Post-flight investigation revealed that City of Edinburgh's problems had been caused by flying through a cloud of volcanic ash from the eruption of Mount Galunggung. Because the ash cloud was dry, it did not appear on the weather radar, which was designed to detect the moisture in clouds. The cloud sandblasted the windscreen and landing light covers and clogged the engines. As the ash entered the engines, it melted in the combustion chambers and adhered to the inside of the power-plant. As the engine cooled from inactivity, and as the aircraft descended out of the ash cloud, the molten ash solidified and enough of it broke off for air to again flow smoothly through the engine, allowing a successful restart. The engines had enough electrical power to restart because one generator and the on-board batteries were still operating; electrical power was required for ignition of the engines.