Been much discussion about Compasses & sensors, Magnetic Interference, Detecting, Toilet Bowl Effect & Crashes, and methods to avoid.  Comments from discussions, how T.B.E. can occuor, and previously testing I had done came idea that DJI's drones startup process might be part of problem.  In that powering up drone when it was sitting in area effected by Magnetic Interference was not being detected, because of process establishing compass norms for general area.  


Idea was after picking a takeoff area, to move 2 to 3 meters away.  Sit drone down, and power it up.  While watching GO-4 App main screen's ''Green'' (good to go), pick drone up and carry to chosen takeoff area.  If there was any magnetic interference at power up spot, there should be a brief message about magnetic interference or need to calibrate compass; due sudden change to Earth's unaffected magnetic field.  Same as what happens to drone's compass that results in T.B.E.  Followed by placing drone at chosen takeoff area.  If there is any magentic interference there, again a message about magnetic intereference or need to calibrate compass should appear because of reverse, sudden deviation to Earth's normal magnetic field.

Cutting to chase, testing with my Mavic Pro proved this idea will not work.  However, from failure of idea comes a warning.  

Mavic Pro (likely other DJI drones) will not indicate Magnetic Interference on Main Screen's ''Green'' (good to go) when source of magnetic intereference is in a narrow range.  Range, which may lead to T.B.E. and likely crash, because pilot is not alerted.  From this came a possible way to watch out for magnetic intereference that eliminates virtually all previous objections, such as having to carry a Smartdevice, need to download 3rd-Party App, not DJI equipment or software, briefly walking around area, etc.  

And idea that does alert pilot to magnetic interference in narrow range and strong interference.


The workable Idea:


How to get ready:

Under main menu of GO-4 App - in middle at Top of Screen - Tap on QuadCopter Icon
At bottom of MC Settings screen - Tap on Adance Settings
At bottom of Advance Settings screen - Tap on Sensor State
At top of Sensor State screen - Tap on Compass

Below Interference, you should see two compasses for Mavic Pro, and one compass for Mavic Air
Blue dot next to Compass (1 or 2) indicates which one is being used in case of Mavic Pro

Checking for magnetic interference:

After powering up your drone, and it completes startup.  What you want to see {and not see} at your chosen takeoff spot

You want to see short 'Green' bar(s).  {You do not want to see 'Red' bar(s) at all.  And it is best you do not see 'Yellow' bar(s).}

An example of two short green bars:


If you are wondering why this shows green, it is because compasses on drone are just far enough away from sheetmetal.


An example of red bar and yellow bar.


Time to find another takeoff place!

An example of a Yellow bar and Green bar.
With DJI system picking compass with Yellow to use for flight; instead compass with Green.


DJI's Main screen showed ''Green'' (good 2 go)
But I would move drone to another location.


For those curious,

How testing was setup


The testing involved two phases.  

Critical to both phases, was finding source for Magnetic Intereferece that would cause slight interference, but not enough interefernce that resulted in 'Red' Error messages about Compass Calibration or Magnetic Inteference when Mavic Pro was placed on or about source.  Goal was to come as close as possible to a case of interference resulting in Toilet Bowl Effect.

Some of materials used.  Minus cardobard strips used in place of sheet metal strip.


Square sheet metal, insuficient magnetic interference

3/16" thick metal bar stock, far to much magnetic intererence
Narrow strip of sheet metal, slight magnetic intereference (chosen for testing)
1 of 4 Pieces of Plywood used to simulate ground, and to hide cardboard strips vs. narrow strip of sheet metal.

First phase was to Test the idea of powering up drone away from source, then moving drone to source.  That idea as mentioned was failure.  As part of this testing came workable Idea.  Along with worrisome fact, it was possible to have magnetic interference yet get 'Green' (good 2 go) on GO-4's main menu.

Second phase was to Test the idea (now workable Idea) in blind manner.  Drone pilot (me) would not know if there was or was not the source of slight interference.  Naturally, I had to involve a friend, which is going to cost me a few Brewskies.

Final setup for blind testing.  Four plywoods pieces, 3 with cardboard, and one with narrow strip of sheet metal.


As seen from by Mavic Pro after a successful detection and move.

If you are wondering why strips of cardboard, it was to ensure I would not be able to visual see there was nothing under a piece of plywood.

I plan to do some more testing.  Along with testing a suggestion made by SparksBird in another post.

Excellent my friend this is great stuff.  I am looking forward to the further results.  These kind of tests are very interesting.

SparksBird Posted at 2018-6-5 18:20
Excellent my friend this is great stuff.  I am looking forward to the further results.  These kind of tests are very interesting.

Your idea for further testing in regards to smaller drones being closer to source is a good one.  
It will several days before I have a chance.  Responsibility unfortunately get in way of playing.  

You know I’ve been saying this to you for months now along with a very experience pilot Ardvark, and I’m glad to see it has eventually got through to you. Checking compass values in your dji go app will result in less problems.

Compass error on its own shouldn’t cause a TBE..  however as a proof of concept to the “blind spot” that these drones and software have its a good concept to prove a point.   

The biggest mistake I think happen is newbie pilots see compass error and says to calibrate and they just calibrate it on location, right where they generated the compass error... this usually leads to the drone getting itself confused... which is stage one of TBE...

HereForTheBeer Posted at 2018-6-6 03:51
Compass error on its own shouldn’t cause a TBE..  however as a proof of concept to the “blind spot” that these drones and software have its a good concept to prove a point.   

The biggest mistake I think happen is newbie pilots see compass error and says to calibrate and they just calibrate it on location, right where they generated the compass error... this usually leads to the drone getting itself confused... which is stage one of TBE...

You are correct, if you get warning to calibrate or move, you should always move.

Very rarely see request to calibrate compass on products after P4, I always check compass(s) before flight. On the other hand my P3P requires compass calibration before every flight if the Go App is shut down between battery swaps. If I leave the Go App up and running during a battery swap then no calibration required.

Thanks for the work  you put in.  Don't you just hate it when responsibility gets in the way?

I'm looking forward to the next set of tests and results!