Conformal fuel tanks (CFTs) are additional fuel tanks fitted closely to the profile of an aircraft which extend either the range or "time on station" of the aircraft, with a reduced aerodynamic penalty compared to external drop tanks.
Conformal fuel tanks have the disadvantage that, unlike drop tanks, they cannot be discarded in flight because they are plumbed into the aircraft and so can only be removed on the ground. As a result they may impose a drag-penalty on the aircraft, even when the tanks are empty, without any benefit. They can also impose g-load limits and they add a slight, though negligible, weight gain to the aircraft even when empty. While the weight gain and drag penalty are always present, g-load limitation is not always an absolute issue. The CFTs on the F-15E actually allow the same maneuverability without g-limitations.
Other advantages CFTs provide is that they can free up hardpoints that would normally be used to carry weapons or fuel tanks and that they do not significantly increase an aircraft's radar cross-section.
F-15C entered service with CFT capability. Initially known as FAST packs (Fuel And Sensor Tactical), each unit carried an additional 849 US gallons (3,213.8 L) of fuel, while retaining hardpoints for four AIM-7F Sparrow missiles or bombs, some on the FAST packs. They were first tested on the F-15B in 1974. All U.S. F-15Es, and Strike Eagle export variants such as the IAF and Singapore models, are fitted with CFTs under the wing outside the engine intake and require modification to fly without them. The FAST pack was originally intended to carry a navigational and targeting infrared sensor system (thus "Fuel And Sensor"); however, the F-15 simply began carrying LANTIRN pods for ground-attack missions instead.
Israeli F-16I Sufa, based on the F-16D Block 50/52+, with 2 attached CFTs on top of the aircraft near the wing root and 2 wing-mounted drop tanks.
Export aircraft for Greece, Chile, Israel, Poland, Turkey, Singapore, Morocco, Egypt and the UAE all plumbed for carriage of two CFTs mounted on top of the aircraft near the wing root. Each is capable of holding 450 US gallons (1,703.4 L)
Conformal fuel tanks are mounted above the wings to replace the drag of underwing tanks. Combined they carry 3,500lbs of extra fuel, while adding extra lift and expanding combat radius by 260nmi with a small transonic acceleration penalty.