Aircraft Accidents and Lessons Unlearned XLI: Atlantic Southeast Flight 529

Atlantic Southeast Airlines Embraer EMB-120RT

As per National Transportation Safety Board (NTSB) accident report AAR-96/06, on August 21, 1995, Atlantic Southeast Airlines flight 529 (ASA529), an Embraer EMB-120RT, registration number N256AS, crashed during an emergency landing attempt near Carrolton, Georgia, 31 minutes after departing Atlanta Hartsfield International Airport. The flight, operating as a scheduled flight to Gulfport, Mississippi, had experienced a separation of five feet of a single left (#1) engine propeller blade during climb through 18,000 feet of altitude. The blade, one of four, departed the engine propeller, immediately introduced an out-of-balance condition in the still turning engine, which contributed to existing damage on the engine, cowling, remaining propeller assembly and wing until the propeller came to a stop.

The NTSB determined, “… that the probable cause of this accident was the in-flight fatigue 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. The fracture was caused by a fatigue crack from multiple corrosion pits that were not discovered by Hamilton Standard because of inadequate and ineffective corporate inspection and repair techniques, training, documentation, and communications.” It further stated, “Contributing to the accident was Hamilton Standard’s and FAA’s failure to require recurrent on-wing ultrasonic inspections for the affected propellers.”

The NTSB’s decision to blame the Federal Aviation Administration (FAA) for reasons not-accident-related, diverted necessary attention away from the accident’s root causes. The NTSB’s probable causes demonstrated their fundamental misunderstanding of the FAA’s responsibilities of active oversight of the manufacturer/repair station, as well as Hamilton Standard’s own obligations. The NTSB’s repeated allegations that the FAA was directly to blame in many other accidents remains a problem and demands note. The theory that ‘casting a wide net’ generates more causes only confuses the Findings and does nothing for safety. This NTSB practice inhibits actions that can lead to improvement by placing blame where it serves no benefit, in other words, a counterproductive ‘cry of “Wolf”’. It is not because the FAA had no culpability for its certificate holder’s actions – the FAA did. But to categorize all safety issues as “FAA failures” is irresponsible and passive. It would be just as foolish to blame the NTSB for getting past probable causes wrong, which would be a more accurate Finding. Aviation safety would be better served with useful recommendations. Instead, an important issue was missed.

Recommendation A-96-143 stated, “… the need to require inspection (“buy back”) after the completion of work that is performed by uncertificated mechanics at Part 145 repair stations …” Federal Aviation Regulation (FAR) Title 14 Code of Federal Regulations (CFR), Part 145, Subpart E, Section 145.211 (c): Quality Control System addressed this topic for decades, yet the NTSB failed to research that fact. Since AAR-96/06’s publication, this Part had been revised twice as shown in Federal Register (FR) 41117 (8/6/2001) Volume 66 and FR 9176 (3/5/2018) Volume 79. Section 145.211 was not revised; the NTSB recommendation A-96-143 served no purpose.

This is the problem: NTSB engineers that investigate aircraft maintenance issues do not work with the FARs that deal with aircraft maintenance, in this case Title 14 CFR Parts 65 or 145. Engineers would not know how a repair station run by a manufacturer is divided into two separate entities: the manufacturer and the repair station. NTSB engineers do not comprehend the terms uncertificated mechanics, repairmen, inspectors, the roles each plays in a repair station or their limitations.

A second recommendation, A-96-149, stated, “Evaluate the necessary functions of the aircraft crash ax, and provide a technical standard order or other specification for a device that serves the functional requirements of such tools carried aboard aircraft.” This baffling recommendation referred to the first officer’s futile attempts to break through the cockpit window with the aircraft’s crash ax when the traversing window jammed from structural damage. The ax’s handle broke. The cockpit’s available space was not designed for swinging an ax with a longer handle. Furthermore, local first responders also failed to break through the window with larger axes and more swinging room. A-96-149 was irrelevant.

Was the unfortunate propeller blade’s departure the main contributor to this accident? It did contribute directly to the tragedy. Was the propeller’s overhaul to blame? Again, yes, this led to the tragedy. However, the NTSB should have studied whether the plane could have successfully landed with the damage it incurred, particularly how to survive a propeller blade separation at climb. The NTSB’s probable guesses did not address training and, worse, the NTSB never pointed to this training omission in the recommendations. The NTSB failed the industry.

Was this the first time that a propeller blade was thrown?  No, and probably would not be the last. The truth was confirmed nine pages into the cockpit transcript. The NTSB knew when it happened, why and how. What the NTSB did not do was to make this type of event survivable in the future.

The focus of an accident investigation is to prevent, not only the circumstances that led to an accident, but the repetition of history; that is the ‘product’ of an accident report. A key NTSB investigatory team on a major accident is Survival Factors, a group of professional investigators who determine what should change to guarantee the survival rate goes up in the next event, e.g. seat design, fire resistant materials. The purpose of the Operations investigators and the aircraft-specific investigators, e.g. Powerplants, Aircraft Maintenance, is to work towards surviving an accident when the unexpected happens. The ASA529 accident was similar to other accidents, like United flight 232, in that they were the victim of catastrophic failures, next to impossible for the pilots to anticipate.

Before United 232 crashed in Sioux City, Iowa in July 1989, the #2 engine failure had jettisoned engine metal, which cut into all three of the plane’s hydraulic systems, making the DC-10 uncontrollable. The solution was to assure that another #2 engine failure would not simultaneously damage the three hydraulic systems in the future; a hydraulic fuse was placed in the #3 hydraulic system. Simple, yet effective.

After the ASA529’s #1 engine propeller came apart, the pilots flew ASA592 for nine minutes before impact; the pilots made heroic efforts to maintain control and save the plane and all aboard. To protect future flights, the NTSB had a duty to lead the industry in analyzing the final minutes of ASA529 and generate solutions for how to survive a propeller failure in the future.

To do this, the NTSB should have taken the facts of the aircraft’s condition during the last nine minutes. What other damage was incurred when the propeller came apart? Were the flight control systems victims to the propeller blade’s damage? Why would that be important? With Southwest flight 1380, when the #1 engine threw a fan blade; the blade could have exited at any point within 180 degrees of travel, yet the blade was launched at the exact degree that resulted in tragedy; the plane remained manageable, but a passenger died.

AAR-96/06 did not report fuselage damage, e.g. flight control cables, pushrods, etc. affected by the propeller blade’s trajectory; the aircraft did not depressurize; damage was limited to the #1 engine, nacelle, propeller and surrounding wing structure, which was harmed extensively. Despite this, the pilots managed to continue on to Carrolton regional airport for nine minutes. The circumstances of the propeller coming apart in flight would be impossible for pilots to anticipate. Although aviation safety dictates that all is done to prevent a repeat of this event, it could be duplicated; it would be just as unpreventable.

The pilots successfully flew the wounded aircraft under extreme conditions; they did their best. But even with all they did, there were lessons to learn that could have been incorporated into pilot training. A good recommendation would have been for manufacturing, industry and the FAA to build a training program that, after studying the accident, would have figured out proactive measures to survive a similar event using knowledge taken from ASA529. It was encouraging for the industry to know that, in the aftermath of ASA529, the FAA, industry and aircraft manufacturers of both propeller and jet aircraft have categorized the circumstances, analyzed the contributing events of their actions and improved pilot training to learn from ASA529 and survive.

It would be hoped that the NTSB would focus on improving aviation safety by concentrating on solutions while avoiding casting subjective aspersions. The cost in people and machines is too great to waste time impugning the reputations of valuable organizations; it only serves to divert attention away from facts. ASA529 never should have happened. But will it happen again?

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