Oh Hail!

Credit: Adobe Stock/CDSTOCK

An airliner pulls into a gate exhibiting dramatic structural damage to its nose cone, wing leading edges and windshields. A stressed passenger proclaims, “It was the tensest 10 min. of my life.” Pictures of the damage caused by hail to Delta Air Lines Flight 1889 on Aug. 7, 2015, while en route from Boston Logan International Airport (KBOS) to Salt Lake City International Airport (KSLC), were jaw-dropping and resulted in an official NTSB investigation that revealed decision points not uncommon for pilots attempting to navigate around lines of convective activity.

The dispatch release packet contained SIGMET 1C, which warned of areas of severe thunderstorms moving from 250 deg. at 30 kt. with thunderstorm tops above FL 450. Tornadoes, hail up to 2 in. and wind gusts to 60 kt. were all possible in these severe thunderstorms. The Minneapolis Center controller informed the flight crew of the large area of moderate-to-extreme precipitation at their 12 o’clock position and 130 mi. out, and advised them of the decisions of other flight crews headed in that direction to deviate well north or south around that area. The Delta flight crew chose to continue toward the gap in the weather, and both pilots believed they would be able to maintain at least 20 mi. lateral separation between each individual thunderstorm cell (as required by Delta procedures for thunderstorm avoidance). With the flight still more than 50 mi. from the thunderstorms, the gap in thunderstorm activity had closed to 12 mi.

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 Denver Center controller indicating that the hole was starting to close and that he did not think anybody should go through it behind them. However, the Denver Center controller did not provide this PIREP to the Delta flight crew.

At the time that the Delta flight entered the gap, the distance between the areas of thunderstorm activity had reduced to less than 10 mi. and was continuing to decrease. According to the incident captain, he also noted a temperature increase at cruise altitude, followed by a "rough ride" and "static discharge," and he was briefly unable to hear the radio due to the static interference. The captain decided to turn away from the weather, and the Denver Center controller provided him a heading to avoid the weather. As the airplane began to turn left, its 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. In addition, the wing leading edges, engine cowlings, parts of the aircraft skin, and especially the radome sustained surface damage. The nose cone damage also resulted in damage to the airborne weather radar antenna, which rendered the system unusable. The flight crew declared an emergency, diverted to Denver and requested an ILS approach due to the visibility problems through the shattered outer panes of the windshield. The aircraft landed safely on Denver's Runway 35L.

The NTSB determined that 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 resulted in the airliner's encounter with hail and subsequent airplane damage.

Hail Encounters

Despite onboard radar, aircraft are still experiencing substantial damage from hail encounters. For this report BCA examined an international aviation incident database (avherald.com) for inflight hail encounters between 2017 and 2019. We found a total of 20 incident summaries that reported substantial damage. The aircraft types involved included two turboprop transports, one regional jet, 15 medium jet transports ( Boeing 737 and Airbus 320, for example) and two widebodies. Seven of the incidents occurred in the climb phase of flight, seven during cruise and six during descent.

Seven of the incident reports indicated that the aircraft’s altitude at the time of the hail strike exceeded FL 300 (nine reports were missing the altitude information). This is problematic because upper levels of a towering cumulonimbus are far above the freezing level, where any form of precipitation is likely to be in the form of “dry” hail. Dry hail is a pellet that is completely frozen without a thin layer of water on the surface, and is a very poor reflector of energy. Thus, it can lead to a flight crew underestimating a storm’s intensity.

Windscreens and radomes take a battering in hail encounters. Thirteen of the 20 hail incidents caused substantial damage to the windscreen. Windscreens are designed under the damage tolerance concept to withstand much larger impacts from bird strikes, thus the damage is usually confined to the outer ply while the inner plies maintain sufficient structural integrity to keep the entire windscreen from imploding inward onto the flight crew. Nonetheless, a high-speed encounter with hail is likely to damage the outer ply of the windscreen bad enough to completely obscure forward visibility for the landing.

Eleven of the 20 aircraft experienced substantial damage to the radome. Damage to the aircraft’s nose can be severe enough to cause structural failure of the radome whose debris then causes a cascading chain of structural damage to other portions of the aircraft. This occurred on June 9, 2006, to Asiana Airlines Flight 8942, an Airbus 321-100, while en route from Jeju International Airport (RKPC) to Gimpo International Airport (RKSS) in South Korea. Hail pummeled the aircraft for 36 sec., causing enough structural weakening to the radome that it detached from the front of the aircraft. Components of the weather radar were destroyed due to exposure from the hail. The noise generated by the detaching radome seriously hindered conversations between the flight crewmembers and with the approach controllers. Fifty-two separate failure messages saturated the flight crew’s attention span. Not only were the captain’s and first officer’s airspeed indicators showing large differences from each other, but the aircraft also exhibited a banking tendency that further complicated control. While dealing with those emergency conditions, the aircraft was flying toward mountainous terrain. ATC provided instructions to climb to avoid getting any nearer to terrain.

Hail damaged windscreen
A high-speed encounter with hail is likely to damage the outer ply of the windscreen bad enough to obscure forward visibility for the landing.  Photo credit: Aviation and Railway Accident Investigation Board, Korea Ministry of Construction and Transportation

The Airbus’ cracked windshield seriously impeded the flight crew from having visual contact with the runway and making a landing. Flight 8942 executed two missed approaches due to difficulties encountered with speed and altitude control as well as the inability to see the runway environment through the severely cracked windshield. A more-experienced air traffic controller was then utilized to provide a PAR (precision approach radar) whereupon the aircraft was landed safely without further incident. The accident report lauded the captain for “completing his mission by safely landing the aircraft under the exceptional situation.”

Post-landing inspection of the aircraft found substantial damage to the radome, windshield, nose structure, wing leading edges, wing skins, engine cowlings, engine nacelles and horizontal stabilizer leading edges. Debris from the radome struck the bottom of the left wing, causing skin damage. Forty indentations were found on the forward pressure bulkhead. The left engine also was damaged by debris from the radome, causing rivet heads to break off. Total damage to the aircraft was estimated to be about $3.5 million.

Radome damage from hail
The radome of this Airbus A321 was destroyed during an encounter with hail. Debris from the radome struck the bottom of the left wing causing skin damage and the left engine. Photo credit: Aviation and Railway Accident Investigation Board, Korea Ministry of Construction and Transportation.

Editor's Note: This is the first of a three-part article on hail.