The recent rains and snows in California have left a lot of image ready landscape out there.  Today while driving in the desert I saw an image I had to get, the snow capped Sierra Nevadas on one side and the equally snow capped Panamints on another.

  

  

I got the P4 out of the case, inspect the aircraft, insert a battery, calibrate the compass, and get ready to fly for a few minutes.  The battery had been in the case for a bit over a week and the temperature was cold, so it was not 100% but still showed over 55% and this was going to be a short hop.  The intent was to climb to 100 feet, do a couple slow 360's with video running and then grab some stills in multiple directions, climb to about 200 and do it all again, and maybe up to 300 after that.  Land and go home.

  

  

The launch point was about 40 feet north of my vehicle.  I was just going to go straight up from that to do the imaging.

  

  

When I launched I ran it up to about 15 feet to check things out.  I did notice the quad seemed to be less stable than I was accustomed to, drifting a bit in location and altitude.  Not bad, just noticeably less stable.  There was no wind.  The quad seemed to stable out, so onward I went.

  

  

Up to 100 feet, take the video.  Up to 180 feet and do the same thing.  Most of the time eyes on the drone, but sometimes head down to see how the vid / pics were going.  During one point while eyes on the drone I got 6 beeps from the tablet / controller, but when I looked down there was nothing unusual on the screen.

  

  

Just before I was going to land, and still hovering at 180 feet, I went head down for about 30 seconds, looking at the image on the tablet.  At about the time I started to go eyes out to visually find the drone I realized it was getting louder, fast.

  

  

I looked up in time to see it plummeting downwards, level flight, all propellers turning, coming down very fast, faster than I have ever been able to make it descend.  Further, it was centered right over my truck, which was behind me in relationship to the launch point.  I went full up on the stick, but this did not slow the descent in the least.

  

  

The P4 impacted the center of the roof of my truck hard enough to dent the roof.  Two of the blades got tangled in one of the antennas on the roof of the vehicle, causing a Motor Obstructed warning, the other two blades still turning.

  

  

After getting everything shut off I started looking at damage.  Surprisingly there did not appear to be much, other than the dents in the top of the truck.  One of the P4 blades was broken, from the marks apparently on impact with the antenna.

  

  

After an inspection of the P4 I put 4 new blades on it and turned everything on to see if anything worked.  The battery still showed over 40%, and all indications were fine.  After convincing myself the drone was, maybe, fine I launched it again and flew it, at low level, for a few minutes.  There was no sign of abnormal operation.

  

  

On playback of the flight on my tablet I can see that at about the time the P4 shot up in altitude about 40 feet, without stick input form me, to about 220 feet on the data (if it can be believed).  It then slid sideways (to the position over my truck) while visually rapidly descending on the video, and showing a slight descent in the data.  On impact with the top of the truck, and the Motor Obstruction warning, the altitude in the data still shows about 120 feet above launch point.  Obviously it could not be at 120 feet and still be putting those dents in my roof, but either the data is wrong or the World moved.

  

  

It might be worth noting the downwards collision sensors appear to have done nothing to stop the impact.

  

Further analysis of the flight data, both the DJIFlightRecord for that flight and the .DAT file data, seem to show that initially all was well, but about 77 seconds into the flight the GPS altitude data and the barometer altitude data started to diverge.  The barometer data seems to slow and lag behind what the P4 was actually doing.  For example at the time it impacted the top of my truck, and the first motor obstructed indication, the barometer altitude data showed the drone in a descent, but still over 35 meters above the top of the vehicle.

  

Since then I have had two more uneventful flights, showing no abnormal indications.

  

Have other people had similar failures?

  

T!

Did you get the memo, and were you flying during the military's test times?  https://www.faasafety.gov/files/ ... Flight_Advisory.pdf

fansa84fe8a4 Posted at 2017-1-25 20:40
Did you get the memo, and were you flying during the military's test times?  https://www.faasafety.gov/files/notices/2017/Jan/NTTR_17-01_GPS_Flight_Advisory.pdf

That's completely irrelevant.
GPS is not used by the Phantom for altitude data.
GPS issues would only affect horizontal position holding - not vertical.

fansa84fe8a4 Posted at 2017-1-25 20:40
Did you get the memo, and were you flying during the military's test times?  https://www.faasafety.gov/files/notices/2017/Jan/NTTR_17-01_GPS_Flight_Advisory.pdf

Yes, I was slightly over 6 hours outside of the time window for the date of the incident.

As Labroides pointed out, the GPS altitude is not used for altitude hold on the P4, rather the barometer is what is used until the quad is close to the ground.

Further, the GPS altitude was the more accurate of the two at the time of the crash, since at the time of the first Motor Obstruct warning the GPS altitude showed the altitude slightly over 2 meters above the launch altitude (about the correct altitude for the location, on top of my truck) while the barometer data shows the quad still over 35 meters above that point.  But for the first about 77 seconds of the flight the two data tracked well.

Also keep in mind the barometer data was still present, the barometer did not stop sending data so the sensor did not fail, however the barometer data lagged GPS altitude data by several seconds.

T!

Which firmware/software are you using ?

My initial thought is the battery, you say it was cold, had been sitting for over a week and was only showing 55%.

You could try posting the flight data to :-

http://www.phantomhelp.com/LogViewer/Upload/

That may give some idea as to what happened.

Interesting about the deviation and separation between the GPS altimeter and the barometer sensor separting in the OP's. post.

In another forum where the guys build these things they were discussing the GPS altimeter and barometer sensor (Some part number for the barometer.) where they differed.  As the barometer warmed up over 10 minutes the reading wavered before it stabilized.  GPS was providing altitude, location, time, and something else I think.  Personally, I don't think GPS data is excluded in the altitude reporting for the drone as the barometer is more for redundancy checks to stabilize flight in ATTI mode.  We don't know what the engineer's programming is, but I doubt they'd exclude the data from the GPS sensor for altitude.

With mine, the altitude drifts as it warms up in flat-field flying by as much as 50 feet.  It might be that the barometer is warming up and stabilizing in the 10 minutes whereas the GPS altimeter is giving out data although not as accurate (I think they mentioned the altitude error could be as much as 1.5 times the horizontal error.).  I haven't tried shoving in a new battery and seeing if a resume of same flight is any more correct on the second flight once the unit has warmed up.

I've wondered if the combination of all the data in the OP"s unit, especially the separation of the GPS and barometer altitudes mentioned "Further analysis of the flight data, both the DJIFlightRecord for that flight and the .DAT file data, seem to show that initially all was well, but about 77 seconds into the flight the GPS altitude data and the barometer altitude data started to diverge.  The barometer data seems to slow and lag behind what the P4 was actually doing" put it into a RTH mode so it climbed and descended over him in P mode?  Speed of fall is disturbing too.

There could be as many as four different sensors running for altitude and how they tabulated and work the data so we don't know unless an engineer for DJI tells us.  DJI made some altitude mods several versions back to correct the drift, but just what sensor they made it to or in combination with is unknown.

Aardvark Posted at 2017-1-26 06:09
You could try posting the flight data to :-

http://www.phantomhelp.com/LogViewer/Upload/

That may give some idea as to what happened.

I did that already for a thread in another forum, the link is here http://www.phantomhelp.com/LogViewer/0FGYX8OJVQANVY9AHNX2/

The battery, while not topped up, was fine for the duration of the flight.  Looking at the data from the .DAT file shows that the quad was able to draw all the power it asked of the battery, right up until the end.

T!

Here is a plot of the GPS altitude, Barometric altitude, Commanded RC Throttle position, and Power used.



GPS Altitude = Dark Blue
Barometric Altitude = Magenta
RC Commanded Throttle = Yellow
Power drawn = Cyan

You can see the power draw increase when the motors throttle up, whether commanded by me or the auto pilot.  This is a fair indication of if the quad is trying to climb or descend.

You can see that the barometric altitude and the GPS altitude track each other well until about 67 seconds into flight.  At about 86 seconds into flight the GPS shows the altitude under the commanded alt and also shows a small step down in altitude on the barometric altitude.

At this point (when the baro alt steps down about 2 meters or so at about 86 seconds) the autopilot attempts to raise the quad, you can tell this by the increased power draw consistent with throttling up the motors.  The GPS altitude shows the quad rising during this time, but the barometric altitude slows a much slower rise.

At about 108 seconds into the flight the baro pressure reaches the original commanded altitude and the current draw relaxes.  GPS altitude shows the craft starting to descend.  Barometric altitude also starts to rise, coming in line with the real altitude of the craft.

From about 108 seconds until impact the aircraft oscillates, porpoises, up and down.  The GPS data looks to be responding faster than the barometric data.

At about 134 seconds you can see my throttle inputs trying to stop the descent, and the associated power increase, indicating the craft was responding, but it was too late.

At about 135 seconds the GPS data shows the drone at the final altitude, on top of my truck, while the barometric data shows the altitude at about 39 meters up and falling.  It also shows an increased power draw for several seconds as two of the blades were obstructed but trying to turn.

To me it looks like the autopilot got behind because of late barometric pressure data.  It was responding, but several seconds behind what was happening in the real world.

T!