Some pilots think the "new wave" in engine management is not to use EGT to set the mixture but instead to keep the CHTs under some generic maximum. Their engines won't last very long.
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instantly pulled the MP back to 23" (from 30.9), killing the preignition event. Well done, but too late to save the engine.
To repeat: Use 380 °F to avoid nuisance warnings at 400 °F and set your warning to 400 °F so you have time, precious time, to take action before reaching 420 °F, so that you don't
get end up where you really don't want to be on CHTs.
Now here's the disconnect. This is not a blanket suggestion that it's OK to run all these engines at 380 °F CHT all the time. These issues up to this point have largely been related to
material properties of the cylinders (metallurgy).
There are some engines out there that apparently run very cool, and it's not clear if that's because they are truly cool, or because of some artifacts in the CHT measurement process. Many of these
engines are capable of producing very high power, and thereby very high internal cylinder pressures (ICP) at very low CHTs -- especially during cold ambient conditions!
It just so happens that there are some engines that can use 380 °F as a "cruise CHT" and maintain ICPs within reasonable limits. However, even these engines shouldn't be operated at 380
°F in seriously cold air.
"Waitaminnit," you say. "Cooler is better, so how can this be true?"
Think it through. You take off from Florida with 70 °F weather, and you set up the mixture on a "large-displacement" engine at high MP and 380 °F on the hottest cylinder, with about 17.5 GPH
fuel flow. For some engines (mine is one) this is an "aggressively good" high power setting and nicely lean-of-peak (LOP) EGT with reasonable internal cylinder pressures. We call it the "Go Fast Mode"
at about 260 HP, which is about 84 to 87% of rated power on a 300- to 310-HP engine. (When an engine with a compression ratio of about 8.5 is flown LOP, fuel flow times about 14.9 equals HP.
Warning: This calculation is good only when lean of peak!)
Now, you launch from Minneapolis, where it's -10 °F. You do the big mixture pull to your usual 17.5 GPH, and you notice, "Wow, the CHT is only 305
°F!" The colder air flowing over your engine is giving you more cooling, but the HP is still the same. For those who may have been taught "The Target 380 Method," the temptation is to increase
fuel flow to bring the CHT up closer to the 380 °F target. You might end up with 21 or 22 GPH, nearly 330 HP, out of your 300 HP engine. You just became a test engineer. That is not a good idea.
But it sure illustrates this is not a trivial issue.
Here's a graph of this condition:
Engine Monitor Plot -- winter, high-power condition. (Click here for larger version -- 207 KB.)
The vertical yellow line at about 08:08 is the EGView sliding marker that points to the data shown at the right side of the chart. The power is
very high (about 94%), cylinder pressures are probably 800 to 900 PSI, and the airplane is bumping up into the yellow airspeed arc.
Why are the CHTs so very low? Well, first, we're a very low altitude, where the air is dense, and maximum cooling is available. Second, the airplane is haulin' ... er ... going very fast, also good
for cooling. Finally, that cooling air is very cold.
I'm not so sure this is a good thing to do to our engines. We've become very comfortable with 85% when 80 to 100 °F LOP. We've run our own engines there a lot, hundreds or thousands of
other pilots are running their engines there, many have gone to and beyond TBO, and we've got pressure data from the test stand that shows ICPs well within our "comfort range." We are not so sanguine
about higher power settings. If you choose to do this, please report back in a few years!
The real point here is that any attempt to bring the CHT up to some mythical "target" will result in HP beyond 100%, and ICPs over 1,000! We know that's harmful; we've seen the test stand engines pop
spark plugs right out at only slightly higher pressures!
So what good are all these EGTs and CHTs, then? Just why did you get an engine monitor? Glad you asked. While some engines under some conditions may peak at 1400 °F EGT, and others at 1750
°F, and CHTs may peak at 280 to 450 °F (just grabbing numbers here), there is one number that will serve you well: the difference from peak EGT. No matter what inaccuracies there may
be in the absolute value of the measurement, no matter what the conditions are, if you know peak EGT today, under these conditions "right now," then some incremental number of degrees from that peak
EGT value will be a repeatable and reliably useful parameter for a large number of different engines under all conditions for routine operations.
What are these numbers? They're not quite as simple as a flat, fixed, "universal" number like 380 °F CHT, but they're quite usable. At and below 60% of rated power, no mixture setting will harm
your engine, and 10 to 20 ºF LOP EGT is very close to the most efficient setting (best brake specific fuel consumption -- BSFC). The more power you set, the further from peak EGT you need to be.
At 85% power, best BSFC is found across a broad range of LOP mixtures (think "flat curve," and see the charts in my last column with the big, red boxes), but
keeping the EGT at 80 or 90 ºF LOP works really well to both keep the engine cool and to mitigate the peak internal cylinder pressures.
So, use 20 ºF LOP at low power (60%) and 90 ºF LOP at high power (85%).
Use a straight-line variation to connect those two points for all your LOP settings. If that mixture setting drives the CHT up near 380 °F, then lean a bit more. It usually doesn't take much.
Leaning by an additional 0.5 GPH will normally drop the CHT's by 10 to 15 degrees after five minutes or so.
Here's a different view of those dangerous mixture settings that illustrates this:
Mixture Setting "Red Triangle." (Click here for larger version -- 105 KB.)
The lower (light green) line is where we'd suggest you should operate when LOP with the normally-aspirated engines (no turbo). The darker green line represents a more aggressive mixture setting that
might be used when needed. Remember, all these lines and numbers are a bit "fuzzy!" We can argue all day over the precise placement of the lines, and the red triangle, and where the breakpoints are,
but in the end, this is just an illustration of the general idea.
With a turbo-normalized engine, the engine thinks it's at sea level all the way to its critical altitude, so most of the chart goes away, like this:
Mixture Setting "Red Triangle" for Turbonormalized Engine.
The turbonormalized engine is so much easier to manage! Either run it full rich, or 90 ºF LOP (or a bit leaner if CHTs go above 380 °F).
The TSIO (and TIO) engines can attain much higher manifold pressures for takeoff and climb, but if the manifold pressure is limited to about 32" in cruise, the rules for the turbonormalized engines
work pretty well.
How Much Power Is This, Anyway?
One common mistake is that people will check the POH and find the MP and RPM for a given percent of power, and then they'll lean from there to some LOP setting. That doesn't give you that same percent
of power, it will be somewhat less. The chart in your POH is almost always drawn at "Best Power" (usually specified in very tiny print). As mentioned before, for any given MP and RPM (LOP
only!), you can calculate horsepower by multiplying fuel flow in gallons times 14.9 for most normally-aspirated and turbonormalized engines, and 13.7 for most of the TSIO and TIO engines. There is
some engine-specific variability on some of those engines, but you won't go seriously wrong with these numbers.
Remember, no one can specify any power setting with MP and RPM alone. All three parameters must be known and mixture is arguably the most important.
(For real engine-heads, the big variable is the compression ratio [CR]. 14.9 is based on the usual TCM normally-aspirated [and turbonormalized] engines at CR of 8.5 and 13.7 is the number for many of
the TCM TSIO and the Lycoming TIO engines, with superchargers and CRs of about 7.5.)
On the other hand, if you are operating at high power and rich of peak (ROP, as in climb), you want the mixture set so that the EGT is 250 °F ROP (normally aspirated, sea level) or even as
much as 350 °F ROP (for turbocharged engines). This will give you a bit more fuel flow than the charts and manuals call for, and that's good.
Once you've gone through the above drill a few times in a specific airplane, you will have a pretty good idea of the fuel flow that works. This is another number upon which you can hang your hat. On
my engine -- a TCM IO-550 with Millennium cylinders and the Tornado Alley Turbonormalizer -- 17.5 GPH is my magic number. That's very close to 260 HP, a good, aggressive, "go fast mode" power setting.
I've got about 600 hours on that engine now, all of it at that power setting. Compressions are fine, borescope looks great, and the engine is running smoothly and well. There have been no
issues, no cylinder work.
What percent power is that, anyway? Ah, now we get another "slippery number." Nominal HP on this engine is 300, so that would be about 87%. But we know the Millennium cylinders move air just a bit
better than the factory cylinders, so full power is probably 310, or 315, so that number could be 84%, or 83%, depending on which "max. HP" you use in the calculation.
In summary, 380 °F is a useful number to keep in mind for metallurgical purposes. It may not be a good number upon which to fixate for a cruise mixture setting for the long term.
Be careful up there!
More from AVweb's Pelican is available here.
Ice claims another victim as a twin Commander crashes miles short of the runway. But could the accident have been avoided?
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Runway 5R approach. Aware of the altitude deviations he had seen earlier, the controller also provided vectors requiring turns of less than 30 degrees. Radar data retrieved after the accident
indicates that the airplane's speed and altitude remained constant during these vectors.
As the pilot neared KPVD, the controller issued a descent to 2,000 feet and told the pilot there had been wind shear of minus 10 kts reported at 500 feet on the approach into Providence. The pilot
responded normally and for the moment it seemed like everything was under control.
At 5:48 p.m., the controller said, "Aero Commander 99N, six miles from RENCH [the final approach fix], turn right heading 020, 2,000 until established on the localizer, cleared ILS Runway 5R approach,
Providence. Report established." He then called the Providence tower on the landline and advised the controller to "keep an eye on [the aircraft]. He diverted from Newport and gave us a bit of a
A minute later the pilot contacted the controller to say he had the localizer. When the controller asked the pilot if he was established, the pilot replied, "I sure hope so." At that point, the
controller observed the airplane descending below the glide slope intercept altitude. He instructed the pilot to climb and maintain 2,000 feet until intercepting the glide slope, adding that he was
still outside the outer marker, when the pilot came back and said, "Son of a [expletive], I got problems." The controller asked the nature of the problems to which the pilot responded, "I'm all over
the place, I have no idea I (unintelligible). I think I'm iced up."
The controller then issued a low-altitude advisory, telling the pilot to climb to 3,000 feet as his radar indicated that the airplane had descended to 1,200 feet. At that point the airplane made a
tight left turn and began descending again.
The controller called the pilot again urging him to climb to 3,000 feet. The pilot responded, "Hey, I'm trying like hell." At that point, Mode C data indicated the aircraft was at 1,000 feet.
A few seconds later the Mode C read 800 feet. The controller told the pilot that the Quonset State Airport (KOQU) was off to his right at three miles. The pilot said, "Give me something, would you?"
The controller instructed the pilot to fly eastbound. He wasn't sure what heading the airplane was flying because of the turn he had seen it make to the southwest. A few seconds later, the Mode C
readout dropped off the radar screen.
The controller's supervisor, who had been nearby throughout the incident, called the Quonset tower and asked the controller there to turn the runway lights to full bright and watch for the airplane.
At one point, the tower controller thought he saw the airplane in the darkness to the west of the airport, but he later realized that he was looking at the lights of a construction crane.
There was no further contact with the Commander. It crashed 11 miles south-southwest of the Providence airport in the town of Exeter. The airplane impacted trees and small boulders before coming to
rest upright. The front left portion of the fuselage was crushed, and the pilot, the sole occupant of the aircraft, was killed.
Investigators sifted through the wreckage for anything that would give them an indication as to what had happened. There was no mention of finding any ice on the aircraft, but it is possible that the
NTSB investigators did not arrive on the scene until the following morning.
The airplane was equipped with de-ice boots on the wings and tail. The switches for the de-ice boots were found in the "auto" position. The switches for the propeller de-ice and windshield anti-ice
systems, which used alcohol, were found in the "off" position. The reservoir for the windshield anti-ice system was empty. The right hand pitot heat switch was in the "on" position, while the left
hand pitot heat switch was in the "off" position. It is not known if any of the switch positions were changed due to the ground impact.
Investigators examined the aircraft's directional gyro and found that it functioned within tolerances even though it had sustained some minor impact damage. The de-ice distributor valve and boot
timers were also examined and found to be operational.
The NTSB blamed this accident on the "pilot's failure to maintain control after encountering icing conditions while on approach for landing. Factors in this accident were the night conditions and
pilot's failure to select the airplane's propeller de-icing switches to the 'on' position."
The pilot held a private pilot certificate with ratings for single-engine aircraft, multiengine aircraft and rotorcraft. The pilot's logbooks were not recovered, but six months prior to the accident,
when he renewed his medical, he claimed 860 hours of flight experience and 47 hours in the previous five months. When the pilot obtained his multiengine rating in December 1999, two years and two
months before the accident occurred, he reported 656
hours of flight time, 435 hours in airplanes and 142 hours of instrument time. Investigators estimated that he had about 200 hours in the Commander, but his proficiency and currency were not
There were several witnesses to the crash who observed the airplane at an extremely low altitude "wobbling" from side to side before turning and descending in a left turn into the ground.
What Went Wrong?
We don't know for sure what took place in the Rhode Island sky that evening but there are a few possibilities that we can point to in our efforts to prevent a similar accident from happening again.
The reservoir for the airplane's alcohol system was found dry and the switches for the windshield anti-ice and the propeller de-ice systems were in the off position. While it is possible that one or
both of the switches were knocked into that position as a result of the impact, let's suppose for a moment that they were not.
What if the pilot switched off both of the switches when the alcohol in the tank ran dry? Perhaps turning off the propeller de-ice system was inadvertent, but he probably would have switched off the
windshield anti-ice system to save the pump. If he was distracted from the instruments when he attempted to do that he might have hit both switches at the same time.
The altitude deviations that were observed on radar appear to be more in line with the pilot losing control of the aircraft as a result of instrument failure or misinterpretation rather than ice
building up on the wings and tail. If it was the latter, it is doubtful that he would have been able to climb the aircraft back to 3,000 feet, which he did after the first deviation. So, what if the
left pitot heat was off while the aircraft was in flight?
The facts seem to back up this theory. If the pitot tube iced up while the pilot was being vectored into Newport he might have been having a hard time controlling the airplane or making sense of what
he was seeing on the panel. The temperature on the surface was above freezing, and it is possible that when the aircraft descended the first time to 1,100 feet that the air was warm enough to clear
the pitot tube. If that was the case that would explain why the pilot was able to climb to 3,000 feet and fly the assigned headings until it iced up again. Then the trouble repeated itself.
There was only one PIREP in the vicinity of the Providence VOR that afternoon and evening. At 3:25 p.m., about 2-1/2 hours before the accident occurred, a regional jet reported a trace of rime ice
between 4,000 and 4,500 feet while on descent into Providence. The crew reported that the temperature was 3 C at 3,000 feet.
A trace of rime ice on a jet can mean more coverage on a smaller airplane that stays in icing conditions longer. It also is possible that the conditions deteriorated further as time went by so there
was more ice in the clouds by the time the pilot arrived in the Providence area.
Another possibility is that the pilot's pitot heat did not work. That and the potential icing conditions may have acted in concert and perhaps the pilot never did understand what was happening. A
couple of his transmissions could be interpreted to suggest this scenario. Or, it is possible that his comment about being iced up referred to the pitot tube rather than the airframe.
The lessons from this accident are clear. Before you begin any IFR flight make sure your equipment is functioning properly. If you have an alcohol anti-icing system, make sure it is full and use it
sparingly. Not only do you not want to run out in flight, but alcohol may be hard to get at smaller FBOs. I flew a Baron that had an alcohol propeller and windshield system. Every fall the aircraft
owner would order a stock of alcohol that he would keep in his hangar locker, because the FBO he was based with did not sell it.
Check your pitot tube to make sure it is heating up before you take off into IFR conditions. Just turning on the switch and looking for a bump in an ammeter might not be sufficient for some systems.
Don't forget that you have to maintain currency on partial panel operations. Practice recognizing instrument failures that result from a blocked pitot tube or static system. Know which is which so if
you encounter that type of problem you will know which instruments to believe and which should be disregarded.
Finally, don't fly in conditions your airplane is not equipped to handle.
More accident analyses are available in AVweb's Probable Cause Index. And for monthly articles about IFR flying including accident reports like this
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