F-111D was the designation given to a more advanced version of the F-111. It was powered by a pair of Pratt & Whitney TF30-P-9 engines, each rated at 12,000 lb.s.t. dry and 18,500 lb.s.t. with afterburner. The aircraft were equipped with Mark II microprocessor avionics with improved air-to-air capability. This system was a first generation version of what later came to be known as a "glass" cockpit.
In addition, the F-111D was provided with Triple Plow 2 air intakes, which were intended to correct the F-111's seemingly chronic problems with compressor stall. The Triple Plow 2 intakes were mounted four inches farther from the airframe in order to improve the boundary layer "plow", and the translating cowl was replaced by a series of blow-in doors. These blow-in doors were a set of auxiliary inlets which enabled extra airflow to reach the inlet duct during takeoff or when the engine is at full power but the aircraft is moving slowly. They are normally sealed closed by spring-loaded doors which are pushed open by air pressure when additional air flow is needed.
The Mark II avionics system included 7 major components--an inertial navigation set and attack radar built by the Autonetics Division of North American Rockwell, an IBM computer system, converter and panels by the Kearfott Division of Singer-General Precision, Inc., an AN/AVA-9 integrated display set by the Norden Division of United Aircraft Corporation, a Doppler radar by the Canadian Marconi Company, a horizontal situation display by the Astronautics Corporation of America, and a stores management set by the Fairchild Hiller Corporation. The main forward-looking attack radar of the F-111D was the APQ-130, with MTI, Doppler beam sharpening, and illumination for radar-guided AAMs.
The F-111D was ordered on May 10, 1967. The first F-111D (68-0085) flew on May 15, 1970. It was equipped with the new P-9 engines but did not have a complete Mark II system. It was delivered to the Air Force on June 30, 1970, only one day after the lifting of the F-111 delivery hold order imposed after the F-111A crash of December 11, 1969.
The F-111D went through a rather protracted development cycle before it was deemed fit for service. There were difficulties in integrating the various complex electronic components with each other. The Autonetics attack radar needed several improvements in its initial design, and the Norden integrated display set required extensive changes. The radar problems required that the radar doppler unit be redesigned, which in turn caused interface problems with the Norden integrated display set. By late 1969, the Mark II system was still not ready. By mid-1970, the problemms with the Norden integrated display set were still not resolved. Several months of acrimonious arguments between Autonetics and Norden followed, Norden claiming that the IDS's original specification was beyond the state of the art.
Development problems with the F-111D's advanced avionics caused so many delays that the Air Force decided to acquire the simpler F-111E as an interim version.
It was not until November 1, 1971 that the first F-111D was delivered to the 27th TFW at Cannon AFB in New Mexico, the third TAC Wing to receive the F-111. This aircraft was the sixth F-111D produced (68-0090). It was equipped with a full Mark II avionics system, featuring one of Norden's early IDS productions. The initial operational capability with the 27th TFW was in September 1972. Eventually, the F-111D equipped the 522nd, 523rd, and 524th Squadrons of the 27th TFW.
Throughout the rest of 1972, TAC's few F-111Ds continued to be crippled by avionics problems. The horizontal situation display was prone to frequent failures, delivery of field ground equipment was late, and depot support was poor. There were excessive reliability and maintenance problems with the Mk II avionics, so severe that at times line mechanics were forced to resort to buying parts at Radio Shack. Operational readiness remained low all throughout 1973, and the abort rate of the F-111D was higher than that of other F-111s. It was not until January of 1974 that the F-111D was finally declared operationally ready.
96 F-111Ds were delivered between June 30, 1970 and February 20, 1973. The serials were 68-0085/0180. Beginning in 1991, surviving F-111Ds were retired to AMARC. The last F-111D went to AMARC in December of 1992. Substantial number of these machines remain there in storage.
The RF-111D was a proposed but unbuilt reconnaissance version of the F-111D with very sophisticated avionics. The program was abandoned in September of 1969 because of lack of funds. Cheaper RF-111As were to be acquired, which were in turn cancelled as well.