Passage: Torpedo bombers first appeared immediately prior to the First World War. Generally, they carried torpedoes specifically designed for air launch, which were smaller and lighter than those used by submarines and surface warships. Nonetheless, as an airborne torpedo could weigh as much as 2,000 pounds (910 kg), more than twice the bomb load of contemporary single-engined bombers, the aircraft carrying it usually needed to be specially designed for the purpose. Many early torpedo bombers were floatplanes, such as the Short 184 (the first aircraft to sink a ship with a torpedo), and the undercarriage had to be redesigned so that the torpedo could be dropped from the aircraft's centerline.

Question: Airborne torpedos are not capable of being carried by what?
contemporary single-engined bombers