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RTH Crash Flight Log Analysis
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fans52dac3f5
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Hi All,

I experienced a RTH malfunction while inspecting a roof recently which ended in having a P3P stuck 125ft up a tree.

Luckily the tree was climbed safely and the drone was undamaged, but I need help understanding the cause of the crash.

Flight Log: http://www.phantomhelp.com/LogViewer/4Y9F1RH0Z9340G1RL6QJ/

I had recognized the tree next to the structure as a potential hazard and to mitigate that risk I manually set the RTH Altitude at the start of the flight to well above the structure so that I could regain control and cancel RTH without the drone moving laterally.

The incident starts at 10m41.5s with a Downlink Restored message.

Immediately RTH begins Heading Alignment and seconds later (10m50.1s) starts RTH Cruise towards the home point

Around 10m54s it becomes clear to me on the ground that the drone is moving laterally and not ascending as programmed so I pull up and back hard from the tree, gaining a few feet of altitude and reducing speed from 20mph to 3mph.

At 10m56s the drone gently introduces itself to the tree.

According to this link https://forum.dji.com/thread-38269-1-1.html the drone should have ascended to the preprogrammed altitude before performing heading alignment.

According to the log, the heading alignment started the exact moment downlink was restored.

The log shows no mention of any attempt to ascend.

My main questions is this:

Why did the drone initiate the RTH Heading Alignment and RTH Cruise before ascending to the RTH Altitude?

Thank you for any insight, comments, or suggestions you might have.
2018-2-24
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Nigel_
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Flight distance : 388642 ft
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You cancelled the climb to RTH altitude by pulling the throttle stick down when it was above 20m altitude.   Read the manual if you didn't know you could do so.
2018-2-24
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straylight
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I have read the manual, thank you.

From my reading it seems that any throttle input, not just stick down, will cause the drone to immediately RTH at the current altitude.

Am I to understand that the drone will always disregard the RTH Altitude setting if signal loss occurs during throttle input above 20m?

I was thumbs-off-stick from when the controller started beeping the RTH warning up until it became apparent the aircraft was headed for the tree.

I have pulled the on-board flight recorder .dat files hoping to get more clarity into what the drone was thinking while disconnected but I haven't figured out how to read them yet.

2018-2-24
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Mark The Droner
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Am I to understand that the drone will always disregard the RTH Altitude setting if signal loss occurs during throttle input above 20m?

Yes - provided the AC senses the controller signal.  When you're on the edge of range, the control signal can become intermittent and can cause a host of problems which inexperienced pilots aren't equipped to deal with.  

I was thumbs-off-stick from when the controller started beeping the RTH warning up until it became apparent the aircraft was headed for the tree.

Since you had apparently lost the AC to controller signal, it's possible the AC was in RTH before you knew it and before you heard the controller beeping the RTH warning.  Meanwhile, the AC regained the controller signal, sensed the stick control input, and stopped the RTH ascent.  

Another possibility is your controller is out of calibration, but it's not likely.

This kind of crash has happened a number of times before - to other pilots.  
2018-2-24
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Labroides
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straylight Posted at 2018-2-24 15:51
I have read the manual, thank you.

From my reading it seems that any throttle input, not just stick down, will cause the drone to immediately RTH at the current altitude.

Here's the relevant section from the manual:
Aircraft will stop ascending and immediately return to the Home Point if you move the left stick if the aircraft reaches 65 feet (20 meters) altitudes or beyond during Failsafe.

Signal was lost at 10:43.7, RTH would have started 3 seconds after that.
Signal was restored at 10:47.2
Here's the relevant part of your flight data.
It shows you working the left stick when signal was restored at 10:47.7 (highlighted in yellow and pink).
That is what cancelled the climb part of the RTH operation.

Working close to buildings and trees is always going to require caution.
Losing signal and having RTH initiate in that environment is an additional complication.
The optimum way to fly such a mission would be to take things very slowly and ensure you are in position to maintain a clear line of sight for al parts of the mission, pausing and relocating the controller when necessary.
RTH issue.jpg
2018-2-24
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DJI Susan
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I'm sorry to read the crash. If the drone is still in the warranty period, please kindly start a case and send it in for diagnosis: https://www.dji.com/support
2018-2-24
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Nigel_
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straylight Posted at 2018-2-24 15:51
I have read the manual, thank you.

From my reading it seems that any throttle input, not just stick down, will cause the drone to immediately RTH at the current altitude.


"Am I to understand that the drone will always disregard the RTH Altitude setting if signal loss occurs during throttle input above 20m?"

No, it needs to loose the connection for 3 seconds and then receive a throttle input after that, which can't happen if there is no connection.

" I was thumbs-off-stick from when the controller started beeping the RTH warning up until it became apparent the aircraft was headed for the tree."

From a quick look at the log file, that statement doesn't look very accurate, but given the very short loss of connection maybe it is correct.

Your mistake was probably that you flew the drone out of sight, which meant that you lost the connection and then couldn't see what was happening so weren't able to respond quickly enough when you took back control.  It was you that guided it into the tree, there are 4 seconds where you could have been throttle up to avoid the tree, or if that wasn't enough then you could have just pressed the pause button and it would have stopped where it was while you took your time to work out how to get out of the situation.
2018-2-25
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straylight
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Thank you all for your insight.

@DJI Susan:

This drone is definitely outside the warranty period (I pre-ordered it immediately after it was announced).

However I still think there are some unresolved questions concerning this log that may warrant your interest.

@Labroides, @Nigel_, @Mark:

The log does indeed show that the throttle was below neutral for 200 milliseconds starting half a second after RTH was initiated.

However it also shows that the aircraft was in the "heading alignment" phase of RTH (10m47.2s) even before the throttle was touched (10m47.7s), which according to the manual should have happened after the ascend phase.

Am I reading that right?

Even stranger is that the GoHomeStatus was set to "cruise" before the signal was lost.

The signal lost event that happened at 8m33.9s is also suspect because the drone never even entered "heading alignment" or ascended and instead went straight for cruise.

An additional point that I would like clarified, does the remote controller start beeping the RTH warning after being out of contact for three seconds or does it start beeping after reconnecting and learning from the drone that it is in RTH mode?

Thanks again all!
2018-2-25
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Mark The Droner
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No ....

The first thing that happens during RTH is the AC spins around and points to the home point.  

Regarding your last question, your latter guess is correct - the controller won't warn you that the AC is in RTH until it receives the RTH data from the AC.

2018-2-25
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straylight
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@Mark

Quoting from https://forum.dji.com/thread-38269-1-1.html

"
“Master is commanding me to return home.” (When receiving the RTH command)
“Alright, I’m ascending to the preset RTH altitude! Nothing can stop me from ascending!’ (To the predetermined height)
‘Ok, now I’m adjusting my nose towards master.’ (Adjusts direction)
‘I’m on my way back now!’ (Returns to the Home Point)
‘I’m going to land now!’ (Descends after reaching the Home Point)
"

Is that post incorrect?

And the first time the signal was lost there was no heading alignment at all!

Is this step skipped when the drone is already pointing vaguely in the right direction?

I just want to make sure I understand this clearly.

Thanks!
2018-2-25
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Nigel_
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straylight Posted at 2018-2-25 12:45
Thank you all for your insight.

@DJI Susan:

"Even stranger is that the GoHomeStatus was set to "cruise" before the signal was lost."

That log variable had been left in "Cruise" after the last lost signal RTH that you cancelled.  Note that in that one it completed the heading alignment phase and began the climb before you got the signal back, you also cancelled that climb using the throttle before cancelling the RTH.

984ft seems rather high for an RTH altitude, thought you were limited to 400 in the USA...
2018-2-25
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Nigel_
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straylight Posted at 2018-2-25 13:26
@Mark

Quoting from https://forum.dji.com/thread-38269-1-1.html

Maybe it was correct at the time?

Best thing to do is find an open area and get some practice, find out how it works and how to control it.
You can use your car to block the signal to simulate loosing it.
2018-2-25
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Mark The Droner
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Hi Straylight,

There was a guy who had this same type of crash who posted a thread a while back.  In his flight, he flew his AC out across some oddly shaped finger lakes quite a distance rather low, then turned right and headed up another finger lake and ended up blocking the signal with an island and some pine trees.  He lost contact with the drone and it did not return.  He lost his drone but eventually found it and posted the video.  The video showed what happened.  The AC lost signal, came to a stop and hovered for the required 3 secs, and then turned.  It then rose a bit, but not to the minimum RTH height.  Then it headed straight for home but unfortunately hit the trees on the little island.  

The AC didn't rise first and then turn.  It turned first.  Then rose.  

We determined that as it rose, it sensed stick movement because it regained signal strength due to the increased height, and so abandoned it's rise and headed home at the given height.  The pilot couldn't see it and didn't have FPV either, so was trying to control it blindly which is the wrong thing to do.  Better thing to do is let go of the sticks and wait until you see it returning.  Best thing to do is turn the controller completely off because that eliminates the intermittent signal problem which might keep it from initiating RTH in the first place.  

Anyway, that's why we know the AC spins first, then rises, then heads home.

However, in the thread you linked, that situation is an RTH command from the controller, not a lost signal situation.  Is it possible it behaves differently with an RTH command?  I suppose it's possible but it seems unlikely.  I believe it rotates first, then rises, regardless.  

The video is probably still posted on here somewhere but it would be hard to find.  

I sympathize with you re this crash because I really don't care for the way DJI allows us to cancel the height rise with stick movement.  It's too easy to do it in error as you've done and as others have done.  The older P2s didn't have this problem.   We could move the sticks all we wanted and the AC would continue to rise.  We had to cancel RTH to stop the rise when flying with the P2s.  I find that method to be just fine.  

The other argument is we're not supposed to fly out of VLOS anyway, so it shouldn't matter - we can see what's happening and act as needed.  





2018-2-25
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straylight
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@Mark

Thank you for that example. Practical experience is better than the manual in this case.

I too think that DJI having programmed any amount of throttle input to completely cancel the fail-safe ascent is a recipe for disaster.

Especially considering that the RTH alarm doesn't sound until after it is too late to take your thumbs off stick.

A better solution might be similar to how the sticks behave during descent in RTH, as additive input.

This sort of roof inspection scenario where I can see it but the shallow angle causes the signal to be dicey compounds the issue, but now I'll at least know that if I even suspect signal has been lost I should go hands off.

The manual also says there should be some sort of chime when the signal is initially lost, I don't recall hearing it, but I'll definitely be keeping an ear out for it in future flights.

Thanks all!
2018-2-25
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Mark The Droner
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What you could do during a roof inspection flight is set your fail-safe to Hover instead of RTH.  
2018-2-25
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straylight
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Perhaps this is another instance of the manual being incorrect.
2018-2-25
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Mark The Droner
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Yeah I see that.  I have a P3P and I just don't remember the chimes on the disconnect/reconnect.  Could it be they're talking about the intial linking process?  Or my problem could be I'm half deaf and miss it.  Thanks
2018-2-25
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KedDK
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Mark The Droner Posted at 2018-2-25 14:13
Hi Straylight,

There was a guy who had this same type of crash who posted a thread a while back.  In his flight, he flew his AC out across some oddly shaped finger lakes quite a distance rather low, then turned right and headed up another finger lake and ended up blocking the signal with an island and some pine trees.  He lost contact with the drone and it did not return.  He lost his drone but eventually found it and posted the video.  The video showed what happened.  The AC lost signal, came to a stop and hovered for the required 3 secs, and then turned.  It then rose a bit, but not to the minimum RTH height.  Then it headed straight for home but unfortunately hit the trees on the little island.  

Many of them but would this eventually be the thread  you had in mind?
https://forum.dji.com/forum.php?mod=viewthread&tid=114480&page=2#pid987594
2018-2-25
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Mark The Droner
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No that's not the one I had in mine - the one I was thinking of was in a more remote location with very few structures and included a video of the crash ... close though!
2018-2-26
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