Italian investigators believe insufficient use of available weather information contributed to a Delta Air Lines Boeing 767-300ER’s flying into severe hail, and suffering substantial damage, shortly after departing Milan Malpensa.

The twinjet – bound for New York JFK on 24 July 2023 – encountered hail as it climbed through 13,000ft while following the DOGUB 6T departure pattern.

It sustained a shattered cockpit window on the first officer’s side and a punctured radome, along with 20 hits to the weather radar antenna, plus structural damage to the horizontal stabiliser, leading edges of both wings, both engine spinners, and several lights.

The 767’s crew initially sought to return to Milan but after the windshield cracked they opted for Rome Fiumicino instead, where the aircraft made an overweight landing.

Italian investigation authority ANSV states that standard forecast reports such as METAR and TAF showed weather instability, but nothing precluded a take-off.

Available satellite radar images indicated a “high probability” of encountering intense weather along the standard departure route, it adds, but these were not presented to the crew. Nor did the crew receive an updated threat plot from the operator’s meteorology department which indicated worsening conditions – although this covered too large an area to be practical.

At least eight other aircraft took off from Malpensa in the same 30min window as the Delta 767, either side of its departure, but ANSV points out that none of these other flights headed west or southwest – all were directed north or northeast.

When the 767 turned southwest to follow the DOGUB 6T pattern, the weather radar showed “significant returns”, says the inquiry, and the crew sought to take evasive action.

But while the most effective deviation would have been to the north, the presence of terrain from the Alps prevented such a manoeuvre, and forced the crew instead to bear to the left, and enter the hail zone.

ANSV says its efforts to determine the precise circumstances of the occurrence, particularly in relation to human factors, have been hampered by the absence of cockpit-voice recorder information.

None of the 226 occupants was injured. Four pilots were in the cockpit at the time, including a relief pilot and a line-check pilot assessing the captain and first officer.

Delta put a number of safety measures in place after the event, raising awareness about the importance of thunderstorm and hail avoidance. The airline is also upgrading weather radars on its 767 and 757 fleet with a 3D system which includes greater sensitivity, with completion expected in April 2027.

Scientific evidence suggests the potential of increased frequency and severity of meteorological hazards to aviation, including turbulence, icing and the likelihood of large hailstones, says ANSV.

Among the papers submitted to the recent ICAO Assembly was a proposal for the explicit inclusion of climate-related operational safety risks in future Global Aviation Safety Plan editions, given the concerns over atmospheric instability on long-haul routes.