'We changed 70 cylinders in 70,000 flight hours'
Anyone who has ever kept an ear cocked while sitting around the pilots' lounge or talked to other pilots about their powerplant management techniques will agree that there are as many differing opinions on this subject as there are airplanes in the air.
One school of operators says that engines should be run easy, while a less vocal, but still firm, group points out that wear is occurring any time the engine is running so it's only logical to run it hard.
And then there's the debate about leaning. How much should the engine be leaned? Leaning saves fuel, but is there a point where aggressive leaning ends up costing more money than it saves?
Pilots also seem to be split in their opinions about the issue of extreme leaning — leaning so much that the engine can no longer produce peak combustion temperatures.
Leaning 101
There are three zones that pilots should be aware of when the subject of leaning comes up. The reference point for all leaning discussions is peak exhaust gas temperature (EGT) — the temperature of the gases as they leave the cylinder of a reciprocating engine or the turbine of a gas turbine engine. The peak is the hottest combustion temperature for a given power setting, and it occurs when the oxygen and the fuel molecules completely come together in combustion — if there's more fuel than oxygen, the mixture is rich of peak, and if there's more oxygen than fuel, the mixture is lean of peak. Both rich of peak and lean of peak mixtures result in EGTs that are cooler than peak.
Running an engine on the lean side of peak, or simply lean of peak (LOP), is almost always restricted to engines that are equipped with fuel injection systems, simply because the carburetors and induction systems on our engines don't do a very good job of evenly distributing the fuel to all the cylinders. Uneven distribution of air or fuel can cause one or more cylinders to run hotter than others.
The AOPA Pilot magazine editors who flew the 2001 Sweepstakes Bonanza, which was equipped with Tornado Alley Turbo's turbonormalized IO-550, learned the LOP method and successfully used it during the 80-plus hours we flew the airplane.
The rich-runners of the leaning debate argue that fuel is cheap and running rich of peak keeps temperatures down. The lean-runners say that running an engine lean of peak is not only less costly than running rich of peak, but also reduces the temperatures and stresses on engines so much that it should be the preferred engine operating method. Who's right?
Both sides are. There are good reasons for operating engines using both of these methods, simply because not all flying is the same, and probably more important, a pilot's experience level plays a huge part in which technique to use.
Common ground between these two camps revolves around the idea that controlling heat is the key to dependable engine operation and long engine life.
This month we will listen to a few operators who run rich of peak with very good results. We will discuss the pluses and minuses of running lean of peak next month.
Airplane operating manuals and pilot's operating handbooks lay out a bare-bones outline of operating techniques and leaning methods. Few of these manuals discuss the many reasons that go into selecting a power setting or mixture setting. Even when these variables are discussed, often the sentences are as cryptic as these from the information manual of a modern composite four-place airplane: "The primary issues are time, range, and fuel consumption. High cruise speeds shorten the time en route, but at the expense of decreased range and increased fuel consumption." Until fairly recently, the practice of running a fuel-injected general aviation engine with the mixture lean of peak was a nonissue; it just wasn't done. None of the manufacturers recommended it and it wasn't referred to in any manuals. Running rich of peak was so readily accepted that many manuals stated that leaning shouldn't be attempted below 3,000 feet msl.
"We never leaned our engines"
Dan Volberding, who formerly held the post of director of maintenance for Scenic Airlines in Las Vegas, told me that its pilots were directed to never lean the turbocharged Continental engines of the fleet of Cessna 402 Businessliners.
"We traded gasoline for cylinder life," says Volberding, who now works in material acquisition for Scenic. Not too many years ago Scenic operated 16 Cessna 402s and four Cessna 207s to fly tourists on scenic flights of the Grand Canyon. Considering that these 2.4 hour flights launched from McCarran International Airport in Las Vegas (elevation 2,181 feet msl) and climbed to an altitude of 8,500 feet, leaving the mixtures full rich seems like a prehistoric operating technique. The main reason that Scenic keeps the red knobs all the way forward is to keep the cylinders cool. Rich fuel flows cool cylinders. This technique guaranteed high fuel flows, but it produced spectacular results.
TBO extensions
The normal factory-recommended TBO for the Continental TSIO-520 engines installed on the 402s is 1,400 hours. FAA regulations (FAR 135.421) allow the agency to extend these TBO periods under certain conditions when an extension is applied for and supporting data from engine teardown inspections is submitted as proof an extension is warranted.
Scenic was granted amazing extensions — a full 1,000 hours on its fleet of 402B (2,400-hour TBO) models and 1,100 hours above and beyond the factory TBO figures for its 402C (2,500-hour TBO) model twin Cessnas. Before everyone runs down to their flight standards district office demanding a 1,000-hour TBO extension because they read about Scenic in AOPA Pilot, it should be pointed out that TBO extensions are usually granted in 10-percent step increases — Scenic's management team worked with their FSDO for decades to achieve these numbers.
These turbocharged engines held up extremely well in spite of the fact that many of Scenic's pilots stayed in Las Vegas only long enough to log enough twin-engine time to move on. "If we have a pilot here longer than a year, they're a lifer," says Volberding.
Another Las Vegas Grand Canyon tour operator, King Airlines that operates out of nearby Henderson Airport, echoed these suggestions. Chief Pilot Vince Stapleton said that open cowl flaps and full rich are the rule in the summer, but the pilots are permitted to lean during winter months.
"We see young pilots that firewall the engines because they want to get there fast, and it shows up in cracked cylinders. I've tried to set up a one pilot-one airplane schedule so I can track who's hot-rodding my planes, but it hasn't worked out. We do insist that our pilots use a stage cooling process during letdown. This prevents rapid temperature changes in the cylinder heads."
Since 100-degree-plus ambient air temperatures are the rule in Las Vegas during summer months these two operators have adopted procedures that keep their engines cool and are easy to follow. One operator that runs a large fleet of airplanes in Florida has a different set of rules — and changes very few cylinders.
The Flight Express method
John Kirchhoefer smiles a lot. His business is growing and he's found people who think the way he does about running a company that is dependent on a fleet of piston-powered airplanes. Their secret? Keep an eagle eye on costs, give the pilots a simple powerplant management technique, and simplify the airplanes.
Kirchhoefer is the president of Flight Express, an air courier company based at Orlando's Executive Airport. When Director of Maintenance Pat Hawk was asked how often Flight Express changed cylinders on its fleet of 62 Cessna 210s and 25 Beechcraft Barons he answered, "Last year we changed 70 cylinders in 70,000 flight hours."
Hawk's estimate was close. Last year his engines accumulated exactly 81,144 hours and there were 76 cylinder changes, for an average of one cylinder every 1,067 hours of engine time.
Flight Express doesn't apply for TBO extensions, reasoning that following the manufacturer's advice on this issue provides extra liability protection. But a company that's this fiscally savvy has a plan. It's going to remove all the 520- series engines and replace them with Continental IO-550P engines and Hartzell Scimitar propellers. "Then we will have an engine and a prop with a 2,000-hour TBO on each airplane," says Hawk.
Engine health tips
Years of experience have shown Hawk that a few steps are essential for long engine life. "We've learned that these engines don't require too much maintenance if the [fuel injection system] fuel flows are set correctly. We set them [in accordance with TCM SID97-3] after a new engine is installed, after the first flight, and then again at 100 hours. We've had the fuel pressures change so much in that first 100 hours that the engine would hardly run. After the first hundred hours we recheck them every 300 hours. The other thing we found that's critical is the baffle condition and baffle seals. We install new seals at every engine change."
24/24 and 100 degrees rich
Chief Pilot Matt Harper explains that new hires, who typically have 1,200 hours' total time with commercial and instrument ratings, are put through a weeklong ground school. The students have to learn hazardous material (hazmat) handling, regulations governing Flight Express operations, weather, FAR Part 91 and Part 135 operations, and flight planning. "Our more recent classes have had high pass rates — in the past we usually washed out 50 percent of each class," said Harper. When asked about powerplant management, Harper said, "We teach our pilots to use 24/24 and 100 degrees F rich of peak, meaning 24 inches of power, 2,400 rpm, and just barely leaned." Asked about these rich settings, Hawk explained, "We do this for two reasons. We don't want to configure our airplanes to break. That means that none of our airplanes have engine monitors. Since we can't tell if the factory EGT gauge is installed on the leanest cylinder, the 100-degree-rich setting gives us a cushion." That doesn't always mean these rules are followed. "We're a courier business and if our guy gets behind, he'll push the power up. These engines seem to be happy running at almost full throttle as long as you don't lean them too much," says Harper.
Kirchhoefer explained his philosophy. "I like to hire people who are curious. That helps us financially. Our computer tracking system helps us make good decisions on everything from tires to spark plugs. We recently studied the labor and maintenance associated with spark plugs and found it was cheaper in the long run to install platinum plugs, so we're making that change."
Hawk and Kirchhoefer spoke frankly about the company's debt-to-capital ratio. Rarely is this a topic of conversation around the airport — but Flight Express knows it's running a business, and the bottom line is factored into every maintenance and operating decision. For this growing company, success goes hand in hand with a simple powerplant management philosophy that is conservative — and works.
Simple, rich, and cool
The Grand Canyon touring companies have adopted engine operating and leaning philosophies that are dictated by high ambient air temperatures. These deviations from manufacturers' suggested practices have proven to be extremely effective.
Flight Express, under different operating conditions, has decided that a conservative leaning schedule works best for it. This decision is based on years of experience. Opting for simplicity in equipment and procedures has provided Flight Express with engine reliability, and that translates into happy customers.
What does this mean for you? There are times when leaning in accordance with the manufacturer's suggestions is warranted, but it's OK to operate these engines on the rich side of the suggestions if conditions (or your own personal preferences) indicate it would be prudent.
E-mail the author at [email protected].
What's Stage Cooling?
Vince Stapleton of King Airlines in Las Vegas teaches his pilots that power changes should be limited to one inch of manifold pressure every two minutes during descent, which translates into a technique known as stage cooling.
Stage cooling is a procedure in which the pilot slowly reduces power during descents. The purpose is to prevent rapid cylinder-head temperature changes, which can create stresses and contribute to cylinder-head cracking.
John Hazlet, vice president of maintenance and director of operations for Ameriflight in Burbank, California, says his pilots are taught to pitch down to get a 500-foot-per-minute descent, then reduce power two inches and note the time. At two-minute intervals power is reduced in two-inch steps until the manifold pressure has been reduced to 23 inches (from cruise power settings of 30 inches). This procedure takes planning, but these professionals guarantee it's worth it. — SWE
Related Articles
