The US Navy has linked the deaths of four F/A-18 Hornet pilots to “physiological episodes” that occurred while the aircraft was airborne, according to an in-depth review of oxygen system failures that are plaguing the service’s primary fighter jet and T-45 training aircraft. Released on Thursday, the Navy report details findings of an investigation that was launched in March after more than 100 T-45 instructional pilots refused to fly in protest at continuing issues with the aircraft’s oxygen system. That protest occurred just days after Navy officials described a rising rate of “physiological episodes” (PEs) affecting pilots who fly all models of F-18 aircraft, especially the Boeing-built Super Hornet, during testimony on Capitol Hill. Pilots reported that physiological episodes resulting in dizzyness or even blackouts have been caused by oxygen contamination, human factors – including air sickness and vertigo, failure of the on-board oxygen generation system and the failure of other key systems – according to March testimony from Rear Adm. Michael Moran. According to the Navy, the four F-18 pilot deaths, which occurred over a span of 10 years, are not all the direct result an oxygen system failure but are linked by the fact that pilots experienced various symptoms that fall within the scope of what is described as a physiological episode. Although FA-18 aircrews have experienced PEs attributed to breathing air problems, the majority of recent serious incidents have been attributed to issues related to the aircraft’s environmental control system, which supports air quality in the cockpit and cabin pressurization malfunctions, according to the report. Physiological episodes in T-45 pilots have not led to a fatality, according to the report but a recent spike in symptoms reported by training pilots mirrors the troubling trend observed among F/A-18 Hornet pilots. Investigators have documented a steady rise reports of physiological episodes among T-45 and F/A-18 pilots since 2012. T-45 incidents rose from 13 in 2012 to 38 in 2016 while reports among F-18 pilots increased from 57 in 2012 to 114 in 2016. F-18 pilots have already reported 52 incidents to date in 2017. While investigators were able to offer several steps to mitigate the risk of physiological episodes, they were not able to identify the root cause of a problem that has also been observed in Navy pilots who fly EA-18G Growlers and at least five members of an Air Force F-35 fighter airwing. “To date, finding a solution … has proved elusive,” the report said. “The complexity of aircraft human-machine interfaces and the unforgiving environment in which aircrew operate will continue to generate PEs whenever systems do not operate as intended or human physiology is a factor.” According to Moran’s March testimony, determining the cause of these incidents has been complicated by the fact that “symptoms related to depressurization, tissue hypoxia and contaminant intoxication overlap.” Hypoxia results from deficiency in the amount of oxygen delivered to cells and can induce potentially fatal complications. The exact causes of the four F/A-18 deaths are redacted in the Navy’s review, but it does note that the service has now implemented mitigation procedures that could prevent similar incidents from resulting in fatalities going forward. “Subsequent to these mishaps, training to recognize the symptoms increased and procedures now stress the importance of selecting emergency oxygen as a first step. Correct application of emergency oxygen would have likely prevented these mishaps,” according to the investigation report. The Navy has also implemented several training and procedural actions for the T-45. These risk mitigation procedures are intended to ensure aircrew safety while the Navy continues to investigate the root cause of the problem within both aircraft, the report said. Often described as the backbone of naval aviation, the various F-18 aircraft make up most of the service’s strike fighter fleet. The F/A-18 Super Hornet is the newest model and was designed to have a lifespan of roughly 6,000 flight hours. But today, jets are being stretched to fly between 8,000 and 9,000 hours to fulfill mission expectations as a result of fewer operational aircraft, budget restrictions and delays to the fifth-generation F-35 Joint Strike Fighter. Boeing, which has continuously updated the Super Hornet with new electronics, bigger fuel tanks and new stealth features, has pitched the Navy jet as a more affordable alternative to the F-35. The Navy included funding for 14 Super Hornets in its initial 2018 budget and has asked for an additional $739 million as part of its “unfunded requirements” list to add another 10 aircraft. The Navy estimates a portion of their current F/A-18 fleet will remain in use through 2030.