Descent and climb rate limit when near ground


We are currently flying the Freefly Systems Alta X which is equipped with a PX4 and uses QGC.

We use this drone for purely cinema applications and some of the baked in functions geared for autonomous flight are detrimental if not down right dangerous for the way we use this drone.

Problem 1: when the drone is within 20ft of take off altitude the climb and descent rates are drastically limited when in altitude hold and GPS modes. For cinema flying any loss of throttle authority especially at low altitudes is extremely dangerous. Also when smoothly descending from an altitude above 20ft when the drone breached 20ft it instantly slows down which absolutely kills any cinema shot.

I cannot for the life of me figure out which parameter if any can get rid of this limit. I have been told this is linked to the auto land descent speed but this doesn’t seem to affect this issue.

Problem 2: when flying below your take off altitude throttle authority is drastically limited in altitude hold and gps modes. I have been told this is something that cannot be changed.

If anyone has any input on how to fix or change this this would be amazing.

I also think that if I fact these parameters are not changeable in the current software I think it should be a top priority as these limits when flying fast and low are extremely dangerous.


Hey @Papacopter

It’s interesting to read your feedback.
The vertical slow down is a feature you can configure to your liking.

It is defined by 4 parameters: If you fly below the altitude MPC_LAND_ALT2 in meters your downwards speed is limited to MPC_LAND_SPEED and your upwards speed to MPC_TKO_SPEED. If you fly above an altitude of MPC_LAND_ALT1 meters your vertical speeds have the usual limits and if you fly between AL1 and ALT2 the limits gradually transition such that there’s no abrupt setpoint change.

I’ll make sure to add this information to the altitude mode documentation:

There’s no single switch to turn off the feature.

I don’t expect you to run into safety issues except for very fast autonomous takeoffs when raising the takeoff speed to the value of your maximum speed MPC_Z_VEL_MAX_UP. But there is definitely an issue with increasing the land speed to MPC_Z_VEL_MAX_DN or lowering ALT1 and ALT2 below the ground level because if you ever fly an autonomous RTL for example in a failsafe situation like RC loss the drone might not slow down but slam into the ground.

As a conclusion for Problem 1 you can lower ALT1 and ALT2 to an altitude you’re comfortable the drone can still slow down when landing. For Problem 2 there’s no safe goto solution at the moment.

I took note of the requirement to completely disable slow down close to the ground when flying manually in altitude controlled modes. Probably Freefly could incorporate changes once available. I suggest you inform them about your request.

I would point out some safety issues with changing those-

if you increase MPC_LAND_SPEED, if the drone executes failsafe RTL, it will slow down only to MPC_LAND_SPEED which could lead to hard ground impact when landing.

Also, Setting the LAND_ALT parameters will give the aircraft less time to slow down in an autoland and could hit the ground hard.

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Thank you so much for your feedback.

I think the ultimate fix would be to uncouple the settings for automated flight and manual flight modes so we can have a safe fail safe RTL action as well as full control in any manual flight mode. Do you think this change could be made in future updates?

Since myself and my other pilots have been flying drones on set for 10 years and we have never had a full fail safe RTL we are going g to take the risk and change these settings to achieve full throttle authority at any altitude. We will test fly these settings tomorrow so please let me know if there are any other warnings other than the know issues of a RTL being too fast.

Here is how we have adjusted our settings:

MPC_Land_Alt 1 - 33ft (default)

MPC_Land_Alt 2 - 16ft (default)

Mpc_land_speed - 18mph (default 1mph)

Mpc_Z_vel_max -DN - 17.9mph

MPC_TKO_SPEED - 18mph (default 3.4mph)


Thank You for feedback, MaEtUgR !

I think the ultimate fix would be to uncouple the settings for automated flight and manual flight modes


Do you think this change could be made in future updates?

Yes definitely, I absolutely see your use case. The default will not change but the configuration option will be there. No timeline atm.

we are going to take the risk
Mpc_land_speed - 18mph

You definitely would definitely take a very high risk with that configuration. Since any autonomous modes like RTL, Land, Descend use the land speed to descend the drone to the ground and experiencing any autonomous failsafe will almost certainly destroy the drone and possibly what’s underneath it. You have been warned and it’s strongly discouraged to do fly like that. It’s definitely out of specifications, I’m not liable in any way and I’d assume the manufacturer will not support issues arising from that configuration.

if there are any other warnings other than the know issues

A possible implication could be different landing detection behavior. This can be safety critical depending if and depending on what issue you might see. In any case you should be very careful.