The first F-100D (54–2121) flew on 24 January 1956, piloted by Daniel Darnell. It entered service on 29 September 1956 with the 405th Fighter Wing at Langley AFB. The aircraft suffered from reliability problems with the constant speed drive which provides constant-frequency current to the electrical systems. In fact, the drive was so unreliable that the USAF required it to have its own oil system to minimize damage in case of failure. Landing gear and brake parachute malfunctions claimed a number of aircraft, and the refueling probes had a tendency to break away during high speed maneuvers. Numerous post-production fixes created such a diversity of capabilities between individual aircraft that, by 1965, around 700 F-100Ds underwent High Wire modifications to standardize the weapon systems. High Wire modifications took 60 days per aircraft at a cost for the entire project of US$150 million. In 1966, the "Combat Skyspot" program fitted some F-100Ds with an X band radar transmitter to allow for ground-directed bombing in inclement weather or at night.