The British adopted "effective ceiling", meaning the altitude at which a gun could deliver a series of shells against a moving target; this could be constrained by maximum fuse running time as well as the gun's capability. By the late 1930s the British definition was "that height at which a directly approaching target at 400 mph (=643.6 km/h) can be engaged for 20 seconds before the gun reaches 70 degrees elevation". However, effective ceiling for heavy AA guns was affected by non-ballistic factors:
Along with other factors, how long must a target be engaged to have effective ceiling?
for 20 seconds

Estonia produces about 75% of its consumed electricity. In 2011 about 85% of it was generated with locally mined oil shale. Alternative energy sources such as wood, peat, and biomass make up approximately 9% of primary energy production. Renewable wind energy was about 6% of total consumption in 2009. Estonia imports petroleum products from western Europe and Russia. Oil shale energy, telecommunications, textiles, chemical products, banking, services, food and fishing, timber, shipbuilding, electronics, and transportation are key sectors of the economy. The ice-free port of Muuga, near Tallinn, is a modern facility featuring good transshipment capability, a high-capacity grain elevator, chill/frozen storage, and new oil tanker off-loading capabilities.[citation needed] The railroad serves as a conduit between the West, Russia, and other points to the East.[citation needed]
What percentage of electricity was produced with local oil shale in 2011?
about 85%

Season six began on Tuesday, January 16, 2007. The premiere drew a massive audience of 37.3 million viewers, peaking in the last half hour with more than 41 million viewers.
What year did the sixth season of American Idol first air?
2007