and radar processor (WARP). The weather is displayed via WARP for a view of weather targets on the controller's radar display at ARTCCs and was viewable by the ZDV controller. Composite reflectivity is also the weather radar data that goes into the weather radar information available at the Delta OCC and was viewable by the company dispatcher. The imagery is available to the user after all of the elevations scans from the weather surveillance radar-88D are complete, which takes about 5 to 6 minutes. Delta Air Lines pilots are required to maintain a lateral distance of 20 miles between individual thunderstorms when operating above 25,000 ft. A review of the composite reflectivity image that started at 0133 and was available to ZDV and Delta OCC at 0138 showed that the total gap distance between the areas of thunderstorm activity was 30 miles. The 0138 the composite reflectivity image, which was available to ZDV and Delta OCC at 0143, showed that the total gap distance between the areas of thunderstorm activity had reduced to 26 miles. The 0143 the composite reflectivity image, which was available to ZDV and Delta OCC at 0149, showed that the total gap distance between the areas of thunderstorm activity had reduced to 23 miles with the incident airplane between about 80 and 100 miles east-northeast of the thunderstorm activity. The 0149 composite reflectivity image, which was available to ZDV and Delta OCC at 0149, showed that the total gap distance between the areas of thunderstorm activity had reduced to 12 miles with the incident airplane about 55 to 75 miles east-northeast of the incident site. The 0153 composite reflectivity image, which was available to ZDV and Delta OCC at 0158, showed that the total gap distance had reduced to 9 miles between areas of thunderstorm activity with the incident airplane about 30 to 40 miles east-northeast of the incident site. Composite reflectivity images starting at 0158 and 0203, which were available to ZDV and Delta OCC at 0203 and 0207, respectively, showed that the total gap distance between thunderstorm activity had reduced to 7 and 4 miles, respectively. The flight crew consisted of two pilots and three flight attendants. The flight crew was based in Minneapolis, St. Paul, Minnesota. The incident occurred during the first leg of the third day of a four-day pairing. The captain, age 57, held a valid Federal Aviation Administration (FAA) airline transport pilot (ATP) certificate with a type rating for the Airbus A320. The captain held a first-class medical certificate with a limitation to have glasses for near vision, and he had his glasses for the inc