REALLY DANGEROUS BUG, IF CONFIRMED: RTH Failsafe for lost signal.

[Lukappaseidue]

I am sorry for errors on translation: If something is not clear, please tell me:
This post is also on DJI official forum.

I do not want to create alarmism but it is right that the circumstance is taken into account both by the MAVIC 2 pilots and by DJI whom I am forwarding this message to.

I'll tell you what happened to me with two mavic Zoom (but I read in the network of other cases similar to mine)

In early October, purchase the Mavic 2 Zoom from a well-known shop.

I update drone and remote controller to the most recent version of that time, v. 1,000,000 and installed the latest DJI GO V4 application for Android (v. 4.3.2.) on my XIAOMI MI PAD 4 LTE 4gb / 64gb

I turn on the AIRCRAFT and RC, made the pre-check list (RTH at 50 meters, RTH in case of loss of signal, battery status and cells etc. etc.) and after hooking 18 satellites start flying without any anomaly.

At a certain distance from me (but no long range), I lose the signal between Radio and Drone and on the display of the radio control, the word "Connecting" appears to me.

After a few seconds, seeing that nothing happens, I start to move and hang up the connection. The Mavic had stood still in the air for a good 40 seconds. It never started the Return To Home. Obviously the application was (as always) configured to return to the take-off point and not hovering.

I landed the drone, immediately I go to analyze the flight data and I am disconcerted, since having seen that the Mavic 2, without any connection with radio control, has NOT STARTED the RTH. It was me, as soon as the connection was reestablished, to activate the RTH.

Since the drone was less than 6 days from the purchase, I went to the dealer who replaced it with another sealed one.

I take the new drone, I upgrade to the FW 1,000,000 and yesterday, with a day without the sun, I finally managed to fly.

All apparently working, apart from an error message VPS Obstacle avoidance sensors errors, immediately gone ... however I was flying in P mode (as always) and without any other automation.

Taken up a lot of courage (mindful of the previous experience), I bring the drone in an uninhabited area, about 35 meters away from me and about 40 meters high and I decide to test the FAILSAFE of Mavic 2 for loss of signal, so I turn off the radio control: in this case, like all drones, the Mavic 2 would have to return to the starting point: but in reality it was not. At first the Mavic 2 turned towards my direction and started to approach the take-off point (and so far everything is ok); then suddenly it stopped, turned around 15 ° and moved from a take-off point; then it turned again and started to rise rapidly. At that point I immediately switched on the remote controller and gained control over the drone: As soon as the connection appeared on the display appeared the word gohome and then also on the tablet, with 19 hooked satellites that I had left shortly before, at take-off. On the display map the H was correctly shown and placed; in all this few time the drone had already reached the height of 125 meters, although I had set the RTH to 50 meters (I also checked after landing and the set value was always 50 meters). There was no sun and therefore no interference with the sensors.

I can understand that at most it may seem that I have made a madness (and maybe it was) but according to me, in a drone the first thing that must work, without any doubt, it is the FAILSAFE RTH.

That is, if I was flying a little more distant (but respecting the limits) and suddenly for any reason, if the connection between drone and radio control had been lost, what would have happened?

After what has happened to me, of course, I will not be able to fly calmly.

At this point, it would be advisable to verify that this is a (serious) bug in the firmware or software and especially to bring it to the attention of DJI so that, if confirmed that it is a bug, fix it.

I fly always with more than 14 satellites
checking RTH height
drone behaviour in case of lost signal
the "H" home point is well recorded and no interferences near the place
I also check sensors status, health of the battery
The issue occur randomly amd only when the SIGNAL CONTROLL IS LOST, and the only way to check what the drone do in case of signal lost, is switching off the remote controller
Its all.
The strange was that, when switched on the remote controlle, the GOHOME message appeared on remote controller display and device too, and the drone start return home. So only when signal is lost, RTH is not so safe

here my logs

RTH FAILURE - Google Drive

@sar104 , @BudWalker

 

[DanMan32]

I can't explain your first experience, other than you might have had Failsafe on Hover when you thought it was on Return Home.

The other part might be able to be explained: After signal loss, the AC will retrace its steps for 60 seconds. If it still doesn't get its signal back, then it will return home as you'd expect.
If it does get its signal back within the 60 seconds, then it will hover and wait 10 seconds for you to tell it to do something or cancel RTH. If after those 10 seconds it receives no input from you, then it will resume going straight home.

Please read the section of the manual regarding RTH thoroughly, as it can be confusing.
One point of confusion that gets everyone is that if the AC is not far enough from you, it will land where it is, or it will return, but at its present altitude rather than rising to the set height. Personally, I think it's designed actions less than 20m/approx 60ft is rather dumb. Then again, that action could be beneficial if you happen to be indoors and unexpectedly trigger RTH. It isn't too likely you'd have the AC further than 60ft when indoors.

The 20m boundary definitely makes it a challenge to safely test RTH.

 

[DanMan32]

One other thing to consider: you might think you lost connection, but you might have only lost video and/or telemetry downlink but not control uplink. Since the AC is still receiving your signal, it won't RTH. That's happened to me with my P3 where I waiting 4 minutes for it to come back, only to find out it came back due to low battery. It was still receiving me and was hovering, waiting for my next command. The M2 seems to be less prone to this but it can happen. I believe but haven't personally confirmed or seen in any documentation that the RC will consider the AC is still receiving and offer you to send RTH.

So, when it appears you've lost signal, either try RTH or try raising altitude if you know there aren't any obstacles above it since signals go further when the AC is higher with less signal obstructions.
If those fail, be patient and let failsafe do what it was designed to to. If you really did lose all signal, turning off the RC isn't going to improve failsafe RTH as the signal is already lost, and you may actually delay your ability to recover your signal and control of your M2.

 

[Lukappaseidue]

One other thing to consider: you might think you lost connection, but you might have only lost video and/or telemetry downlink but not control uplink. Since the AC is still receiving your signal, it won't RTH. That's happened to me with my P3 where I waiting 4 minutes for it to come back, only to find out it came back due to low battery. It was still receiving me and was hovering, waiting for my next command. The M2 seems to be less prone to this but it can happen. I believe but haven't personally confirmed or seen in any documentation that the RC will consider the AC is still receiving and offer you to send RTH.

So, when it appears you've lost signal, either try RTH or try raising altitude if you know there aren't any obstacles above it since signals go further when the AC is higher with less signal obstructions.
If those fail, be patient and let failsafe do what it was designed to to. If you really did lose all signal, turning off the RC isn't going to improve failsafe RTH as the signal is already lost, and you may actually delay your ability to recover your signal and control of your M2.

Thank you for your reply.
Do you know other way to test RTH Failsafe.
I only know that when i switched off the remote controller, the AC does not start the RTH, ma ascend really fast and also over the RTH altitude limit (set in 50 meters) I recover the DRONE, only because i switched it on and it was 125 meters high. But I don't know what would happen if switched on later ..

 

[Speedmeister]

Went out today and tested true RTH. From 20 meters above homepoint, turned off my controller and the M2Z landed. From 30 meters altitude, 100 meters away from home point, turned off RC and M2Z travelled back to homepoint, my RTH altitude is set at 30 meters. I tested my RTH button but never tried lost signal RTH. Now I know it works like it suppose to.

I have been testing flight since I received my M2Z 3 weeks ago. I have been paranoid from the horror stories of M2Z dropping from sky with no apparent reason. Been keeping an eye on battery status and ESC errors. All seems normal. Now I can start enjoying the M2Z.

 

[Meta4]

Do you know other way to test RTH Failsafe.
I only know that when i switched off the remote controller, the AC does not start the RTH, ma ascend really fast and also over the RTH altitude limit (set in 50 meters) I recover the DRONE, only because i switched it on and it was 125 meters high. But I don't know what would happen if switched on later ..

Your Mavic lost signal at 12:58.7 and signal was restored at 13:37.1 (after 0m 38.4s)
When signal was restored, the drone was in RTH.
In the missing 38 seconds it had turned from facing 306° to face 190° and had climbed from 41 metres to 101 metres.
Your RTH height was set at 50 metres.

The usual explanation for an unexpected climb during RTH is the obstacle avoidance identifying a low, bright sun as an obstacle and trying to avoid it by climbing.
Was the return path toward the sun?

It should be simple to test your Mavic by flying in a large, clear area where there's nothing to hit.
Go out 30 metres and switch off the controller to see what happens when signal is lost.
Do it so that the return flight is not facing into the sun.

If you ever get into this situation, the solution is to disable obstacle avoidance or fly home manually but avoid flying directly toward the sun.

 

[ac0j]

What we learned from this.
It is NOT a bug. It is a lack of understanding your Mavic and its limitations.

 

[Meta4]

What we learned from this.
It is NOT a bug. It is a lack of understanding your Mavic and its limitations.

It's always a mystery the first time you encounter it.

 

[Lukappaseidue]

Your Mavic lost signal at 12:58.7 and signal was restored at 13:37.1 (after 0m 38.4s)
When signal was restored, the drone was in RTH.
In the missing 38 seconds it had turned from facing 306° to face 190° and had climbed from 41 metres to 101 metres.
Your RTH height was set at 50 metres.

The usual explanation for an unexpected climb during RTH is the obstacle avoidance identifying a low, bright sun as an obstacle and trying to avoid it by climbing.
Was the return path toward the sun?

It should be simple to test your Mavic by flying in a large, clear area where there's nothing to hit.
Go out 30 metres and switch off the controller to see what happens when signal is lost.
Do it so that the return flight is not facing into the sun.

If you ever get into this situation, the solution is to disable obstacle avoidance or fly home manually but avoid flying directly toward the sun.

Thank you for deep analisys of my log.
As far as regarding the unexpected climb, that day was really cloudy and not sunny day.

 

[Lukappaseidue]

What we learned from this.
It is NOT a bug. It is a lack of understanding your Mavic and its limitations.

Yes, it isn't a bug: Anyway I have not understood why mavic 2 unexpected climb to 125 meters high, even tought with couldy day.

 

[sar104]

Apologies for the delay in responding. The failsafe behavior was not caused by a bug - you have simply misunderstood the failsafe RTH procedure on the Mavic 2, which is similar to what was introduced with the Mavic Air.

Failsafe RTH​

​

The Forward Vision System allows the aircraft to create a real-time map of its flight route as it flies. If the Home Point was successfully recorded and the compass is functioning normally, Failsafe RTH automatically activates after the remote control signal is lost for more than two seconds.​

​

When Failsafe RTH is activated, the aircraft starts to retrace its original flight route home. If the remote control signal is re-established within 60 seconds of Failsafe RTH being activated, the aircraft hovers at its present location for 10 seconds and waits for pilot commands. The user may tap in the DJI GO 4or press the RTH button on the remote controller to cancel Failsafe RTH and retake control. If no pilotcommand is given, the aircraft flies to the Home Point in a straight line. If the remote control signal is still lost 60 seconds after activating Failsafe RTH, the aircraft stops retracing its original flight route and fliesto Home Point in a straight line.​

​

You had just descended from around 215 m relative altitude, and so as part of the retrace procedure it was climbing back to that height. The aircraft behaved as programmed.

 

[Lukappaseidue]

@sar104 I THANK YOU very much, and I am happy that no issue or bug was found on (my) Mavic 2
really really happy

 

[Speedmeister]

RTH failsafe different from button RTH? Where do the RTH altitude and RTH at current altitude parameters apply?

 

[sar104]

RTH failsafe different from button RTH? Where do the RTH altitude and RTH at current altitude parameters apply?

There's an instruction manual. Start there.

 

[Speedmeister]

That 60 seconds after the failsafe RTH is activated would be scary to see because depending on how u where flying before the failsafe, it would look like the MAvic is not returning home.

 

[Speedmeister]

There's an instruction manual. Start there.

Thanks!

 

[Speedmeister]

I just read page 18 of the manual pertaining to RTH. Sar104 posted what failsafe RTH does and below that from the manual, list the RTH procedure that all RTH follows. So if I understood the manual, the 60 seconds after a failsafe RTH is for retracing original route and after that it performs RTH procedure.

 

[Roc Rat]

I had a very similar experience as Luka... last summer and now reluctant to fly. I was in the middle of a forest with great satellites contact and no other interference and lost total contact with drone and it failed to RTH. When the battery was almost dead it regained connection to the controller and I landed it. Still a puzzle to me what happened.

 

[sar104]

I had a very similar experience as Luka... last summer and now reluctant to fly. I was in the middle of a forest with great satellites contact and no other interference and lost total contact with drone and it failed to RTH. When the battery was almost dead it regained connection to the controller and I landed it. Still a puzzle to me what happened.

Why do you think that was similar to this case? In this case the aircraft responded exactly as expected, initially retracing its flight path prior to uplink loss.

 

[Kombucha]

Thanks to the original question and to this detailed answer. Really helps build confidence in a new pilot!

-Gary