Fly-By-Light (FBL)

What Is Fly-By-Light?

Fly-By-Light (FBL) is an advanced flight control architecture that uses fiber-optic cables — rather than the copper electrical wiring used in fly-by-wire systems — to transmit pilot control inputs and sensor data between cockpit controls, flight control computers, and actuators. Light pulses traveling through glass fibers are inherently immune to electromagnetic interference (EMI), lightning-induced transients, and high-intensity radiated fields (HIRF) — vulnerabilities that all electrical fly-by-wire systems must address through expensive shielding and redundancy.

FBL represents an extension of the fly-by-wire philosophy pioneered in systems like the fly-by-wire system of the Airbus A320 family. While FBL has been extensively researched and demonstrated, it has not yet entered commercial production due to challenges in high-speed optical signal processing, the cost of ruggedized fiber-optic connectors, and the maturity and reliability of established fly-by-wire systems.

How It Works

In an FBL system, sensors convert physical measurements (stick position, pressure altitude, angle of attack) into light signals using electro-optical converters. These signals travel through single-mode or multimode fiber-optic cables at the speed of light to flight control computers, which then command hydraulic or electro-hydrostatic actuators via optical-to-electrical converters at the actuation end.

• Bandwidth advantage: Fiber-optic cables can carry data at rates exceeding 100 Gbps — orders of magnitude beyond the ARINC 429 (100 kbps) or AFDX (100 Mbps) data buses used in current glass-cockpit aircraft.
• Weight reduction: Fiber-optic cables weigh approximately 70% less than equivalent copper wire bundles. On a large transport aircraft, replacing all signal wiring with fiber could save 200–500 kg (440–1,100 lb).
• EMI immunity: No electromagnetic emission means FBL aircraft would be less detectable by radar — a key driver for military FBL programs, particularly the U.S. Air Force's Advanced Tactical Fighter program in the 1980s.
• Integration with composites: Composite materials used in modern airframes are non-conductive, making it difficult to use the aircraft structure as an electrical ground return — a problem FBL avoids entirely.

Key Examples

The Boeing X-36 (1997) and the NASA F/A-18 Systems Research Aircraft (SRA) were among the first aircraft to fly with fiber-optic flight control signal transmission. The Eurofighter Typhoon uses a partial FBL architecture for its digital flight control system's sensor data buses. Rockwell Collins demonstrated a full FBL architecture on a modified Lockheed C-130 in 2003 as part of an USAF research program, achieving MTBF (Mean Time Between Failures) exceeding 20,000 hours for fiber-optic harness assemblies.

Aircraft Examples

• Eurofighter Typhoon: Partial FBL for sensor data transmission; primary flight control commands remain fly-by-wire electrical.
• Boeing F/A-18E/F Super Hornet: Fiber-optic data buses for MIL-STD-1773 avionics interconnect, separate from primary flight control wiring.
• Future commercial programs: Airbus has explored FBL for the A220 family successor; Boeing's New Mid-Market Airplane (NMA) concepts included FBL architecture studies before the program was suspended.
• Unmanned Aerial Vehicles (UAVs): The General Atomics MQ-9 Reaper and similar MALE UAVs use fiber-optic connections within the aircraft to isolate sensor payloads from flight-critical EMI-sensitive avionics.