My P3P crashed, DJI not cooperating

[msinger]

I have already answered that question.

I'm just trying to help you understand what happened. Please hang in there

Are you suggesting the altitude is incorrect in the entire log? If not, can you help me find the location where it started to report the incorrect altitude?

 

[Chris_P]

The only method I have for assessing the accuracy of the altitude is the photos taken by the Phantom. I can post more photos if that helps, maybe others are better at estimating altitude than I am. As it was coming in to land, I am better able to estimate altitude and I can say for sure it was approximately 5' above my head which I estimate to be 15' +-

 

[MikeyOnline]

That said, I do see the point cancelling RTH ascent if you have 20% battery remaining over water and a long flight home. Sometimes you want it to come home now, not in 30s. Should never happen but if it does... I like knowing I can do that. If you read the friendly manual, it's worth while information to know, however dangerous to those that don't.

I do see the logic in that, however, I have to disagree. You have to weigh how often that scenario will happen versus the scenario where you're not over water, or you can't guarantee that a sailboat mast won't be in your way on the way home... versus how often an RTH is automatically initiated and then the throttle stick is bumped for a fraction of a second, making the AC come home much lower than specified. You could have loss of telemetry or intermittent telemetry which initiates an RTH and while the AC is ascending to the RTH altitude, it gets an intermittent signal and during that time, you happen to have the throttle stick off center. And potentially without consistent telemetry, you might not even be able to tell that the RTH altitude is lower than it should be (until it hits something). I would argue that if you are in range enough to cancel the RTH altitude, you should just cancel RTH and fly it home lower if you like, rather than having it take off automatically at a low (and potentially unknown) altitude. I just see more opportunities for failure than success with that code or "feature". And again, if you have enough information (through LOS or telemetry) to be able to decide that it's OK to come home at 20 ft rathern than 65 ft, you should have enough information to just cancel RTH, turn the bird on the home line, and fly it back.

Mike

 

[msinger]

The only method I have for assessing the accuracy of the altitude is the photos taken my the Phantom. I can post more photos if that helps, maybe others are better at estimating altitude than I am. As it was coming in to land, I am better able to estimate altitude and I can say for sure it was approximately 5' above my head which I estimate to be 15' +-

I understand your side of the story, but I'd like to dig into the flight data some more. This data was logged by your Phantom throughout your flight -- and, DJI is assuming it's correct (hence their 20% offer). Can you take a few minutes, comb over it, and help us pinpoint the exact time in the log where you feel the altitude started to be reported incorrectly?

 

[Luap]

Could you post the actual Fly46.Dat as opposed to the converted csv?

 

[Chris_P]

Please correct me if I'm missing something but I've already combed through the logs to the best of my ability. I see at takeoff it was a 0 and climbed steadily to the 50-60' range. It increased up to 106' at one point I believe but I don't see any abrupt unexplained changes in altitude throughout the flight. The only anomaly is the continued altitude of 62 feet while landing and the reported altitude of 54 feet upon impact. I certainly see where your tree suggestion comes from, thank God for the site photos that show no trees in the crash location. Ive mapped it on Bing and Google and there are no 54' tall objects within 50+ feet.

 

[Chris_P]

Could you post the actual Fly46.Dat as opposed to the converted csv?

Done. I tried uploading it but it exceeded the max size, Here is a dropbox download link ...

Dropbox - FLY046.DAT

 

[msinger]

I don't see any abrupt unexplained changes in altitude throughout the flight.

Me neither. So, where exactly in the log did your Phantom descend from ~62 feet down to ~10 feet?

 

[Chris_P]

After impact, agreed. But there was no tree so we are back to the unknown ..... The logs show 54 feet at impact then 10 feet seconds later. Looking at the crash site, there is no other explanation than an altimeter error that may have been corrected on impact.

I'm willing to send the Phantom in DJI for their inspection. They never asked ! Surely they can tell from the damage or forensic residue what the Phantom hit. This really isn't that difficult.

 

[MikeyOnline]

I think the real mystery is to explain how your Phantom descended 49 feet without you touching the throttle stick.

In the log, at about 944.8 seconds, I see the throttle stick being held down. Looks like about 60% to 70% down. During that time, the Phantom descended about 10 meters while the stick was held down (about 14 seconds). I believe this is the point where the OP was bringing it down to the boat. There's more down stick at 960.33 through 961.8 and another batch of 10% to 30% down throttle from 965s through about 969s although those last ~5 seconds of down stick only resulted in a ~1.5m drop in altitude according to the baro. This would be consistent with more controlled, slower descent necessary to initiate a hand catch. RTH was initiated while the throttle was being held at about 10% down throttle in those last down-stick events which apparently caused it to cancel the RTH set altitude and just fly toward home at the current altitude. I see no indication that the AC tried to rise at all once RTH was engaged, meaning that all that was required to cancel the RTH altitude was to be holding a little down stick on the throttle when RTH is first initiated.

And I know I sound like a broken record, but this is the reason I think canceling the set RTH altitude based on the position of the throttle stick is a bad idea. I'll give you one of my own personal examples. I sometimes (manually) orbit some of my tall palms inspecting for damage due to storms like broken fronds and such. I don't want to be surveying damage on the opposide side of a palm tree and I just happen to be descending a little to survey a little lower on the tree, when RTH kicks in while I'm down a little on the throttle and then decides to just immediately drive the AC into the tree at the current altitude. I'd never have enough time to take manual control if I'm 20 feet from a tree doing surveys when this happens. Sure, you could argue that I shouldn't be doing that if my battery is close to 30% when RTH could kick in, but there are other circumstances that could initiate RTH while (or immediately followed by) a little down throttle. So my 2 cents is that canceling RTH set altitude based on stick input is going to be a bad idea more often than a good one.

Mike

 

[msinger]

After impact, agreed.

Now, I'm confused

If your Phantom was about 10 feet in the air when it was next to your boat, how was it 54 feet at the crash site? Your log does not show it increasing in altitude after it headed toward the crash site.

 

[Chris_P]

I never said it was at actually at 54'. I said the logs incorrectly reported 54' at impact

 

[Chris_P]

In the log, at about 944.8 seconds, I see the throttle stick being held down. Looks like about 60% to 70% down. During that time, the Phantom descended about 10 meters while the stick was held down (about 14 seconds). I believe this is the point where the OP was bringing it down to the boat. There's more down stick at 960.33 through 961.8 and another batch of 10% to 30% down throttle from 965s through about 969s although those last ~5 seconds of down stick only resulted in a ~1.5m drop in altitude according to the baro. This would be consistent with more controlled, slower descent necessary to initiate a hand catch. RTH was initiated while the throttle was being held at about 10% down throttle in those last down-stick events which apparently caused it to cancel the RTH set altitude and just fly toward home at the current altitude. I see no indication that the AC tried to rise at all once RTH was engaged, meaning that all that was required to cancel the RTH altitude was to be holding a little down stick on the throttle when RTH is first initiated.

Thank you Mike ! Thats exactly what happened ......

 

[msinger]

In the log, at about 944.8 seconds, I see the throttle stick being held down. Looks like about 60% to 70% down. During that time, the Phantom descended about 10 meters while the stick was held down (about 14 seconds). I believe this is the point where the OP was bringing it down to the boat. There's more down stick at 960.33 through 961.8 and another batch of 10% to 30% down throttle from 965s through about 969s although those last ~5 seconds of down stick only resulted in a ~1.5m drop in altitude according to the baro. This would be consistent with more controlled, slower descent necessary to initiate a hand catch.

Well, let me ask you the same question then. At what point in the log does the altitude jump from the correct altitude to the incorrect altitude?

 

[msinger]

I never said it was at actually at 54'. I said the logs incorrectly reported 54' at impact

Ah, okay.

So, please tell me. At what time in the log do you first notice an incorrect altitude? I apologize for sounding like a broken record, but you seem to be dodging my question.

 

[Chris_P]

Ah, okay.

So, please tell me. At what time in the log do you first notice an incorrect altitude? I apologize for sounding like a broken record, but you seem to be dodging my question.

I'm not dodging your question at all .... Ive already stated that the only anomaly is the reported altitude of 54 feet on impact. Mike has stated that he sees an altitude reduction of 10M with no rise in altitude as it flew off and crashed. Simple math says that 54' at impact is impossible.

 

[MikeyOnline]

It seems most of the flight I was between 65-100 or so feet which sounds about right. It would seem that at the end of the flight, when I was attempting to bring it in to land, it was still registering 60+ feet. I was standing on the swim platform of the boat reaching for it when it took off by itself in the auto RTH mode. That certainly wasn't 60, 50 or even 40 feet. It may have been at 15-20 ft in reality. So it seems the altitude was correct all along but didn't decrement as I was attempting land it, it retained the original altitude reading.

I agree this part is confusing. I don't think your altitude was correct for any of it, at least the last (most important part) of the trip, including before you started to descend. It looks to me like it was indicating about ~40 feet higher the whole last part of the trip. For example, it reported you were at about 105 feet before you started descending to the boat. You used the throttle stick and descended to about 63 feet. At that point, it indicated 63 feet when you said about 15 feet. So I don't see how it was correct before the descent and then just didn't register the descent. By your acount, you must have been at about 70 feet before your descent and then you descended to 15, where the log indicated you were at 105 and descended to 63. Looks like a steady ~40 foot offset to me.

Mike

 

[msinger]

I don't think your altitude was correct for any of it

Was it correct at the takeoff point? If so, when did it stop being correct?

 

[msinger]

I'm not dodging your question at all .... Ive already stated that the only anomaly is the reported altitude of 54 feet on impact.

I understand your viewpoint and explanation of what you saw happen. I'm looking for the anomaly in the flight log.

 

[MikeyOnline]

I understand your viewpoint and explanation of what you saw happen. I'm looking for the anomaly in the flight log.

Good point. I'd like to know that too. To that end, I graphed throttle versus altitude for the whole flight. I thought I'd solve the mystery that way but for me at least, it only deepens the mystery. Looking at this graph with throttle (percent) versus baro altitude (meters) with altitude being the red line, it looks to me like the AC responded (in altitude) with each move of the throttle stick. I was expecting to see a jump (up) in altitude somewhere along the way where the pressure dropped or something altered the altitude. Both seem consistent. What am I missing?

Edit: the takeoff point shows 0 altitude which I assume would be close to correct since it was launched from the boat.

Edit2: both the GPS and barometer indicate that the impact occurred about 16 meters (52 feet) higher than the takeoff point.

Mike