Turbine Pilot

Checking up on the combustion chamber

Aviators learn early that engine reliability is like money in the bank — no matter how much you have, more is better. Since an engine begins to wear the very first time it's run, the way to maintain reasonable powerplant peace of mind is to operate, inspect, and maintain it as the manufacturer recommends. For operators of turbine aircraft this means the engine hot section must be periodically inspected — and, if necessary, serviced — to meet the manufacturer's design specifications.

So what exactly is a hot section, anyway? It isn't any one particular component; rather, it is a collection of parts, all of which share a common but not surprising characteristic — they are the hottest operating components in the engine. The hot section includes the combustion chamber, the turbine wheels, the stationary vane rings (also called stators) which direct hot gases to the turbine wheels, and any associated ducting through which the fiery gases of combustion are routed. These components are roughly analogous to the valves, pistons, and cylinder heads of a piston engine. As one would expect, they are the parts most susceptible to heat damage.

When a typical turbine engine is started, relatively small quantities of fuel and air are combined and ignited in the combustion chamber. The resulting hot gases expand and are directed onto one or more turbine wheels — which, in turn, drive a compressor. The compressor forces additional air into the combustion chamber, allowing even more fuel to be burned. The engine spools up and soon stabilizes at its designed idle speed. From that point, the power output of the turbine is directly proportional to the amounts of air and fuel that are ignited in the combustion chamber. The more fuel burned, the faster the turbine spins, and therefore the larger the mass of air that is drawn through the compressor and routed to the combustion chamber. Manufacturers match the output of the compressor and turbine sections for optimal efficiency throughout the normal operating range of the engine.

As with all things mechanical, however, a turbine engine experiences various kinds of wear and tear which affect how efficiently this process occurs. Although a pilot may detect such clues as higher engine operating temperatures or reduced power output for a given fuel flow, not all engine deterioration is immediately obvious. Problems can develop anywhere in an engine, but the hot section is where the gremlins are most likely to congregate. Periodic hot section inspections are intended to catch little problems before they can grow into big, expensive ones.

To get an idea of just what a hot section inspection entails, we visited Prime Turbines at the Barnstable Municipal Airport in Hyannis, Massachusetts. Prime Turbines is an FAA certificated repair facility that specializes in Pratt & Whitney PT6 turbine engines. Company founder Les McIsaac has spent his entire 21-year career working on PT6s, the engine Pratt & Whitney describes — with some justification — as the world's most popular and thoroughly proven powerplant in its class. More than three decades old, the design began life as a 550-shaft-horsepower powerplant. Over the years it evolved to become a whole family of engines, including a 1,600-shp version that powers regional and commuter airliners. Along the way, its power-to-weight ratio has grown steadily with its overall reliability.

According to McIsaac, the PT6 was designed with an eye towards on-the-wing hot section inspections, which on most variants of the engine are required every 1,250 hours. Pratt & Whitney says it takes about 10 hours to accomplish such an inspection in the field, presuming that no serious defects are discovered. McIsaac concurs and notes that anyone utilizing good operating and maintenance procedures should have no difficulty achieving the inspection interval without a hitch.

In fact, many hot sections will operate for 4,000 hours or more without requiring significant repair. But if other than a mere inspection is required, the hot section must be removed from the engine and sent to a repair or overhaul facility. The complete power section of the PT6 is removable by unbolting one flange, which in effect splits the engine in half.

Turnaround time and cost are the operator's two biggest concerns once it becomes apparent that the hot section must be separated from the engine. These two items can vary widely, depending upon the extent of damage that has occurred and the manner in which the repairs are executed. Customers usually can choose between repair or replacement of damaged components. Replacement is the quicker but more expensive route.

One of the most common problems uncovered during a hot section inspection, says McIsaac, is deterioration of the vane ring or stator. Every turbine engine has both a moving and a stationary component in each of its power-generating stages. The moving component is the turbine wheel itself, while the stationary portion is known as the stator. A stator is a circular component surrounding the turbine wheel, through which combustion gases are routed. The stator accelerates the gases through a series of small exhaust nozzles and directs the gases onto the turbine wheel. When a stator begins to wear, the exhaust nozzles tend to enlarge; gases therefore exit the stator at a slower speed, causing the turbine to spin more slowly. The result is a deterioration of engine power. A hot start can cause instantaneous stator damage, but normal wear and tear will eventually take its toll as well.

Another shortcoming commonly unearthed during hot section inspections is out-of-tolerance turbine blade tip clearance. Blade tips in the PT6 are designed to clear the turbine wheel housing by a mere 0.013 of an inch. When that clearance enlarges for some reason, either from the deformation of the turbine shroud or from blade tip erosion, the turbine no longer produces its rated power. Some gases which would otherwise be driving the turbine wheel bypass it altogether through the enlarged gap, much like the action creating loss of compression in a piston engine. From the pilot's perspective, the detrimental effects of improper blade tip clearance on engine performance are the same as those resulting from a worn stator. If allowed to progress far enough, either problem will cause noticeably higher exhaust gas temperatures and fuel flows for a given power setting.

The combustion chamber itself is highly susceptible to heat damage, which can be caused by poorly maintained fuel nozzles. Burning fuel should never contact a PT6's combustion chamber liner. Instead, an insulating blanket of air flows through the chamber, protecting the liner from the hot gases. When the normal fuel spray patterns in the combustion chamber are disrupted by clogged or dirty fuel nozzles, however, the gases can contact the liner and might actually burn through it. Clogged nozzles also can prevent the engine from producing its rated power.

There are many variables that affect the cost of a hot section inspection, but by far the biggest customer sticker shock arises when replacement or repair of major hot section components is needed. For instance, a factory-new stator for the 750-shp PT6A- 36, the engine found in the Beech C99 Airliner, ranges in price from $14,000 to more than $27,000 depending upon the design selected. Need a new turbine disk? Expect to drop another $13,000. The high cost of purchasing new replacement parts is one reason most customers elect to repair damaged components if at all possible, says McIsaac. A stator or turbine wheel often can be repaired and recertified for less than half the cost of a new one. The flip side to this option is longer aircraft downtime. Customers unable to wait for damaged components to be repaired and unwilling to pay for new parts have a third option, a hot section exchange. Under this scenario, the damaged hot section is traded for a reconditioned one. The aircraft is back in the air in minimum time; and the customer still chalks up significant savings, compared to using factory-new parts.

Not surprisingly, good preventive maintenance and engine operating practices can go a long way toward minimizing the cost of a hot section inspection. One of the best things a PT6 operator can do is ensure that fuel nozzles are inspected regularly. Pratt & Whitney recommends doing so every 400 hours for most versions of the engine. McIsaac recommends that operators have them checked more regularly if inspection reports warrant. Doing so can greatly reduce the cost of subsequent hot section inspections.

Since heat cycles are the biggest enemy of a turbine engine, anything that can be done to minimize temperature extremes during a flight will pay big dividends in the way of reduced repair costs, too. For instance, using a good external power source to start a PT6 can reduce starting temperatures by as much as 75 degrees, compared to using the ship's battery. Likewise, opting for lower power settings during cruise flight will have a positive cumulative effect on the condition of the hot section. A regular engine trend monitoring program will help operators spot subtle changes in an engine's performance over time and will allow for earlier — and therefore usually less expensive — repair work. Trend monitoring becomes especially important when an aircraft is regularly flown by a variety of pilots, none of whom may fly the aircraft frequently enough to notice gradual deterioration of engine performance.

Aircraft flown in parts of the world where humid, salty air is the norm require special preventive maintenance. Under these conditions, a process known as sulphidation can take place inside the engine. Sulphidation refers to a chemical interaction of contaminants in the fuel and air that occurs during combustion. It can cause accelerated turbine-blade corrosion, with damage sometimes occurring after just a few flights. In such extreme environments, operators may need to wash the turbine blades with water every day the aircraft operates, usually by connecting a hose through an engine igniter plug port.

A hot section inspection doesn't have to be the equivalent of opening Pandora's box. With a little engine care, savvy operators need find no unpleasant surprises lurking beneath the cowling.