Mixture at high density alt. airports

[pauldan181]

After flying around the upper midwest for 60 hours since first flight I decided to make the trip to New Mexico last Sun. Upon arrival at my destination I did my normal pattern routine including pushing up the prop and mixture. After extending downwind a bit to allow a Cessna full of jumpers to take off, I needed some throttle on short final and the engine stumbled and popped for a few seconds before responding. DA was over 8K feet. This carburated O-360 hasn't missed a beat since it came out of the box.

Question is, should I just leave the mixture where it was in cruise all the way to touchdown? I was at 10.5K feet leaned 50 deg. ROP. Also how are you high DA C/S prop guys handling the mixture on take-off?

Paul Danclovic RV-8A

Still learning how to operate this machine

[Mathew Sharp]

Hello Sir,

Lean at take off and climb at high altitudes for an EGT reading of:

Sea Level EGT Reading + 25 Deg. F / 1000' (Or 1350, which ever is less)

If the full throttle sea level EGT readings are unknown, the approximate values can be derived using the following formula:

Sea Level EGT at Full Throttle = Peak EGT reading at 70% power -300 Degrees F.

During descent maintain the same leaning method used during cruise (other than the power recovery method). The manifold pressure will need to be reduced during descent to maintain proper power levels for a lean mixture.

When entering a pattern for landing, richen the mixture (as much as possible) for a possible full throttle go arround.

Do you have a multi-point analyzer that will allow you quickly (visually) identify how far from peak EGT you are? A system like that makes this process a lot easier.

Good luck!

Matt Sharp

[nucleus]

Originally Posted by Mathew Sharp

(snip
When entering a pattern for landing, richen the mixture (as much as possible) for a possible full throttle go arround.
(snip)

Full rich on some planes at high density altitude will yield black smoke, coughing, sputtering, and low power, so be careful!

The 172 I learned on was like this. Full rich was for under 2500 ASL and below only.


Hans

[Phil]

This is a well timed question. We're headed into Taos or Santa Fe (Depending on DA) at the end of July and I've been wrestling with the same question.

Not having the full power at Sea Level is one thing, but at least your prop and wing is operating efficently. However it's a completely different thing when your prop and wings are struggling to find enough air molecules to do their job. Low, slow, and restricted power is recipe for disaster at high DA's.

We're bring a Piper Arrow (Non-Turbo) and the POH says mixture full rich at landing - lean on Take-Off. I have a hard time believing this isn't an oversight in the POH in this case. I've pretty much convinced myself to keep it lean on the approach. If I have to go around 20ft AGL and rapidly approaching stall speed, I want the power to be there.

[RVbySDI]

I also think this a timely discussion. I have recently been reading about using the carb heat at high DA. Can anyone expand on this idea of applying carb heat to increase performance when flying around high DA airports?

[Mathew Sharp]

Originally Posted by nucleus

Full rich on some planes at high density altitude will yield black smoke, coughing, sputtering, and low power, so be careful!

The 172 I learned on was like this. Full rich was for under 2500 ASL and below only.


Hans

"When entering a pattern for landing, richen the mixture (as much as possible) for a possible full throttle go arround."

This statement was intended to be very aircraft specific. It was not meant to read: "full rich". Even on our field at 3500' we can't be full rich. The equations mentioned are science supported and if you do them you will identify the mixture point you should target.

[morlino]

I learned to fly in Boulder, CO and recently moved to Fairbanks, AK.
I found a 172 to to rent and yesterday was the first time I've ever flown with the mixture knob firewalled...

All of the rentals I flew in Boulder had some sort of leaning procedure built into the runup as part of the before takeoff checklist.

Basically the first thing you did during the runup was to back out the mixture knob and watch the RPMs go up until they peak and start going back down, then you turn the knob back in to get back to peak and normally give it another 1/2 or full twist in.

I think most of the owners and FBOs add that last richening part to keep inexperienced people from flying too lean at high power.

To answer somebody's question about carb heat: the carb heat air is warmer and therefore less dense so it enriches the mixture because the amount of fuel stays the same but there is 'less' air (same volume but fewer molecules) . So the ratio changes (more rich) because you are mixing the same amount of fuel with less air.

Another thing to keep in mind is leaning for taxi. Normally on carbed airplanes you would start the engine at full rich and then pull out 3/4 inch or so once it is running and on injected planes you would jut not go all the way to full rich once the engine starts.

Most people just guess at the taxi mixture setting. I don't think you'll hurt anything by being slightly too lean or too rich at such low power settings but if you are way too rich then you might foul your plugs.

I never went from sea level and landed up high so I can't help with the best way to set the mixture for landing, although if you need to go around you don't want to be full rich.

[Guy Prevost]

I live in Albuquerque, NM. In 172's and T-41's (A 172 with 210hp), I usually pull the mixture knob out about 3/4" for takeoff and landing. This amount was found by using the RPM method mentioned in another post above. If you're coming into a high density altitude airport, definately pull a bit of mixture. It's better to be rich than lean, but everything will work better if you're not full rich.

Guy

[az_gila]

...some stuff in the first few pages of this Lycoming Flyer....

http://www.lycoming.com/support/tips...Operations.pdf

My home base is 5000 ft. DA at 81F (104F right now......) so I use the OAT as the point to lean for take-off.

One or two clicks rich of the peak rpms does keep the CHTs down quite noticeably though. (FP prop and carb. on my O-360 Tiger)

[Phil]

Originally Posted by lcnmrv8r

Full rich will usually be too much up there. I usually just richen up a 1/4 inch or so on decent. What will you be cruising at? I find that often this time of year I end up cruising LOP at 8.5k or 9.5k. With DA at the destination not being much different. So it doesn't usually take much to richen it up.
Have fun and Get there early.

Hi Seth,

Even if it's forecasted for VFR, we'll probably be coming in IFR around 10,000 MSL (~2500 AGL). I have absolutely '0' mountain flying experience and want to do everything I can to stay out of trouble in case the Wx takes an unexpected turn. It's a whole lot easier to be IFR going into the Mountains, than it is to search through paper work and file in the middle of a descending ceiling and rising terrain. This is one of those situations where I don't mind being called a pansy. That's synonymous with the word 'smart' for a mountain rookie like me.

It shouldn't take that much of push on the mixture knob to richen it for a 2500' decent to land.

I'm glad I wasn't off base for not taking the POH at it's word. Sometimes common sense needs to replace the words (or at least recognize the omissions in the manual).