In the search for the root cause of a recent spike in hypoxialike cockpit incidents that leave student pilots disoriented and short of breath, the U.S. Navy is beginning to look more closely at the quality of the air that comes off the McDonnel Douglas T-45 Goshawk’s engine and feeds into the oxygen generator system.
Fleets across the Navy and U.S. Air Force have been plagued by a sharp increase in these so-called physiological episodes in recent months, where pilots experience symptoms that could be related to lack of oxygen during flight. The issue is a complex one that the Pentagon has so far been unable to resolve. It is not even clear that what the pilots are experiencing is actually hypoxia—defined as an insufficient supply of oxygen—or something else that causes similar symptoms, such as a change in cabin pressure, contaminated air, or even too much oxygen.
For T-45 pilots, who were grounded in April, the Navy believes the problem lies with the airflow through the Cobham-built Onboard Oxygen Generation System (OBOGS), a complex process that begins in the Goshawk’s
F405 engine. The aircraft “is not equipped to continuously provide clean, dry air” to OBOGS, according to the Navy’s comprehensive review of the recent incidents; the result is contaminants can enter the pilot’s breathing air, potentially causing hypoxia.
The Navy’s Hypoxia Mystery
Navy still struggling to find root cause of hypoxialike cockpit incidents in T-45
One possible cause is a toxin entering the airflow from the T-45 engine
Navy to begin testing sample of uninstalled engines using comprehensive contaminant detection methods
Commander, Naval Air Forces hopes to lift T-45 restrictions in July
The Navy says it is now looking “very closely” at the quality of the bleed air coming off the T-45’s engine as a possible source of the problem, and plans to begin testing a sample of uninstalled engines using “comprehensive” contaminant detection methods this month, the service confirmed to Aviation Week.
Rolls-Royce is cooperating closely with the Navy on the investigation, according to company spokesman George McLaren. He stressed that the service is examining “many potential root causes.”
Although there are many possible reasons for the incidents, aircrew reports seem to support the theory that the symptoms seen in the T-45 are caused by a toxin in the airflow. In incidents of true hypoxia—known as hypoxic hypoxia, or a lack of oxygen flow to the lungs—symptoms abate once pilots use the emergency oxygen. But many T-45 pilots had persistent symptoms that sometimes lasted for hours after landing—in one instance, the pilot could not even remember landing, said one congressional staffer familiar with the issue.
These symptoms fall in line with what is called histotoxic hypoxia, which occurs when the body’s tissues are not able to use the oxygen that has been delivered to them, and is often caused by a contaminant in the airflow.
The rapid onset or the delayed recognition of the symptoms in some cases led many T-45 pilots to speculate that what they experienced was histotoxic hypoxia due to a contaminant, according to the Navy’s report.
“Histotoxic hypoxia symptoms are highly variable across individuals, may not be immediately recognized, and 100% emergency oxygen may not quickly alleviate the symptoms,” the report states.
The problem is that there are countless opportunities during the OBOGS cycle for contaminants to enter the airflow. In the T-45, the bleed air from the compressor section of the engine flows through a cooling heat exchanger and enters the OBOGS, where it first passes through a heater, particulate filter and pressure reducer. The air is then directed to a sieve bed material loaded into two identical canisters. Each canister’s sieve bed material absorbs nitrogen, passing the concentrated oxygen to a mixing plenum—or box—and then to the pilot’s regulator and mask. The nitrogen absorbed in the sieve bed is purged from the system.
The ability of the OBOGS to produce usable oxygen depends on clean, dry air, delivered to the air crew in the right pressure and volume, flowing into the system. Moisture is a particular problem; due to the sieve bed’s high affinity for water, any contaminants trapped there could be exchanged for moisture and then released from the OBOGS into aircrew breathing air.
The Navy has identified several modifications it hopes will fix the problem, according to the report, including installing a water separator into the OBOGS bleed air line to filter out any moisture from the airflow.
Vice Adm. Mike Shoemaker, Naval Air Forces commander, recently indicated instructor pilots will resume flying the T-45 with OBOGS in July, with students resuming flight training later in the month. Currently, students are grounded; instructor pilots are flying with a modified mask configuration that doesn’t pipe in air from the OBOGS. Instead, the pilot breathes the ambient air in the cockpit, restricting altitude.
“The safety of our aircrew remains my number one priority,” says Shoemaker. “After months of using a modified mask and configuration that circumvented the OBOGS, new mitigation measures have been put in place that give us the confidence to safely resume flight training using the system. These mitigations monitor the breathing gas and alert and protect our aircrew, as well as incorporate new maintenance procedures to ensure the systems are clean and working properly prior to flight.”
But despite extensive testing, so far the Navy has been unsuccessful in finding a contaminant at any point in the airflow. The service has already brought several aircraft from the training squadrons to NAS Patuxent River, and torn them apart in search of a potential source of the problem, Vice Adm. Paul Grosklags, commander of Naval Air Systems Command, told Congress in June.
“We took every component, every single component in that [breathing gas pack] out of the aircraft, starting with the engine and going through the entire system, inspecting all the piping in between, all the way up to the mask and the vests that the air crew wear,” said Grosklags. “We’ve subjected each one of those individual components to extremes of testing, extremes of environmental conditions, in excess of what we would ever expect to see in the aircraft. And we still have not been able to find what we would consider a proximate cause of contamination or something being released into that gas pack.”
The Navy’s
Hornet and
Growler fleets have also been plagued by incidents of hypoxia, as well as the Air Force’s
at Luke AFB, Arizona.
