HK-VTOL refers to the upgraded Skynet twin-engine airborne Hunter-Killers. Derived from the original HK-Drone, it retains the form and a similar maneuvering system but on a much larger scale. Sleeker and more organic in appearance than the HK-Aerial the VTOL is an agile hunter with a single underslung Plasma cannon, the HK-VTOL is fearsome and terrifying to behold. Often operating in support of ground sweeps by swarms of Series 888endoskeletons, they are one of the more dangerous targets for Resistance fighters to engage.
HK-VTOLs can be brought down by shoulder fired surface to air missiles, captured plasma weaponry, and even anti-materiel rifles. 
Continually engineering upgrades to the original design, the latest air unit is a sleeker, more organic design powerful enough to carry a Boeing 747 jet engine in a cargo sling.
The HK-VTOL is another design variant in a long line of VTOL capable craft manufactured or co-opted for use by Skynet. The massive tri-thruster units employed (and the front of the HK) as depicted in the first two film of the franchise dwarf the sleek styling adopted in the TSCC timeline, but show a clearly distinguishable design trend.