Hi. I’m building a balsa model that is a combination of Mini Talon and Krossblade SkyProwler.
I see the problem after the close of the multirotor engines in the fuselage.
I wonder how to set Pihxawk to not run the multirotor engines if they are not extended…
I am going to control the opening and closing of motor arms with an external microcontroller (probably STM32F103) with my firmware, however, I have to connect it with Pix. to properly start engines without destroying the plane 
With control from Taranis, I can handle the appropriate channels (parameters and delays), but what about Mission Planner?
It might be easiest to simply use the Taranis Flight Modes for triggering the boom extension. Each flight mode can use a channel mix that sets high or low PWM on a spare channel of your choice.
In this manner, you don’t need to involve the Pixhawk or Mission Planner.
Keep in mind that having extended booms eliminates the benefit of using the quad motors to help for forward flight stalls.
Hi Greg! Thank you for your answer in the topic. I was thinking about setting the RC radio and flight modes the way you wrote. We also think in a similar direction - control of booms not by Pixhawk… But what about an autonomous mission? There will be no flight modes from Taranis…
I saw that Pix sends in mavlink text info like “conversion complete” after switching to fixed wing (this is a good signal to hide the booms). Does switching to quadrocopter mode precede any mavlink message?
I remember - there is no quadro support in airplane mode - but in the concept of this model, the plane is to be an airplane - without support
Yes, the Taranis Flight Modes will not work for autonomous missions. You might consider trying the binary-tiltrotor configuration where the tilt mechanism can only be in one of two positions. In your setup, you would trigger the boom extension instead of the binary tilt mechanism. This setup is used on a model like the CL-84 where a retract servo was used for determining up or forward. In your setup, it would be used for in or out.
You can read about this configuration below.
If you control the opening and closing with a separate uC, why not pass through the motor PWM signals and inhibit starting if they are not fully opened ?
Hello, Thank you for the next hints… Pass through the motor PWM signals is provided for the protection of the fuselage structure 
However, I would like to receive additional information from FC about the current mode.
Thanks to you, I have basic project assumptions. In a few days I will start to inform about the progress. Thanks!