At The Drive‘s “War Zone” (and, before that, Gizmodo‘s “Foxtrot Alpha”) Tyler Rogoway has been regularly posting first-hand reflections on flying military jets. The latest, by Richard Crandall, covers the F-111 Aardvark, probably the leading all-weather strike aircraft of the Cold War. Designed around bleeding-edge 1960s avionics that would take the guesswork out of high speed, low-level navigation, the F-111 ended up being caught between the limits of its technology and the development of a newer generation of weapons. At the same time, its combat debut pointed to the limits of the entire system of navigational tools that military aircraft have been using ever since.
The F-111 was built to penetrate enemy airspace at low level regardless of rain, fog, or darkness. How low? Crandall recalls that “sometimes we would be flying low through the mountains of New Mexico or southwest Texas and the jet’s external rotating beacon would flash off the terrain that we were flying by and it would seem to be right next to the wingtip. Some aircrew would turn it off as it unnerved them.”
The airplane’s avionics were supposed to be interconnected to make the attack process practically automatic. While the F-111’s terrain-following radar (TFR) kept the plane 200 feet above the ground, the autopilot flew the plane from waypoint to waypoint. Upon reaching the target, the ballistics computer automatically released the F-111’s bombs when the plane reached the correct parameters (position, airspeed, delivery angle, etc.). Or, if the crew put it into manual mode, the computer showed the pilot a “continually computed impact point” (CCIP) that adjusted for wind and drift to indicate where the bombs would land if they were released at that moment. In the planned ultimate version of the F-111, the F-111D, all this equipment would be integrated in to a fully digital computer system complete with “glass cockpit” multi-function electronic displays. Even the other versions of the F-111 that flew with fully or partly analog systems included similar systems.
At the heart of the system was the airplane’s inertial navigation system (INS), a package of gyroscopes and accelerometers that provided an ongoing track of the airplane’s location. Because the INS drifted by about half a nautical mile every hour, the F-111 required regular position updates to keep itself on course. The problem of correcting for INS drift wasn’t unique to the F-111. Ballistic missile submarines updated their INS using a fix from the Loran-C radio or Transit satellite network. Tomahawk missiles used an onboard terrain-matching system called TERCOM.The Strategic Air Command’s variant of the F-111 carried an Litton ASQ-119 Astrotracker that took position fixes based on the locations of up to fifty-seven stars, day or night (as well as a more accurate INS that used an electrostatically suspended gyro).
The tactical F-111 usually took position updates by locking the non-TFR attack radar onto a pre-selected terrain feature with a known position and good radar reflectivity (called an offset aimpoint, or OAP). When everything worked, that gave the F-111 remarkable accuracy. As F-111 WSO Jim Rotramel remarked to writers Peter E. Davies and Anthony M. Thornborough for their book on the F-111, “if the coordiantes for various offset aim-points (OAPs), destinations and targets were all derived from accurate sources, the radar crosshairs were more likely to land neatly on top of them.”
Even if some parts of the system failed, the remaining elements were good enough to let the F-111 complete the mission. As Crandall explains:
Our inertial navigation system was nice, but we trained to use dead reckoning and basic radar scope interpretation to get to the target even without the INS. We had backups to the backups to the backup. INS dead? Build a wind model and use the computers without the INS. That doesn’t work? Use a radar timed release based on a fixed angle offset. We practiced them all and got good at them all.
The navigator could even use the raw, unprocessed data from the TFR, a truly terrifying process that Crandall calls a “truly a no-kidding, combat-emergency-only technique.”
Trouble with the F-111Ds avionics, which proved too ambitious for the time, meant that the US Air Force flew three other versions of the plane while trying the debug them. The first was the F-111A, which had an all-analog cockpit; the -E (Crandall: “basically an F-111A with bigger air inlets”), which added a few features; and the -F, which added more digital equipment but not the full suite of the features designed for the -D. The last of these was also upgraded to operate the PAVE TACK pod that let the plane drop laser-guided bombs (LGBs). LGBs put the terminal “smarts” for precision bombing into the weapon itself, with the bomb following laser energy reflected off the target from a beam in the PAVE TACK pod.
The US Air Force Museum in Dayton, Ohio, has a 360-degree photo of the cockpit of the F-111A in their collection here. It’s all switches and gauges, with no screens apart from those for the radar.
However, even with laser-guided bombs, the F-111F still needed the airplane’s avionics to get to the target, and those systems remained dependent on maps and geodetic information in order to ensure that INS updates and OAPs were accurate. In 1986, that would prove to be the weak link during the F-111’s combat debut.