Today I was trying to fly down a drive which is canopied with trees with my new MPP. For some reason the Mavic kept descending - despite there being no forward pressure on the left stick. I made several attempts, but kept losing the shot, because the Mavic would after flying straight and level for a period, the start to descend. Anyone else encountered this? This is my third Mavic and I’ve never encountered this sort of behaviour with my previous a/c.

Confused: You meant despite no BACKWARD pressure on the left stick (descending), right?

There are at least a couple of flight options that come to mind which could cause this.  There is a Terrain Follow mode that is especially  useful going uphill - i presume it works the same for a downhill run.  The other possibility is that the VLS system is attempting to maintain you height above ground for stability.  Try turning  OFF  VLS..  I assume your starting your forward flight at around 5 ft AGL,

Upload the DAT file from the flight if you can access it or the TXT file if you can't; that may shed some light on what's happening.

Griffith Posted at 2018-1-12 06:00
There are at least a couple of flight options that come to mind which could cause this.  There is a Terrain Follow mode that is especially  useful going uphill - i presume it works the same for a downhill run.  The other possibility is that the VLS system is attempting to maintain you height above ground for stability.  Try turning  OFF  VLS..  I assume your starting your forward flight at around 5 ft AGL,

There is a Terrain Follow mode that is especially  useful going uphill - i presume it works the same for a downhill run.

It only ascends -- never descends -- in Terrain Follow mode.

If light is not good enough VPS won’t work correctly, did you have good gps?

I might be tempted to calibrate the visual sensors using DJI Assistant 2.

In my armchair analysis, there only two (or 3 or 4) systems that could be used to maintain altitude - and I am assuming that the Mavic actively tries to maintain altitude  - absent elevator movement :-)  This would be the case when hovering, for example.

(1) barometric pressure altitude - should be fairly stable.  Weather related pressure changes may fool the Mavic into altitude changes.

(2) GPS-derived altimeter - less accurate, so the Mavic probably defers to barometric altitude.

(3) VPS height (above surface) - much more accurate if the drone is below about 30 ft. AGL   (Fairly sure VPS is used in Terrain-Follow, and most definitely in low-altitude hovering.) VPS utilizes a downward directed ultrasonic measuring system for its primary height measurement; and a visual system for lateral stability when stationary (assuming good optical conditions)  Note that the ultrasonic VPS height measurement can be inaccurate and erratic over rough terrain.

(4) The only other system which may exert some control is the inertial system  -  3-axis accelerometer and compass (compass not involved here)

So, if OP wants to fly straight and level over a falling terrain, the Mavic would need to: (1) ignore VPS height, (2) try to maintain a zero Z-axis acceleration, and (3) maintain a constant barometric pressure altitude.  (I'm tossing out GPS for lack of elevation accuracy.)  

Things that could effect OP's  outcome - wind, barometric pressure changes, and functioning VPS height.  

The other possibility is that the Mavic doesn't even attempt to maintain height, only stability. However, I've done a fair amount of low flying (4-5 ft) over flat level ground and haven't noticed much, if any, altitude change. Flying level at higher altitudes is obviously harder to evaluate - but I know altitude stays fairly constant.

So I'm still guessing that VPS is involved in OPs issue - or wind or weather change.  Try turning off VPS.

Good Luck! - Try flying straight and level over a level surface and see if that works.

This happened to me once when I was filming with a canopy of trees overhead.  It was like flying a toy drone without any assistance from GPS.  I later realized that the trees was not allowing a good signal to come through and the mavic was relying on the down sensors.  Of course the trees was also not providing enough lighting to reach the floor.  Typical I think!

M Stuart K Posted at 2018-1-12 12:44
This happened to me once when I was filming with a canopy of trees overhead.  It was like flying a toy drone without any assistance from GPS.  I later realized that the trees was not allowing a good signal to come through and the mavic was relying on the down sensors.  Of course the trees was also not providing enough lighting to reach the floor.  Typical I think!

Interesting. Thanks!

Yes i have had this happen a few times. I haven't been able to workout why it happens. But I have found tapping the up joystick just slightly seems to stop it.

PilotC Posted at 2018-1-12 21:37
Yes i have had this happen a few times. I haven't been able to workout why it happens. But I have found tapping the up joystick just slightly seems to stop it.

True, upping the stick will correct it, but it spoils the evenness of the shot.

Sir, have you tried to calibrate the RC? I'll attach the tutorial video for double check.

DJI Susan Posted at 2018-1-13 23:57
Sir, have you tried to calibrate the RC? I'll attach the tutorial video for double check.

https://www.youtube.com/watch?v=psFeOF1lBsI

Thanks, Susan. Is it safe to expect recalibration to work if it was occurring all the time? The issue only seemed to arise as I was flying along the canopied drive.  

GMG Posted at 2018-1-14 04:02
Thanks, Susan. Is it safe to expect recalibration to work if it was occurring all the time? The issue only seemed to arise as I was flying along the canopied drive.

If the issue appears all the time, calibrating RC will be helpful. Regarding your issue, may I know how many GPS satellites at that time? Could you sync your flight records and offer your DJI Account here? We'd like to check the exact status.