Digital Flight Control System

[daviddewilliams]

When the F-16 first entered the production line, conventional aircraft surfaces were combined with a number of advanced aerodynamic features and incorporated into a fly by wire flight control system (FLCS). This new concept in aerodynamics and highly developed FLCS produced a highly maneuverable multirole weapon system. The flying qualities of the aircraft enabled pilots to quickly develop a confidence in the aircraft's superior maneuverability. One of the F-16 squadrons from Hill Air Force Base praised the flyability of the F-16 when they returned from a very successful engagement called, "Desert Storm."

The block 40 and beyond F-16 is equipped with an advanced digital flight control system (DFLCS) which is a computer controlled, four channel, fly by wire control operation which will hydraulically position control surfaces. The pilot generates the electrical command through a side stick controller, rudder pedals and a manual trim panel. These signals are sent to the digital flight control computer (DFLCC). These pilot generated commands, along with signals from the air data system, flight control rate gyros and accelerometers are all processed by the DFLCC to become transmitted signals to the flight control surfaces.

This quadraplex redundant DFLCS has fewer components than the analog system, is highly reliable and has a quick and thorough built-in test (BIT) which allows a complete BIT run of the entire system in less than 30 seconds. In an effort to reduce the pilot's cockpit work load, the DFLCS incorporates various intelligent functions. The primary intellect feature is the installation and safe implementation of an automatic terrain following system.

Most of the components of the original FLCS were retained, however, the four flight control converters were deleted, along with the electronic component assembly. This resulted in reduced weight and volume requirements. The new or modified components are the flight control computer (FLCC), the flight control panel and the angle of attack (AOA) transmitters.

The most visible change to the user is the integration of the pilot fault list display, which permits reporting of FLCS faults in the same manner utilized to report avionic system messages. This has permitted the addition of a new autopilot mode and steering select. The new cockpit now contains: a flight control panel with a BIT switch, a digital back up switch, a terrain following flyup disable switch, an advanced modes switch and the steering select mode of the roll autopilot switch on the miscellaneous panel. Additionally, the pilot will receive cautions and warnings concerning the DFLCS on the new pilot fault list display.
The predominant benefit of the DFLCS is the growth potential it offers to future block aircraft. Only 70% of the available memory and real time have been utilized. Since the FLCS is so critical, a backup system has also been incorporated. This backup system provides all the basic control requirements for a "return to base" situation.

The backup mode provides fail operative redundancy for the DFLCS. To provide this mode of operation the system must assume that no hardware malfunction exists when the backup system engages. It is automatically engaged by the FLCC in the unexpected event three branches trip their watchdog timer. The same quad-redundant hardware is used by both the primary and the backup system, only the software is different. Once the backup system has been engaged, it can only be disengaged by a command request from the pilot.

[mor10]

I am glad to see that the converters were removed, as they could at times be really awkward to replace. Especially the two mounted in front of the air intake, as my 6’3” height made it impossible to find a good position to get at them. My assumption at the time was that these 4 converters were an after thought and they had to find a place for them anywhere they would fit. However, although they at times created more problems than they solved, are there any other additional power sources available now?

It was relatively common that the WOW switch failed, leaving the FLCS thinking it was still in the air, refusing to turn off power to the FLCS when the Pilot shut down main power after engine shutdown. This would lead to all four converter batteries discharging below the manufactures specifications and they had to be replaced and often discarded. The only way to stop them was to open up the panels and short two of the pins on the converter plug. I can’t remember if it was enough to do this on only one, or if all four had to be stopped manually. To make it easier for the crew chiefs to stop the converters discharging the batteries, I made a dummy plug where the appropriate two pins where shorted. It also had a nice red “REMOVE BEFORE FLIGHT” tag linked to it. All the crew chief then had to do was opening the panels, disconnecting the front plug and connect the dummy to the converter and it would stop instantly. This stopped crew chiefs trying to figure out which pins to short, often bending pins, forcing a replacement of the converter even if the batteries was saved.

[IDCrewDawg]

They fixed the flcs battery discharging problem after a jets brakes failed due to low flcs power and a batt fail, subsequetly the jet rolled off the end of the runway (don't know the tail number or dates). I know this as I read the findings a few years ago. I don't remember what the fix was.

For some of you fellas who work on the older blocks, when the jets are comming back from CCIP are you getting the new maintenance free batteries? This new battery is supposed to eleminate batt fails, as when you remove the battery you have to do a DC gen check every time it's disconnected.