AP4ATCO - Turboprop Engine
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- The following SKYbrary Articles:
a) article description Turboprop engine structure, operation principles and basic terms. Advantages and disadvantages in terms of traffic flow and ATC.
b) source (IANS) 3.3
c) additional sources FAA Airplane Flying Handbook (FAA-H-8083-3A) – chapters 11, 14
- FAA Pilot’s Handbook of Aeronautical Knowledge – chapter 6
CAST (The Commercial Aviation Safety Team) - Airplane Turboprop Engines Basic Familiarization CAST (The Commercial Aviation Safety Team) - Introduction to Turboprop Engine Types SKYbrary Article: “Engine Failure After Take Off – Light Twin Engine Aircraft” (http://www.skybrary.aero/index.php/Engine_Failure_After_TakeOff_-_Light_Twin_Engine_Aircraft) SKYbrary Article: “Engine Failure: Guidance for Controllers” (http://www.skybrary.aero/index.php/Engine_Failure:_Guidance_for_Controllers)
A turboprop engine propels the aircraft using a propeller, as well as, an air jet. The air jet is produced in the same manner as the jet of the turbofan. The main driving force comes from the propeller. The turbine in the engine extracts more energy compared to a jet because it must power the propeller as well as the compressor.
The amount of propeller thrust varies up to 90% of the total thrust depending on type. The air, compressed and burnt with injected fuel, is discharged through a turbine transmitting its energy to the compressor and via a gearbox to the propeller. Reduction gearing, in the ratio of 5 – 20 to 1 is needed to prevent the speed of the propeller blades reaching supersonic levels.
Turboprop engines are commonly found on smaller, shorter range, commercial aircraft. Older turboprop aircraft tend to be slow and this can cause complications when they are mixed with jets during descent and landing. Turboprops also tend to be more vulnerable to en-route icing problems due to operating altitudes which can often find them in clouds.
Principle of operation – turboprop engine
The turboprop uses a gas turbine (jet engine) to turn a propeller. There are two main parts to a turboprop propulsion system, the jet engine and the propeller. The jet engine found in turboprops is very similar to a jet engine, except that instead of expanding all the hot exhaust gases through the nozzle to produce thrust, most of the energy of the exhaust gases is used to turn the turbine.
There may be an additional turbine stage present, which is connected to a drive shaft. The drive shaft is connected to a gearbox. The gearbox is then connected to a propeller that produces most of the thrust. The exhaust velocity of a turboprop is low and contributes little thrust because most of the energy of the core exhaust has gone into turning the drive shaft.
In the matter of takeoff and initial climb performance, the reciprocating engine is superior to the turbojet engine. Turbojet engines are most efficient at high speeds and high altitudes, while propellers are most efficient at slow and medium speeds (propellers become less efficient as the speed of the aircraft increases). Propellers also improve takeoff and climb performance. The development of the turboprop engine was an attempt to combine in one engine the best characteristics of both the turbojet, and propeller driven reciprocating engine.
Turboprop engines are most efficient at speeds between 220 and 350 knots and altitudes between 18000 and 30000 feet. They also perform well at the slow speeds required for takeoff and landing, and are fuel efficient. The minimum specific fuel consumption of the turboprop engine is normally available in the altitude range of 25000 feet up to the tropopause.
The power of a turboprop engine is measured in shaft horsepower (shp). Shaft horsepower is determined by the RPM (revolutions per minute) and the torque (twisting moment) applied to the propeller shaft. Since turboprop engines are gas turbine engines, some jet thrust is produced by exhaust leaving the engine. This thrust is added to the shaft horsepower to determine the total engine power, or equivalent shaft horsepower.
Although the turboprop engine is more complicated and heavier than a turbojet engine of equivalent size and power, it will deliver more thrust at low subsonic airspeeds. However, the advantages decrease as flight speed increases. In normal cruising speed ranges, the propulsive efficiency (output divided by input) of a turboprop decreases as speed increases.
The propeller of a typical turboprop engine is responsible for roughly 90 percent of the total thrust under sea level conditions on a standard day. The excellent performance of a turboprop during takeoff and climb is the result of the ability of the propeller to accelerate a large mass of air while the airplane is moving at a relatively low ground and flight speed. All turbine engines, turboprop or turbojet, are defined by limiting temperatures, rotational speeds, and (in the case of turboprops) torque. Depending on the installation, the primary parameter for power setting might be temperature, torque, fuel flow or RPM (either propeller RPM, gas generator (compressor) RPM or both).
In cold weather conditions, torque limits can be exceeded while temperature limits are still within acceptable range. While in hot weather conditions, temperature limits may be exceeded without exceeding torque limits. In any weather, the maximum power setting of a turbine engine is usually obtained with the throttles positioned somewhat aft of the full forward position.
An overtemp or overtorque condition that lasts for more than a very few seconds can literally destroy internal engine components.
Advantages and disadvantages of the turboprop engine
Advantages: • in dense air, i.e. lower levels, a propeller has a higher efficiency than jet exhaust; • generally turboprop aircraft can operate into shorter runways than jets; • the propeller can be feathered to minimize drag in the event of engine failure, which is not possible for jet or turbofan engines. • mechanical reliability due to relatively few moving parts; • light weight; • simplicity of operation; • high power per unit of weight;
Disadvantages: • propellers lose efficiency at high altitudes; • vibration levels can cause slight passenger discomfort; • en-route weather (icing/turbulence) can cause problems and additional passenger discomfort due to operating altitudes (often in clouds); • older generation turboprops are slow.
TURBOPROP ENGINE TURBOCHARGING
The turbocharged engine allows the pilot to maintain sufficient cruise power at high altitudes where there is less drag, which means faster true airspeeds and increased range with fuel economy. At the same time, the powerplant has flexibility and can be flown at a low altitude without the increased fuel consumption of a turbine engine. When attached to the standard powerplant, the turbocharger does not take any horsepower from the powerplant to operate; it is relatively simple mechanically, and some models can pressurize the cabin as well.
The turbocharger is an exhaust-driven device, which raises the pressure and density of the induction air delivered to the engine. It consists of two separate components: a compressor and a turbine connected by a common shaft. The compressor supplies pressurized air to the engine for high altitude operation. The compressor and its housing are between the ambient air intake and the induction air manifold. The turbine and its housing are part of the exhaust system and utilize the flow of exhaust gases to drive the compressor.
The turbine has the capability of producing manifold pressure in excess of the maximum allowable for the particular engine. In order not to exceed the maximum allowable manifold pressure, a bypass or waste gate is used so that some of the exhaust will be diverted overboard before it passes through the turbine.
The position of the waste gate regulates the output of the turbine and therefore, the compressed air available to the engine. When the waste gate is closed, all of the exhaust gases pass through and drive the turbine. As the waste gate opens, some of the exhaust gases are routed around the turbine, through the exhaust bypass and overboard through the exhaust pipe.
The waste gate actuator is a spring-loaded piston, operated by engine oil pressure. The actuator, which adjusts the waste gate position, is connected to the waste gate by a mechanical linkage.
The control center of the turbocharger system is the pressure controller. This device simplifies turbocharging to one control: the throttle. Once the pilot has set the desired manifold pressure, virtually no throttle adjustment is required with changes in altitude. The controller senses compressor discharge requirements for various altitudes and controls the oil pressure to the waste gate actuator which adjusts the waste gate accordingly. Thus the turbocharger maintains only the manifold pressure called for by the throttle setting.
First and foremost, all movements of the power controls on turbocharged engines should be slow and gentle. Aggressive and/or abrupt throttle movements increase the possibility of overboosting. The pilot should carefully monitor engine indications when making power changes.
Because of the high temperatures and pressures produced in the turbine exhaust systems, any malfunction of the turbocharger must be treated with extreme caution. In all cases of turbocharger operation, the manufacturer’s recommended procedures should be followed. This is especially so in the case of turbocharger malfunction. However, in those instances where the manufacturer’s procedures do not adequately describe the actions to be taken in the event of a turbocharger failure, the following procedures should be used.
- Overboost condition
If an excessive rise in manifold pressure occurs during normal advancement of the throttle (possibly owing to faulty operation of the waste gate):
- the throttle should be retarded smoothly immediately. It is in orded to limit the manifold pressure below the maximum for the RPM and mixture setting. - the engine should be operated in such a manner as to avoid a further overboost condition.
- Low manifold pressure
Although this condition may be caused by a minor fault, it is quite possible that a serious exhaust leak has occurred creating a potentially hazardous situation:
- engine should be shut down in accordance with the recommended engine failure procedures, unless a greater emergency exists that warrants continued engine operation. - if continuing to operate the engine, the lowest power setting should be used, demanded by the situation and airplane should land as soon as practicable.
TURBOPROP ENGINE CONTROL
Powerplant (engine and propeller) control is achieved by means of a power lever and a condition lever for each engine. There is no mixture control and/or RPM lever as found on piston engine airplanes.
On the fixed shaft constant-speed turboprop engine, the power lever is advanced or retarded to increase or decrease forward thrust. The power lever is also used to provide reverse thrust. The condition lever sets the desired engine RPM within a narrow range between that appropriate for ground operations and flight.
Powerplant instrumentation in a fixed shaft turboprop engine typically consists of the following basic indicator:
• torque or horsepower; • ITT – interturbine temperature; • fuel flow; • RPM;
Torque developed by the turbine section is measured by a torque sensor. The torque is then reflected on a cockpit horsepower gauge calibrated in horsepower times 100. Interturbine temperature (ITT) is a measurement of the combustion gas temperature between the first and second stages of the turbine section. The gauge is calibrated in degrees Celsius. The fuel flow indicator indicates fuel flow rate in pounds (or kilograms) per hour. Propeller RPM is reflected on a cockpit tachometer as a percentage of maximum RPM. Normally, a vernier indicator on the gauge dial indicates RPM in 1 percent graduations as well. Propeller feathering in a fixed shaft constant-speed turboprop engine is normally accomplished with the condition lever.
In a free power-turbine engine, such as the Pratt & Whitney PT-6 engine (used in Beechcraft King Air, Beechcraft 1900, Let L-410 Turbolet, Pilatus PC-6 Turbo Porter, Embraer EMB 110, Piper PA-42 Cheyenne III, Piper Meridian, Piaggio Avanti, Pilatus PC-12, Cessna 208 Caravan, etc.), the propeller is driven by a separate turbine through reduction gearing. The propeller is not on the same shaft as the basic engine turbine and compressor. Unlike the fixed shaft engine, in the split shaft engine the propeller can be feathered in flight or on the ground with the basic engine still running.
A typical free power-turbine engine has two independent counter-rotating turbines. One turbine drives the compressor, while the other drives the propeller through a reduction gearbox.
Powerplant (engine and propeller) operation is achieved by three sets of controls for each engine: the power lever, propeller lever, and condition lever.
The power lever serves to control engine power in the range from idle through takeoff power. Forward or aft motion of the power lever increases or decreases gas generator RPM (N1) and thereby increases or decreases engine power. The propeller lever is operated conventionally and controls the constant-speed propellers through the primary governor. The propeller RPM range is normally from 1500 to 1900. The condition lever controls the flow of fuel to the engine. Like the mixture lever in a piston-powered airplane, the condition lever is located at the far right of the power quadrant. But the condition lever on a turboprop engine is really just an on/off valve for delivering fuel. There are HIGH IDLE and LOW IDLE positions for ground operations, but condition levers have no metering function. Leaning is not required in turbine engines; this function is performed automatically by a dedicated fuel control unit.
Engine instruments in a split shaft/free turbine engine typically consist of the following basic indicators:
• ITT (interstage turbine temperature) indicator; • Torquemeter; • Propeller tachometer; • N1 (gas generator) tachometer; • Fuel flow indicator; • Oil temperature/pressure indicator;
The ITT indicator gives an instantaneous reading of engine gas temperature between the compressor turbine and the power turbines. The torquemeter responds to power lever movement and gives an indication, in foot-pounds (ft/lb), of the torque being applied to the propeller. Because in the free turbine engine, the propeller is not attached physically to the shaft of the gas turbine engine, two tachometers are justified—one for the propeller and one for the gas generator. The propeller tachometer is read directly in revolutions per minute. The N1 or gas generator is read in percent of RPM. The ITT indicator and torquemeter are used to set takeoff power. Climb and cruise power are established with the torquemeter and propeller tachometer while observing ITT limits. Gas generator (N1) operation is monitored by the gas generator tachometer. Proper observation and interpretation of these instruments provide an indication of engine performance and condition.
Reverse thrust / feathered position
The thrust that a propeller provides is a function of the angle of attack at which the air strikes the blades, and the speed at which this occurs. The angle of attack varies with the pitch angle of the propeller.
So called “flat pitch” is the blade position offering minimum resistance to rotation and no net thrust for moving the airplane. Forward pitch produces forward thrust—higher pitch angles being required at higher airplane speeds.
The “feathered” position is the highest pitch angle obtainable.
The feathered position produces no forward thrust. The propeller is generally placed in feather only in case of in-flight engine failure to minimize drag and prevent the air from using the propeller as a turbine.
In the “reverse” pitch position, the engine/propeller turns in the same direction as in the normal (forward) pitch position, but the propeller blade angle is positioned to the other side of flat pitch.
In reverse pitch, air is pushed away from the airplane rather than being drawn over it. Reverse pitch results in braking action, rather than forward thrust of the airplane. It is used for backing away from obstacles when taxiing, controlling taxi speed, or to aid in bringing the airplane to a stop during the landing roll. Reverse pitch does not mean reverse rotation of the engine.
TURBOPROP ENGINE MALFUNTIONS AND EFFECTS OF FAILURE
Hot start / false start
Turbine engines are extremely heat sensitive. They cannot tolerate an overtemperature condition for more than a very few seconds without serious damage being done. Engine temperatures get hotter during starting than at any other time. Thus, turbine engines have minimum rotational speeds for introducing fuel into the combustion chambers during startup. Hypervigilant temperature and acceleration monitoring on the part of the pilot remain crucial until the engine is running at a stable speed. Successful engine starting depends on assuring the correct minimum battery voltage before initiating start, or employing a ground power unit (GPU) of adequate output.
After fuel is introduced to the combustion chamber during the start sequence, “light-off” and its associated heat rise occur very quickly. Engine temperatures may approach the maximum in a matter of 2 or 3 seconds before the engine stabilizes and temperatures fall into the normal operating range. During this time, the pilot must watch for any tendency of the temperatures to exceed limitations and be prepared to cut off fuel to the engine.
An engine tendency to exceed maximum starting temperature limits is termed a hot start. The temperature rise may be preceded by unusually high initial fuel flow, which may be the first indication the pilot has that the engine start is not proceeding normally. Serious engine damage will occur if the hot start is allowed to continue.
A condition where the engine is accelerating more slowly than normal is termed a hung start or false start. During a hung start/false start, the engine may stabilize at an engine RPM that is not high enough for the engine to continue to run without help from the starter. This is usually the result of low battery power or the starter not turning the engine fast enough for it to start properly.
In modern turboprop engines, compressor surge is a rare event. A surge from a turboprop engine is the result of instability of the engine's operating cycle.
Compressor surge may be caused by engine deterioration, it may be the result of ingestion of birds or ice, or it may be the final symptom from a “severe engine damage” type of failure.
The operating cycle of the turbine engine consists of intake, compression, combustion, and exhaust, which occur simultaneously in different places in the engine. The part of the cycle susceptible to instability is the compression phase. In a turbine engine, compression is accomplished aerodynamically as the air passes through the stages of the compressor. The air flowing over the compressor airfoils can stall just as the air over the wing of an airplane can. When this airfoil stall occurs, the passage of air through the compressor becomes unstable and the compressor can no longer compress the incoming air. The high-pressure air behind the stall further back in the engine escapes forward through the compressor and out the inlet. This escape is sudden, rapid and often quite audible as a bang. Engine surge can be accompanied by a visible flash forward out the inlet and rearward out the tailpipe. Instruments may show high Inter-Turbine Temperature (ITT) and Engine Pressure Ratio (EPR) or rotor speed changes; but, in many stalls, the event is over so quickly that the instruments do not have time to respond.
A flameout is a condition where the combustion process within the burner has stopped. A flameout will be accompanied by a drop in ITT, in torque, in engine core speed and in engine pressure ratio. The first symptom noticed by the pilot may be a yaw as the propeller becomes a source of drag, or autofeather of the propeller accompanied by a drop in propeller RPM. The engine ignition light may come on.
The flameout may result from the engine running out of fuel, severe inclement weather, a volcanic ash encounter, a control system malfunction, or unstable engine operation (such as a compressor stall). Momentary flameout may be perceived as a short-term power fluctuation accompanied by an ignition light.
No pilot action is necessary provided the engine recovers within a few seconds.
“Engine fire” almost always refers to a fire outside the engine but within the nacelle. A fire in the vicinity of the engine should be annunciated to the flight crew by a fire warning in the flight deck. It is unlikely that the flight crew will see, hear, or immediately smell an engine fire. Sometimes, flight crews are advised of a fire by communication with ATC (control tower).
It has been shown that, even in incidences of fire indication immediately after takeoff, there is adequate time to continue climb to a safe altitude before attending to the engine.
Flight crews should regard any fire warning as a fire, even if the indication goes away when the power lever is retarded to idle. The indication might be the result of pneumatic leaks of hot air into the nacelle. The fire indication could also be from a fire that is small or sheltered from the detector so that the fire is not apparent at low power. Fire indications may also result from faulty detection systems.
One of the most alarming events for passengers, flight attendants, ground personnel and even air traffic control is to witness is a tailpipe fire.
Fuel may puddle in the turbine casings and exhaust during start-up or shutdown, and then ignite. This can result in a highly-visible jet of flame out the back of the engine. Passengers have initiated emergency evacuations in these instances, leading to serious injuries.
Some airplanes have overtemperature detectors installed around the tailpipe; others may give no indication of an anomaly to the flight crew until the cabin crew or control tower draws attention to the problem.
If notified of an engine fire without any fire indications in the cockpit, the flight crew should accomplish the tailpipe fire procedure. It will include motoring the engine to help extinguish the flames, while most other engine abnormal procedures will not. The normal engine fire procedure is not effective in controlling a tailpipe fire.
Birdstrike / Foreign Object Damage (FOD)
Airplane engines encounter birds most often in the vicinity of airports, either during takeoff or during landing. Encounters with birds occur during both daytime and nighttime flights. By far, most bird encounters do not affect the safe outcome of a flight. In more than half of the bird ingestions into engines, the flight crew is not even aware that the ingestion took place. When a large bird is involved, the flight crew may notice a thud, bang or vibration. If the bird enters the engine core, there may be a smell of burnt flesh in the flight deck or passenger cabin from the bleed air.
Birdstrikes can damage an engine or propeller. Foreign Object Damage (FOD) from other sources, such as tire fragments, runway debris or animals, may also be encountered, with similar results.
Severe engine damage
Severe engine damage may be difficult to define. It is important for flight crews to know that severe engine damage may be accompanied by symptoms such as fire warning (from leaked hot air) or engine surge because the compressor stages that hold back the pressure may not be intact or in working order due to the engine damage. In this case, the symptoms of severe engine damage will be the same as a surge without recovery: - there will be a loud noise; - EPR will drop quickly; - torque, NP, NG and fuel flow will drop; - ITT may rise momentarily; - there will be a loss of power to the airplane as a result of the severe engine damage.
Engine seizure describes a situation where the engine rotors stop turning in flight, perhaps very suddenly. The static and rotating parts lock up against each other, bringing the rotor to a halt. In practice, this is only likely to occur at low rotor RPM after an engine shutdown.
Seizure cannot occur without very severe engine damage, to the point where the vanes and blades of the compressor and turbine are mostly destroyed. This is not an instantaneous process – there is a great deal of inertia in the turning rotor compared to the energy needed to break interlocking rotating and static components.
Symptoms of engine seizure in flight may include vibration, zero rotor speed, mild airplane yaw, and, possibly, unusual noises.
No power lever response
A “no power lever response” type of malfunction can be completely overlooked, with potentially serious consequences to the airplane.
If an engine slowly loses power – or if, when the power lever is moved, the engine does not respond – the airplane will experience asymmetric thrust. This may be partly concealed by the autopilot’s efforts to maintain the required flight condition.
As is the case with flameout, if no external visual references are available, such as when flying over the ocean at night or in IMC, asymmetric thrust may persist for some time without the flight crew recognizing or correcting it. In several cases, this has led to airplane upset, which was not always recoverable.
Vibration is a symptom of a wide variety of engine conditions, ranging from very benign to serious. The following are some causes of tactile or indicated vibration: - propeller unbalance at assembly - propeller blade icing - birdstrike/FOD - bearing failure - blade distortion or failure - internal engine failure
It is not easy to identify the cause of the vibration in the absence of other unusual indications. Although the vibration from some failures may feel very severe on the flight deck, it will not damage the airplane. There is no need to take action based on vibration indication alone, but it can be very valuable in confirming a problem identified by other means.
Engine Failure After TakeOff - Light Twin Engine Aircraft If a multi-engine aircraft suffers engine failure when airborne, there are two immediate aerodynamic effects. The initial effect is the yawing that occurs due to the asymmetry of the thrust line. The size of this initial yawing moment depends upon the engine thrust and the distance between the thrust line and the aircraft centre of gravity. The yawing moment is also affected initially by the rate of thrust decay of the ‘dead’ engine and ultimately by its drag. In addition, the yaw is aggravated by the drag effect of the windmilling propeller. The total moment can be very large, particularly when the airplane is at high power and low speed. The second effect is roll. This occurs when the aircraft continues to yaw towards the failed engine resulting in a decrease in lift from the ‘retreating’ wing and a yaw-induced roll towards the failed engine. This roll is reinforced by the offset of the wings and the loss of the lift from the slipstream in aircraft with the propeller in front of the engine. As well as the aerodynamic consequences of the failure, the performance penalty is very significant. While the failure of an engine represents a 50% loss of available power, it can result in as much as an 80% loss of performance.
Engine Failure: Guidance for Controllers
What to Expect • Deviation from SID - if the engine failure occurs at take-off or after rotation, the crew might not follow the published SID and any associated noise abatement procedures • Intermediate level-off - if the engine failure occurs during climb out or descent, the crew might elect to level-off the aircraft in order to assess the situation • Descent - the crew might decide to descend (gain airspeed and re-start the engine) or to descend due to pressurisation problems connected with the engine failure • Course deviation - the crew might decide to divert to the next suitable or to the alternate aerodrome • Long and high speed approach and landing - due to performance limitations attributed to the engine failure the approach speed might be higher than prescribed, which could consequently may result in non-stabilised approach, runway excursion and blocked runway • Slow turn rates - The turn rate is expected to be slow if it is executed on the inoperative engine side.
1. [Question type: true or false, based on AirQuestions ENG-TP/177 ]
Q: Because propellers become less efficient as the speed of the aircraft increases, turboprops are suitable for low speed aircraft. A1: True A2: False
Correct answer: A1
2. [Question type: true or false, based on AirQuestions ENG-TP/189 ]
Q: Turboprop aircraft operate at altitudes where the weather is never a significant factor for passenger comfort. A1: True A2: False
Correct answer: A2
3. [Question type: multiple response, based on AirQuestions ENG-TP/180]
Q: Select the statements describing the disadvantages of a turboprop engine. A1: Limited forward speed of the aircraft A2: Power changes are slow to take effect A3: Low power to weight ratio A4: Relatively high vibrations A5: Inefficient at high altitudes A6: Economic at low levels and low airspeeds
Correct answers: A1, A3, A4,A5
4. [Question type: multiple choice ]
Q: In case of engine failure after take-off, in a light twin engine aircraft, two immediate aerodynamic effects are: A1: roll and stall A2: yawing and roll A3: yawing and descent
Correct answer: A2
15. Jet engine