Chapter 10: ASA Flight 529
Most aircraft have several backup systems for when mechanical failure happens in flight in order to keep the failure from proving fatal. Sometimes despite these backup systems, an aircraft can have a mechanical failure that is so awful the mishap still proves fatal. This is what happened on American Southeast Airlines flight 529.
American Southeast Airlines specialized in being a regional airline that flew short routes from large airports to smaller ones (Walters, J., & Sumwalt, R., 2000, page 215). Flight 529 was an EMB-120 type aircraft. This plane is a small 29 passenger turbo-propeller aircraft that has an aircrew that consists of a captain, a first officer, and a flight attendant.
This flight happened on August 21, 1995. That day there was rain and fog that limited visibility. Immediately after takeoff the aircraft experienced a fair amount of turbulence due to weather, but later it stabilized out. About twenty minutes after takeoff the crew heard several loud thuds. The pilots had autopilot disconnect and got warnings for engine control and engine oil. The captain then feathered the left propeller and cut off fuel to the left engine.
The EMB-120 became uncontrollable, but the pilots couldn't figure out why. The propellers are designed to do what is called feathering. This process moves the propeller blades flat so they are inline with the air stream to help prevent excess drag from the engine. Normally if you feather the propeller and yaw trimmed the aircraft can be managed. Unfortunately this engine failed so badly that it got knocked askew in its mount and three of the blades were wedged into the wing causing large amounts of drag. The pilots had to throttle down the right engine a considerable amount to be able to maintain some sort of control of the aircraft. Because of this they were descending rapidly.
The crew tried to coordinate with ATC to get the nearest runway to land on. Unfortunately the ATC agent wasn't able to give them clearance to the nearest airport due to lack of information from that airport. Their aircraft was four miles out from the runway that they were coordinated from when they started receiving altitude warnings from the Ground Proximity Warning System (Walters, J., & Sumwalt, R., 2000, page 214). The pilots requested that ATC have the fire department waiting, but they were not called in time.
At this point the flight attendant was preparing the cabin for the crash and instructing the passengers on how to brace for the impact. While demonstrating what to do she looked out the window and saw the tops of trees passing by so she quickly strapped in. (Walters, J., & Sumwalt, R., 2000, page 215) The pilots somehow managed to keep the aircraft from striking a bunch of trees and crash landed the plane into a field. The plane slid five hundred feet and broke in half. It had only nine-and-a-half minutes from when the crew first started hearing thuds in the cockpit at 18,000 feet to when the aircraft struck the ground (Walters, J., & Sumwalt, R., 2000, page 215).
Most of the passengers were able to escape immediately through the large opening in the fuselage. Others were not so lucky. In less than a minute some cut electrical lines started a fire and smoke was engulfing the area. Passengers were catching fire as they were leaving the scene. Though seriously injured the flight attendant was doing her best to put out the fires. All together there were nine fatalities including the captain.
The NTSB determined the cause of the accident was "the in-flight fracture and separation of a propeller blade resulting in distortion of the left engine nacelle, causing excessive drag, loss of wing lift and reduced directional control of the airplane" (Walters, J., & Sumwalt, R., 2000, page 223). The propeler blade that broke off was a Hamilton Standard 14RF-9 design. These blades are made of aluminum with a conical shaped hole called a taper bore drilled into them that is used for weight reduction and balancing. The mechanics place different amounts of lead wool inside to help balance the blade.
The NTSB found that the blade failed because of fatigue cracking. This was likely caused by corrosion and mechanical damage from tools that were used to insert the lead wool. Though mechanics had noticed these flaws before, they never removed the blade from service. It was also determined that ultrasonic inspections should have been required that would have found the cracks in this blade and taken it out of service.
It's sad that nine people had to die to learn the lessons from this accident, but I think a lot was learned. Altogether the aviation industry is a little safer because of the findings from the NTSB, and I think that is the point of aircraft safety investigations.
References
Walters, J., & Sumwalt, R. (2000). Aircraft accident analysis final reports. New York: McGraw-Hill.
Most of the passengers were able to escape immediately through the large opening in the fuselage. Others were not so lucky. In less than a minute some cut electrical lines started a fire and smoke was engulfing the area. Passengers were catching fire as they were leaving the scene. Though seriously injured the flight attendant was doing her best to put out the fires. All together there were nine fatalities including the captain.
The NTSB determined the cause of the accident was "the in-flight fracture and separation of a propeller blade resulting in distortion of the left engine nacelle, causing excessive drag, loss of wing lift and reduced directional control of the airplane" (Walters, J., & Sumwalt, R., 2000, page 223). The propeler blade that broke off was a Hamilton Standard 14RF-9 design. These blades are made of aluminum with a conical shaped hole called a taper bore drilled into them that is used for weight reduction and balancing. The mechanics place different amounts of lead wool inside to help balance the blade.
The NTSB found that the blade failed because of fatigue cracking. This was likely caused by corrosion and mechanical damage from tools that were used to insert the lead wool. Though mechanics had noticed these flaws before, they never removed the blade from service. It was also determined that ultrasonic inspections should have been required that would have found the cracks in this blade and taken it out of service.
It's sad that nine people had to die to learn the lessons from this accident, but I think a lot was learned. Altogether the aviation industry is a little safer because of the findings from the NTSB, and I think that is the point of aircraft safety investigations.
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