On the 14 July 1995 a F-14D test aircraft fitted with a GMAv Digital Flight Control System (DFCS) made 'a highly successful one and a half hour first flight at the Naval Air Warfare Center, Patuxent River. Designed in the late 1960s and fitted with an analogue Automatic Flight Control System (AFCS), the F-14 had undesirable flying qualities -primarily in the high Angle of Attack (AOA) and approach/landing portions of the flight envelope. These characteristics led to a number of out-of-control flight incidents and several aircraft losses. The DFCS, developed by GMAv's Flight Systems Division, was a form, fit, function replacement for the analogue system. The new system enabled additional control laws to be implemented which considerably improved aircraft safety. An extensive System Integration and Test Programme was performed both at Rochester and Patuxent River (PAX) prior to flight trials. At PAX this included many hours flying the DFCS in the F-14 simulator which provided very high confidence in the system before the flight test phase began. The first flight in July was followed by a six month flight test programme, during which DFCS performance was assessed within the aircraft in all flight conditions. Extensive instrumentation on the test aircraft, designated "230", together with data from the DFCS, enabled a vast number of parameters to be monitored in real time during the flight. The data was transmitted to the Real-time Telemetry Processing Station (RTPS) during each flight for the Engineering Flight Test Team. A typical team consisted of fourteen engineers from the Navy, GMAv and Northrop Grumman, who monitored the data throughout the flight.
Several years passed before funding constraints permitted the Navy to develop the Automatic Rudder Interconnect system within plans to equip the F-14 fleet with a new advanced digital flight control system (DFCS). Following further refinements during Navy flight evaluations at Patuxent River Naval Air Station in Maryland, the Navy implemented the DFCS with the ARI. The first F-14 deployments with the ARI occurred during the 1999 Kosovo operations, and glowing reports from the F-14 squadrons indicated that the new system was a success.
The Navy decided to incorporate the GEC Marconi Digital Flight Control System (DFCS) into all F-14 aircraft to significantly improve flight safety. The system was designed to protect against unrecoverable flat spins and carrier landing mishaps. DFCS also incorporated a lateral stick-to-rudder interconnect designed to improve less than desirable flying qualities in the powered approach configuration. Pilots agreed that with the DFCS the Tomcat was more manoeuvrable and had a more crisp response to pilot control inputs. The new system also improved performance and safety during carrier landings. The Foreign Comparative Test (FCT) demonstrated that DFCS drastically decreased the chance of entering out-of-control flight and improved the F-14's ability to recover, if a spin was entered. Departure from controlled flight had been a primary factor in 35 F-14 mishaps. Also significant is its ability to improve carrier approach line-up control addressing a problem often cited as a contributing factor in carrier landing mishaps. The incorporation of DFCS increases safety, both during "edge-of-the-envelope" manoeuvring flight and carrier landings.
The new the Digital Flight Control System [DFCS] provides enhanced manoeuvrability for the F-14. The DFCS control panel replaces the current AFCS panel in the front cockpit, the analogue system in use since the aircraft's inception. It contained the modified SAS switches, and also displayed maintenance codes for system failures identified during IBIT and in flight. The DFCS system lived up to its promise of enhanced controllability and performance in the high AOA regimes and in the landing configuration. However, the structural issue raised by the enhanced roll rates achievable with the DFCS is a potential factor affecting the crucial problem of F-14 fatigue life.
The F-14 DFCS began fleet introduction in July 1999 with the initial fleet release software version OFP 4.1.1 in F-14A squadrons. An aggressive transition schedule has seen the subsequent incorporation of the DFCS into all F-14A, B, and D variants. Every F-14 squadron has commenced or completed DFCS integration and the last AFCS squadron deployment was completed in April 2000 with the VF-102 Diamondbacks. The DFCS has been a tremendous success story. The system greatly increases departure resistance and provides enhanced recovery capability as well as significantly improves landing approach handling qualities. The F-14 operational and maintenance departments have been pleasantly surprised not just by the significant improvements in flying qualities of the DFCS, but also the improvements in the safety, reliability, and maintainability of the DFCS versus the older analogue AFCS it was designed to replace. This modification was carried out on 211 active duty and 16 reserve F-14 aircraft.
The final DFCS software version (OFP 4.4) released in August 2000 included improvements to the existing control laws for the automatic carrier landing system (ACLS), roll SAS control laws easing manoeuvring flight envelope restrictions, and expansion of system self-test fault reporting capability. Flight test of the new ACLS flight control laws was completed in April 2000 onboard the USS Enterprise (CVN-65) for both the F-14D and F-14A aircraft with very favourable results. The final ACLS configuration included hardware and software modifications that replaced the old pitch attitude command system in favour of a new vertical velocity or "h-dot" command system with integrated direct lift control (DLC) for smooth glideslope control. The new design even corrects many maintenance limitations of the old system including pitch feel switch failures and force link disconnects.
