Bf 109 G6/AS erratic RPM and ATA behaviour at high altitude

[DanielZockt]

The Bf 109 G6/AS RPM & ATA starts jumping all over the place at higher altitude (+7km) when on continuous power and a low speed (below 250-350kmh depending on how high you are). This does not happen in Combat or Emergency mode and stops once you either throttle up past 70% or go fast enough.
 

 

[Gutholz]

I think this might happen because of the automatic engine controll system. 

In the 109 the pilot only operates one "power/throttle" lever but a mechanical computer constantly and automatically adjusts engine parameters like mixture/propeller pitch/rpm etc. In the Fw190 the "Kommandogerät" looked like this:

 

In the 109 it was less compact, instead the system was spread all over the engine/cockpit but basically same function.

I am not sure what inputs/outputs the 109's system has, but some parameters are likely based on each other and the system can enter such feedback loop. It changes one parameter but that influences two other parameters and then it needs to adjust the first thing again but now the second parameter is a bit off etc repeat. For most conditions it should of course stabilize but maybe sometimes it does not.

I have no idea how a real 109 would beheave but it does not seem implausible behaviour. Especially because such low speeds at high altitude might be outside the design limits? The conditions in the video also do seem not stable, always climbing or falling a bit or maybe even stalling. The 109's manual tells to turn the propeller pitch to "manual" on take-off and landing and maybe that is also to avoid such unpredicabtle behaviour.

[DanielZockt]

On 9/13/2026 at 4:09 AM, Gutholz said:

I have no idea how a real 109 would beheave but it does not seem implausible behaviour. Especially because such low speeds at high altitude might be outside the design limits? The conditions in the video also do seem not stable, always climbing or falling a bit or maybe even stalling. The 109's manual tells to turn the propeller pitch to "manual" on take-off and landing and maybe that is also to avoid such unpredicabtle behaviour.

Problem is that this also happens if you switch to manual prop pitch and fly perfectly level, no matter what throttle setting you put it at, it will have fluxuating power outputs as long as its in Continious mode. Also this issue doesnt happen on any other 109 even under extreme situations, so It would be somewhat odd if that was an issue irl that only occured on a variant specific to fly at those altitudes.

 

Edited Wednesday at 01:13 AM by DanielZockt