Pelican's Perch #66
Where Should I Run My Engine?
(Part 4 -- Descent)

  • E-Mail this Article
  • View Printable Article
  • Text size:

    • A
    • A
    • A

After a short discussion about whether running engines the factory way or the skydiving way will hurt or help engines, AVweb's John Deakin settles in for the descent. And, yes, there are more old wives tales to be debunked, and better control settings to use.

Pelican's Perch

First, the usual "housekeeping."

One alert reader pointed out that I had referred to the in-flight mag check, but never explained it. Let me do that in my next column.

Some of the excellent "reader responses" (by e-mail and a couple of forums) deserve mention here.

Those Skydiving Airplanes

First, I don't think I've ever covered the situation with airplanes used for skydiving. One person wrote:

One thing (name omitted) and I bring to this discussion is firsthand knowledge of numerous TCM engines in skydiving aircraft that are always climbing, then being shock cooled, never have GEMs or JPIs, rarely have pilots who pay any attention to temperatures, if the temp gauges are even working, and are flown by a variety of pilots who all fly them different, and all lean them differently.

The result is that the engines almost always make TBO.

This very well-intentioned and knowledgeable reader is absolutely correct, and that's a great way to run these engines.

However, he continues with:

And it just doesn't jive with the "GAMI boys" saying that high temps dramatically reduce the chance of making TBO.

In this, he has gone astray, because it DOES "jive"! Here's why.

Look at precisely what the skydiving airplanes do, day in and day out:

  1. They generally fly a LOT, and often (that's good);
  2. They operate at full power, "balls-to-the-wall," for all climbs, with moderate peak cylinder pressures from proper mixture control. The rich mixtures slow the combustion event, putting the peak pressure further from top dead center. Less pressure means less heat transferred to the cylinder head.
  3. They throttle back for a short time during the jump run, providing a transition and some cooling, leading into ...
  4. A descent at very low power.

To a powerplant engineer, that's a PERFECT recipe for long, trouble-free engine life! That's ALL outside "the red box"!

Heck, if I owned an airplane used exclusively for skydiving operations, I might even keep it simple, too. I might even omit the GAMIjectors and the engine monitor! What sacrilege!

Well, let's not get too carried away here. I still think the injectors will pay for themselves, because without them, the jump pilot should lean in the climb for the leanest cylinder (to protect them), which leaves the other cylinders a bit too rich. An engine monitor will pay for itself in reduced troubleshooting costs and engine reliability.

I'd have the takeoff fuel flow set up right (never mind how I'd know that, without an engine monitor), and just tell the pilots to operate is as I suggest here in these columns for the climb, and as I'm about to suggest in this column for the descents, skip the cruise portion.

The same is generally true of training aircraft, they're flown often, and they usually operate full rich (outside "the red box").

So, in the final analysis, aircraft engines used for training and skydiving tend to prove our points.

Run It the Factory Way?

We get a few comments from people who report they used to run as the factory says, and usually enjoyed good engine life, some even making TBO. That's true, but there's more to the story.

Unfortunately, we need to separate all the "flat" (horizontally opposed) engines into two groups. One group is all TCM engines manufactured since about 1991, and the other group is "all others."

Something changed in TCM engines in about 1991. I cannot tell you for sure what it was. It may have been the "choke," it may have been a change in the machining processes. It may have been changing to a different method of installing valves. TCM essentially threw away all the old machinery that had served so well (if inefficiently), and installed all-new, modern machinery. They had a strike that resulted in management assembling engines for a time, and they lost a lot of their senior engineering talent. But one thing seems clear: It's a very, very rare TCM engine that lasts more than about 400 to 700 hours without major cylinder work, if the cylinders were made since 1991.

These cylinders WILL require work, REGARDLESS of how they are operated. Some mechanics are quick to blame LOP operations for this effect, but LOP operations were almost unknown before 1998 or so, and this started a long time before that.

Now, with that subset of cylinders out of the way, let's take a closer look at the rest. Most pilots have used some variation of the 65% power setting for decades. All the magazines show the data at 65%, the power charts all seem to suggest 65% is "the way to go." Everyone is comfortable with "good old 65%."

Folks, note that's right on the edge of our "red box"! We suggest that you can safely set the mixture anywhere you please at 60% power, and 65% is probably "close enough." Remember, just because we teach the concept of the "red box" doesn't mean the engine will instantly explode if you get near it, and we cannot begin to state with assurance EXACTLY where the red box begins and ends. There's room for error here, and we're trying to be conservative in this. We DON'T want pilots to look at our "red box," and say too themselves, "Well, if 125 ROP is the edge of the 'red box,' I'll add a margin and use 150."

These are "fuzzy numbers," folks! You may be able to operate well within the "red box" in some cases (65%), but if you do, be aware you may not be treating your engine with quite the care it deserves.

So, what does "The Factory Way" (65%, 50 ROP) get you? At 50 ROP, we suggest you run at 60%, and certainly not more that 65%. We can say, then, that at 50 ROP, 65% is your LIMIT on power. At that setting, you will probably be burning up to 3 GPH more fuel than you need to. You will be running "dirty" enough to eventually foul your valve guides and encrust the tops of your pistons with deposits. Finally, you will be making enough carbon monoxide to be lethal if you develop an exhaust leak into the cabin.

Until recently, speaking in broad general terms, we operated the whole fleet that way, industry-wide. Have you ever looked at the "for sale" ads in Trade A Plane, and the engine numbers in them? How often have you seen "1,200 since new, 600 since top overhaul." If TBO is 1800 or so, WHY did they do that "top overhaul" at 600? Ever think about that? Now, how many of those ads that don't mention "top overhaul" are about engines where the mechanics/owners/operators just sorta forgot to report the removal of one or more jugs during that run, for valve work, or a damaged piston? How many of those "events" just get quietly forgotten when it comes time to make that required entry in the logs? Certainly, no one would EVER omit such work intentionally! I would be aghast at such dishonesty! Sure I would. Riiiiiight.

So, if most of the ads have that verbiage in one form or another, and there are significant numbers that have had the same problems without recording them, I can only leave it to the reader to make the judgment. Have we been operating these engines properly? I think not.

Do we really NEED to do more of this, to prove a point? No, that experiment has been ongoing since World War II, and the results have been dismal at best, even with "conforming" engines.

We have become so accustomed to "early top overhauls," or frequent jug work, that we forget these engines should make TBO and beyond, assuming they are "conforming" engines to start with, and they are operated in any reasonable manner. We overhaul a jug here, and another there, maybe one per annual (when the compression test is done), and we shrug it off as the normal cost of doing business. It isn't.

The jug problems so evident since about 1991 have made this situation much worse, and have confused the issue. That will continue, as jugs manufactured since 1991 continue to fail. Even if TCM fixed the problems today, it would be 10 years before all those jugs were finally purged.

In spite of all that, we still think ROP operation is a viable part of any pilot's bag of tricks, provided it is rich enough, and used properly. Climbs are probably best done well ROP, for example. For normally aspirated engines cruising at altitudes above about 9,000 feet msl, 80 ROP will produce the most possible power, and 50 ROP will produce almost as much power, but on less fuel.

But for normally aspirated engines cruising below about 9,000 feet (and for turbos cruising at all altitudes), LOP operations, properly done, will give the needed power, will burn less fuel at the same power, will operate with lower peak combustion pressures and temperatures, and therefore run cooler (in all parts of the engine, valves included). They will run cleaner, making no deposits on the pistons or the valves, and without making measurable carbon monoxide, with less stress on the engine for any given power. It will remove that 60% or 65% limit, and allow normal (LOP) cruise at higher power settings, even up to 85%.

It seems intuitively obvious that all that will extend the service life of the engine, but we'll probably never see hard data for that. For everything else, there is hard data, already.

We Left You Last Month ...

... with a couple of charts. Click each one for a larger version.

Mixture Example Chart Mixture Example Chart -- Annotated

We use these charts in our seminars, and they are a variation of those showing "the red box." Ok, so these are triangles, so sue me!

Both charts assume WOT ("Wide Open Throttle"), and a fairly high (or full) RPM.

The left edge represents sea level, and note that to stay outside the red box at sea level, you must be somewhere around 250 degrees ROP (slightly off the chart), or you must be very close to 100 LOP. Both are completely safe settings for the engine, with the LOP setting running much cooler, but producing less power.

Reminder, this assumes full throttle, full RPM (or nearly so)! We are fooling around with the mixture only, here.

Move across the chart to about the 5,000-foot level, and note the red box has gotten smaller, because altitude has taken away a lot of MP, and there's no recovery from that in a normally aspirated engine. Now our red box is around 30 LOP to about 90 ROP. At roughly 8,000 feet, the red box goes away, and that's the area where you can't get 65% due to altitude.

Again, let me remind you, these are approximations; don't be a slave to precise numbers! Don't yell at me because I say "80" in one place, and "90" in another. That's "close enough"!

The same chart can be used for illustrating the climb, if you climb as I suggested a couple of columns ago. Leaning to that target EGT as you climb produces pretty much what you see on the second chart, just rich of the red box. Once you get to your altitude (the example is at 4,500 feet), do the "big mixture pull," and set LOP. The green lines show a good area to be in when running LOP, up to about 9,000 feet altitude. At and above that point, you probably want to switch over to ROP to keep the power up as much as possible.

At Last, the Descent!

Well, you've been on this flight for two whole months now, and you're probably ready for a pit stop.

I don't know about you, but I was taught (back in the dark ages) to begin the descent by just going full rich on the mixtures, pulling a whole lot of power off, and starting down at 500 (or even 300) fpm to "save the passengers' ears."

I sure don't do THAT, anymore! Consider. You push the mixture to full rich, dumping a LOT more fuel in, slowing the combustion rate, delaying the peak pressure pulse, which has the effect of cooling the engine a whole lot. Consider also, that fuel may have been cooling off in the wing tanks at 12,500 feet on a cold winter day, and now you're spraying that cold fuel into hot intake ports.

You push the nose down, increasing the airspeed, cooling the engine even more. You pull the power off, cooling the engine a lot more.

I'm not a big fan of the various "shock cooling" theories, and I get a case of the giggles when I hear some of the elaborate methods of handling this "problem." But if you do all these improper things, maybe there is such a thing!

For example, suppose you do that climb in a airplane used for skydiving, and the mixture is not set rich enough for takeoff? Say you further lean too much in the climb, and you manage to really run the CHTs up to the insane factory limit of 460 degrees by the time you throw your jumpers out.

Now, you pull the power off, and dive for the next load. It hurts me to think about that, and I'll sure call that "shock cooling!" Please, don't do that!

But if you climb with a mixture set rich enough, and your CHTs are in the low 300 range, and you gently pull the power off and keep the CHTs up with mixture control, that's just fine. You cannot shock cool an engine that is already cool!

First, you have to decide what type of descent you want, and the first consideration is those pesky ears. Most of my passengers are pilots, or very experienced travelers, and most are very familiar with the various techniques for clearing the ears during descent. For them, 300 to 500 fpm is not necessary; they can easily handle 2,500 fpm, or more. If you've got that baby with a cold, it may be another story, but you shouldn't have put the baby in that situation. You may have a need to stay high, then "dump it," for freezing levels, or to stay in smooth air as long as possible. It's really nice to use this technique on those summer days in Florida, where the turbulence tops out at 7,000 to 9,000 feet! Stay high as late as possible, then descend at 2,500 fpm to the airport, and get it over with. JFK controllers used to call this a "crowbar," as it simulates tossing a crowbar out the window, then trying to beat it to the ground.

On the other hand, if you're at the end of a long, high-altitude trip, and you want to make up for the long, hard climb back there after takeoff, you might do a "cruise descent."

The Cruise Descent

Just drop the nose a little, start your desired descent rate, and allow the IAS to build. I don't like going into the yellow, but this is another individual decision. When you have your descent rate set the way you want it, wait until your IAS builds to your "maximum comfort level," doing nothing with the engine controls.

The additional speed will cause a slow drop in CHT, which is fine. This even makes the "shock-coolers" happy.

As you hit your "comfort speed," reduce the RPM a little to keep the speed there, leaving the throttle and mixture alone. When the RPM gets to the lower part of the green arc (1800 on mine), leave it there. The reduced RPM will pump less air, and it will also reduce the RPM of the engine-driven fuel pump proportionately. You'll see the fuel flow drop, but the fuel/air ratio will stay pretty constant. The CHTs will continue a gentle decline.

A few minutes of this, and your CHTs will be well under 300, and it doesn't matter what you do after this, so reduce the throttle as needed to give the speed you want.

With my engine (before and after the turbo), that's the configuration I use to land. Mixture still leaned the way I had it in cruise, prop in low RPM. Some engines will get rough doing this, just enrich until they get smooth again.

To many, landing this way is more heresy, and most will scream, "But, what about a go-around?"

Well, what about it? Everyone should train properly for any major increase in power. The sequence is "mixture, prop, then throttle." That should be a mantra for setting takeoff power, for go-arounds, for missed approaches, and transitions to the climb configuration. It must be your habit pattern. You should practice touching all three controls on EVERY TAKEOFF. On EVERY GO-AROUND. On EVERY transition to a climb. "Red Knob, Blue Knob, Black knob."

What if you panic and forget, and just shove in the throttle? Contrary to popular belief, that won't hurt a thing, because you'll be pretty well set for LOP, high-power operation! Even if your engine is a bit different, shoving the power in will give you more than enough power for the maneuver, and there's plenty of time to shove the other levers up, as needed. Try this for yourself, at altitude, and see what your engine does. There is nothing you can do to your normally aspirated engine that will harm it in the short term (30 seconds or so). Suppose you shove the throttle in, and the engine quits? I haven't seen one do this, but there may be one, somewhere. Fine, then this technique may not be for you during a last-second go-around, and you either need to preset your mixture, or remember to push it in first. But doing that is NOT going to hurt your engine!

I know this one is going to drive some old-timers up the wall, so let me add this. Manifold pressure (in a normally aspirated engine) is not pressure at all, but suction. Manifold pressure is not going to blow anything up. It is only the combustion event that creates heat and pressure. With the low RPM and LOP mixture, you can't get that much power, and the peak combustion pressures will be less than those at takeoff power. You won't hurt the engine, short-term. Finally, it takes time for the heat to build up, if it builds up at all, so detonation is not a factor.

If you simply cannot bear to do it this way, then go ahead and enrich the mixture on downwind or before, as needed for the go-around. By then, your cylinders will be cool enough that it won't matter. Full rich at sea level, less rich for altitude airports. But in any event, please don't run that prop up over populated areas! That creates horrendous noise, and noise is our biggest enemy. If you must land with the prop full forward, shove it up on short final, after the RPM has dropped out of the governing range.

So much for the gentle descent -- on to the crowbar.

The Crowbar Descent

For whatever reason, you've decided to stay high, and descend rapidly. My airplane has a 156-knot gear speed, so the first thing I do is slow down by pulling off RPM to minimum, then putting the gear down at some speed lower than the limit.

(Note: Don't continually mistreat your airplane by extending anything right at the limit, all the time. It can create fatigue issues in the long term. Get that airspeed 10, 20, or even 30 knots below the limit, and be gentle with the equipment when you can. On the other hand, if you need it, use it.)

Nose down gently to maintain the speed, running it back up to the limit speed if necessary, pull the prop right back to 2100 or less, then do the other big pull, the "Big Throttle Pull" (BTP). Go on, just pull off anywhere between 10 and 15 inches of manifold pressure, all in one fell swoop. (What the heck IS a fell swoop?) Anyway, one smooth pull, over some seconds, perhaps five or ten.

First couple times, after you've done this, watch the TREND on the CHT. If you've been cruising LOP, this will often put you just slightly ROP, and the CHTs may not change at all! In fact, if I hit it just right, my CHTs will RISE just a bit, after the BTP! The engine goes from high power, LOP, to low power and slightly ROP, leaving the CHT stable. This demonstrates the enormous effect mixture can have on an engine. Your engine may be different, the various linkages can leave you "too rich," or "too lean," but with a couple tries, you'll know what your engine does, and what it takes to keep those CHTs up in any descent. Setting the mixture to about 50 ROP will maximize the CHTs at this low power setting. But on many engines, it won't even be necessary, and you'll just see the CHT slowly tick, tick, tick down a bit.

After you do this a few times (from a LOP cruise), you'll be able to pull the prop back to 1800, the throttle back to 15 inches MP, and go for that crowbar. Maybe move the mixture a little bit, if needed.

Coast into the pattern that way, decelerating, gear down, turn to final, final flaps, and land. If you have a sudden need for lots of power, it's "Mixture, Prop, Throttle." Red knob, blue knob, black knob.

First action when you get off the runway and stop is to reset the mixture for "aggressive" ground leaning. On some airplanes, you'll already be there, if you landed with the mixture leaned.

Repeating here, you MUST train yourself to make major power changes a three-step process! MIXTURE, PROP, THROTTLE. RED knob, BLUE knob, BLACK knob.

This ends the three-part column on power management in your normally aspirated engine, with fuel injection, and a few new pointers for those with carburetors.


Use full power on all takeoffs, with a "rich enough" mixture. FORCE your mechanic to set that fuel flow high enough to get roughly 1300 degrees EGT or a bit less, and CHTs in the low to middle 300s.

If you have that full-power mixture set properly, determine your "target" EGT right after takeoff, and lean in the climb to keep that same absolute value on the digital EGT. As you climb, that EGT will drop a little. Lean until it comes back up. Drops a little, lean and bring it back up.

For cruise, first determine your range needs, and set a power setting to maintain the AIRSPEED that will do the job you want. Set WOT, then the RPM and mixture you need to maintain that. LOP is much better, if it will do the job, but use ROP if needed. Cruise outside the red box, or at worst, on the fringes of it.

For descent, use mixture control and RPM to get the desired descent, switching to slightly rich of peak EGT, if you wish to keep the CHTs up. (Remember, 50 ROP on the EGT may be the same absolute value of EGT as 50 LOP, but the CHT will be much hotter when 50 ROP!)

None of this takes a lot of effort, once learned. Most of it will fry your CFI's mind, but it may do him some good in the long run, and force him to examine the issues. You may teach him something.

Finally, and very sadly, unless you have a truly enlightened check pilot or examiner, do it the old way when taking a check ride. It'll feel like you're abusing your engine, but once won't hurt a thing. You will never profit from doing "something different" with the FAA or a check pilot on board.

Be careful up there!