March through 22 March. On 24 March, an explosive eruption created an ash plume and a small pyroclastic flow. In March 1944, the United States Army Air Forces (USAAF) 340th Bombardment Group was based at Pompeii Airfield near Terzigno, Italy, just a few kilometres from the eastern base of the volcano. The tephra and hot ash from multiple days of the eruption damaged the fabric control surfaces, the engines, the Plexiglas windscreens and the gun turrets of the 340th's B-25 Mitchell medium bombers. Estimates ranged from 78 to 88 aircraft destroyed. The eruption could be seen from Naples. Different perspectives and the damage caused to the local villages were recorded by USAAF photographers and other personnel based nearer to the volcano. Future Large Vesuvian eruptions which emit volcanic material in quantities of about , the most recent of which overwhelmed Pompeii and Herculaneum, have happened after periods of inactivity of a few thousand years. Sub-Plinian eruptions producing about , such as those of 472 and 1631, have been more frequent with a few hundred years between them. From the 1631 eruption until 1944, there was a comparatively small eruption every few years, emitting 0.001–0.01 km³ of magma. For Vesuvius, the amount of magma expelled in an eruption increases roughly linearly with the interval since the previous one, and at a rate of around for each year. This gives an approximate figure of for an eruption after 75 years of inactivity. Magma sitting in an underground chamber for many years will start to see higher melting point constituents such as olivine crystallizing out. The effect is to increase the concentration of dissolved gases (mostly sulfur dioxide and carbon dioxide) in the remaining liquid magma, making the subsequent eruption more violent. As gas-rich magma approaches the surface during an eruption, the huge drop in internal pressure caused by the reduction in weight of the overlying rock (which drops to zero at the surface) causes the gases to come out of solution, the volume of gas increasing explosively from nothing to perhaps many times that of the accompanying magma. Additionally, the removal of the higher melting point material will raise the concentration of felsic components such as silicates, potentially making the magma more viscous, adding to the explosive nature of the eruption. The government emergency plan for an eruption therefore assumes that the worst case will be an eruption of similar size and