DJI obstacle avoidance: Bypass, Brake or Off, and where the sensors cannot see

Bypass, Brake and Off

The setting is in DJI Fly under Settings > Safety. The Mini 4 Pro manual calls it Obstacle Avoidance Action; the Mini 5 Pro manual lists it as Manual Obstacle Avoidance.

• Bypass turns on APAS (Advanced Pilot Assistance Systems). The drone keeps following your stick inputs and plans its path around obstacles, going over, under, left or right of them. If there is no way around, it hovers. Pressing the Flight Pause button on the remote controller makes it brake and hover for three seconds while it waits for your next command.
• Brake stops the drone and holds a hover when it detects an obstacle. What happens next is up to you.
• Off means the drone cannot sense obstacles at all. Dan Bayne of Droneblog says he uses Off when flying in tight spaces.

With Bypass you also choose between two Bypassing Options, Normal and Nifty. In Nifty mode the drone flies faster, more smoothly and closer to obstacles, which gives better footage, and DJI states plainly that the risk of crashing into them goes up. Nifty does not work properly when the drone's orientation changes quickly near obstacles, when it flies through narrow gaps such as tree canopies or bushes at high speed, near obstacles too small to detect, or with the propeller guard fitted.

The setting does more than pick a reaction. On the Mini 4 Pro the omnidirectional vision system only switches on in Normal or Cine mode with Bypass or Brake selected, and DJI's takeoff procedure for that drone includes setting one of the two before every flight.

When the setting does nothing

There are several situations where the drone will not avoid anything, whatever the menu says.

• Sport mode. Obstacle sensing is disabled in Sport mode on the Mini 4 Pro and Mini 5 Pro. On the Mini 4 Pro, Sport mode raises the top horizontal speed to 16 m/s, and DJI asks for at least 30 m of braking distance in calm air and plenty of room to manoeuvre.
• Flying faster than the sensors. Every sensor has an effective sensing speed. On the Mini 4 Pro that is 12 m/s forward, backward and sideways, and only 5 m/s upward and downward. The manual also reminds you that inertia means the drone needs room to stop, and that braking is your job.
• Night. The Mini 4 Pro and Mini 5 Pro manuals say the vision systems become unavailable when flying at night.
• ATTI mode. When the vision systems are unavailable and the GNSS signal is weak or the compass has interference, the drone switches to ATTI mode. It can no longer hover or brake on its own, and DJI tells you to land as soon as possible. The article on DJI compass and IMU calibration on this site covers what causes it.
• Vision positioning switched off. Under Advanced Safety Settings you can disable Vision Positioning and Obstacle Sensing. The drone then hovers on GNSS alone, has no omnidirectional obstacle sensing and no longer slows down automatically when descending close to the ground. On the Mini 5 Pro this only applies to manual flight, not to RTH, auto landing or the intelligent flight modes, and it switches back on after a restart.

The intelligent flight modes follow their own rules. On the Mini 4 Pro, MasterShots and QuickShots brake and hover when they detect an obstacle, and during a waypoint flight the drone brakes and hovers if avoidance is set to Bypass or Brake. Point of Interest bypasses obstacles whatever the setting is, as long as the vision systems are working. On the Mini 3 Pro, APAS is off during intelligent flight modes and also when you record at high frame rates: 1080p at 120 fps and 2.7K or 4K at 48, 50 or 60 fps.

What the cameras do not see

DJI's vision systems judge distance from camera images, so they need light and a surface with visible texture. The manuals list the situations where they do not work properly:

• large structures made of frames and cables, such as tower cranes, high-voltage pylons and lines, and cable-stayed or suspension bridges;
• obstacles with a small surface area, such as tree branches and power lines;
• plain single-colour surfaces (pure black, white, red or green), surfaces with no clear texture and surfaces with a repeating pattern, such as identical tiles;
• highly reflective surfaces, water and transparent surfaces such as glass;
• moving surfaces or objects;
• areas where the light changes often and sharply;
• very dark or very bright surfaces: below 10 lux or above 40,000 lux on the Mini 4 Pro, below 1 lux on the Mini 5 Pro;
• surfaces that strongly reflect or absorb infrared light, such as mirrors (the Mini 5 Pro manual adds asphalt).

Branches and wires are the obstacles you are most likely to meet. DJI's advice for RTH follows from this: the drone cannot avoid small objects such as branches or power lines, so fly into open space before you trigger RTH. If there are power lines or towers on the way home, set Advanced RTH to Preset with an RTH altitude above everything on the route.

APAS has its own warnings. It may not work properly over water or snow, near flight limits or inside a GEO zone, and DJI asks for extra care in very dark or very bright light (below 300 lux or above 10,000 lux on the Mini 4 Pro). On the Mini 5 Pro, when the light drops and the vision system is only partly available, the drone switches from bypassing to braking and hovering, and you have to centre the sticks before you can carry on. The Mini 5 Pro also has a forward-facing LiDAR, and it has its own limits: it does not detect obstacles with a reflectivity below 10% or reflective objects such as glass, and it does not work properly in light above 20,000 lux.

The weather and the sensor glass matter too. DJI says not to fly in rain or smog or when visibility is under 100 m. Before each takeoff, check that nothing covers the sensor windows, including stickers, and wipe off dirt or water with a soft cloth. Do not use a cleaning product that contains alcohol.

Blind directions

"Omnidirectional" on the Mini 4 Pro means forward, backward, sideways and upward. The manual's footnote says that system senses obstacles horizontally and above, while downward vision and the 3D infrared sensor on the bottom handle positioning and the distance to the ground. The precise measurement ranges are:

Older models can see less. The Mini 3 Pro only has forward, backward and downward vision. Its manual says APAS works only when flying forward, backward or down, and that there is no bypass or avoidance at all when it flies left, right or up. Answering a DJI forum thread about obstacle avoidance in the QuickShots Circle mode, DJI support wrote that the Mini 3 Pro has no left and right sensing and recommended keeping the path of the circle clear of obstacles. The Mini 3 Pro also has a Sideways Flight switch in the safety menu; with it disabled, the drone cannot fly left or right at all.

Before you rely on avoidance, look at the Sensing section of your drone's specs and find out which directions it covers, at what range and up to what speed.

Radar map, Vision Assist and collision warnings

Display Radar Map, in the same Safety menu, shows a live radar map of detected obstacles. The Mini 4 Pro also has Vision Assist, a small window with the view from the horizontal vision system in the direction the drone is moving. It shows a yellow collision warning when an obstacle is 2.2-5 m away and a red one at 2.2 m or less. The manual adds three limits: the view covers about 80 degrees, glass walls and thin objects such as branches, wires and kite strings do not show accurately, and the collision warning only appears while Vision Assist is shown in the small window.

Landing Protection

With avoidance set to Bypass or Brake, Landing Protection starts when you pull the throttle down to land. The drone checks whether the ground below is suitable. If it decides it is not, the Mini 4 Pro stops and hovers 0.8 m above the ground. Hold the throttle stick down for at least five seconds and it lands without obstacle sensing. The manuals also warn that downward vision may not work over water, so the drone may not avoid water below it when landing.

Which setting to fly with

For normal flying, the choice is between Bypass and Brake, as DJI's own takeoff procedure suggests. Brake is the more predictable of the two: the drone stops and you decide what to do. Bypass suits filming where you want the shot to keep moving past obstacles, with Nifty as the faster and riskier version. Off is for pilots who deliberately fly close to things and accept that nothing will catch a mistake.

Whatever you choose, the setting only helps in daylight, below the sensing speed, in a direction the drone can see and with obstacles big and textured enough for the cameras. Branches, wires and glass fall outside that, and so do Sport mode and night flights. In those conditions, keep your distance from obstacles yourself.