M5.3 Turbines and Props

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Embry-Riddle Aeronautical University *

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Mechanical Engineering

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Jan 9, 2024

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Answer the following questions. 1. Describe the term “feathering” as it pertains to propeller function and why it’s essential to be aware of this dynamic during an accident investigation such as the Zonk Air accident. 2. What is meant by the general investigative expression, "Turbine parts may have a memory?" ( Note: the word “memory” does not refer to a human factors issue.) 3. What is meant by "critical flows" needed by a turbine engine to operate properly, and is there a cause-and-effect relationship in regards to these flows and an accident involving turbine power plants? Hi all, Describe the term “feathering” as it pertains to propeller function and why it’s essential to be aware of this dynamic during an accident investigation such as the Zonk Air accident “Feathering a prop” refers to the action of twisting the propeller blades so that the chord line, or “narrow side,” is facing into the wind. This reduces the drag created by the propellers. When propellers are working correctly, they create lift horizontally and towards the rear of the aircraft, moving the aircraft forward. To be more effective, the pitch of the propeller can be adjusted so that a larger cross-section of the blade moves through the air. If the engine is not working properly, the blade pitch angle is increased to the point where the propellers are parallel with the wind and create the least amount of drag possible. In any accident investigation, it would seem a feathered prop would indicate that the engine failed before the accident and the pilot feathered the prop to reduce drag, whereas a prop with reduced pitch angle and not feathers would indicate the engine may have been operating during the accident. However, because blades are free to move upon impact investigators must use a variety of additional indicators to determine their exact position during the accident. But overall, once the prop pitch angle is determined, they can indicate whether the engine failed in flight or was alive and well during the accident. What is meant by the general investigative expression, "Turbine parts may have a memory?" The memory referred to in terms of turbines is the ability of turbine engines to retain their condition in the face of adverse conditions. For example, metal turbine components that have been elongated or stretched due to extreme temperatures can show evidence of that effect even after cooling and returning to their original shape or size. These components may continue to function currently, but may eventually fail. A good example of this is the Southwest 1380 number 1 engine failure due to a slowly progressing cracked fan blade dovetail resulting from stresses that were higher than expected at regular operating loads (NTSB, 2018).

What is meant by "critical flows" needed by a turbine engine to operate properly, and is there a cause-and-effect relationship in regard to these flows and an accident involving turbine power plants? Critical flows refer to the controlled movement of air, and later air and fuel, through the engine. This includes the direction for air to enter the series of sections within the engines and properly progress through the compression stages before getting mixed with fuel and heading for combustion. The different sections of the engine, especially in many of today’s advanced fanjets (high-bypass engines), require different pressures, temperatures, and/or correct air distribution. There is a very real cause-and-effect relationship between these stages and accidents involving powerplants that utilize these sections and require critical flows. For example, at the very basic level, a blocked inlet restricts air from entering the engine and its compressor stage, possibly leading to a compressor stall. Admittedly, this can be a difficult cause to detect because ice can melt or get broken off on impact. Another important flow is the perfectly measured atomized fuel spray in mixing fuel with the compressed air before combustion. A reduction or cutoff to the fuel nozzles within the engine can lead to an engine flameout. This incident may be easier on investigators because the cause for the fuel restriction will likely still be present after the accident since the fuel used for turbine aircraft usually contains an icing inhibitor additive. References National Transportation Safety Board. (2018, April 17). Left engine failure and subsequent depressurization: Accident report (Report No. PB2019-101439). https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR1903.pdf

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