What happened
On 26 May 2003, an Airbus A321-231, registration G-MIDJ, was cruising at FL340 approximately 70 nm southeast of Vienna during a public transport flight from Cyprus to Manchester. While the crew had previously monitored thunderstorm activity via weather radar, they had switched the system off after observing what appeared to be benign returns ahead of the aircraft. Shortly after entering a region of high cirrus cloud, the aircraft encountered a sudden transition from light turbulence to severe turbulence and intense hail.
The impact of the hail was so loud that the crew could not hear the master warning audio when the autopilot disconnected. The aircraft experienced rapid vertical deviations, with rates of climb and descent exceeding 5,900 feet per minute. During the encounter, several passengers and cabin crew members who were not yet seated were thrown to the floor, though no injuries were reported.
Upon exiting the weather cell, the crew discovered that the commander's windscreen and the first officer's direct vision window had been severely crazed. The aircraft was subsequently descended to FL230 to comply with emergency procedures regarding cracked windscreens. Despite the damage, the crew determined they had sufficient fuel to continue to Manchester and landed safely.
The investigation
The AAIB investigation examined the flight recorders, weather radar data, and the physical damage to the airframe. The investigation found that the aircraft had sustained extensive damage, including dents to the radome, engine nacelles, wing leading edges, and the horizontal stabiliser. The radome's outer layer had been compromised, causing the inner honeycomb structure to implode.
Investigators also reviewed the meteorological conditions, noting that while the flight crew's radar showed minimal returns, a nearby Boeing 757 was able to clearly identify the thunderstorm activity on its own radar display. The investigation also looked into the limitations of the aircraft's weather radar system in detecting precipitation types like dry hail or ice crystals at high altitudes.
Findings
- The primary cause of the encounter was the failure to detect the full extent of the thunderstorm cell due to the weather radar being switched off and the inherent limitations of the radar in detecting certain types of precipitation at high altitudes.
- The weather radar was set to a downward tilt of 2 to 3 degrees, which, combined with the lack of active scanning, prevented the crew from identifying the severe weather ahead.
- The aircraft's radar system was unable to reliably detect ice crystals or dry hail at altitudes above 30,000 feet, leading the crew to believe the path ahead was clear.
- The crew was unaware of the full extent of the airframe damage, including the damage to the stabiliser and engine components, until after the aircraft had been vacated at the destination.