Originally Posted by
Steve6443
Big Pistons Forever wrote
Hi BPF, must admit that the instructors at the club I'm a member of still teach that, they claim that the excessive fuel entered into the cylinder aids internal cooling. At the club we have a couple of 172s with fuel injection and PA28s with carbs so would love to hear your reasoning.
Could this be something to do with the heights, the club trainers only instruct at heights around 2000 feet so when descending to a circuit height of, say, 1000 feet, would pushing the mixture full rich have such an impact as say, descending from FL95 down to 1000 feet and pushing mixture full rich, to avoid the need to look at EGT, fouling the plugs with an over rich mixture, etc?
Sigh, another example of a flying school urban myth mindlessly passed on from instructor to instructor. The excess fuel from full rich will indeed cool the cylinders which is good for long climbs at full power. However when you are descending you are increasing airspeed and/or reducing power both of which will lead to cooler cylinder temps. Dumping a whole bunch of fuel will exacerbate this cooling and potentially lead to cracked cylinders from shock cooling.
For the scenario posted by the original poster I tell my students to leave cruise power on lower the nose to an attitude which gives a rate of descent of 500 feet/min and slightly enrich the mixture from the cruise lean (ie push the knob in a quarter of the the distance from where it is to the full rich mixture). For aircraft with fixed pitched props you will need to occasionally reduce the throttle because the engine RPM will increase as you descend. The aim is to maintain cruise RPM throughout the descent.