Fly-By-Wire Low Altitude Limit¶
This function of Plane allows you to set a minimum altitude for FBWB and CRUISE modes (even if you don’t have airspeed sensor) which your airplane will try to stay above.
When ALT_HOLD_FBWCM is enabled, if your airplane goes lower than defined altitude relative to HOME, it will level up and/or climb back to this altitude. After it is reached you will regain altitude control.
the minimum MSL altitude target will be the higher of: this altitude, or the current altitude target above terrain in FBWB or CRUISE modes if Terrain Following is enabled, or the FENCE_ALT_MIN above HOME if that fence (See GeoFencing) is enabled.
this parameter is in centimeters, not meters!
Use for R/C training¶
This function will help those of us who doesn’t have that much flying skills to fly safely. It is really useful for FPV flying as FBWB or CRUISE modes provide easy control and the ALT_HOLD_FBWCM limit will help you avoid crashing into the ground ;-)
Here is a video demo. APM is in FBW-B mode and I hold pitch all the way DOWN! until the very end of video. Minimum alt was set 30 meters above home altitude, so you can see ground.
How to setup Alt Limit¶
Using your APM Mission Planner you will have to set parameter ALT_HOLD_FBWCM to the desired alt in centimeters! If it is 0 then it is turned off.
Choosing the right altitude¶
Desired altitude should be set very carefully. As we know our planes can’t react immediately, so we have to choose altitude with this in mind. Using 3000 (30 meters) is recommended for flights over pretty flat terrain.
In order to land your plane you will need to disable this feature by changing your flightmode to anything other than FBWB or CRUISE. FBWA, STABILIZE, or MANUAL modes are always the preferred modes for a manual landing, in any case.