I'm able to fly my Mavic Pro out from my second floor balcony, then fly back in and land just fine. However, horizontally out about 20 feet and hovering, there's suddenly some kind of "MAX ALTITUDE REACHED" warning, and it immediately disallows flying up or down. What's wrong?

Change your max altitude on the app or check if there's flight restrictions in your current fly area. I doubt theres a fly restriction if you can already get it up. or your sensors are not allowing you to fly any closer to obstructions

Max altitude is set to 120m (about 400 ft). But the problem is so severe that I cannot even go up or down 2 feet from the hover position. Another clue is that the "max altitude" warning disappears when I approach the balcony to land again.

Also not in restricted area. I've flown around same location when taking off from the ground outside. Flew as high as 300 feet.

Check the GPS Satellites, sometimes if they are less than 6 or 7 you’d get a limited altitude warning  

It showed 7 satellites before takeoff on the balcony after about half hour waiting for more satellites. I thought it would be sufficient because I was not using RTH. Is this a built-in limitation (by design)? Nothing I can do to bypass it? I suppose I could risk hovering 20 feet out for a few minutes in hopes of gaining more satellites.

There have been quite a lot of fly aways with this situation.
It is very risky to perform this procedure as both the compass and the GPS position will probably get in conflict when they receive different values because of the change in environment.
I can't check myself since I sold my MP, but I would never perform this procedure myself.
I know it looks cool (and many have made this type of shot) but you don't want to lose your drone for a cool shot

PS, minimum nr of sat that I wait for is 12 usually.

Montfrooij Posted at 1-11 06:32
PS, minimum nr of sat that I wait for is 12 usually.

If true that the number of satellites would so severely limit flying, literally locking it at hover height, then it means no flying is possible through long tunnels or large caverns!

In fact, this means no flying indoors whatsoever with the Mavic Pro?!  A large hangar or warehouse can easily block all satellites. Same for under-canopies of dense trees.

It makes no sense.

There has to be a workaround or this can't be true.

veryken Posted at 1-11 23:14
If true that the number of satellites would so severely limit flying, literally locking it at hover height, then it means no flying is possible through long tunnels or large caverns!

In fact, this means no flying indoors whatsoever with the Mavic Pro?!  A large hangar or warehouse can easily block all satellites. Same for under-canopies of dense trees.

You can fly indoor without problems.

What MIGHT cause your problem is that all at a sudden the compass, gps and altitude (visual) meters give different readings when you fly away from the balcony.
I can imagine this causes weird behavior.
And it has been the reason for many fly aways in the past here on the forum.

Not saying this is not possible, but just warning you that you would not be the first to lose a drone with this type of flying.  

Montfrooij Posted at 1-12 00:51
You can fly indoor without problems.

What MIGHT cause your problem is that all at a sudden the compass, gps and altitude (visual) meters give different readings when you fly away from the balcony.

So this means totally not possible to start flying from indoors of a large hanger or warehouse or forest where there’s no satellite signal, then continue to outdoors or open space beyond?! Seems to make no sense. This would be HUGE limitation.

veryken Posted at 1-12 01:16
So this means totally not possible to start flying from indoors of a large hanger or warehouse or forest where there’s no satellite signal, then continue to outdoors or open space beyond?! Seems to make no sense. This would be HUGE limitation.

It makes perfect sense, that is why your drone can fly so super stable without much effort from your hands.

The drone relies on 3 main sensor types to 'know' where it is flying and how fast / what direction.

• GPS
• Compass
• Altitude meter (both from compass and from bottom sensor)
  

The drone combines these readings to calculate what it needs to do to remain stable (or fly, depending on what you tell it to do)
If one of the sensor types all off a sudden makes drastic changes in readings, bad things can happen.
For example, if the GPS tells it it is on location B instead of A in a matter of seconds, the drone 'thinks' it is flying way too fast and compensates for this (fake) movement by dropping power.

Now there are various failsafes that prevent this from happening, so it most likely won't fly away.
BUT
If you push the limits too far, it could do just that.
And there are numerous cases where this is exactly what happened.
People start complaining their drone flew away or crashed without a reason.
While in fact they were the reason this happens.

There are a lot of warnings in the manual to stop you from making these mistakes.
I'm just making sure you know what is going on inside the drone.
Just to be clear.
I'm not saying you can't make such a move. You can. But it is a risk.
Nor am I 100% sure it is why the drone behaves like you described.
It is just an explanation why this kind of move can result in fly aways and weird behaviour.

PS, one more question.
Are you in beginners mode?
That is heavily restricted.

Montfrooij Posted at 1-12 01:32
PS, one more question.
Are you in beginners mode?
That is heavily restricted.

The drone is not in beginner's mode. But I understand the failsafes.

It's just really odd for me to imagine a Mavic Pro flying around just fine indoors like in a large empty warehouse, then suddenly act like it's hitting a virtual wall when it goes out through a large open door to the outside and open sky. Maybe I shouldn't complain if this is indeed the common mishap of drones flying away suddenly, which I've never experienced.

Thanks for your extended replies.

veryken Posted at 1-12 06:27
The drone is not in beginner's mode. But I understand the failsafes.

It's just really odd for me to imagine a Mavic Pro flying around just fine indoors like in a large empty warehouse, then suddenly act like it's hitting a virtual wall when it goes out through a large open door to the outside and open sky. Maybe I shouldn't complain if this is indeed the common mishap of drones flying away suddenly, which I've never experienced.

No problem at all.

Still I'm not 100% sure this is the reason why you are seeing this behavior.
That requires some more testing.

(if you are up for that)

Just film what happens and share it on YT  (and here afterwards)

If you use the 'products' section of this forum, you might get (a lot) more response than just me.
That part is way more active

Montfrooij Posted at 1-12 06:37
No problem at all.

Still I'm not 100% sure this is the reason why you are seeing this behavior.

Thanks for the tip about the other section of the forum.

It seems the workaround in my situation, though totally risky, is to switch to S-mode — if I dare to try.

veryken Posted at 1-12 06:51
Thanks for the tip about the other section of the forum.

It seems the workaround in my situation, though totally risky, is to switch to S-mode — if I dare to try.

Sports mode?
I'm not sure if that makes a difference.
The mode that could help is 'ATTI' mode, but you can't select that (and probably don't want to , since a drone in ATTI mode requires very good piloting skills)