The EASA formula
The dashed black curve in the VLOS estimator comes from guidance published by the European Union Aviation Safety Agency (EASA). It is the only widely published formula that turns a drone’s size into a VLOS distance, which makes it a useful reference point for the acuity model.
The formula
EASA’s Guidelines for UAS operations in the open and specific category (Issue 03, 17 July 2025) compute VLOS from two parameters and use the smaller one.
Attitude line of sight (ALOS) is the farthest distance at which the pilot can still see the drone’s position and orientation, based on its size:
\[ \text{ALOS}_{\text{multirotor}} = 327 \times CD + 20\ \text{m} \qquad \text{ALOS}_{\text{fixed-wing}} = 490 \times CD + 30\ \text{m} \]
\(CD\) is the characteristic dimension in meters, which EASA defines as the drone’s maximum dimension. For a multirotor that is usually the prop-tip-to-prop-tip distance; for a fixed-wing it is usually the wingspan.
Detection line of sight (DLOS) is the distance within which the pilot can spot other aircraft in time to avoid them:
\[ \text{DLOS} = 0.3 \times GV \]
\(GV\) is ground visibility. EASA recommends at least 5 km, which gives a DLOS of 1,500 m (about 4,920 ft).
\[ \text{VLOS} = \min(\text{ALOS},\ \text{DLOS}) \]
EASA’s guidelines credit additional detail to a guidance document from Germany’s Federal Aviation Office (LBA). They also state that the guidelines are not legally binding.
The estimator plots ALOS only. For the drones on this page, ALOS is far below a 5 km DLOS, so DLOS would not change the result in good visibility. In poor visibility it can: at 1 km ground visibility, DLOS drops to 300 m (about 984 ft).
Straight-line vs. horizontal distance
EASA measures VLOS as the straight-line distance between the pilot and the drone. The estimator’s x-axis is horizontal distance, so the ALOS limit is converted at each altitude \(a\):
\[ d_{\text{horizontal}} = \sqrt{\text{ALOS}^2 - a^2} \]
That is why the dashed curve bends toward the y-axis as altitude increases. Drawing it as a vertical line would overstate how far the drone can go horizontally at altitude.
Worked examples
| Drone | CD | ALOS | Horizontal at 400 ft AGL |
|---|---|---|---|
| EVO II Pro V3 (multirotor) | 0.558 m | 202.5 m (664 ft) | 530 ft |
| eBee X (fixed-wing) | 1.161 m | 598.8 m (1,965 ft) | 1,923 ft |
The EVO II’s CD uses the largest dimension listed on retailer spec sheets. Measure your own aircraft if you can.
How this compares to Part 107
US rules don’t set a VLOS distance. 14 CFR 107.31 instead requires the pilot or visual observer to see the drone well enough to:
- know its location;
- determine its attitude, altitude, and direction of flight;
- observe the airspace for other traffic or hazards; and
- determine that it does not endanger people or property.
EASA’s two parameters map loosely onto these abilities. ALOS relates to item 2 (seeing attitude), and DLOS relates to item 3 (seeing other traffic). This is an interpretation for comparison, not an FAA position. Part 107 also sets a minimum flight visibility of 3 statute miles (about 4.8 km) in 14 CFR 107.51.
References
- EASA, Guidelines for UAS operations in the open and specific category, Issue 03, 17 July 2025. Section “Calculation of the VLOS distance.”
- 14 CFR 107.31, Visual line of sight aircraft operation.
- 14 CFR 107.51, Operating limitations for small unmanned aircraft.