# Optimization engine failure behaviour (coaxial octocopter)

**URL:** <https://discuss.px4.io/t/optimization-engine-failure-behaviour-coaxial-octocopter/7947>\
**Category:** PX4 Autopilot\
**Created:** [September 28, 2018, 1:07pm UTC](https://discuss.px4.io/t/optimization-engine-failure-behaviour-coaxial-octocopter/7947 "2018-09-28T13:07:29Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![VertLim](https://discuss.px4.io/letter_avatar_proxy/v4/letter/v/96bed5/32.png) [@VertLim](https://discuss.px4.io/u/VertLim)\
**Post date:** [September 28, 2018, 1:07pm UTC](https://discuss.px4.io/t/optimization-engine-failure-behaviour-coaxial-octocopter/7947/1 "2018-09-28T13:07:29Z")

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Dear all,

I am running PX4 on a coaxial/X octocopter and I am very interested in optimizing the flight behaviour in case of a single engine failure. The problem is, that the system has little power reserves as the hover throttle under normal conditions is already around 60%. I know that my current configuration can fly stable and maintain height as the same UAV is flying better with 7 engines on a DJI flight controller.

I assume that the main “problem” is, that attitude control (roll, pitch and especially yaw) is more important than vertical thrust. So in order to reach the roll/pitch/yaw setpoints, thrust is reduced in order to avoid mixer saturation. This is also explained here: [https://docs.px4.io/en/advanced\_config/pid\_tuning\_guide\_multicopter.html](https://docs.px4.io/en/advanced_config/pid_tuning_guide_multicopter.html) (_“If mixing becomes saturated towards the upper bound the commanded thrust is reduced to ensure that no motor is commanded to deliver more than 100% thrust. This behaviour is similar to the Airmode logic, and is applied whether Airmode is enabled or disabled”_). It seems, that this high thrust strategy cannot be disabled like for low thrust with the MC\_AIRMODE parameter.

Reducing thrust on a copter with little reserves and an engine failure obviously means that it is not able to maintain its height any more, which is what I want to avoid. Is there any possibility to optimize the default attitude control / mixing strategy / flight behaviour to reach my goals:

- leveling of the copter is important in order to avoid horizontal drifting
- thrust in Z-direction is more important than reaching roll/pitch angle setpoints which are unequal zero
- reaching the yaw setpoint is least important, but of course the UAV should not start spinning

Any help or suggestion is very much appreciated!

Many thanks,  
VertLim