An alarm was blaring.  It was loud and unfamiliar.  My drone was well out over the lake, a couple hundred feet high.  I was standing on the shore, trying to make an immediate assessment of what was going on.  I’m used to hearing a low battery warning.  I have the drone set to alert me at 30% battery remaining.  That’s still about 7 or 8 minutes of flight time to finish up and think about landing.  But this wasn’t that familiar “fries are done” sort of beeping.  Looking at the remote, I saw the battery indicator illuminated red.  On the screen, a warning told me that the remote battery was critically low.  I wasn’t sure how much time I had to get it back before my controller died, leaving me with a lost link.  In an airplane, certain circumstances warrant an emergency descent.  Basically, it’s the fastest yet safest path to the ground.  I think I executed an emergency descent with the drone, because I wasn’t wasting a second bringing that thing back to the ground.  I know that in a lost link situation, the drone should “Return To Home;” that is, fly back and land on it’s own at the point it took off.  Or wait.  Maybe it hovers until the battery is low and then returns to home.  Or maybe it just lands right where it is.  Which would be over the water.  In this case, I got the drone on the ground without incident, but it made me think about the systems and how well I knew them and prepared for them.  Let’s walk through the scenario and dig deeper into the Return To Home Feature.  These particular procedures are for the DJI Phantom 4 at the time of writing, so keep in mind that this is for illustration purposes, not a user manual.

 

Return To Home

When the drone takes off and the GPS and compass are on-line (a normal takeoff), that home point is recorded.  Pressing the Return To Home (RTH) button on the remote will automatically bring it back to that point and land.  It’s a great feature that can get you out of a number of situations, but a little forethought is required to make it successful.  A specific height for the drone to fly as it returns is selected in the settings.  I have mine set to 40 meters (131 feet).  This is enough height to clear most obstacles before beginning a descent over the home point.  What this means, though, is that I shouldn’t just take off anywhere.  I recently took off near some high pine trees.  It was a hot day, so I was setting up under there trees, in the shade.  The tree canopies were high enough for me to take off and get significant height before maneuvering out into the clear, but if a Return To Home had to be executed, the drone would descend through those nice shady canopies, likely to disaster.  When I launch, I make sure that I have a nice clear path straight up to 40 meters and beyond, with a nice radius around it to allow for a margin of error.

DJI Go App Return To Home

This is the popup when selecting Return To Home from the button in the app.

 

Too Close To Home

Unless you read further into the inner workings of Return To Home, you might have a false sense of security thinking that RTH will save you anytime you need.  Even with a low battery, the drone will automatically begin a RTH procedure before it gets too low to make it back.  Nothing to worry about, right?  Except this little line in the procedure: “Aircraft automatically descends and lands if RTH is triggered when the aircraft flies within a 20 meters (65 feet) radius of the Home Point.”  I seemed to remember something about that line when that alarm was blaring out over the lake.  Basically, if the drone is flying within 65 feet of my location, it won’t actually return to the home point.  It just lands right where it is.  Not good over the lake.  At the time, I couldn’t remember the exact distance to put it all together.  The lesson here?  First off, I’lll remember 65 feet. Thankfully, the flight controller shows the distance between me and the drone. Second, I’ll be aware that when I’m within that radius, my Return To Home safety net might not be a safety net – depending on what’s under me.  If I’m out over water, I’d do well to be more than 65 feet out.

Lake Oconee

This was the shore I was standing on, flying out over the lake.

 

Loss Of Control

Back to the original scenario.  My remote battery is critically low and the drone is out over the lake.  What happens if the battery dies?  Well, that’s effectively the same as a lost link.  On the Phantom 4, after three seconds of broken communication between the drone and controller, a Return To Home will be executed.  That’s comforting for the most part, but now that we know a little more about the nuances of RTS, consider this scenario.  I begin to fly the drone back in but halfway there, the remote battery dies completely.  I’m now within that 65 foot radius of immediate landing, and I’m still slightly over the water.  My remote is dead, and I have nothing but automation to bring my drone back.  What does that automation do?  It lands on the water.  A better solution?  When the emergency first began out over the lake, had I verified that the drone was further than 65 feet away and then simply activated the Return To Home button, I would have had the confidence that the drone would ascend to 40 meters, fly back to the home point, and land on it’s own.  The remote, at that point, could die whenever it likes.  Here is the whole procedure in checklist format:

  1. Verify Return To Home altitude setting is adequate to clear local terrain
  2. Verify home point (launch point) is clear above with a safe radius around it
  3. When flying within 65 foot radius, minimize flight over hostile landing terrain
  4. In the event of critical remote or drone battery, fly to or verify more than 65 feet away before activating RTH

 

Take a watch at the video I captured during the emergency descent:

Conclusion

This is just one scenario using a specific drone.  Please consult your particular flight manual to make sure you are up to date on the latest procedures, programming and nuances.  In the commercial drone test, certain questions refer back to a drone’s particular flight manual.  While airplane flight manuals tend to have many of these scenarios addressed in a checklist format, you might have to develop your own checklists for your drone like the one I did above.  As with any troubleshooting, your best line of defense is knowing the systems thoroughly.  I tend to fly manually most of the time, skipping any auto takeoff or auto land features.  But I do like to test and monitor my Return To Home from time to time, just to make sure I’m on top of it.  I’d encourage you to go out and spend a little time with your Return To Home settings and even testing things like the lost link and 65 foot radius RTS procedures.  A word of advice, though: find a nice, clear, open area.  Not something like a lake, but something you can land on.

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