Crash. Now what?

[Cheech Wizard]

Per earlier suggestions did a Compass, IMU and RC calibration this morning. Ran some test flights near my home and all went well. Took the MPP to a home building site about 5 miles away to take some video. All looked good at takeoff. As it was rising it got to about 2m and then suddenly began going backwards and crashed in to some brush. I found it upside down in dirt.

Airdata indicates good GPS at start and then declining as it went backwards.

View attachment 37072

I assume vacuum off good (dirt in vents) and do a visual check? Turn on and see what Go4 says?

I'd really like to know what happened. I don't think I pulled back on the right stick but may have. Download .DAT files from it and post here?

Thanks all,

I would suggest turning the props by hand while holding it upside down before just firing it up. I have had grit in the motors from a low height crash which didn't hurt anything but a couple broken props. When I was taking the props off, I noticed one of the motors was not turning properly. Just a couple tiny pieces of grit will kill your motor.

 

[RoboticTundra]

Just to update. This problem appears to have been from the nearby truck. I have since done a number of tests (set MPP down on landing pad facing various directions to see if actual matches display, test flights, etc.) that indicate that the rebar in the concrete foundation does not appear to ever present any problems. Interestingly, being near vehicles has not seemed to be a problem either so at this point this incident appears to be a one-off.

This MPP still seems to have a consistent problem maintaining heading (consistently off by about -20°) which has been an issue since day one (Correcting Yaw w/ Litchi ?)

 

[Mossiback]

... that indicate that the rebar in the concrete foundation does not appear to ever present any problems...

You still need to be cautious around concrete. Your test area works but others may not. One place I fly has concrete that causes errors in some places and not others. The problem goes away if I hand launch.

 

[ac0j]

This whole theory of nearby metal causing errors has me a bit confused. I have been scolded for questioning the theory. I understand it gets the blame for a lot of compass errors.
I have been told that the magnetic force of earth is a very weak thing easily overcome by nearby ferrous metal. And I agree, IF it is a car, or mobile metal of any kind. But the magnetic fields of earth is a VERY strong force.
BUT REBAR in concrete? wouldn't rebar, or other stationary metals eventually polarize with magnetic north? I have done some testing with a mechanical compass and cannot see that being in a normal metal heavy area has any affect on its readings. Maybe in a Faraday cage it would.
So if rebar that has been in concrete for more than a year can foul up BOTH compasses in the Mavic, one would have to say the compasses are JUNK. Isnt it more likely in these cases that the PILOTS are too quick to take off, and the compasses have not had a chance to orient themselves, and GPS hasnt locked?

 

[sar104]

This whole theory of nearby metal causing errors has me a bit confused. I have been scolded for questioning the theory. I understand it gets the blame for a lot of compass errors.
I have been told that the magnetic force of earth is a very weak thing easily overcome by nearby ferrous metal. And I agree, IF it is a car, or mobile metal of any kind. But the magnetic fields of earth is a VERY strong force.
BUT REBAR in concrete? wouldn't rebar, or other stationary metals eventually polarize with magnetic north? I have done some testing with a mechanical compass and cannot see that being in a normal metal heavy area has any affect on its readings. Maybe in a Faraday cage it would.
So if rebar that has been in concrete for more than a year can foul up BOTH compasses in the Mavic, one would have to say the compasses are JUNK. Isnt it more likely in these cases that the PILOTS are too quick to take off, and the compasses have not had a chance to orient themselves, and GPS hasnt locked?

The earth's magnetic field is weak - that's why you need a freely suspended compass needle to detect it. It is far too weak to re-magnetize ferrous metals that have existing magnetization on any relevant timescale. The earth's field is easily distorted in the proximity of magnetized metals. For metal objects the size of rebar the range of influence is only going to be a few inches, but an aircraft placed on a concrete surface may well be that close. Note, however, that not all rebar is magnetized.

Faraday cages block electromagnetic fields, not magnetic fields, and have no effect here.

I'm confused by your comment that if rebar affects the magnetic field then that implies that the compasses are junk. You understand, presumably, that the compasses can only measure the magnetic fields that they are in. If the earth's field is distorted then that is what they see, and measure. They have no way to subtract out the unknown contribution of the rebar, or anything else.

As for questioning the "theory", I don't know what to say. This is not some untested hypothesis that cannot be confirmed. The magnetometer and IMU fusion data in the log files for multiple events of this kind clearly show the magnetic distortion and the effect that it has on the subsequent flight. There is no mystery here.

 

[ac0j]

The earth's magnetic field is weak - that's why you need a freely suspended compass needle to detect it. It is far too weak to re-magnetize ferrous metals that have existing magnetization on any relevant timescale. The earth's field is easily distorted in the proximity of magnetized metals. For metal objects the size of rebar the range of influence is only going to be a few inches, but an aircraft placed on a concrete surface may well be that close. Note, however, that not all rebar is magnetized.

Faraday cages block electromagnetic fields, not magnetic fields, and have no effect here.

I'm confused by your comment that if rebar affects the magnetic field then that implies that the compasses are junk. You understand, presumably, that the compasses can only measure the magnetic fields that they are in. If the earth's field is distorted then that is what they see, and measure. They have no way to subtract out the unknown contribution of the rebar, or anything else.

As for questioning the "theory", I don't know what to say. This is not some untested hypothesis that cannot be confirmed. The magnetometer and IMU fusion data in the log files for multiple events of this kind clearly show the magnetic distortion and the effect that it has on the subsequent flight. There is no mystery here.

Why wouldnt the compass correct for this in the first few seconds of flight? I have to assume it would like any other compass would. The electronic compass in my car never loses its direction, even in long tunnels. The GPS drops out, but returns upon leaving the tunnel.
I guess my point was, EVEN IF there is magnetic interference for a "few inches" off of buried rebar near the takeoff point, what causes this to crash a Mavic if not pilot error? It seems the pilots are fighting the incorrect compass causing the crashes, when if they just waited a few seconds it would be a non issue.
Is it incapable of making the correction in flight within milliseconds of leaving the interference? I have never had the problem, so I am just curious.

 

[sar104]

Why wouldnt the compass correct for this in the first few seconds of flight? I have to assume it would like any other compass would. The electronic compass in my car never loses its direction, even in long tunnels. The GPS drops out, but returns upon leaving the tunnel.
I guess my point was, EVEN IF there is magnetic interference for a "few inches" off of buried rebar near the takeoff point, what causes this to crash a Mavic if not pilot error? It seems the pilots are fighting the incorrect compass causing the crashes, when if they just waited a few seconds it would be a non issue.
Is it incapable of making the correction in flight within milliseconds of leaving the interference? I have never had the problem, so I am just curious.

The compass heading at the takeoff point is used to initialize the IMU heading (yaw) value. After it is initialized, the heading is updated by the sensor fusion scheme that uses, as its primary data source, the fast rate gyros. The subsequent compass data are used only as low gain corrections to correct for drift and bias in the rate gyros. If the aircraft takes off with an incorrectly initialized yaw value, then as soon as the aircraft leaves the area of distortion the compass reading will become correct, but now there is a discrepancy between the compass reading and the calculated IMU yaw because the rate gyros will not have detected any rotation. That either causes the aircraft to drop into ATTI mode (preferable) or results in uncontrolled flight when pitch and roll produce aircraft motion in unexpected (by the FC) directions.

 

[ac0j]

The compass heading at the takeoff point is used to initialize the IMU heading (yaw) value. After it is initialized, the heading is updated by the sensor fusion scheme that uses, as its primary data source, the fast rate gyros. The subsequent compass data are used only as low gain corrections to correct for drift and bias in the rate gyros. If the aircraft takes off with an incorrectly initialized yaw value, then as soon as the aircraft leaves the area of distortion the compass reading will become correct, but now there is a discrepancy between the compass reading and the calculated IMU yaw because the rate gyros will not have detected any rotation. That either causes the aircraft to drop into ATTI mode (preferable) or results in uncontrolled flight when pitch and roll produce aircraft motion in unexpected (by the FC) directions.

OK, I think I get it. It cannot correct itself after take off then? So If I take off with the compass arrow on the app pointing in the wrong direction, I am pretty much expecting a crash unless I can land it and start over?

 

[sar104]

OK, I think I get it. It cannot correct itself after take off then? So If I take off with the compass arrow on the app pointing in the wrong direction, I am pretty much expecting a crash unless I can land it and start over?

That's right - and that's why that check is important. The best outcome if it does take off with an incorrect IMU yaw value is a quick switch to ATTI, but the Mavics seem to take longer than the Phantoms trying to reconcile the error, which leads to uncontrolled flight.

 

[Mossiback]

I have been told that the magnetic force of earth is a very weak thing easily overcome by nearby ferrous metal....BUT REBAR in concrete?...

A compass is a magnet that is affected by steel (ferrous materials). It is easy to test. Just place a steel item next to a compass and watch the needle move. That is why boat compasses have adjustments to compensate for nearby metal. Concrete floors have more than rebar. They also include steel mesh to tie the cement together and in places this mesh may be close to the surface. When I get a compass error while sitting on concrete, the error will go away when I lift the Mavic up a few feet. In this case I just hand-launch and land.