Mixture setting/leaning during decent

Frank Smidler · #1 04-18-2010, 06:16 PM

Last Friday I was heading home to Indiana from Iowa at 11,500 ft with a good tailwind pushing me along at over 200 kts ground speed. I couldn't believe it when the vnav on my Garmin 296 notified me that it was time to start my decent to KLAF when I was still 70 nm out. I have it set for 500 ft/min decent. Gotta love these planes.

As I started my decent I began to think about the best way to handle the high altitude leaned out mixture during a long decent. I have a 0-360 A1A carburated engine and I usually push the mixture full rich as I start the decent. Is this the best way to manage the mixture during decent or are there other proven procedures that I should be using?

Thanks

Radomir · #2 04-18-2010, 06:43 PM

I don't enrich in descent at all.. I usually pull the power back every so often to keep manifold pressure from building up too much.. I don't have exact numbers that I follow every single time.. I kinda go by ear/feel instead.. You can go with pull 1" of MP every 1-2k ft.. either way, no need to enrich mixture downhill.

Also, as a general guideline, your mixture should never be full rich at 11.5k... that's way too rich...

flyboy1963 · #3 04-18-2010, 06:45 PM

Hi guys,
I may be one of many who was never properly 'trained' in the use of mixture, and what the heck the MP really MEANS to operation of our engines.
Can anyone suggest a good source of practical info on this? There seems to be as much ( or more) opinion than fact out there.

thx

fstringham7a · #4 04-18-2010, 07:04 PM

These articles at AVWEB has some prety good info that may help.

http://www.avweb.com/news/pelican/182084-1.html

rank @ 1L8 ...RV7A... Flying

Ron Lee · #5 04-18-2010, 07:09 PM

I enrich further periodically as I descend. If you know the setting on the ground and where you are at altitude, just make small changes as required to get to your ground based setting near pattern altitude.

My airport is near 6860 feet MSL so I rarely see full rich.

L.Adamson · #6 04-18-2010, 07:15 PM

Quote:

Originally Posted by Ron Lee

I enrich further periodically as I descend. If you know the setting on the ground and where you are at altitude, just make small changes as required to get to your ground based setting near pattern altitude.

My airport is near 6860 feet MSL so I rarely see full rich.

At our 4200-4600' airports, we seldom see full rich either. You can even tell the difference when taxiing with fuel rich. Normal procedure is to start the engine at full rich, then immediately pull the knob out a bit. Usually 1/2 to 5/8".

I never go full rich at landing, with these altitudes either. It has a tendency to foul plugs, which is easily noticeable on the mag check.

L.Adamson --- RV6A

Ron Lee · #7 04-18-2010, 09:11 PM

I do not even start full rich. I know about where the mixture should be and start there. Then lean out more to taxi.

Verify correct mixture during runup.

DakotaHawk · #8 04-18-2010, 10:05 PM

Quote:

Originally Posted by Ron Lee

If you know the setting on the ground and where you are at altitude, just make small changes as required to get to your ground based setting near pattern altitude.

Which "setting" should you know? Are you talking about EGT? CHT? Fuel Flow? MP?

I am getting a better understanding of leaning as I climb, but descent still has me somewhat confused. I guess my saving grace on descent is that I have already reduced power (2300 rpm and 18" MP gives me a relaxed descent), and I go over-rich as I descend.

What I'm thinking (haven't tried this yet) is that I should maintain altitude and pull power/prop to 2300/18" and let EGT stabilize - maybe 30 seconds. Then begin descent and maintain EGT at the stabilized temp by adjusting mixture. At low power, that technique should keep my engine leaned for the current altitude and avoid fouling the plugs.

What do you think?

Bill Dicus · #9 04-19-2010, 05:39 AM

Lycoming says it is unnecessary to richen mixture during descent at less than 75% power if engine is running smoothly. We have followed this for years and several Lycomings for more than 2000 hrs each before uneventful o'haul. So there you have it - both authoritative and anecdotal!

Ron Lee · #10 04-19-2010, 06:46 AM

Scott, at my field elevation (>6800 feet), the mixture is pulled out a certain distance at optimal mixture setting as determined during the runup. By this I mean max RPM while varying the mixture setting at around 1700 RPM. At sea level, you may not do that.

As I climb, I lean out more as determined by EGT. So the difference in how much the mixture is leaned out (knob pulled aft) at altitude and where it is on the ground is gradually reduced as I descend. If I land closer to sea level, the mixture knob gets closer to completely in as I approach pattern altitude.

I am not an expert but I would not enter the traffic pattern near sea level with the mixture knob set for 12,500' or higher. My guess is that a go-around full power situation could result in the engine quitting if it does not during the descent. By gradually pushing the mixture in during descent, instead of doing it as part of a pre-landing checklist, I avoid that possible outcome.