uncommanded descent

[tino.v]

A couple of days ago, I had an uncommanded descent with my Mini.

In the log, you can skip to 14 minutes and 10 seconds.
Airdata log file

At that point you can start this video.

I'm flying at 7m height over a little wooden platform bridge in 'tripod' mode.
Then I make a turn to the left.
Al goes smooth, until I see that the ground is coming closer and closer.

The uncommanded descent starts at 14 minutes 29 seconds.
More or less 20 seconds after the video started.

As you can see, no input down from the left stick.

Luckily I could gain height again and even a crash was probably harmless, but .... the question is 'why did it do that ?'


Regards
Tino

 

[timinator]

If the engines are operating at full power and the aircraft can't maintain altitude, the props aren't generating sufficient lift. How is the aircraft stored? Are the rear props being deformed while stored?

 

[FEATHERS]

How do you get these logs?

 

[FoxhallGH]

How do you get these logs?

Mavic Flight Log Retrieval and Analysis Guide

With the introduction of the Mavic Mini there has been a noticeable uptick in the number of posts requesting help with lost or crashed aircraft. While there is plenty of help and advice available on this forum, and detailed guidance and options on several other websites, I thought that perhaps a...

mavicpilots.com

 

[FoxhallGH]

If the engines are operating at full power and the aircraft can't maintain altitude, the props aren't generating sufficient lift. How is the aircraft stored? Are the rear props being deformed while stored?

Good and serious question ... @tino.v - can you please tell us how you store your Mavic Mini between flights, with particular reference to how the prop's are stowed, and if you use anything to keep the prop's from moving?? I'm (and a few others too) wondering if there is a correlation between Mavic Mini 'crashes' like yours, and the prop's being deformed due to stowage????

 

[FoxhallGH]

A couple of days ago, I had an uncommanded descent with my Mini.

In the log, you can skip to 14 minutes and 10 seconds.
Airdata log file

At that point you can start this video.

I'm flying at 7m height over a little wooden platform bridge in 'tripod' mode.
Then I make a turn to the left.
Al goes smooth, until I see that the ground is coming closer and closer.

The uncommanded descent starts at 14 minutes 29 seconds.
More or less 20 seconds after the video started.

As you can see, no input down from the left stick.

Luckily I could gain height again and even a crash was probably harmless, but .... the question is 'why did it do that ?'


Regards
Tino

It appears that something that may have contributed to this, was that while the Mavic was in trouble, it was in 'Tripod mode' - which would have limited the power available for manoeuvre. You entered Tripod mode at 1:20.4 into the flight, and the only indication of it switching out, is at 16:57.0 - when the Mavic switched to Confirm_Landing mode, and then after landing, it returned to 'Tripod' at 17:00.4 ... You may have had more control over the Mavic if you took it out of Tripod when it was misbehaving?

 

[Starz]

I’ve seen these posts regarding “deformed props” many times and as of yet never seen any conclusive evidence that that problem exists. With the flexibility that they have, is that even a possibility?

 

[slup]

I’ve seen these posts regarding “deformed props” many times and as of yet never seen any conclusive evidence that that problem exists. With the flexibility that they have, is that even a possibility?

Yeah ... think it's more complicated then just the props ... we have now seen this for several month, and DJI have been totally quiet about it ... usually just replacing Mini's that have been lost. IF ... this only were a prop problem I'm sure that DJI already had thrown out new designed props every where as they surely aren't expensive for them instead of letting this just go on.

Ask yourself ... why did this problem only manifest it self after 14 minutes of flight if they were deformed due to stowage? Probably the props are a part of the problem but it's more to it ...

 

[sar104]

Yeah ... think it's more complicated then just the props ... we have now seen this for several month, and DJI have been totally quiet about it ... usually just replacing Mini's that have been lost. IF ... this only were a prop problem I'm sure that DJI already had thrown out new designed props every where as they surely aren't expensive for them instead of letting this just go on.

Ask yourself ... why did this problem only manifest it self after 14 minutes of flight if they were deformed due to stowage? Probably the props are a part of the problem but it's more to it ...

One possible explanation for delayed onset of the problem during a flight is that the deformation is exacerbated by high motor speeds. In other words, if a flight maneuver or some other excursion during flight triggers the FC to spool up one of the back motors to around 15 krpm, then that might further deform the props enough to lose lift. The props are extremely flexible.

I've been trying to trigger that effect, without any success so far, but I also haven't yet resorted to deliberately deforming them before flying in the way that has been hypothesized might happen during storage.

 

[Starz]

With the new mini that they sent for a replacement, the first one being obviously a first batch, I wonder if some internal hardware was changed since I haven’t had any issues with the new one.

 

[rscheff]

I see lots of talk about the props and how they possibly get deformed from storage. I am new at owning a drone and I can definitely see that a problem could develop if props were damaged, after all everything about the flight relies on prop condition. I recently bought a set of new props from DJI and installed them, my first flight resulted in the drone, when hovering, listing a couple degrees to the right. It would not move to the right but the angle was not straight. I thought to re-calibrate the IMU but it was still listing about the same. After a few flights it was back to normal, anybody have any ideas why. I have ideas of what is going on but I just want to see if others have had the same problem and came to the same conclusion.

 

[EyesWideShut]

I see lots of talk about the props and how they possibly get deformed from storage. I am new at owning a drone and I can definitely see that a problem could develop if props were damaged, after all everything about the flight relies on prop condition. I recently bought a set of new props from DJI and installed them, my first flight resulted in the drone, when hovering, listing a couple degrees to the right. It would not move to the right but the angle was not straight. I thought to re-calibrate the IMU but it was still listing about the same. After a few flights it was back to normal, anybody have any ideas why. I have ideas of what is going on but I just want to see if others have had the same problem and came to the same conclusion.

You are saying you calibrated the IMU and it still listed. I have a couple of ideas. Just checking if there was possibly some slight wind and it was tilting to hold it's position? Maybe double check by flying indoors.
Also any chance when changing the props that any one of the split props were mixed up and aren't providing the proper lift and are compensating by spinning faster? Any power warnings when giving full thrust?

 

[Seabee23]

I’m curious how many people with the fly more combo this has happened to. If this is still happening to those people with the fly more combo case, then I’m reluctant to think it’s a deformation of the rear props. The fly more case has a hump in the middle of where the mini is stored, thus resting the mini on its belly, and leaving a space for the props to go without touching the bottom of the case. Just wondering. I had a brief uncontrolled decent a couple weeks ago, and I have the fly more case. Props look great, I was hovering, and there was no higher than normal sound or max power warnings. There has to be more to it than props. And it only happened once to me in105 flights and 6.5 hours. I haven’t changed props, so why hasn’t this happened again? The only thing different with that flight vs all my others, I was in a very high WiFi area. It sucks, because I live at the beach, and have so many cool things I could film, but **** if I’m flying this thing over water at this point.

 

[rscheff]

You are saying you calibrated the IMU and it still listed. I have a couple of ideas. Just checking if there was possibly some slight wind and it was tilting to hold it's position? Maybe double check by flying indoors.
Also any chance when changing the props that any one of the split props were mixed up and aren't providing the proper lift and are compensating by spinning faster? Any power warnings when giving full thrust?

I was hovering the drone on my deck and there was absolutely no wind. There was nothing wrong with the original props, in fact I still kept them. I had just received the new props in the mail (not packed very good, just in a plastic bag) and wanted to see if they would be different. If you re-read my thread it says that after a few flights the listing stopped. I suspect the blades were squished in some manner during shipping and must have sprung back to their right shape with use. I wish DJI would ship them in a box or something which offers some protection.

 

[hiflyer201]

AC 35.37-1B - Propeller Fatigue Limits and Evaluation
This PDF may sound way overboard for simple drone props but I'm thinking that in the interest of weight the Mini props as designed are not up to the task.

 

[rehkram]

Pure conjecture but it smells like a software problem to me. I was, still am, a programmer.

If it is software the obvious rhetorical question would be where's the reboot button on this thing? In my fantasy world a quick reboot command issued to drone and controller would force the drone to hover in place until everything came back up and reconnected. Maybe it can already do that, I must try switching off the controller and Fly app and see what happens, does anyone know?

But not so fast... it's likely impossible to force a reboot of the vehicle's onboard control systems without switching off the motors/GPS/IMU. I guess it would depend on how tightly-bound those subsystem modules are, and/or whether there's a fail safe built-in to keep it in the air while the control system is restarting. If it's a bug in one of the subsystems required to hover restarting wouldn't work. Sorry, just thinking out loud here.

 

[GerdS]

As my Mavic Mini increasingly has problems with keeping altitude when flying at full speed I want to ask you to do a little test with yours to check if this problem is affecting all Mavic Minis more or less:

1. Go to a flat location with at least 200m free space in one direction.
2. Take off in P mode and go to a preferred altitude, keep the altitude as shown within the Fly app in mind. Don't touch the throttle stick anymore from now on.
3. Fly straight away approx. 200m with full speed, then slow down for hovering.
4. Read the altitude there, then wait about 10s and check for uncommanded altitude loss.
5. Turn 180° and fly back to your home point with full speed.
6. When back, check the altitude as displayed by the Fly app. Is it still the same as was at the beginning, or how far is it off?
7. Switch to Sports mode and repeat from step 2.

Please let me know your results.

 

[timinator]

So maybe there are actually two issues here. Some have experienced uncommanded descent, changed the props and have never had an issue after that. Others have had multiple instances up to and including aircraft loss and gotten a new bird. So is it possible that even though the logs show max power, not all the motors are actually generating max power or would there be a different error flagged?

 

[FoxhallGH]

This is an interesting post on YouTube, from a guy in Idaho (Idaho Quadcopter), on the 29th March. His video is rather long, but it 'shows' the Mavic Mini problems as per this thread. He has videoed his observations, and also comments about reading posts here on this forum that led him to diagnose that he had to change the rear props to cure the problem. That did in fact work, and he had a completely different (positive) experience with his MM after that prop change.
He has interesting conclusions right at the end of the video, and shows that when the MM is packed into the FlyMore case, the rear props (which end up folded up under the front half of the MM), should cross & point directly left & right, and sit in an open area of the case - that is - if they are aligned as per the DJI label stuck to the bottom of the case. If however, the props swing back toward the tail of the drone as it is stowed, then the props end up sandwiched between the drone's body, and a support ridge immediately behind the cut-out area for the props. He believes that it was this misalignment of the props as the MM was put into the case, that caused the prop blades to be put under pressure and squeezed flat (reducing their pitch). It is notable that there is a rectangular block shape in the lid of the Flymore case that applies positive pressure onto the top of the drone's body when it is stowed, and this would in turn, apply compression to the blades if they were stowed incorrectly and sitting on top of the support piece in the case.

 

[slup]

... So is it possible that even though the logs show max power, not all the motors are actually generating max power or would there be a different error flagged?

What's happening during a "uncommanded descent" incident is pretty well explored ...everything starts with a sudden up pitch & then stays near horizontal angle, meantime one or both rear motors are at or very near max rpm's, heading speed goes down (as no down pitch can be achieved) ... the AC lose lift & down speed goes positive (descend).

In below DAT log example from February the AC have just started a failsafe RTH ...

-All goes well to start with. At 704,8sec the AC have turned around & start to gain heading speed (green).
-The pitch is around 15 degrees down which is according to spec. (dark green).
-During the whole RTH process the rear motors have been revving pretty high (Black & Blue).
-Then suddenly at 709,7sec (where the markers are in the chart) the AC pitch up to 14 degrees then down again to very near horizontal level ...
-There the right rear motor are at max (around 16000rpm), the heading speed goes down and the down speed (dark yellow) goes up (meaning descend) ---> the AC have lost lift totally.
-Close there after the RC-AC connection is lost & the AC is later found on ground.

The mystery isn't what happens ... it's why.

(Click on the chart to make it larger)