Engine and Prop Systems

Engine Description

The TPE-331 is a lightweight fixed-shaft turboprop engine and caters to a market for efficient and compact turboprop engines. We say fixed because the rotating turbine assembly is permanently affixed to the propeller through a gearbox. When the turbine turns, so will the propeller always. Similar to the reciprocating engine, the fixed-shaft design provides immediate propeller thrust with power application; however, the TPE-331 yields an added bonus of also providing extra jet thrust due to the flow-through design, increasing its efficiency.

The engine is a single shaft type gas turbine engine. The major engine components from front to rear are the prop shaft, air intake plenum, reduction gearbox, a two-stage centrifugal compressor, an annular combustion chamber, and a three stage axial turbine. The combustion process is initiated by a spark ignition system and combustion is self-sustained after completion of the starting cycle. The combustion produced drives the turbine, which is coupled back to the compressor and the gearbox. See figure below of the major components.

Engine Quadrant Controls

All engine controls are contained on the engine / throttle quadrant. Refer to the illustration below for specific control locations.

  • Run-Crank-Stop Switches: Set to RUN prior to engine start. Set to CRANK to turn the engine without ignition. Set and hold to STOP (STOP position is momentary) to shutdown the engines. Stopping the engines via the RCS switches will initiate the fuel purge system described below.

  • Starter Buttons: Used to initiate the start sequence. Button will illuminate at lightoff when the SRL computer takes over the autostart process.

  • Prop Unfeather Button: Activates an oil pump to move the propeller blades from a feather pitch position toward reverse pitch. Used to place the prop "on the locks" for engine starts.

  • Fuel Enrich Buttons: Adds additional fuel to the engine after lightoff, but before idle RPM. Used to aid engine acceleration if required.

  • SRL switches: Enables/Disables the SRL computer above 80% RPM. SRL EGT function disabled below 80% nominally.

  • Power Lever: When in ALPHA range, adds fuel to the running engine. When in BETA range, changes the pitch of the propeller blade.

  • Condition Lever: When above TAXI position, essentially sets the engine speed. See The Real Deal: Prop Speed below.

SRL System

The SRL system consists of a computer and an array of engine and air data sensor inputs. The SRL system has two primary jobs:

  1. Automatically manage the engine start sequence after lightoff and before idle.
  2. Modify the EGT signal for the given conditions and present to the pilot only one value to represent engine redline, 650ºC. (above 80% RPM only)

THE REAL DEAL: EGTs

Turbine blade strength is affected by temperature. Knowing the temperature the turbine blades are exposed to is important to keep from overheating the blades. The first stage turbine experiences the highest temperature of all the turbine blades; however, placing a "turbine stage" temperature sensor adjacent to the first stage turbine is not practical on the TPE-331 design for a few engineering reasons, not the least of which is exposure to very high temperatures which would contribute to the rapid and frequent failure of the sensor. As such, EGT sensors are utilized because it is a practical design choice for both maintenance and part life reasons.

The problem with the turbine EGT indication, is that the temperature relationship between the exhaust plenum and first stage turbine locations is variable and affected by multiple factors. In other words, we cannot tell from the EGT reading alone exactly what the temperature is at the first stage turbine without knowing several other variables. Without the SRL system, the pilot would have to manually reference multiple charts to look up temperatures, pressures and airspeeds to determine what the maximum allowed EGT is for those conditions and keep the EGT under that value so that the first stage turbine does not overheat. This is very cumbersome to the pilot since conditions are constantly changing with altitude and weather, and so the SRL system alleviates this pilot workload. As long as the pilot keeps the SRL modified EGT value below 650º with the SRL system on, then the engine is within temperature limits for the conditions. The SRL EGT indication is higher than the true EGT, so in the event the SRL fails or is turned off, the EGT usually drops a bit and a very simple and conservative EGT schedule is printed on the face of the OAT gauge for the pilot to reference.

Fuel Enrichment

The SRL system manages the auto-start process in order to maintain an EGT of approximately 700ºC during the start sequence. This systems works well in most all cases. In very cold weather however, starting times may be slower due to more rapid heat loss out of the engine components. The RPM range from 18% - 28% RPM is a critical RPM range where the TPE-331 rotating assembly is subject to resonant vibrations and should not be allowed to rotate in this RPM range for prolonged periods. Steady RPM acceleration through this RPM range is required. The fuel enrichment buttons on the quadrant may be used to inject additional fuel into the engine to assist with engine acceleration in such cases. A watchful eye should be kept on the EGT when using fuel enrichment so as to not exceed temperature limits.

Fuel Purge

A fuel purge system is installed in the engine to avoid any unburnt fuel in the manifold from draining overboard to the environment. This fuel purge system works by utilizing the residual fuel in the fuel manifold at shutdown and burning it during the shutdown via the RCS switches. This results in a slight RPM increase during shutdown and also eliminates the emissions of unburned hydrocarbons into the atmosphere.

Propeller Description

The propeller is a constant-speed type and fully feather-able. Pitch actuation is accomplished hydraulically via a propellor governor utilizing engine oil as the hydraulic fluid. The oil pressure is further boosted by a pump within the prop governor. The propeller governor incorporates a feather valve for manual feathering via the condition levers and also features Negative Torque Sensing (NTS) capability, designed to increase blade angle towards fully feathered pitch in the event of an engine failure. The NTS system is not a feathering system as it does not fully feather the propeller, but rather is a drag-reduction system designed to reduce drag at a critical time and allow the pilot valuable seconds to feather the propellor fully manually.

Feathering / Unfeathering

The propeller is manually feathered by moving the condition lever for the engine full aft. Moving the lever to this position activates linkages within the engine that open the feather valve, allowing oil to drain from the propeller governor whereby the feather spring causes the propeller to rotate to the feathered pitch position. Feathering the propeller will also shut off the fuel supply to the engine. In cases where the pilot forgets to put the prop on the locks during engine shutdown, then the unfeather button on the quadrant may be used to activate the unfeather pump and move the propeller from a feathered pitch angle to a low-drag, flat pitch angle needed for engine starts. The propeller is held at flat pitch by the prop locks described below.

Prop Locks

When shutting down the engine, engine oil pressure reduces as the engine decelerates. This causes the prop pitch spring to drive the propeller pitch towards a feathering angle, as is commonly seen on free-turbine PT-6 engines. To overcome this feathering tendency during the loss of oil pressure at shutdown and lock the propellers at flat pitch for the next engine start, a propeller pitch lock pin is utilized to prevent feathering and hold the propeller blade at a flat pitch angle. This is commonly referred to as "props are on the locks". In general, props must be put on the locks during engine shutdown and taken off the locks after engine start and before taxiing. If you do not take the props off the locks after engine start, you will get no thrust when advancing the power levers and go nowhere.

If the engine is shut down with the power lever placed forward of the GROUND IDLE position, then the propeller will feather with decreasing oil pressure and the unfeather pump will be required to put them on the locks before the next engine start. Power levers must be near the REVERSE positions when shutting down the engines in order to have the propellers be set on the locks properly during engine shut down.

After the engines are started, the propellers must be taken off the locks (unlocked). To unlock the props, refer to NORMAL PROCEDURES section of this handbook. The start lock pins are released by centrifugal force and as such, placing the condition levers in the TAKEOFF LAND position to increase engine RPM before aids in releasing the start locks.

Negative Torque Sensing

The NTS system is a combination of hydraulics, springs and valves, whereby if an engine fails in flight, then in this condition, the windmilling propeller is driving the engine turbine, rather than the other way around where the turbine is driving the propeller. Between the turbine and the propeller is a transmission gearbox. If then, a turbine driving the prop through the gearbox is positive torque on the gearbox, then the propeller driving the turbine through the gearbox is a negative torque on the gearbox and the TPE-331 leverages that torque force and in doing so, operates a valve to dump oil from the prop pitch control, thereby increasing prop pitch torwards feather. Why then isn't this full feathering? The REAL DEAL section below explains more for those interested.

THE REAL DEAL: Negative Torque Sensing

TPE-331 procedures require a "Negative Torque Sensor Ground Test" under certain conditions. As you read above, the NTS system generally comes into play during an engine failure when the windmilling prop drives the turbine section, producing negative torque on the torque sensing system. The system can be tested on the ground because when the engine is initially started, but before lightoff, the propeller is driving the turbine, sort of. Actually, the STARTER is driving the turbine and prop via the gearbox, but in either case, the torque sensing system picks this up as negative torque. Certainly the turbine, having not lit off yet, is not driving anything. However, shortly after lightoff, the turbine begins to pick up speed and at some point, now that its producing power, will begin to drive the prop and the torque will reverse. Think of a merry-go-round you start to spin with your hand...and now imagine it took off on its own and began accelerating. Eventually the bars would hit the 'back/negative' side of your hand. In this example, your hand is the torque sensing system.

The way the pilot can check the torque sensing system is by pressing and holding the unfeather pump button during engine starting (see Command List for 'unfeather'). Pressing the button just before engine start will pressurize elements of prop governing system and cause the BETA light on the panel to illuminate. As the starter button is hit and the starter accelerates the engine, then negative torque is created, which opens the NTS valve and dumps oil pressure. As such, the BETA light, being driven by a pressure sensor, will extinguish almost immediately after engine rotation begins. The pilot continues to hold down the unfeather button, which pumps oil into the governor, only to flow right back out via the open NTS valve while torque is still negative. As the engine spins faster, somewhere around 30% RPM, the torque reverses, the NTS valve closes, and the unfeather pump, still running while the pilot holds the button, again pressurizes the system and the BETA light comes back on, indicating a successful NTS test. This ON-OFF-ON cycle of the BETA annunciator indicates a properly operating NTS system.

BUT WAIT THERES MORE: NTS LOCKOUT:

Imagine you just came in for a landing and right after touchdown, you bring your power levers rapidly aft into the BETA region towards reverse. Now also imagine those big propellers out there with all their momentum. For a brief period, your small diameter turbine is producing little power/torque. Those big spinning props however, have lots of stored energy like a big flywheel and this manifests itself briefly as negative torque with the props driving the turbine again...and at a time where you want some negative thrust perhaps and a prop pitching towards feather is the last thing you want. Well for this reason, the NTS system is locked out when the Power Levers are in the BETA region. You can read about the BETA region below. This means that when you do your Negative Torque Sensor ground test, procedures call for you to move the Power Levers to just above flight idle, because you can't test the NTS system if its "locked out" with the power levers in the BETA region.

Engine and Prop Operation

A single TPE-331 engine have 2 levers to control it. They are the power lever and the condition levers. The power levers are on the left side of the MU2 quadrant and the condition levers, with blue handles are on the right side. The condition levers control both propeller RPMs and manual feathering and the power levers command thrust. I specifically say "command" vs "control" for reasons explained below. We will discuss each lever functionality separately. Also, I will frequently refer to lever position as a percentage or ratio where 0% lever travel means a lever pulled all the way aft (0) and 100% lever travel means a lever pushed all the way forward (1.0) and 50% (0.5) means halfway between the two extreme positions of lever travel.

Power Lever Operation

The range of power lever travel are divided into two distinct regions, with the crossover between the regions at about 50% of lever travel. There is a physical lift-gate detent between the two regions where you must pull up on the power levers to move them into the beta region. Refer to the image below as we continue.

The beta range of lever travel is that range where moving the lever will directly control the propeller pitch. Moving the lever aft will move the propeller pitch towards reverse thrust. This is called the BETA region because in the world of aeronautical engineering, the greek letter, beta is used to denote propeller pitch angle, and since the power lever directly controls the propeller pitch angle (beta) in this region, it gets that name.

You will note in the illustration that lever movement within the beta range yields both forward and reverse thrust. This is simply because just below the 50% point of lever travel, the prop blade exhibits a few degrees of positive pitch that yields forward thrust. As you pull the levers aft, the prop pitch angle moves towards a negative pitch angle, thereby producing negative thrust. There is no detent or gate between forward and reverse thrust.

The BETA region is used for ground operations only. The levers are not permitted to be in the BETA range while airborne. You CAN physically move the levers into the BETA region while airborne by pulling the levers over the ALPHA region lift-gate detent, but doing so would result in significant drag and rapid deceleration and could dangerously upset the aircraft stability, particularly at low speeds common during approach.

It is common for pilots to move the power levers into the ALPHA region when taxiing to get more thrust and increase taxi speed, and then pull the levers back into the BETA region in order to keep from moving too fast. Towards the forward area of the BETA range, a small amount of positive thrust is produced for low-speed taxiing, but for some pilots in a bit more of a hurry, it may not be enough.

The ALPHA region is used for flight operations. When the power levers are in this region, they control the amount of fuel sent to the engine and the propeller governor mechanism controls the prop pitch in order to hold the engine at a constant RPM that is set by the condition lever position (which we'll cover shortly).

The alpha region is quite simple, more forward lever travel = more thrust. The 50% position is labled "flight idle" and is the farthest aft you may pull the power levers while in the air without pulling the levers over the lift-gate detent.

Condition Levers Operation

The term condition levers are sometimes used to describe levers that simultaneously control multiple functions using a single lever, for example: engine speed / fuel-cutoff / feathering etc. You may note that PT-6 powered aircraft have 3 levers per engine while the TPE-331 only have the two. Refer to the following illustration as we continue.

If you recall earlier, we said that if a fixed-turbine turns, then so must the propeller because they are permanantly connected. In general, turbine engines idle at at or above 60% of their peak RPM at minimum. They are not designed to run at lower RPMs. So if you need to feather the propeller for some reason, which effectively stops it, then you will have to shut down the turbine engine as well or risk damage to the turbine. This is why the condition lever has a single feather + shutdown position all the way aft at 0% of lever travel. This feather + shutdown action would be done only in an emergency and indeed this area of condition lever travel on the quadrant is labeled EMER STOP.

Before we move on to describing the nominal range of condition lever operation, I bring attention to that vague area of lever travel between the FEATHER and IDLE lever positions. Moving the lever to feather from the taxi position manually actuates a valve called the feather valve and also manually actuates a fuel cutoff valve. This lever travel region simply actuates those physical mechanisms, which requires a bit of lever travel to move all the linkages inside the engine cowling. There is a lift-gate detent at the TAXI position of lever travel, such that one does not accidently feather/shutdown the engine when pulling the levers aft.

Nominal operation of the condition levers involve 3 distinct lever positions.

  1. The IDLE position, labled TAXI on the throttle quadrant
  2. The MIN CRUISE position
  3. The TAKEOFF / LAND position

It is not terribly common to place the condition lever at intermediary points between TAXI and MIN CRUISE positions for typical taxiing, but may be required, for example when heavily loaded or taxiing uphill and a bit more engine RPM (but not max) is easier on the engine. The condition lever is generally moved from TAXI to TAKEOFF/LAND right before the takeoff roll and after the landing rollout, moved from TAKEOFF/LAND back to the TAXI position . While in flight, the lever can be modulated between the MIN CRUISE and TAKEOFF/LAND positions as desired for power/economy considerations.

For the most part, the IDLE lever position results in around 70-72% engine RPM on the "dash 10" variant of the TPE-331, the MIN CRUISE lever position results in 95-96% RPM and the TAKEOFF/LAND position, 100% RPM. I say 'for the most part' because there is a bit of a caveat to the condition lever setting. The condition lever does not really control engine speed directly, but rather sets operating points/limits of various governors. If the power lever is advanced into the ALPHA region while the condition lever is still in the taxi-position, then the operating RPM range in Alpha is 96% to 100% and the condition lever/underspeed governor is ignored below 96% RPM. In short, the TAXI condition lever position is not designed to be used in conjunction with the ALPHA range of the power lever, though it is possible. It will simply result in reduced power/RPM, certainly not desirable for takeoff. The REAL DEAL: Prop Speeds section below discusses speed governing in a bit more detail

To recap, the BETA range of power lever travel is used in conjunction with the TAXI condition lever position (or condition lever position anywhere below the MIN CRUISE lever position as required) and the ALPHA range of power lever travel is used in conjunction with the MIN CRUISE - TAKEOFF/LAND condition lever range. We'll now discuss flight sim hardware setup to best simulate these lever movements and functions.

THE REAL DEAL: Prop Speeds

This topic can be quite involved, but we'll try our best and keep it brief. Turbine engines have an optimal range of speeds it operates within. When the engine speeds up, then the prop governor increases prop pitch and drag to slow it back down to some point, but what keeps the engine from going too slow and stalling, usully less than 60% of max RPM? The answer is the "underspeed governor". This is a fuel governor that adds fuel to the turbine to keep it spinning at some minimum speed where it was otherwise slow down. The condition lever actually sets an underspeed governor lower operating limit. At the TAXI speed setting, this is approx 70% RPM at idle. When the condition lever is moved to MIN CRUISE, say while on the ground, then the fuel govenor low-limit is recalibrated to 96%. The underspeed governor must then add fuel to maintain 96%. Now imagine we move the power levers forward into the ALPHA region, which adds more fuel to the engine. In this case, the engine starts to speed up, but as we noted a few moments ago, the prop governor then kicks in to increase prop pitch and slow the engine back down...but to what speed? As it turns out, the condition lever sets two operating points at the same time. it sets the underspeed governor limit, and also the prop-governining speed limit. The underspeed governor limits speed on the low-end and the prop governor limits speed on the high side. This is why you can adjust the engine speed between 96% and 100% when in cruise.

So that begs an interesting question. What happens if you pull the condition lever back to taxi while the power levers are pushed forward in the ALPHA region? In this case of ALPHA lever travel, the Power lever, via cams and linkages, essentially sets the underspeed governor to 96% and that will be the low-limit whenver the Power Levers are in the ALPHA range.

A final interesting question then. What happens when the props are on the locks and cannot rotate pitch to increase drag, yet the power levers are moved into the ALPHA range, adding fuel. In this case, there also exists an overspeed fuel governor, specifically designed to keep the engine RPM from running away and destroying itself in a case like this. TPE-331 procedures call for testing the overspeed governor for the first flight of the day. It involves moving the condition levers full forward and then the power levers forward and observing the engine stablize around 105-106%. Any higher and problems will be quick to follow.