If the SR20 is equipped with an altitude compensating fuel pump, what is the purpose or use of a mixture?
- lean brutally for taxi
- the “compensation” is only for the climb. Once at cruise altitude, do the Big Mixture Pull to achieve Lean of Peak operation.
The Altitude Compensating Fuel Pump reduces the fuel flow by a set formula based on perceived altitude; the fuel pump has a aneroid all its own to make this determination.
During initial climbs, you’ll notice a slight reduction in fuel flow, typically less than .5 GPH per thousand feet of altitude gain. It also gradually increases the mixture by the same amount as you descend.
While this is useful for takeoffs from high density altitude airports, it does not take the place of the mixture control for setting cruise power, especially for operating at “Best Economy”, which requires that you lean the mixture significantly more than the compensator does.
If you use the WOTLOP techniques that many espouse in COPA, your mixture control essentially becomes your power control; there is a VAST amount of information about this topic covered elsewhere on this site.
The compensation in the pump also makes LOP descents a little more challenging, since at that mixture setting the increase in fuel flow by the pump brings you closer to peak EGT, rather than decreasing it, effectively gradually increasing power as you descend. In practice, the increased airflow from even a moderate descent will act to cool the cylinders, so in many cases, it’s not a perceptable issue, but some might argue that because we use temperature as a proxy for pressure in the cylinders, a reduction in power by reducing the mixture or MP to maintain the same LOP fuel flow as you descend is also a good idea.