This past week I was flying and went through my normal checks, finding all in working order.  I flew for about 15 mins and had no issues.  An hour later I swapped batteries and went to fly again.  This time the app indiated there was no GPS signal.  I thought this odd, but was under the shade of a tree so i thought that might be the cause for slow lock.  I was trying to catch a shot that I had limited time to get, and since I was planning on heading only ~100 feet away and 40-50 feet high with all else looking good, I took off.

The aircraft took off normally, I flew out over the ocean toward where i wanted to shoot from, and after less than a minute it started to descend.  As I wasn't very high (13 feet), I knew I had little time to act.  Noticing that my pitch and yaw were well under control (it was only the altitude that was the problem) I saw a safe path to return and ran full-speed toward the shore.  I also noted that the DJI app indicated I'd reached maximum flight altitude (again, while at 13 feet per the app, though the app was set to a max altittude of ~375 feet).  The aircraft got to the shore near me just in time to tumble hard though moist sand and onto the beach.  

I can't rule out that I did something wrong - but I can't imagine what it would be, as even if the max altitude was set to 13 feet (and it *certainly* was not), an un-overidable descent would hardly seem the expected behavior.  I'd imagine many will say I shouldn't have flown without a GPS lock - and that may be a valid statement, but still doesn't explain the behavior.  I've gone through the recording of the flight (data, I wasn't yet rolling video when this happened), and don't see anything past what I already knew to be the case, and what I described here.  

You can see the log of this flight here, but I guess I still need to figure out the following:

1) Is there something I did that directly caused this which I've not figured out?
2) What will DJI need to see if this can be handled as a warranty claim? (they're closed right now but I want to be prepared)
3) Are there other log files helpful to analyzing this?

Thanks!
BH

Hey BH

Please upload your flight log here
Post the link back to this thread and someone with greater skills than I will be along to take a look.

Hi BH,

Wow, lucky to read that your MA landed ashore!

What happend:

Takeoff in OPTI mode ; 7 GPS sats and no GPS reception.
- In OPTI mode max height is limited, that`s why you had the max height warning.
- No HP was set, due to lack of GPS signals
- After 18 seconds less visual reference for OPTI, so ATTI mode alive.
- glad you were able to bring your MA back to the sand. much better than go for a swim....

Not sure why it start to descend, but in ATTI mode with full aft and up stick...guess hard for the MA to keep height. If baro sensor is getting variations in air press (due to low flying, false wind, high attitudes of MA) this is seen in height keeping (or not).

My advice is to wait for proper GPS lock, HP set and start to fly away from your HP.

Mayby this checklist will help for the next flights.

cheers
JJB

Hello and good day BH. I am sorry to know what happened to your previous flight with your DJI Mavic Air. This is not the experience we want you to have with DJI. Since this crash happened the best thing you need to do is to contact DJI Support at https://www.dji.com/support and open a ticket for your aircraft. Please sync the flight data as well using your DJI Go 4 application for further analysis. Thank you.

JJBspark Posted at 11-25 03:39
Hi BH,

Wow, lucky to read that your MA landed ashore!

Thanks for your detailed analysis and advice.

"Not sure why it start to descend, but in ATTI mode with full aft and up stick...guess hard for the MA to keep height."

That's the key thing I'm trying to understand.  As you mentioned, perhaps pressure fluctuations made the aircraft think it needed to descend below max height, the only flaw there is that as we see in the logs, it thought it was below takeoff altitude yet continued to descend.  Perhaps there is zero hard-stop for that behavior, which would seem flawed.

CruiseHabit Posted at 11-25 08:32
Thanks for your detailed analysis and advice.

"Not sure why it start to descend, but in ATTI mode with full aft and up stick...guess hard for the MA to keep height."

HI,

A DJI drone can fly below takeoff height, there is no limit for that.

If you take off from the roof of your house, you can fly down (minus height) and land in your garden at minus - 6 meter.
So if in ATTI the height lowers, no software stop to kill that descend.
Height is hold in P-GPS mode as well in ATTI, but mayby not in large attitude movements while in ATTI.

cheers
JJB

JJBspark Posted at 11-25 09:09
HI,

A DJI drone can fly below takeoff height, there is no limit for that.

"A DJI drone can fly below takeoff height, there is no limit for that."

Flying below takeoff height I realize - it's that during an automatic descent it kept going.  I suppose I'd understand this circumstance if the aircraft thought (b/c of a perceived change in atmospheric pressure) that it had climbed or was climbing when in reality it was descending, but we can see in the logs that it knew it was descending (well below the max 16 foot limit for ATTI mode).  In researching, I can't figure out why the aircraft would force descent well below any max altitude setting (ATTI or whatever one has set for GPS mode).

Again, I appreciate you helping me think through this.  I want to make sure I fully understand to avoid further issues.  Additional research gives me something to do while trying to figure out how to remove sand from inside the device...

CruiseHabit Posted at 11-25 10:27
"A DJI drone can fly below takeoff height, there is no limit for that."

Flying below takeoff height I realize - it's that during an automatic descent it kept going.  I suppose I'd understand this circumstance if the aircraft thought (b/c of a perceived change in atmospheric pressure) that it had climbed or was climbing when in reality it was descending, but we can see in the logs that it knew it was descending (well below the max 16 foot limit for ATTI mode).  In researching, I can't figure out why the aircraft would force descent well below any max altitude setting (ATTI or whatever one has set for GPS mode).

Hi,

"Flying below takeoff height I realize - it's that during an automatic descent it kept going"  [ not an automatic descent in the way you are thinking (i think), AC keeps height in GPS and ATTI mode, so just correcting if height differs from setting (ac actually up/down; steering down/up by software OR baro sensor indicates different height so ac is going offset initial height, thus up or down)

".. but we can see in the logs that it knew it was descending (well below the max 16 foot limit for ATTI mode)"  [ yes, has nothing to do with any height limitation ]

I see an error at my ID 260 wich says "Motor Obstructed", so at that point the drone touched the sand or....?


In GPS mode the height is corrected perfect, even if we apply pitch, roll and/or yaw. Any input has influence of the height but software corrects this quickly by correcting in pitch, roll and /or yaw (mostly pitch and roll). Keeping height is for sure not a straight line but fluctuating around this. How faster the software reacts the flatter the line is.

I think that in ATTI mode the height hold mode is not that fast, with high pitch and roll angles you see more height fluctuations. Not to be noticed flying at 100 feet, but flying low....

cheers
JJB

CruiseHabit Posted at 11-25 10:27
"A DJI drone can fly below takeoff height, there is no limit for that."

Flying below takeoff height I realize - it's that during an automatic descent it kept going.  I suppose I'd understand this circumstance if the aircraft thought (b/c of a perceived change in atmospheric pressure) that it had climbed or was climbing when in reality it was descending, but we can see in the logs that it knew it was descending (well below the max 16 foot limit for ATTI mode).  In researching, I can't figure out why the aircraft would force descent well below any max altitude setting (ATTI or whatever one has set for GPS mode).

There is no 16ft height limit in Atti mode, height aircraft is limited to 98ft when vision sensors are inactive, so aircraft is in Atti mode, you should have been able to climb 98ft above take off point, aircraft should also have been able to maintain horizontal position just the same as it does with P gps mode, gps works as hold from top of Aircraft to satellite and baro pressure works from bottom of aircraft to the ground, the only difference in your aircraft when flying in Atti mode is your aircraft no longer has auto breaking, so craft will drift on the wind and breaking will only work if operator breaks by counteracting his own stick movements.

I could not download your log from google so am not sure what stick movements you made, on thing I can tell you is because you went into Atti mode the correct procedure was to land, whether you did this by choice or by luck, it was the safest and right thing to happen for your aircraft. This is also the recommended procedure written in your manual.

JJBspark Posted at 11-25 12:54
Hi,

"Flying below takeoff height I realize - it's that during an automatic descent it kept going"  [ not an automatic descent in the way you are thinking (i think), AC keeps height in GPS and ATTI mode, so just correcting if height differs from setting (ac actually up/down; steering down/up by software OR baro sensor indicates different height so ac is going offset initial height, thus up or down)

"I see an error at my ID 260 wich says "Motor Obstructed", so at that point the drone touched the sand or....?"

Actually that obstruction was my finger.  I mentioned I flew it to an area where there weren't people (I'd much rather sink a drone than hit a bystander), and when the aircraft tumbled it startled a young girl.  I was so concerned that something happened (even though I saw that it didn't crash near her) that I ran over to see if she was okay.  In doing so, I picked up the drone while I, and evidently the motors, were running.  It should be fun to type with nine fingers for the next week or two.

hallmark007 Posted at 11-25 14:20
There is no 16ft height limit in Atti mode, height aircraft is limited to 98ft when vision sensors are inactive, so aircraft is in Atti mode, you should have been able to climb 98ft above take off point, aircraft should also have been able to maintain horizontal position just the same as it does with P gps mode, gps works as hold from top of Aircraft to satellite and baro pressure works from bottom of aircraft to the ground, the only difference in your aircraft when flying in Atti mode is your aircraft no longer has auto breaking, so craft will drift on the wind and breaking will only work if operator breaks by counteracting his own stick movements.

I could not download your log from google so am not sure what stick movements you made, on thing I can tell you is because you went into Atti mode the correct procedure was to land, whether you did this by choice or by luck, it was the safest and right thing to happen for your aircraft. This is also the recommended procedure written in your manual.

"There is no 16ft height limit in Atti mode"

My error, you're correct.  This makes it all the more curious that I got a max altitude warning.  Here is another source for the log file: https://www.dropbox.com/s/jmishn ... 2-18-22%5D.txt?dl=0

CruiseHabit Posted at 11-25 15:48
"There is no 16ft height limit in Atti mode"

My error, you're correct.  This makes it all the more curious that I got a max altitude warning.  Here is another source for the log file: https://www.dropbox.com/s/jmishnla26dztw8/DJIFlightRecord_2018-11-19_%5B12-18-22%5D.txt?dl=0

Ofcourse there is a height limit of 16 feet, see the manual, and #3.

16 feet:  GPS signal weak + Vision active // your situation flying in OPTI  (as you did) not in ATTI.
98 feet: GPS signal weak + Vision not active // well, your vison was on.
If GPS is weak, and downward vision is ON but no visual reference: craft flying in ATTI mode with no height limit.

OPTI mode is not in the phantomhelp viewer, so guess that`s why some are confused....or did not see the log at all.

There is a big difference in stick behaviour in GPS and ATI mode!.
GPS > if stick up is applied, craft wil ascend. No matter of the attitude (ps not altitude) it is flying.
ATTI > if stick up is applied and craft attitude is not level this stick input will strengthen the current attitude ; so i.e fwd pitch force will become even bigger.
This is one of the reasons why it is so hard to fly in ATTI mode. (first level craft than up input, not that digital but with 40 degrees down and pitch up....nope)


BTW i have tested this myself (opti mode + 16 feet max height) and its working like this.

see my vid. (watch the GPS count + reception)



BTW I always fly with a screen recorder active, good for fun to see later what i have missed on the screen while watching the sky + really usefull if something out-of -the-box did happen.

cheers
JJB

In Atti mode aircraft will hold horizontal it will not change its height unless operator commands it to, it will not drop height, in Atti mode only difference is aircraft can no longer Auto break.