My previous post about the side-slips being prohibited in C172 with Thielert 135 engine started a debate in comments, so I checked everything again today, and here is what the AFM and my flying club decided.

The risk of non-coordinated flight is such a plane fuel starvation. As the diesel engine needs very high fuel pressure (about 3′000 bars), it has a low pressure and high pressure pumps. If the pumps run for more than 15 seconds without being fed, they will get damaged.

As prolongated side slips (15 seconds or more, which is a very long side-slip) can lead to such a starvation if the tanks contain less than 1/4 of their capacity, the AFM discourage side-slips when fuel quantity is below 1/4 in the tank on the outside of the slip, if it is the selected one.

As a conservative measure, my flying club did prohibit all side slips. This has been done after a bad experience by a member, doing a long side-slip to show someting to a passenger, not to manage and approach. After 13 seconds, he experienced a serious loss of power, which could be shown in the logfiles of the plane when examined by Thielert.

I don’t want to draw a conclusion here, and I’m obviously open to comments.