Here's a link to an ill-fated phantom 4 pro flight. We wanted to get a shot of the talent from above while on a (metal) railway bridge. The railway bridge turned out to be a really good faraday cage, and the drone failed spectacularly. With no pilot input, the drone accelerated to 19m/s, lurched hard right and crashed into a girder. You can see all the guidance and avoidance systems fail - compass, GPS, forward, rear and downward facing vision systems. Everything died. If you are wondering, yes, the drone survived, with a few scuffs, and bumps, and required a new set of props and recalibration of the vision systems.

You can see all the guidance and avoidance systems fail - compass, GPS, forward, rear and downward facing vision systems.

The only thing that failed was the pilot.
When you fly into a steel cage, the compass has no way to tell where north is any more.
All that steel is telling it that north is everywhere.
Once you have crazy compass data, all bets are off.
Your Phantom defaults to atti mode and your sensors won't help much because the Phantom has no position holding and no way to fly straight.

Labroides Posted at 2017-8-21 19:27
You can see all the guidance and avoidance systems fail - compass, GPS, forward, rear and downward facing vision systems.

The only thing that failed was the pilot.

I would have to agree.  I have several large bridges in my area I though hmm should I fly near one.
I could not even launch within 50m of most of them or get compass error. So there was no way I wanted to fly under one either.  I just fly over them or not too close.

Yikes.  Not what I needed to see right before bed.

WHAT MODE were you flying in?  

Smart flying, Not getting to close.

Never had a problem with a bridge :-)

After retouching...

Why would a compass error disable GPS?
Orientation (compass data) and position (GPS data) are different but related.
When the Phantom is moving and getting position information from the GPS that tells the Phantom that it is travelling in a different direction from what it's getting from compass data, The Phantom programming can't work with the data conflict.
The solution is to drop data from one source and since the Phantom can't fly without a compass but can without GPS, it's the GPS data that gets dropped.