Medical Analysis in NTSB Accident Investigation
Insights from Dr. Nicholas Webster, NTSB Medical Officer
Recently, NTSB Member Weener initiated a series of articles about the Board’s GA investigative techniques. The second Compass posting is by Dr. Nicholas Webster.
Nicholas Webster, MD, MPH
Dr. Nicholas Webster is an aerospace medicine physician who has served the NTSB for the past year as one of two medical officers. He currently works with investigators from all modes of transportation to identify problems and develop mitigation strategies for transportation medical issues. Before joining the NTSB, he was a Federal Aviation Administration research medical officer who helped develop and manage the FAA fatal accident medical case review program. In that role, he worked to identify medical hazards in fatal accidents and supported FAA and NTSB aviation accident investigators. He is a retired naval flight surgeon who earned a bachelor’s degree from the University of Tennessee at Knoxville, a Master of Public Health degree from the Johns Hopkins University, and an MD from the University of Tennessee at Memphis.
Here are his insights into this aspect of GA accident investigation:
By Dr. Nicholas Webster, NTSB Medical Officer
The NTSB investigates every aviation accident in the United States. In each investigation, we look at the roles of the human, the machine, and the environment. By learning about the factors that cause an accident, we can make recommendations to prevent similar accidents in the future.
I am one of two medical officers (physicians) at the NTSB who work closely with investigators in all modal offices. When an investigator-in-charge (IIC) is concerned that operator medical issues, drugs, or toxins may have affected performance, he or she coordinates with us to study the medical aspects of the event. The medical officers review medical documents, toxicological testing results, and sometimes autopsy reports of those involved in accidents. In conjunction with the investigative team, we help determine if operator impairment or incapacitation contributed to the cause of the accident, then we help craft language to explain to the public the nature and significance of the medical issues and how they affected the operator and contributed to the accident’s cause.
We also work closely with the Board’s biodynamics and survival factors experts to help evaluate accident-caused injuries and determine what changes could be made to prevent future injuries.
The resulting information is presented in a medical factual report, which documents all pertinent medical issues and any potential hazards that the medical issues posed. To ensure accuracy, these fact-based scientific reports are peer reviewed by the investigative staff before they are published as part of the public docket. The medical, factual, and operational details of each event are then analyzed by the investigatory team, which determines probable cause by consensus, peer review concurrence, and Board authority. The probable cause represents the most likely explanation for the event given all available evidence.
Two recent cases have garnered some attention in the general aviation (GA) community, both involving fully functional airplanes operating in manageable weather. In these cases, both pilot action (error or impairment) and pilot inaction (incapacitation) can lead to an accident. In these cases, we found that the pilots were operating in a relatively low-workload environment and had the skill and experience necessary to safely complete the flights. On the other hand, medical data showed that both pilots had severe heart issues that could cause sudden incapacitation without leaving a trace.
The first accident occurred on April 11, 2015, when an experimental Quad City Challenger II airplane crashed into terrain near Chippewa Falls, Wisconsin. The 77-year-old pilot died and the airplane was substantially damaged. The pilot had the skill and experience to operate the airplane in visual conditions. According to witnesses, while the airplane was on the downwind leg of the traffic pattern at the pilot’s home airport, it entered a steep dive that continued until it struck the ground in an open field. Investigators found no evidence of preexisting mechanical concerns and, based on the propeller damage, determined that the engine was producing power at impact. Operational evidence also strongly supported pilot incapacitation.
The pilot had a history of coronary artery disease, which was treated by multivessel bypass surgery. He also had high blood pressure, elevated cholesterol, and hypothyroidism, which were controlled with medications. The autopsy showed that the pilot had an enlarged heart; severe multivessel coronary artery disease (greater than 80-percent occlusion of all vessels), with coronary artery bypass grafts and complete occlusion of two bypass vessels; scarring of the ventricular septum, indicating he had had a previous heart attack; and active inflammation of the anterolateral wall of the left ventricle of his heart. These findings, particularly the large scar and active inflammation of the heart muscle, placed the pilot at high risk for an irregular heart rhythm, which can easily cause decreased blood to the brain and result in fainting without leaving further evidence at autopsy.
Additionally, according to the Chippewa County Coroner Death Report, the cause of death was blunt force trauma. However, the examining pathologist further stated, “the most likely scenario to explain [the pilot’s] death is that he suffered an arrhythmia secondary to myocarditis.” These findings are discussed in detail in the medical factual report. Based on the available evidence, we determined the probable cause of the accident to be the pilot’s incapacitation due to a cardiovascular event, which resulted in a loss of control and subsequent impact with terrain.
The second accident of note was the crash into terrain of a homebuilt Europa XL airplane on June 26, 2015. As in the previous case, the pilot died and the airplane was substantially damaged. In this accident, the 72-year-old pilot also had the skill and experience needed to successfully complete the flight, especially given that it was a clear day and he was operating under visual flight rules.
The airplane crashed under power in a steep, nose-down, slightly inverted attitude in an open field about a half mile from the end of the runway, slightly to the right of an extended centerline. According to the IIC, there was no evidence of preexisting mechanical concerns, the engine was operating at impact, and the operational evidence suggested pilot incapacitation.
The pilot had a history of severe coronary artery disease, which was treated with multivessel bypass surgery, stents, and medication. Additionally, he had elevated cholesterol and high blood pressure, which were treated with medications. Since his last medical certification examination, an exercise stress test showed no significant changes, but a cardiac catheterization report documented that his coronary artery disease had progressed, resulting in 90‑percent occlusion of the left anterior descending coronary artery and impaired blood flow to a part of the heart muscle. Additionally, the autopsy identified multivessel coronary artery disease treated with patent coronary artery bypass grafts, and documented up to 70-percent occlusion of the left anterior descending coronary artery.
These findings are discussed in detail in the medical factual report. The pilot’s severe progressive coronary artery disease and the impaired blood flow to an area of his heart muscle placed the pilot at high risk for an acute cardiovascular event such as a heart attack, anginal attack, or acute arrhythmia. Any such event would likely cause a sudden onset of symptoms such as chest pain, severe shortness of breath, palpitations, or fainting, and would leave no evidence visible on autopsy if death occurred in the first few minutes.
The Mahoning County Coroner Autopsy Report cited multiple blunt force injuries as the cause of death, with coronary artery disease and chronic hypertension contributing to the cause of death. Again, although the pilot died of blunt force injuries, the evidence supports our finding that the accident sequence was likely initiated by his incapacitation due to a cardiovascular event.
These cases illustrate how we integrate medical findings into our investigations. We also provide interested parties with links to publicly available, detailed information that supports our findings.
In both of the cases described here, the medical factual reports document significant medical issues in pilots who were operating under sport pilot rules; however, we only determined the medically related probable causes after thorough, scientific, peer-reviewed analysis of all the available facts concerning the human, the machine, and the environment.
Our goal is to identify medically related hazards that may be causal to or resultant from the accidents we investigate, and then work with the experts on the investigative team to develop mitigation strategies, which take the form of safety recommendations, that target and eliminate these hazards and improve transportation safety.
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