Engine RPM bog down occurs as the engine is loaded beyond its torque-producing capabilities. This occurs when engine RPMs are allowed to decay below the design specification. Bog down can occur during ground start, taxi, and the landing roll. Except for grossly misrigged or maladjusted engines or other component failures, the occurrence of an RPM bog down can generally be categorized as an operationally induced event. The purpose of this article is to review the engine bog-down phenomenon. We will review the nature of bog down and its causes, as well as what we as pilots and technicians can do to reduce the chances of encountering a bog down and how to deal with it when it occurs.
The nature of a bog down
As a review, the TPE331 is a single-shaft, constant-speed-type turbine engine that derives its entire range of available power within a narrow range of engine speed. Depending upon the airframe application, this range typically lies between 65 to 78 percent (ground idle) and 100 percent (maximum) RPM. In other words, between 65 and 100 percent RPM, the gas generator produces enough power to be self-sustaining and it is capable of balancing the load generated by the gearbox and the propeller. About two-thirds of the power generated is needed to drive the compressor and one-third is for useful work. During ground operations in Beta Mode, an increase in propeller load (an increase in positive or negative blade angle) causes the engine speed to droop. Although imperceptible to the pilot, the slightest droop (or slow down) is detected by the Underspeed Fuel Governor (USFG), which immediately increases fuel flow to augment existing power. Thus, the USFG balances the increased load and restores the preselected engine speed before it can drop below the designed minimum RPM.
At this point, it is important to emphasize that any exceedance of the engine’s load bearing capability will lead to a sub-minimum engine speed. Regardless of whether the exceedance of the engine’s load bearing capability was caused by mechanical problems or operationally induced, a sub-minimum speed condition or an insufficient acceleration during starting is not a sustainable condition. This means the engine will suffer a bog down if exposed to a sub-minimum engine speed. What’s more, if allowed to go on, the bog down will lead to further RPM decay, result in localized high and excessive turbine temperature, and can cause internal engine damage.
What causes the excessive turbine temperature during a bog down can be explained as inadequate cooling air. For that reason, let us look at the consequences of insufficient internal (turbine) cooling during a bog down. Normally, only one-third of the intake air is being used for combustion while two-thirds of the remaining air is being used for cooling. Furthermore, within the normal RPM range, the intake air (mass airflow) is proportional to engine speed. Yet, an excessive propeller load that causes the engine speed to droop below minimum RPM results in a disproportionately larger decrease in airflow. Sensing only the RPM droop, and not being affected by the excessive decrease in airflow, the USFG continues to increase fuel flow in a futile attempt to balance power against the excessive propeller load. The Constant Speed concept states that in order to hold RPM constant, turbine power must equal propeller load. If prolonged, this sub-minimum RPM condition will cause too much fuel flow and too little air, air that is needed for combustion and for cooling. Of course, combustion prevails, taking as much air as required to feed the flame (physical law) thus shortchanging the cooling process. The result is a rapidly increasing turbine temperature with damaging effects.
What causes a bog down
Ignoring the signs of an incipient bog down can lead to an irreversible sub-minimum RPM condition. Once the bog down has reached that stage, the pilot or technician operating the affected engine has only one choice, and that is to immediately shut the engine down by closing the fuel valve in order to minimize turbine damage. Consult the applicable AFM or POH on bog down or hot-start procedures for your aircraft.
To be able to recognize the subtle signs of an engine bog down in the developing stage, we must first know what mechanical anomalies or inappropriate pilot actions can cause this event. We can’t cover all possible causes, but the following examples illustrate some of the most common circumstances during which a bog down can develop.
During an engine ground start: The electric starter by itself can accelerate the rotating group from 0 to about 18 percent RPM. Beyond that and up to about 50 to 55 percent RPM, both starter assist and combustion are required to carry the engine to a minimum and self-sustaining speed. In addition, it is important to realize that the start acceleration rate, another important factor, is dependent upon various internal and external factors. Internal factors that can impede engine start accelerations and thus have the potential for a bog down include:
- Premature starter cutout, starter failure, or inadequate starter performance such as worn or damaged starter brushes. Rotating the propeller by hand in the wrong direction can cause damage to the starter brushes.
- Rotational harmonic vibration. Harmonic vibration can occur between 18 and 28 percent RPM.
- Premature start (propeller) locks release.
External factors that can negatively affect engine acceleration during starting include:
- Electrical source failure (battery, cross-generator or generator assist, APU, or GPU failure).
- Strong tailwind, tending to load the propeller against normal direction of rotation.
During ground/taxiing operation: During a powered pushback while using reverse power in order to move or taxi backward while the speed levers are at low can cause an exceedance of normal propeller loads and may result in a bog down. This is especially true when reversing on soft ground, against an uphill slope, or against a strong tailwind.
During landing roll: Several situations and actions during a landing roll can lead to an otherwise preventable bog down. For example:
- Applying full reverse (PL in Full Reverse) at excessive landing-roll airspeed. Typically, airspeed must be below 90 knots calibrated before selecting Reverse (Consult Limitation Section in the applicable AFM/POH).
- Applying full reverse (PL in Full Reverse) during the landing roll with the SL set to Low.
In either situation, the too-high landing-roll speed or a too-low propeller RPM can lead to a very high propeller load as the prop blades move toward the full reverse stop angle. To understand the dynamics of why a higher airspeed or a slower propeller RPM result in higher propeller load during reversing it is necessary to review some basic aerodynamics concepts of lift and drag. The resultant vector (relative wind vector) and the angle of attack increase with an increase in airspeed. Consequently, this causes lift and thus drag to increase as well. In other words, at an excessive airspeed the prop load increases in excess of available power and a droop (bog down) in engine RPM will result.
How to prevent a bog down
Understanding the nature and causes is the first and most important step in avoiding an engine/propeller bog down. Operationally, we can avoid most bog down situations by observing the following guidelines:
- Verify adequate electrical power source and starter performance prior to starting.
- Be aware of any tailwind component during engine start. If in doubt, turn the aircraft into the wind before initiating an engine start.
- Preclude inadvertent propeller locks release by positioning the power lever somewhat forward of the Ground Idle position.
- Monitor each engine start for signs of inadequate acceleration. An acceptable rule of thumb is 1 percent per second. That means you want to see the engine RPM increase by at least 1 percent for every second during the start cycle. This is especially important between 18 and 28 percent engine RPM. Within that range a low-frequency vibration may develop and can bring acceleration to a halt. Consequently, damage to the internal engine components may result and a bog down may develop, resulting in further damage.
- During ground/taxi operations, set speed levers to High before selecting Full Reverse with the power levers.
- During the landing roll, keep the speed levers in High during any reverse application and until the aircraft has slowed to taxiing speed.
- Report insufficient engine acceleration or any RPM droop for corrective actions.
How to deal with a bog down
Typically, the onset of a bog down is indicated by a sub-minimum engine RPM and a rapidly rising turbine temperature. When faced with that situation, the pilot or technician who is operating the affected engine must take immediate steps to reverse the condition. If, for example, a bog down results from applying Full Reverse, the immediate reaction should be to undo the last action that caused the problem. For example, move the power levers out of Reverse to Ground Idle. This will unload the propeller and a recovery to normal ground idle engine speed may be possible. However, the operator should also be prepared to shut down the affected engine if the bog down persists.
Most bog down situations described above are rare occurrences. However, if ignored, a fully developed engine bog down can cause substantial internal engine damage. For that reason, the reader is reminded to familiarize him- or herself with the characteristic causes and indications of an engine bog down and be prepared to take the appropriate corrective actions.
For additional information concerning TPE331 design and operation, please contact Rob Erlick at 480-399- 4007, or send an e-mail to [email protected]. Honeywell’s TPE331 Pilot Tips booklet is available on the Honeywell App or online at https://pilots.honeywell.com; register with name and email, point to “Engines” then “TPE331” then click on “Pilot Tips”.
