Shock Cooling

[Big Pistons Forever]

. I land the 421 with the props at 1800 rpm (cruise) and mixtures leaned, unlike everyone else here. No cracked jugs on the 421 so far.

Well there is at least one other poster that that does the same thing.....me....and I expect lots of other high time and or/well trained pilots who post here do the same.

[Lurch]

. I land the 421 with the props at 1800 rpm (cruise) and mixtures leaned, unlike everyone else here. No cracked jugs on the 421 so far.

Well there is at least one other poster that that does the same thing.....me....and I expect lots of other high time and or/well trained pilots who post here do the same.

I don't touch my mixture or Props from cruise until I'm over the hold line.

Lurch

[StudentPilot]

I don't touch my mixture or Props from cruise until I'm over the hold line.

If you are cruising at altitude (ie 10,000') and descend, wouldn't the fuel-air mixture lean out as you descended to sea level? Why would you leave your mixture as leaned at altitude and not slightly enrich it as you descend and get into denser air?

[crazy_aviator]

Yes the mixtures need to be enrichened a little for the whole descent,,otherwise you may get lean out engine quitting, another thing, on short final, why not put the props to full fine ? one less thing to worry about for go-around AND it doesnt hurt a thing !!!!!

[Hedley]

quod erat demonstrandum

[FlaplessDork]

Yes the mixtures need to be enrichened a little for the whole descent,,otherwise you may get lean out engine quitting, another thing, on short final, why not put the props to full fine ? one less thing to worry about for go-around AND it doesnt hurt a thing !!!!!

Why go full fine when you're just going to bring the props back right away to climb power anyway if you go missed? The Navajo is time limited to 3 minutes at full fine for example.

Lycoming on Leaning
More stuff on Engine Operations from Lycoming

[Hedley]

Yeah, jets are so simple. All you have to do is keep the temp down on the start, and off you go.

As far as jamming the prop controls forward on a piston engine .... with a non-geared engine, as you reduce the throttle, at some point the RPM will fall. The prop is now in full fine pitch, at the stops, and it is now safe to advance the prop control all the way, because it won't make any difference.

However, if you just jam the prop controls in all the way with say 20 inches of manifold pressure, the engines are going to instantly sing up to max RPM where the governor will struggle to keep them from over-revving, at redline. Don't do that. Apart from really pissing off the neighbours, it is hard on your engine to do that to it. If I catch you doing it, I will hunt you down and crazy glue your testicles to your legs.

Now, with geared engines ... you NEVER want the props to be acting as speed brakes at low power settings (and fine pitches) and driving the engines. That will destroy the gearbox.

With the C421 and GTSIO-520 engines, the right way to fly an approach is 21 inches of manifold pressure and 1800 (cruise) RPM on downwind. Drop the gear, then 10 flap. On base and final, add flaps in 10 degree increments: 20, 30, 40. If you time it right, and space your circuit correctly, you NEVER have to touch the throttles - they are at 21 inches MP all the way down. Over the threshold, throttles to idle (with vortex generators - don't try that without them!) and touch down, still with the props at cruise (1800) rpm and the mixtures leaned for 21 in MP slow cruise.

Keep in mind that a constant-speed prop is an incredible speed brake - for NON-GEARED engines ONLY - at full fine pitch.

Here's a trick that works great, sure to enrage many pedants here. With a non-geared engine and a constant speed prop, set the idle so low that on the ground, if you pull the throttle back to idle, the engines will quit. This means that in the air, you have an incredible speed brake for steep approaches, and you will rapidly de-celerate in the flare which really makes the landing easier for the student.

Do they teach that at Flight Safety?

[FlaplessDork]

Here's a trick that works great, sure to enrage many pedants here. With a non-geared engine and a constant speed prop, set the idle so low that on the ground, if you pull the throttle back to idle, the engines will quit. This means that in the air, you have an incredible speed brake for steep approaches, and you will rapidly de-celerate in the flare which really makes the landing easier for the student.

We did that with the Twin Comanche's.

[MichaelP]

Now, with geared engines ... you NEVER want the props to be acting as speed brakes at low power settings (and fine pitches) and driving the engines. That will destroy the gearbox.
Keep in mind that a constant-speed prop is an incredible speed brake - for NON-GEARED engines ONLY - at full fine pitch.

Please explain this in more detail...

We're flying geared engines in our Katanas.... The big advantage of the Rotax with the CSU over the IO-240 with the toothpick FP prop is that the propeller produces a fair amount of braking... The DA20-C1 goes through brake pads quicker than the Katana.

I always want the gear loaded one way or the other and not chattering in between.

Likewise on the subject of shock cooling, the cooling of the IO-240 is not properly set up in the DA20-C1 and we have to make a judgement call whether we put the winterisation baffles in or not in the Summer!
This aeroplane requires descent planning as the temperatures can drop rapidly... The CHT barely gets into the green even with the baffles in and often never with the baffles out!

[Big Pistons Forever]

Here's a trick that works great, sure to enrage many pedants here. With a non-geared engine and a constant speed prop, set the idle so low that on the ground, if you pull the throttle back to idle, the engines will quit. This means that in the air, you have an incredible speed brake for steep approaches, and you will rapidly de-celerate in the flare which really makes the landing easier for the student.

We did that with the Twin Comanche's.

I would suggest that the engine idle should be set to the value specified in the maintainance manual and not some excessively low value. I also feel that if the student flying skills are not sufficiently developed that they can not control the aircraft in a normal flare, hold off, touchdown; than the solution is not a very low idle speed to reduce the hold off time it is more training........

[iflyforpie]

WRT to mixture settings...

Descending from 10,000 to 3000, I will slightly richen mixtures to keep the engine running smoothly (usually passing through 7000), but I use the EGT to try and keep the temps as close to my cruise setting as possible (one thing that is quite hard to do with a large power reduction).

The Continental Engine operating handbooks say that running right on or past peak is perfectly fine at power settings below 65% for most NA engines.


WRT gearboxes...

Most aircraft gearboxes for some reason or another are like boxers with glass jaws. I never could figure out why they couldn't make a simple one-speed reduction gearbox unable to handle dampened reversing loads when an automobile transmission might reverse its load with no damping (on standard or automatics with locking torque converters) hundreds of times a day, plus deal with changing gears.

Perhaps the Katana has finally caught up to automotive technology, but it also doesn't have anywhere near the torque going through it as a GTSIO-520.

Of course, a few flecks of metal is enough to ground an aircraft engine, while I've pulled entire teeth out of my car's gearbox and it still seems to run fine...

[trey kule]

Do they teach that at flight safety?
I dont recall that.

A couple of things..Lurch...on some aircraft such as a navajo you have a limitation on the EGT in the descent, and you have to adjust the mixtures to remain above that figure. I am also not sure that doing a descent from the flight levels where you are running on the lean side of peak without touching the mixtures would be a wise thing to do.

As to the idle thing..not sure what to make of that idea. A few years ago an AME decided that they would set the idle back on a turbine with the intention that it would slow the prop down and thus pick up fewer rocks on the strips we were operating out of. Great in theory, but below 51%, the alternate gens kept kicking in, resulting in a very expensive fix and return to the proper idle speed. I have also seen beavers and otters where the idle was set way back to allow them to approach a dock slower and the resultant panic when the engines quit a little early.
Not saying your idea is a bad one, or doesnt work. I am simply not sure that flying technique is not a better substitute than diddling with idle mixtures.. Each to their own I guess.

[Hedley]

As to the idle thing

I did NOT recommend it for turbines! Only direct-drive piston/prop engines with constant speed props.

Idle speed is a significant factor in landings. In a light training, try leaving on 1200 RPM and see what kind of an excruciating, drawn-out touchdown and rollout that results. Even 1000 RPM is too high - you will notice a significant difference.

This is a significant detail in tailwheel aircraft on wheels. I call it the "expensive speed" - from 25 to 45 mph. Slower than that, you're taxiing, hopefully under control. Faster than that, you're probably flying in ground effect, depending on the exact aircraft.

The "expensive speed" is when a tailwheel aircraft is dancing down the runway at high speed. It can get easily groundlooped and destroyed in this phase, especially on dry pavement. A constant speed prop with a very slow idle will rapidly decelerate a tailwheel aircraft through the expensive speed, leaving less time for things to go horribly wrong.

If you don't fly tailwheel aircraft, or if you think that tailwheel aircraft don't have a problem with groundlooping, please disregard.

[trey kule]

Well, I do have a couple of thousand hours on tailwheel aircraft. Quite a few types.
Some land differently than others.
I have never heard or seen the expression you use but I understand the thinking behind landing an an optimium speed/attitude/power. I just have never really given any thought to the need to have the idle reduced..And again, I am not arguing you are wrong.
When I do give it some thought with a bit of analytic thinking, a few of the taildraggers I have flown we brought them onto the backside of the curve and actually reduced the power to idle almost as the wheels touched.
I only mentioned the idle adjustment on the turbine as an example of something that seems like a good idea but has unitended consequences.
In any event, I cant see any harm in having the idle set back a bit at an inspection and going out and trying it for a couple of circuits to see if a difference can be felt.

[Cat Driver]

Interesting subject.

I am trying to think of a flying machine that I have flown either fixed or rotary wing that " Could not " be landed safely with no power being developed.

Believe me I have flown a lot of stuff and can't think of one that could not be safely landed without power, I never flew the space shuttle but did fly for two years with Patrick Baudry who did fly it and that thing lands without power every time.

[Lurch]

Do A couple of things..Lurch...on some aircraft such as a navajo you have a limitation on the EGT in the descent, and you have to adjust the mixtures to remain above that figure. I am also not sure that doing a descent from the flight levels where you are running on the lean side of peak without touching the mixtures would be a wise thing to do.

Turbo charged engines, the EGTS don't budge more then 10 degrees on the descent.

I don't run my engines LOP, but that's another thread.

Lurch

[FlaplessDork]

I don't run my engines LOP, but that's another thread.
Lurch

Lycoming in the article I posted above recommends that 98% of pilots should not run their engines LOP.

[Hedley]

I lean for max airspeed (both carbureted and fuel-injected) which by definition is max power.

Simple. No EGT required.

Even on the ground, after start and after landing, I lean for max RPM to try to keep the plugs from fouling.

Cruising at 1,000 AGL, lean for max airspeed, especially in the summer with higher density altitudes.

Cruising at 10,000 AGL, lean for max airspeed.

People try to make stuff far more complicated than it needs to be.

[nookie201]

I've owned both PA31 & PA31P's

I paid particular attention in high altitude descents on the P, but still managed to crack two cylinders over time.
Both were found by regular inspection.

Both cracks were right around the fuel injector!, pretty much in my mind the crack hazard is indeed mixture
related and not as much power related.

Ironic, I also always landed mixtures as leaned in descent and cruise prop settings (unless IFR down to min)

Believe it or not of the PA31's I've owned and flown - the P was my favorite (Except for the 85 Gal/hr climb...)

[Hedley]

Both cracks were right around the fuel injector!, pretty much in my mind the crack hazard is indeed mixture related and not as much power related.

Exactly! (sounds of Hedley jumping up and down). Everyone propogates this "inch a minute" power reduction of MP during descent, when in fact it's completely irrelevant. You don't apply power on takeoff using the "inch a minute" rule, do you?

I really wish we could somehow get the word out that cracking cylinders is caused by jamming the mixtures in. Don't do that.

Approach and land with the mixtures leaned, and the props at cruise RPM.