RTL or Rally Point (return height)

Hi, I am just considering what may happen if I fly over some high terrain and my plane goes into RTL or the nearest rally point.
If I set my RTL height to be 100m (relative to my launch/arming position) and I happen to be flying over a terrain that is 300m higher than my launch/arming position, am I at risk of the aircraft trying to descend to 100m and crashing into the terrain? Or is there a way I can program it to fly home at a given height before it descends to said 100m RTL height?

Currently, I have the parameter ALT_HOLD_RTL set to -1 to ensure the plane stays at its current height while it transits back to launch/arming position.

I am now considering the use of rally points, however I cannot see a way I can even set the plane to maintain its current height.

Thanks, Justin


Hi Justin,

One way to get around this is to activate Terrain Following (http://ardupilot.org/plane/docs/common-terrain-following.html), in the documentation it states that this works for RTL mode:

“Being able to come over a hill rather than trying to fly through it when you enter RTL in a hilly area is very useful!”

When planning flight plans in Mission Planner, you also have the choice of setting relative, absolute or terrain altitude for your waypoints. Activating terrain altitude will allow your aircraft to take into account the terrain when navigating waypoints.

The drawback of this is that with the aircraft will waste more energy changing height to maintain altitude over terrain, especially in very hilly places.

Finally, you can use SITL to verify whether the aircraft is flyable in said terrain before going for an actual test flight.

Hope this helps :slight_smile:

You can set your RTL rally points to altitudes that are high enough to clear any part of your terrain. When in RTL, the airplane will climb to the altitude specified for the rally point.

Thank you both for your input. Sorry for the delayed response. Been very busy lately.

Yesterday I did a flight and decided to use the rally point option. This is the best option currently I think.

Thanks, Justin