Hail is a common problem because it is often unexpected – and almost impossible to see before you reach it, especially during a night flight. The way in which hail erupts from the cloud during storms can cause hail to spread in many directions, and the impact of icy hail at high speeds can be significant.
Common problems caused by hail include:
Cracked windshield. It is highly likely that the windshield will be cracked by a hailstorm, and although the damage is mostly cosmetic, as the inner windscreen remains structurally intact, this can cause visibility problems for the pilot and will precipitate an emergency landing in many cases. However, aircraft windscreens are designed to withstand much larger impacts and contain a heating layer to prevent damage to the load-bearing layer.
Damage to the Radome. The Radome on the front of the aircraft often takes a battering in an encounter with hail, and this can look dramatic, especially to those unfamiliar with aircraft components. However, the function of the Radome is only to be transparent to radar signals, and this plastic component has no bearing on the flight of the aircraft. If the Radome was to be significantly damaged or even removed from the aircraft, the only resulting problems would be an increase in noise level and an increased risk of damage to the radar antenna.
Hail entering the engine. Hail that actually gets into the engine could be a significant problem for an aircraft in flight, and could ultimately cause an inlet guide vane or compressor blade to bend or break. This is, however, highly unlikely, and the cowling of the engine is designed to protect the fuselage from any loose components or external objects. Aircraft are, of course, thoroughly tested to ensure that they are not susceptible to problems of this kind, and are designed to withstand the impact of birds and other objects hitting them at speed.