Plotting the details for a long engine life
Perhaps the greatest appeal in stepping up from piston- to turbine-powered aircraft is the promise of longer engine life and better engine reliability. This, in turn, holds out the hope of enhanced economy and safety.
With fewer parts, fewer movements, and less internal friction, turbine engines can seem like models of perfection. Compared to a reciprocating engine's clattering assembly of rods, pins, and valves, a turbine's workings are silky smooth. And where the piston pilot faces a tentative life anticipating prematurely failed cylinders, a top overhaul at midlife, and often feeble hopes of an engine's making it to TBO, the turbine pilot can look forward to confidently zipping along to a TBO double that of a typical piston engine.
But hold on just a minute. Although turbine engines do offer the opportunity of more trouble-free operation, they are by no means infallible. If treated properly, they can live uneventful service lives. However, several conditions and events can conspire to shorten a turbine engine's life and cause it to waste fuel, run abnormally hot, and, in extreme cases, to fail catastrophically.
What can cause all this havoc? Excess heat, foreign object damage, and engine air contaminated by salt, sulfur, and/or pollutants are the three big dangers.
The heat problem usually comes about because of hot starts. That is, overly intense burning of the fuel/air mixture just after a start sequence has commenced. This is usually caused by an insufficient flow of cooling air being drawn through the engine as it spools up. Given a strong battery (or the extra oompf of an externally-powered start), and a pilot's willingness to let compressor speed build high enough before introducing fuel to the combustion chamber, hot starts can usually be avoided.
Engine manufacturers set temperature limits (marked by redlines on inter-turbine or exhaust gas temperature gauges) so that engine components, such as turbine rings, compressor and turbine blades, stators, and combustion chamber parts, are not damaged. While starting, for example, the King Air C90A's Pratt & Whitney PT6A-21 must not exceed an ITT of 1,090 degrees Celsius for any longer than two seconds. ITT redlines for takeoff and cruise are 695 degrees. Let the needles blow past these values, and you could have real trouble — a mandatory borescope examination, at the least; a hot section replacement at worst. This is why pilots often refer to the ITT gauge as the "resume gauge."
When high temperatures burn blades and other components, the result is always a loss of engine efficiency. Turbine and compressor blades are like miniature airfoils, and if they are burned, their surfaces are roughened. Airflow over the blades is disrupted in much the same way that ice accretions disrupt airflow over an airfoil.
Another possibility is heat-induced cracks, or deterioration of metallurgical strength. With compressor or turbine wheels spinning at anywhere from 38,000 to 42,000 rpm, you can imagine the consequences: blade failure, followed by separation, followed by catastrophic failure of the engine.
Foreign object damage is another problem. Virtually all turbine engines have inertial separators, which are supposed to shunt debris like sand, dirt, and ice particles away from compressor and turbine blades. But sometimes, as with hot starts, even the best efforts fail, and the engine ingests material that can nick or crack the blades. (For a worst case, how about a loose bolt, resting on an inlet shroud, being sucked into the works. It's happened, thanks to careless mechanics and cursory preflight inspections.)
Finally, sulfur, salt air, and pollution can wreck the smooth flow of air through an engine. The mechanism is simple to understand. These contaminants build up on the blades, then pit the leading edges by corroding them. Eventually, the blades' surfaces begin to blister and wear away.
Salt contamination is always a threat when flying near ocean environments. For this reason, most manufacturers recommend staying above 5,000 feet msl — the uppermost limit for the heaviest salt concentrations — for as long as possible when flying in these areas.
Sulfur buildups cause sulfidation of compressor and turbine blades. Sulfur is a component of jet fuel, and it's unavoidably released during the combustion process. The sulfidation — a form of corrosion — occurs when this sulfur mixes with the lead present in the fuel and in polluted air, the chlorine in salty air, or certain crop-dusting chemicals. Mix sulfur with these catalysts, add the right amount of engine heat, and, once again, you've got corroded blades and other components.
By now, some of the glitz of turbine power may have worn off. How do you deal with all these problems, you're probably asking.
First, the obvious. Do everything you can to avoid hot starts, keep ITTs or EGTs cool while cruising, avoid marine air as best you can, and perform routine compressor and/or turbine washes.
Second, you must establish a system of engine trend monitoring. This is just a fancy term for keeping track of any changes in an engine's compressor speed, ITT or EGT, and fuel flow. It's the subtle changes in these variables that can give valuable indications of blade deterioration and even forewarn of impending blade failures.
Trend monitoring can be as simple as keeping a diary of written entries, or be as sophisticated as built-in software that makes its own entries and can be downloaded to customized maintenance programs. The software programs, relatively new and aimed at airline operators, is sure to expand to the owner-flown market. But for the time being, most owner- flown turboprops will have to continue to rely on old-fashioned handwritten entries for primary data. This data is then fed into one of the engine trend monitoring software programs offered by several providers.
The important thing is to make entries on every flying day, or every five hours, and to make sure that your entries are as accurate as you can make them. This means eliminating any parallax by obtaining a good, head-on view of the gauge being studied. Trend monitoring software can smooth out any excursions caused by random errors in equipment gauges — that's one of its purposes — but it can't deal with consistently wrong information. That would produce a garbage in, garbage out situation.
To make a reading, first set cruise power at altitude, then wait for five minutes for the engine(s) to settle down. Carefully note your indicated airspeed, outside air temperature, pressure altitude, propeller speed, and torque settings. These are the elements that the trend monitoring software will adjust as constants. Then record the engines' compressor (sometimes called Ng, or gas generator, or N1) speed, ITT or EGT, and fuel flow.
These latter three variables tell the story. For any given constant set of conditions, their fluctuations and trends translate into an engine's internal health.
In general, drops in compressor speed, rises in ITT or EGT, and elevations in fuel flow are bad news. That's because slower compressor speeds mean damaged blades. Higher temperatures and fuel flows mean that you're having to add more fuel — and run combustor temperatures hotter — to maintain a constant power value. In other words, as an engine deteriorates, you have to run hotter and burn more fuel to hold a constant torque setting.
It bears repeating: The changes are subtle. For example, we're talking about fluctuations on the order of 4 percent of compressor speed, 20 to 80 degrees or so of ITT/EGT, and less than 40 pounds per hour of fuel flow. However, when the software is finished with the data, trends and warnings emerge. The accompanying graphs show the plots for several different turboprop engines, along with their interpretations. Although these plots are for Pratt & Whitney PT6A engines, the same basic principles of interpretation apply to AlliedSignal/Garrett turboprop engines.
If a picture tells a thousand words, a close look at the engine trends shown here speak volumes. For example, you can see the beneficial effect of a compressor wash in a correcting rise in compressor speed and a drop in ITT and fuel flow.
Trend monitoring is also useful in other diagnostic applications. An abnormally high fuel flow, for instance, can tip a mechanic off to a faulty fuel nozzle. Similarly, bleed air valve leaks, compressor speed transmitter problems, or poor torquemeter calibration may prove to be the causes of certain anomalies.
Without trend monitoring, the operator of a turbine aircraft is really flying in the dark when it comes to understanding engine health. Anyone considering the purchase of a turbine aircraft ought to ask for trend monitoring information as a condition of the sale. If it doesn't exist, be wary. Once a transaction is completed, get on board with a trend monitoring program as soon as possible. Both Pratt & Whitney and AlliedSignal/Garrett have programs, and there are other, independent trend monitoring programs certainly worth checking out. One, offered by the More Company of Minden, Nevada, offers trend monitoring for various PT6 engines. Its program couples trend monitoring with oil analysis, engine vibration analysis, and periodic inspections in a supplemental type certificate that permits "on condition" maintenance. This means that maintenance that otherwise would have to take place at fixed hourly intervals — such as hot section inspections and complete overhauls — can slide until these procedures are really needed.
Regardless of the route you take, engine condition trend monitoring is the only way to approach turbine engine maintenance. In fact, it may well be the only way you can realistically expect to meet the manufacturer's recommended TBO and beat the kind of maintenance nightmares you'd hoped to leave behind with piston-powered flight.
Which provides more food for thought. Maybe reciprocating engines have more maintenance problems because of a virtual absence of piston- engine trend monitoring.
