Even as the US Air Force’s Lockheed Martin F-35A soared above Le Bourget in July, oxygen issues grounded the same aircraft back home at Luke AFB, Arizona.
Just weeks before the conventional take-off and landing Lightning II was set to make its Paris air show premiere in June, the USAF announced it would cancel F-35A flying operations at Luke AFB following five separate oxygen deprivation incidents over the course of a month.
Pilots had reported experiencing “hypoxia-like symptoms”, although the air force would later characterise the incidents as physiological events that could include hypoxia, hypocapnia or hyperventilation. The Arizona base houses 55 F-35As, but the issue applied to only 48 of those aircraft.
The crews experienced a range of symptoms, from slight dizziness and disorientation to tingling and coldness in their extremities, but were trained to recognise the problems and landed safely using the aircraft’s back-up oxygen system.
When the USAF announced the grounding action on 9 June, it set an optimistic 12 June return-to-flight date. But Luke AFB’s commander did not return the jets to their normal flying operations, at altitudes above 25,000ft, until 30 August.
At the Paris air show, USAF officials insisted the oxygen issue was isolated to Luke's jets. A pilot survey that began at the base in May later expanded across all F-35 pilots. The survey also examined the aircraft’s low-rate initial production (LRIP) numbers as a possible common thread, as well as software variants. The service found that Hill AFB in Utah, Nellis AFB in Nevada and Luke had aircraft from LRIP lots 6, 7 and 8.
When Luke’s F-35As returned to the skies last month, their pilots did so without the root cause of the physiological events (PE) identified. However, officials at the base have been able to rule out some contamination risks, such as suspected higher levels of carbon monoxide on the hot and congested Arizona flightline.
Meanwhile, the US Navy is also attempting to grasp at the elusive cause of PE plaguing its pilots. The issue could threaten generations of pilots, as the service continues to struggle with oxygen issues on its Boeing T-45 Goshawk trainers. Although the navy allowed T-45 instructors to return to flying in April under a restricted envelope that would not require using its on-board oxygen generation system (OBOGS), Naval Air System Command (NAVAIR) officials later said that students would not resume training until an air supply fix was implemented in July.
In June, the USN rolled out a comprehensive review of PE on its T-45 trainers and Boeing F/A-18 fighters, which concluded that the OBOGS on both aircraft are not able to provide clean, dry air to pilots, and can even allow contaminants to escape into their breathing air that can cause hypoxia. The service found that pressurisation issues caused most of the oxygen problems for F/A-18 pilots, while the OBOGS emerged as the culprit on T-45s.
Those findings may have narrowed the scope of the navy’s hypoxia problem, but its report did not single out a specific system or environmental condition that would cause a hypoxia event or pressurisation malfunction.
On 3 August, the USAF began OBOGS testing that will continue until the end of this year. Working with the F-35 Joint Programme Office (JPO) and the Naval Medical Research Unit (NAMRU) in Dayton, Ohio, a team of engineers and statistical analysis experts at the USAF’s 711th Human Performance Wing at Wright-Patterson AFB, Ohio, are currently testing OBOGS that have never been flown before, in order to compare their baseline performance with systems which have experienced incidents at various simulated cabin altitudes, aircraft altitudes, and breathing demand flows. The team is evaluating laboratory results as they are collected and is co-ordinating with the JPO, NAVAIR, the Air Force Research Laboratory (AFRL) and Lockheed.
The two services have long battled, and to some extent accepted, oxygen issues as part of the risk their pilots must take. Between 2003 and 2008, the USAF's F-22 fleet experienced six incidents, followed by another dozen by 2011. After the uptick in reported PE, the service stood down the Raptor fleet and directed its Scientific Advisory Board to study the F-22’s life-support system. Much like the air force and navy’s recent studies, the board could not determine a root cause, but found that the supply or quality of oxygen may contribute to pilots’ symptoms. The USAF returned its Raptors to normal operations in 2013, after modifying the type’s life-support systems.
Historically, the air force has taught pilots to recognise the symptoms of hypoxia, loss of consciousness induced by g forces, or fumes in the cockpit, says Ryan Mayes, a biomedical engineer at the AFRL. Yet the service and the USN keep experiencing unexplained PE that are not necessarily caused by those factors, he says.
“Every flight is a physiological event – we’re putting people in an extreme environment,” Mayes says. “The way to eliminate PE is to better characterise what might be driving PE. We can form a mitigation strategy or inform decision making; part of our function is informing and defining the risk.”
Following a request from the USAF School of Aerospace Medicine, the service awarded Cobham, which manufactures the OBOGS, a contract in 2016 to develop a new method for monitoring a pilot’s breathing and physiology. In addition to the AFRL, the company has briefed NAMRU and NAVAIR on its aircrew-mounted physiologic sensing (AMPS) technology, which connects to the CRU-94 integrated terminal block that sits about 18in from the pilot’s mouth.
AMPS examines the oxygen concentration, gas flow, temperature and pressure in the breathing gas being delivered to the pilot, whether through OBOGS or a liquid oxygen system. In early June, Cobham delivered the first sets to Wright-Patterson AFB, where validation and verification testing began. Preliminary tests show AMPS is still capable of sensing in a high-g environment, which can often stress electronics, says David Burch, a biomedical engineer at the AFRL.
“The oxygen sensor is an optical sensor, so any type of fluctuation in the boards actually changes the optical system because it changes the distance that the light has to go,” he says. “We’re dealing with very sensitive components. So we need to look at this thoroughly, and right now everything looks to be very good with the system.”
Cobham first developed the inhalation sensor block portion of AMPS, and then created the exhalation block by re-packaging the system and adding a carbon dioxide sensor. The inhalation block measures oxygen pressure, gas flow to the pilot and aircraft acceleration to determine whether the life-support system is performing within specifications.
Rob Schaeffer, product director for environmental systems at Cobham, told FlightGlobal at the company’s Orchard Park, New York facility that before AMPS, no sensor looked at the pilot’s breathing or overall health.
“So the pilot – for all intents and purposes – has been the sensor," he says.
On AMPS, a memory card with sensors collects data on the oxygen concentration. The system does not measure contaminants in the OBOGS, Schaeffer notes. Along with the sensor blocks, Cobham created a digital upgrade to the CRU-99 oxygen monitor, called the solid-state oxygen monitor (CRU-123), which delivers information on temperature and oxygen pressure to T-45 pilots.
While the USAF has been able to measure oxygen levels, the service has never been able to quickly record the data for speeds that are relevant to physiology, Burch says. However, even though technology is not yet mature, the service has also faced challenges getting monitoring culturally accepted, according to Mayes.
“We have not really grown up in a flying world where it’s a part of the weapon system,” he says. “So part of us reaching out to the operational community and working with groups like the test pilot school is to start to think about how pilots can use this.”
The USAF put pulse oximeters, which measure the amount of oxygen in the blood, on F-22 pilots, but Cobham wants to go further, by taking a three-phase approach to the PE issue: monitor, predict and protect, Schaeffer says. The monitoring phase starts with the AMPS’ inhalation and exhalation block, which collect data on a large cross-section of pilots that should help the services draw conclusions on oxygen issues. While measuring gas going into the pilot’s mask is critical, the exhalation side measures the amount of carbon dioxide, which helps Cobham understand incidents such as hyperventilation and hypocapnia.
Schaeffer says that based on preliminary data, the PE could be linked to a lack of oxygen at altitude or even cockpit pressure.
“What we’re finding is that if you understand what you’ve read in the recent past about what’s been going on in physiologic events, a lot of fingers are being pointed at the oxygen system,” he says. “I don’t disagree that the oxygen system could be part of it. However, there’s a lot more going on in the cockpit that the oxygen system may not be the root cause of.”
Once the USAF and Cobham amass a library of data, the company can develop a predictive algorithm that could help the pilot make decisions, Schaeffer says. The company plans to integrate the sensing technology into its next-generation emergency oxygen system so that potentially, the systems could communicate with each other, he adds.
Cobham plans to propose that technology for the USAF’s smart aircraft digital breathing regulator (SDBR). In July, the service released a request for information seeking a technology that would enable autonomous control of the regulator using aircraft state, in-flight environment, and pilot physiological state information. The system would notify pilots and allow the breathing regulator to take corrective action before the pilot or jet is compromised.
“The [SDBR] monitoring systems will access, predict, and initiate the flow of oxygen to the pilot using external input parameters from the aircraft cabin pressure and altitude, arterial blood or brain tissue oxygen levels,” the request says. “The tactical version shall account for both altitude and acceleration-induced hypoxia and head-level blood flow and blood oxygen changes.”
Despite decades of testing on oxygen systems, pilots are likely to always experience PE, Mayes says.
“It’s hard for us to characterise and mitigate an unknown something with an unknown cause,” he says. “So the first step in that is trying to define the cause. Once we have done that I’m optimistic AMPS will be an important part of that and we can move on to what’s next, whether that’s mitigation or training.”
Source: FlightGlobal.com
