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How far is 20 meters? RTH. 85 foot crash!

Discussion in 'General Discussion' started by Tenly, Aug 22, 2016.

  1. Tenly

    Mar 27, 2016
    Likes Received:
    Are you comfortable in your ability to estimate how far away from you 20m is? And how about an object that is also 10m above you? Or something 30m above? Is it less than 20m from you horizontally?

    The DJI RTH (Return To Home) feature works differently if the Phantom is within a 20m radius from the home point - regardless of altitude. It doesn't rise to the altitude you configured and travel to the home point before descending, it descends immediately and lands at its current location.

    This should not be news. It's fairly well documented and it's been discussed in other posts. I've even explained it to others in previous posts - yet I crashed yesterday because of it. Not because I wasn't aware of the different behavior - but because apparently I am a poor estimator of horizontal distance. The thought never even crossed my mind that I might be anywhere close to the 20m radius - but it turns out that I was 18m away from the home point and directly overtop a large tree.

    I was using Pix4dcapture and had just finished flying a double-grid mission over an old church with the intent of producing a 3D model of it for fun. At the same time, I was monitoring a friend that was flying for the 2nd time with his new P3P that he had purchased from me about 2 hours earlier. My double-grid mission finished successfully at a height of 97 feet (which provided about 10 feet of clearance over the the tallest trees in the area).

    The default action for Pix4dcapture at the end of a mission is to engage RTH. I saw and heard it engage as expected and for a brief moment I considered cancelling the RTH to take some more pictures of the sides of the church for better detail in the 3D model - but I thought I should probably check on my friend first and decided to just let the drone return home. 10 seconds later, I heard a whining and crunching noise and immediately used the CSC commands to kill the motors. It took about 3 more seconds before my Phantom hit the ground - but by then, I had already realized what had happened.

    Less than an hour earlier I had explained to my friend about the anomalies of the RTH feature both about the 20m radius landing immediately and also that if you apply inputs to the joysticks while the Phantom is still ascending to the RTH height (it stops ascending and returns home at the current height).

    I didn't crash because I was unaware of the different 20m behaviour. I didn't crash because I had forgotten about the 20m behaviour. I crashed because I am apparently so bad at estimating how far away 20m is that it never even entered my mind that I might be "close" to the home point.

    This made me think that perhaps DJI Go and the third party apps could do a little more to help prevent accidents like this in the future. Obviously it wouldn't be possible in every situation - such as lost connection, or even with Smart RTH triggering because of low battery...but <b>if the user engages RTH manually, or if an app engages RTH upon completion of a mission - it would be a nice, safety feature if the app looked at your current distance from the home point and advised/reminded you of the "different behaviour" if you are less than 20m away - and perhaps gave you (battery level permitting) an extra 5 or 10 seconds to cancel the RTH before engaging it.</b> For me, such a feature would have prevented a crash and I can easily see it preventing crashes for others in the future.

    DJI Go and AutoPilot clearly show you your distance from the home point on their in-flight screens, but not all of the 3rd party apps do so. I use Pix4dcapture so infrequently, that if it is there - I don't know where to find it. In fact, I wasn't even able to find where the current altitude is displayed in Pix4dcapture (on the screen after RTH had been engaged) before I had descended into the big tree below where I was hovering. I'm sure it was there somewhere - but as I said, I use that app very infrequently - and regardless as to whether the data is available or not - it would still be nice to have the extra level of protection I mentioned above.

    Some newer pilots may not be aware of the different behaviour of RTH when the Phantom is less than 20m away. Some - like me - may just be bad at estimating what 20m looks like. It would be a very easy feature to add to any of the 3rd party or DJI flight apps - and it could prevent crashes. I'll be formally adding it to the enhancement request lists for all of the apps I use - not because I think it will ever help me again - but because I think it will save others from making a similar mistake in the future. (@autoflightlogic: Please add this feature request to the list!)

    Surprisingly (and very luckily), the fall from about 85' up did much less damage than I would have expected, and most importantly, the camera/gimbal assembly survived unscathed. I'm not sure if that's because I was able to kill the motors before it hit the ground or if I'm just extremely lucky - but here's a picture of how I found it on the ground as I walked up to it.

    The upper and lower shell pieces had popped open on one of the arms and a crack had appeared close to one of the motors - but all 4 props survived and after I was able to pop the shell back into place with a couple of screwdrivers - I powered it back up and was amazed when all 4 motors started and it actually flew! Today, I flew 3 more times for 13-18 minutes each time (over a completely empty golf course) and all seems to be fully operational.

    I've ordered a new shell which arrives tomorrow. If any of you have performed a shell replacement in the past, I would greatly appreciate any tips, advice or things to watch out for during the repair - or links to any video, website or documentation that you found especially useful.

    The following pictures show the remaining damage after I was able to pop the shell back in place.

    This has been a public service announcement! :)

    Attached Files:

    #1 Tenly, Aug 22, 2016
    Last edited: Aug 22, 2016
    KyleMaxxUAV likes this.
  2. KyleMaxxUAV

    Approved Vendor

    Jan 4, 2016
    Likes Received:
    that's not fun :(
  3. joet

    Aug 15, 2016
    Likes Received:
    Sounds to me like the app you were using should not engage RTH if you are within 20m.. At the very least, it should be an option. It obviously should be able to discern the distance.
  4. Mark The Droner

    Aug 26, 2015
    Likes Received:
    IMO, RTH landing is not something one should allow to happen very often. I RTH all the time but I regain control 100x out of 100 and land manually. Maybe that would have helped in this case. Good luck with the repairs.
  5. Tenly

    Mar 27, 2016
    Likes Received:
    No - not at all - because that was my intention. Allow it to RTH until it got close and then take over and land manually. The accident didn't take place at the end, it took place 5-10 seconds after engagement.

    But the previous poster made a good point that the app should not have automatically engaged RTH knowing that it was within 20m of the home point - at least not without a warning. But even if RTH was selected as the "end of mission" action, I think that in cases like this, the 3rd party app should have set one last waypoint to take it back above the actual home point and THEN engaged the RTH rather than engaging within 20m and landing in an incorrect location.

    It could be argued both ways though. One could make an argument that the app does not say it will go back to the home point and land - it says that it will engage RTH. This would make it the responsibility of the pilot to understand what the consequences of RTH are - BUT, since the pilot doesn't have control - and sometimes they don't have advanced notice of where exactly the mission will end - I think the 3rd party apps need to be smarter. They need to consider these situations (which could be common when mapping nearby objects) and do the expected thing so long as it is safe.

    The app could have determined when I first clicked "engage" and it plotted out the grid - that the end point of the grid mission would be less than 20m from the home point - and then asked me how I would like to handle it - before the mission ever started!

    I think I might have initially been too willing to accept full responsibility for this crash - and I still acknowledge that there are a number of things I could have done to avoid the crash - but after a lot of thought, I now also think that DroneDeploy *should* have been able to foresee these types of situations and also should have built-in protection to help avoid them.

    And even DJI Go. Why have a different behaviour for when the drone is less than 20m from the home point? I see a small benefits in that alternate behaviour in a few isolated situations - but also other ways to accomplish the same benefit in those situations. The "land immediately within x metres of home point" should be a configurable option that can be toggled on/off in settings. I'll bet that its existence has caused far more harm than good.
    #5 Tenly, Aug 24, 2016
    Last edited: Aug 24, 2016
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