Forced Landing, almost lost in water - Can someone help troubleshoot my log?

[CaptPete]

Strange, 19 mins into flight, with no warnings and 48ft AGL, forced landing is initiated (not by pilot input), apparently by erroneous VPS?. Log also seems to have strange message regarding motors. The part where it says "upward obstacle detected" was grass tangled in the prop at the site of a virtual crash landing. Direction was controllable in landing, but not altitude.

Drone could have easily gone down in the water a few yards away.

Log:

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MonitorError appears throughout log and VerticalLowLimitLanding appears when ForcedLanding initiates.

I won't be flying until I figure out whether this was a malfunction or pilot error.

Thanks in advance.

 

[msinger]

The aircraft switched to forced landing mode when you held the throttle in the full down position while the downward sensors were detecting the ground was near. This is how forced landing mode is designed to work.

You can prevent this from happening by either:

1) Not holding the throttle in the full down position while the downward sensors are detecting something close to the bottom of the aircraft. This can be tough to do if you're not watching for this in DJI GO.

2) Disable the downward sensors in DJI GO. If this is done, you'll have full control of the aircraft until it touches the ground.

 

[CaptPete]

Thanks for the response msinger.

The aircraft was more accurately at the altitude reflected by the IMU alt +5.5 ft at the time of the event, so it was nowhere near the ground, but about 48 ft AGL. Would you infer that a malfunctioning VPS caused this? It was not over water at the time, but only about 20 ft from the water as shown in map.

 

[Shon]

Sensors sometimes react in unexpected ways. It doesn’t necessarily mean a “defective” sensor, they all do this. Water, sunlight, shadows, etc, sometimes cause errors.

I once had a drone hanging over a cliff and over a waterfall. The sensors refused to allow the drone back over the edge of the cliff so that I could land it.

I eventually had to reach over the edge and catch the drone via forced landing. Another option would have been to disable the sensors and land manually.

The sensors are there for extra safety, but I don’t trust them to be 100% reliable. They have actually caused me more problems than not, but I keep them on in hopes they will save me when I really need it. Some folks keep them disabled.

 

[CaptPete]

Thanks Shon.

So if at all possible will "pause" "left stick up" cancel forced landing (assuming you are high enough to catch it in time)? I searched a little and this was the most recent solution I found.

If you are 10 feet above water would you consider it wise to disable down VPS?

 

[Shon]

Thanks Shon.

So if at all possible will "pause" "left stick up" cancel forced landing (assuming you are high enough to catch it in time)? I searched a little and this was the most recent solution I found.

If you are 10 feet above water would you consider it wise to disable down VPS?

Forced landing: I fly an Air. It is really small. Small enough that it lands on my hand.

When I attempt to land on my hand the sensors detect an object ( my hand) and they refuse to let the drone continue to land.

If I keep pressing down on the left stick, after a few seconds the drone will enter “forced landing” mode, ignoring the sensor telling me there is an obstacle in my way. To me, it is just a landing, but the log data shows this is a “forced landing”.

If a sudden gust of wind were to cause me to abort my landing, I could simply take my thumb off the left stick and the drone would hover in place, or push the left stick up to gain altitude and abort the landing.

A minor aside... in the waterfall incident above, it was the forward and rear sensors that were detecting an erroneous object. It didn’t matter how much I tried to force the drone back over the cliff, the drone did not allow me to “force” an override in the same way I could with a landing.

My personal opinion is that you don’t need to disable the sensors 10 feet over water. I fly over water all the time. It’s when folks are skimming the water or snow where they run into trouble. The sensors don’t work well in monochrome environments ( water, snow) or low light environments. In these situations I would not Expect the drone to stay as level and stable in the air as it normally does , so extra caution is needed to make sure you aren’t actually losing altitude and getting ready to crash.

 

[CaptPete]

I also hand takeoff/land my Mavic 2 when the ground is inappropriate, but as you know this drone was almost 50ft in the air and "forcedlanding" was not pilot activated and unfortunately control was limited to lft, rt, fwd, bkwd, all the while continuing descent.

I would still appreciate someone explaining "MonitorError" that appears in every line of the log, and any varied opinions on cause. This was in daylight at almost 50 ft altitude over clear ground. Until then, I remain grounded.

 

[Mordor]

I only have about 3 flights....but i already notice how trouble can be sensors...specially when i try to land and it says there is a obstacle....
This is stupid.

Last time i try to catch with my hand too, and it did go up....very weird for me. My P3 i always turn off VPS specially because water incidents.

It seems i will turn off them all....specially down sensor.

Very weird.

 

[Meta4]

The aircraft was more accurately at the altitude reflected by the IMU alt +5.5 ft at the time of the event, so it was nowhere near the ground, but about 48 ft AGL. Would you infer that a malfunctioning VPS caused this? It was not over water at the time, but only about 20 ft from the water as shown in map.

Looking closely at your VPS data, there is a problem which is causing VPS altitude to jump up and down significantly where the ground/water does not.
These false readings are related to the issue you reported.
Here are some of the times it jumps:
At 16:24.3, although drone was descending, altitude readings jump to ..
IMU = -2.0ft VPS = 15.7ft
Jumps again at 16:30 VPS jumps from 14.4ft to 2.0ft while IMU stays constant
Jumps again at 16:32.1 VPS jumps from 1.6ft to 14.4ft
Jumps again at 16:33.5
Jumps again at 16:34.4
Jumps again at 16:37.4
Jumps again at 16:38.4
Jumps again at 16:38.8
Jumps again at 16:39.4
Jumps again at 16:39.8
Jumps again at 16:41.9
Jumps again at 16:42.8
Jumps again at 16:44 ... etc, etc.
I would be looking into what is causing problems for the VPS.
Do you have an accessory that is partially blocking the downward looking sensors?
If not, try recalibrating the VPS.

 

[msinger]

So if at all possible will "pause" "left stick up" cancel forced landing (assuming you are high enough to catch it in time)?

Moving the throttle up is the easiest way to cancel forced landing mode.

Would you infer that a malfunctioning VPS caused this?

I like what @Meta4 is thinking above. Got any 3rd part accessories installed near the bottom of the drone?

 

[CaptPete]

Moving the throttle up is the easiest way to cancel forced landing mode.

I like what @Meta4 is thinking above. Got any 3rd part accessories installed near the bottom of the drone?

Thanks @Meta4 and msinger.
No accessories ever - 100% stock DJI Mavic 2.

The 1st thing I did after realising there could be a VPS issue was to clean all positioning sensors. Here is a photo of them before they were cleaned just after the incident. It represents about 30 landings and takeoffs without cleaning them. In your opinion, do they seem dirty enough to cause these highly erratic VPS readings?

I will recalibrate VPS per your suggestion msinger and on my next flight in a safe area I will check the logs post flight to see whether the issue is still present (assuming you experienced pilots believe these mildly dirty sensors are responsible for such significant errors). It will be a few days before I get a chance to do this test and assuming no TFR (Trump visiting Mar-a-Lago) this weekend.

Thanks very much for your guidance and help here. I will report back to this thread with the results either way.

One note: the drone virtually crash landed in a sandy area with long grass while moving horizontally, so some of the debris on the lenses is from that.

 

[msinger]

In your opinion, do they seem dirty enough to cause these highly erratic VPS readings?

I haven't done testing to confirm whether or not dirty sensors can cause this issue. Why not clean them and go out for another test flight? You can then review your new log to see if the same thing happens.

 

[jamesfrankham]

2) Disable the downward sensors in DJI GO. If this is done, you'll have full control of the aircraft until it touches the ground.

Other than black tape, is it possible to disable the downward sensors in DJI Fly for Mavic Mini? Or should I use DJI Go for these flights?

 

[msinger]

should I use DJI Go for these flights?

You should be using DJI Fly if you're flying a Mavic Mini.

 

[jamesfrankham]

So no way to turn off downward sensors?

 

[CaptPete]

I cleaned the sensors and recalibrated. No issues in 20 flights since