What Does VLOS Mean to You? Redefining Visual Line of Sight in a Dynamic Sky

By: Colonel (ret) Bernie Derbach, KR Droneworks, 14 Jan 26

In the rapidly evolving world of Remotely Piloted Aircraft Systems (RPAS), few acronyms are as fundamental—or as frequently misunderstood—as VLOS (Visual Line of Sight). For the aspiring pilot studying for their Basic or Advanced exam, VLOS is a definition to be memorized. For the regulator, it is a boundary of risk. But for the professional aviator in the field, VLOS is something far more profound. It is a dynamic, living concept that shifts with the wind, the sun, and the physiology of the human eye.

To simply define VLOS as "keeping the drone in sight" is to ignore the complex reality of aviation safety. What does VLOS truly mean to you when the cloud deck lowers, or when the winter sun hangs low on the horizon? How does the definition change when applied to a pilot with 20/20 vision versus one battling fatigue?

This article explores the multi-dimensional nature of VLOS, integrating the rigorous definitions from Transport Canada (TC) and the Joint Authorities for Rule-making on Unmanned Systems (JARUS). We will delve into the often-overlooked TC Standard 923, examining how it quantifies the limits of human vision, and discuss why VLOS is never static—it is a personal and environmental contract between the pilot and the airspace.

1. The Regulatory Baseline: More Than Just "Seeing It"

Before we can deconstruct the concept, we must establish the regulatory foundation. In Canada, the Canadian Aviation Regulations (CARs) Part IX provides the legal framework.

The Transport Canada Definition According to CARs, Visual Line-of-sight (VLOS) means:

"Unaided visual contact maintained with a remotely piloted aircraft in a manner sufficient to maintain control of the aircraft, know its location and scan the airspace in which it is operating in order to detect conflicting air traffic and other hazards and take action to avoid them."

There are three critical pillars in this definition that every pilot must apply:

1. Maintain Control: You aren't just watching a dot in the sky; you must be able to orient the aircraft. If you can see the drone but can't tell which way the nose is pointing, you have lost valid VLOS.

2. Know Its Location: This implies spatial awareness relative to the ground and airspace boundaries.

3. Detect and Avoid: This is the most critical safety function. The pilot’s eyes are the primary sensor for detecting manned aircraft (Cessnas, helicopters) or obstacles (cranes, birds).

The JARUS Perspective JARUS, the international body that harmonizes drone regulations (and created the SORA - Specific Operations Risk Assessment methodology), echoes this sentiment but adds a layer of risk management. In JARUS documentation, VLOS is often described as the "primary mitigation" for air risk. JARUS defines the scope of VLOS operations not just by distance, but by the reliability of the pilot's visual scan. In the SORA framework, staying within VLOS allows an operator to claim a lower Air Risk Class (ARC) because the human eye is presumed to be an effective "Detect and Avoid" (DAA) system. However, JARUS also acknowledges that this system is fallible, which leads us to the variable nature of sight.

2. VLOS is Not Static: The Environmental Variable

One of the most dangerous misconceptions in the drone industry is that VLOS is a fixed distance—say, 500 meters. This is a fallacy. VLOS is an environmental envelope that expands and contracts based on the conditions of the day.

The Weather Factor The atmosphere is a lens, and rarely is it a clear one.

• Haze and Humidity: On a hot, humid summer day, atmospheric scattering can reduce contrast significantly. A grey drone against a hazy white sky disappears much sooner than it would on a crisp, cold autumn morning.

• Cloud Cover: Transport Canada regulations (Standard 922/901) and general aviation practices dictate minimum cloud distances for a reason. Clouds not only obscure the drone but create a backdrop that can camouflage aircraft. The "pop-out" effect—where a drone is visible one second and vanishes into a cloud background the next—is a physiological trap for the eye.

The Sun Position: The "90-Degree Quadrant" Trap The position of the sun is perhaps the single biggest disruptor of VLOS. If you are flying in the late afternoon and your drone tracks west, you are staring directly into the solar glare. Your iris contracts, reducing your ability to detect contrast. In this scenario, your effective VLOS might drop from 800 meters to 200 meters. A pilot who ignores the sun's position is flying partially blind. This is where regulatory standards stop being abstract rules and become survival guides.

3. The Science of Sight: TC Standard 923

This brings us to Transport Canada Standard 923 - Vision-Based Detect and Avoid.

Many pilots overlook Standard 923, assuming it only applies to complex Beyond Visual Line of Sight (BVLOS) operations. However, Standard 923 is a masterclass in defining the limits of human visual performance. It provides a scientific benchmark for what "good VLOS" actually looks like.

Standard 923 applies to operations where a Visual Observer (VO) is used as the primary means of "Detect and Avoid" (DAA) without the aid of radar or electronic systems (essentially, the human eye is the certified sensor).

How Standard 923 Quantifies VLOS: While a standard VLOS pilot doesn't need to memorize the text of Standard 923 for a basic flight, understanding its parameters reveals what Transport Canada considers "safe" visual detection.

1. Distance Limits: The standard limits the distance from the VO to the aircraft to 2 nautical miles (approx. 3.7 km). This suggests that beyond this range, the human eye—even under perfect conditions—cannot reliably detect conflicting traffic to ensure safety.

2. Minimum Visibility: It requires a ground visibility of at least 3 miles. If you are flying in 1-mile visibility, even if you can see your drone at 100 meters, you are failing the broader requirement of scanning the airspace for incoming traffic (which travels much faster than you).

3. Cloud Clearance: A ceiling of at least 1,000 feet AGL. This ensures there is enough "clean air" above the drone for a pilot to spot a descending helicopter or low-flying plane against the sky, rather than having it hide in the clutter of the clouds.

The Sun Rule (Crucial for Daily Application): Standard 923 explicitly prohibits the sun from being in the same quadrant as the drone. Specifically:

• The sun must be outside the 90° quadrant centered on the RPA location.

• OR, the sun must be at a high angle (45° or greater elevation).

What This Means to You: If Standard 923 says a Visual Observer cannot reliably ensure safety when looking within 45 degrees of the sun, neither can you. When you plan your flight path, you must apply this "Standard 923 mindset." If your flight plan requires you to stare west at 5:00 PM, you are violating the physics of reliable vision, even if you aren't technically flying under Standard 923. You are introducing a hazard that the regulations have explicitly identified as unsafe for DAA operations.

4. The Human Variable: From Person to Person

VLOS differs from person to person. It is subjective. The regulations say "unaided visual contact" (corrective lenses like glasses are allowed), but they do not account for the brain's processing power.

Visual Acuity vs. Visual Perception You might have 20/20 vision, but do you have 20/20 perception?

• The Stationary Observer: The human eye is designed to detect motion. A hovering drone is much harder to see than a moving one. This is why "staring" at a drone often leads to losing it.

• Empty Field Myopia: When staring into a featureless blue sky, the human eye naturally focuses at a resting distance of only a few meters. You literally stop seeing the distance. Experienced pilots use "saccadic movement"—scanning the horizon or trees and then snapping back to the drone—to keep their eyes focused at infinity.

Physiological State

• Fatigue: A tired pilot has a slower scan rate.

• Hydration and Oxygen: Even at ground level, dehydration affects visual performance.

• Age: As we age, our eyes require more time to adapt to changes in light (accommodation). A pilot over 50 moving their gaze from a bright tablet screen to a dark treeline will take significantly longer to "acquire" the drone than a 20-year-old pilot.

Conclusion on the Human Element: "What VLOS means to you" is a question of your personal limitations on that specific day. If you have a headache, or if you forgot your sunglasses, your personal VLOS range is shorter than it was yesterday. A professional pilot acknowledges this and shrinks their operational volume accordingly.

5. Practical Application: How Pilots Must Apply VLOS

So, how do we take these definitions, weather variables, and standards and apply them to a Tuesday morning flight?

Step 1: The Pre-Flight "Eye Calibration" Before you take off, perform a visual survey—not just of the ground obstacles, but of the light.

• Where is the sun? (Apply the Standard 923 test: Is it low and in my flight path?)

• What is the background contrast? (Will my white drone disappear against those white cumulus clouds?)

• Establish "Hard Limits": Pick visual landmarks (a tree line, a road). Decide, "If the drone passes that tree, I am turning back, because the lighting beyond that point is poor."

Step 2: Dynamic In-Flight Monitoring VLOS is an active task. You are not watching a TV screen; you are scanning a 3D volume.

• The Scan Cycle: Drone -> Horizon -> Airspace -> Tablet -> Drone.

• If the weather changes (e.g., a cloud layer moves in and drops the ceiling), your VLOS definition has changed. You must immediately reduce your range. "Legal" doesn't always mean "Safe."

Step 3: Crew Resource Management (CRM) If you are using Visual Observers (VOs), you must brief them on their VLOS. A VO standing 500 meters away looking into the sun is useless.

• Use Standard 923 as a guide for your VOs. Ensure they are positioned so they are not looking into the sun.

• Establish communication protocols. If a VO loses sight, the operation pauses immediately.

Conclusion

"What does VLOS mean to you?"

It means that Visual Line of Sight is not a static circle on a map. It is a fluctuating bubble of safety that expands and contracts with the environment and your own physiology.

It means respecting Transport Canada Standard 923 not just as a rule for complex operations, but as a scientific warning label about the limitations of the human eye against the sun and weather.

It means understanding that the JARUS and TC definitions are the bare minimum—the starting line, not the finish line. The true definition of VLOS is the discipline to say, "I can see it, but I can no longer ensure the safety of the airspace around it," and bringing the aircraft home.

As pilots, we don't just occupy the sky; we share it. Maintaining a robust, honest, and dynamic VLOS is our rent for that privilege.

References

1. Transport Canada. (2019). Canadian Aviation Regulations (SOR/96-433). Part IX - Remotely Piloted Aircraft Systems.

2. Transport Canada. Standard 923 - Vision-Based Detect and Avoid. [Online]. Available at: https://tc.canada.ca/en/corporate-services/acts-regulations/list-regulations/canadian-aviation-regulations-sor-96-433/standards/standard-923-vision-based-detect-avoid

3. Transport Canada. Standard 922 - RPAS Safety Assurance.

4. Transport Canada. (2019). TP 15263 - Knowledge Requirements for Pilots of Remotely Piloted Aircraft Systems.

5. JARUS. (Joint Authorities for Rulemaking on Unmanned Systems). Specific Operations Risk Assessment (SORA) - Package.

6. JARUS. guidelines on SORA - Annex B - Integrity and Assurance Levels.