The FLARM collision avoidance system and why glider pilots built a traffic solution that works where ADS-B does not

FLARM is a collision avoidance system invented in Switzerland in 2004 that predicts aircraft trajectories rather than simply broadcasting position and speed. Built specifically for glider operations where aircraft circle in close proximity without transponders, FLARM has become essentially universal in European gliding and is expanding into helicopters, general aviation, and unmanned aircraft — filling a critical gap that ADS-B was never designed to close.

Why Doesn’t ADS-B Prevent Collisions Between Gliders?

ADS-B is fundamentally a broadcast surveillance system. It tells the ground and other equipped aircraft where you are, your altitude, and your velocity vector. That works well in the enroute environment and terminal areas where aircraft generally fly in straight lines at known speeds.

The critical weakness emerges when aircraft are turning, climbing, and descending unpredictably within a few hundred meters of each other — exactly what happens in a thermal. Two gliders circling in the same column of rising air, one at a thirty-degree bank and one at forty-five, have different turn radii and different climb rates. A system that only knows current position and velocity struggles to predict where those arcs will intersect.

There’s also an equipage problem. Many gliders, paragliders, hang gliders, ultralights, and drones carry no ADS-B equipment at all. They are invisible to ADS-B-based traffic displays.

How Does FLARM Work?

FLARM takes a fundamentally different engineering approach. Instead of broadcasting only current position and velocity, each FLARM unit calculates the aircraft’s predicted flight path for the next 18 to 20 seconds using GPS position, barometric altitude, and an onboard motion model.

That predicted trajectory is then broadcast to every other FLARM-equipped aircraft in the vicinity. Every unit simultaneously computes its own projected path and compares it against the projected paths of all nearby aircraft. When those predicted paths converge below certain distance thresholds, the pilot gets a graduated alert — starting at roughly 18 seconds before predicted closest approach, with the highest urgency level inside about 8 seconds.

This is the fundamental insight: predict the path, not just the position. In a thermal, FLARM compares arcs rather than vectors, recognizing that two projected circles will intersect in twelve seconds even when a traditional closure-rate calculation might not flag the conflict until much later.

What Are FLARM’s Technical Specifications?

• Range: approximately 3 to 5 kilometers, depending on terrain and antenna placement
• Update rate: one position broadcast per second
• Frequency: 868 MHz in Europe, 915 MHz in the United States
• Power draw: very low, suitable for gliders running on a single 12-volt battery and solar panel
• Alert system: graduated urgency indication showing relative bearing and altitude of threats

The hardware has gone through several generations. The original FLARM Classic was a small box with a GPS antenna and radio module. The current PowerFLARM adds ADS-B In reception, so pilots can see ADS-B-equipped traffic on the same display. PowerFLARM Fusion integrates additional frequency bands for compatibility with different regional standards.

How Widely Is FLARM Adopted?

In Europe, FLARM is essentially universal in gliding. Switzerland, Germany, France, and Austria — the Alpine soaring nations — adopted it first, and most European gliding federations now require or strongly recommend it.

Adoption expanded beyond gliders. Helicopter operators in the Alps adopted FLARM because they face the same problem: aircraft maneuvering in tight mountain valleys where ADS-B ground stations have spotty coverage. European general aviation pilots followed, and FLARM-compatible systems are now being integrated into unmanned aircraft operating in shared airspace.

The Swiss Aero Club published data several years after FLARM’s introduction showing a significant reduction in mid-air collisions and near-misses. The numbers were dramatic enough that neighboring countries accelerated their own adoption. In helicopter emergency medical services, FLARM adoption was driven by actual accidents in mountain valleys where see-and-avoid was failing despite good weather and experienced pilots.

Why Is American FLARM Adoption Lagging?

U.S. adoption has been much slower for several reasons:

1. Institutional momentum behind ADS-B. The FAA’s 2020 ADS-B Out mandate pushed every powered aircraft in controlled airspace to equip, creating a natural bias toward ADS-B as the traffic solution.
2. Different frequency allocations. The European 868 MHz band is unavailable in the U.S., so American FLARM operates on 915 MHz with a much smaller installed base.
3. Smaller gliding community. The grassroots pressure that drove European adoption simply doesn’t exist at the same scale in America.

Consider a specific scenario: flying a Piper Cherokee at 4,500 feet along a ridge in central Pennsylvania on a spring day with thermals popping and six gliders from the local soaring club in the air. Perhaps two carry ADS-B Out. The other four have FLARM only. Your ADS-B traffic display shows two targets. In reality, six aircraft share your airspace within three miles. That gap is not hypothetical.

What Are FLARM’s Limitations?

Proprietary protocol. For years, the collision avoidance algorithm and data format were not openly published, raising concerns about vendor lock-in and limited third-party development. FLARM Technology has gradually opened aspects of the protocol, and compatible devices from other manufacturers now exist. The Open Glider Network receives and redistributes FLARM signals for ground-based tracking. But compared to ADS-B — an open international standard defined by ICAO — FLARM remains an ecosystem controlled primarily by one company.

No ATC integration. ADS-B provides surveillance data to air traffic control and enables ground-based safety nets within a global standards framework. FLARM does not serve that function.

Fragmented cockpit picture. Ideally, one system would display ADS-B traffic, TCAS targets, FLARM-equipped aircraft, and drone remote identification signals together. PowerFLARM combines FLARM and ADS-B In, and some newer avionics platforms are building multi-protocol receivers. But most cockpits today still require pilots to manage multiple traffic inputs or accept blind spots.

Where Is This Technology Headed?

The rise of low-altitude unmanned traffic, urban air mobility concepts, and non-traditional aircraft operating below 18,000 feet is exposing exactly the gap FLARM was designed to fill.

The European Union Aviation Safety Agency (EASA) has been developing U-space, a framework for integrating drones and manned aircraft in low-altitude airspace, with FLARM-type cooperative surveillance as part of the conversation. In the U.S., the FAA’s beyond-visual-line-of-sight drone rules and the evolving advanced air mobility framework will eventually need to address the same multi-protocol traffic problem.

FLARM is not going to replace ADS-B, nor should it. The real question is whether the aviation world can reach a unified traffic picture combining both approaches — predictive trajectory modeling for maneuvering flight and position-velocity broadcasting for the broader surveillance network.

Who Should Equip With FLARM?

• Glider pilots: The case is closed. FLARM is essential equipment.
• Powered aircraft pilots in Europe: If you fly in areas with significant glider or ultralight traffic, a FLARM receiver is immediately useful given the large installed base.
• Powered aircraft pilots in the U.S.: The cost-benefit is harder to justify today due to fewer equipped aircraft, but the calculus changes each year as adoption grows.

Key Takeaways

• FLARM predicts flight paths 18-20 seconds into the future, comparing projected arcs rather than current positions — a fundamentally better approach for maneuvering aircraft in thermals and mountain valleys.
• ADS-B and FLARM solve different problems. ADS-B provides surveillance for ATC and works well for straight-line flight; FLARM addresses close-proximity, three-dimensional maneuvering among aircraft that often lack transponders.
• European adoption is near-universal in gliding and expanding into helicopters, GA, and drones, with documented reductions in mid-air collisions.
• U.S. adoption lags due to the ADS-B mandate’s institutional momentum, different frequency allocations, and a smaller gliding community — but the gap FLARM fills is growing more relevant as low-altitude traffic increases.
• The long-term goal is a unified multi-protocol traffic picture, and both EASA’s U-space and FAA advanced air mobility frameworks are moving in that direction.