The vast majority of aerial attacks are by Antonov aircraft, Russian cargo planes that Khartoum is reliably reported to be adding to its current fleet. It is crucial here to understand what an Antonov "bomber" is: the Antonov is a Russian-made cargo plane, and in no way designed for use as an attack aircraft. There are no bomb sighting mechanisms; there are no bomb racks or bays; typically, crude (and cheap) barrel "bombs" are filled with scrap metal, unusable ordnance, and other shrapnel-producing materials, as well as an explosive medium---and are simply rolled out the back cargo bay. These bombs explode not with a large blast capability (and often do not explode at all), but have enough force to generate a hail of deadly shrapnel in all directions. Moreover, for protection against ground fire and anti-aircraft fire, the SAF Antonovs typically fly at altitudes of about 5,000 meters---far too high to permit any kind of militarily purposeful aerial targeting. They are not by nature a military weapon, but a tool for civilian destruction and terror.
One more post from Eric Reeves, an extract from his Jan 2012 update report on bombings.