U.S. Navy Investigates 12-Fold Increase In F-18 Hypoxia Issues
Fixes are in the works for U.S. Navy F-18 cockpit oxygen problems
Mar 24, 2016 Michael Fabey | Aviation Week & Space Technology
As the U.S. Navy waits longer for its F-35C Joint Strike Fighter carrier versions, the service is being forced to increasingly rely on its F-18 family of aircraft at the very time pilots, air crews and engineers are trying to find and fix the causes of extremely worrisome cockpit breathing issues in the jets.
Continuation of the Boeing F-18 line is one of the critical elements of the Navy’s tactical aircraft inventory. Yet, Navy brass have been telling lawmakers that F-18 pilots are reporting more incidents of breathing problems in their jets, prompting a major combined effort to determine the causes.
“It’s a top priority for the Navy and the Marine Corps,” Rear Adm. Michael Moran, Navy program executive officer for tactical aircraft, testified Feb. 4 at a House Armed Services Committee hearing on naval air operations.
What is alarming both services’ aviation officials is the growing rate of hypoxia symptoms among pilots. Fliers reported just 5.84 hypoxia events per 100,000 flight hours in fiscal 2006 in the F-18 fleet. But from Nov. 1, 2013, through Oct. 31, 2014, F/A-18 and EA-18G fleets logged 70.98 incidents per 100,000 flight hours (see graph).
“Pilots feel dizzy,” says Rear Adm. Michael Manazir, director of Navy Air Warfare. “Confused. A little strange in the airplane.” Pilots have emergency air they can use, he notes.
A Navy fighter pilot since 1982, Manazir says he has personally not felt any hypoxia-related symptoms. “I’ve never experienced it outside of training,” he says.
Investigators from special teams found potential culprits in the cases they have studied: 93 have involved some form of contamination, 90 entailed environmental control system (ECS) component failures, 67 were caused by human factors, 41 were from onboard oxygen generation system component failures, 11 involved a breathing gas delivery component failure, and 45 were inconclusive or entailed another system failure. Some of the events fell under multiple categories.
The services have already made at least 18 changes to pilot breathing systems, Moran says, including tweaks to pressure and control valves as well as sensors. The process is broader than one the U.S. Air Force undertook for its F-22 Raptors after pilots reported increased hypoxia symptoms earlier this decade. But it would be difficult to pinpoint exact similarities until naval officials find the root causes.
Furthermore, there are some significant differences between the F-22 and F-18 families of aircraft as well as different incident-reporting procedures for the two services that make comparison even more difficult.
For example, Navy officials note, the service’s oxygen system concentrators use heater elements, which are not on F-22s. Moreover, Air Force officials had noted major concerns over oxygen concentration levels during the incidents involving their pilots.
“For a majority of our events, there’s plenty of oxygen,” says Dennis Gordge of the human systems division of the life support branch of the Naval Air Warfare Center (NAWC). “It’s not a low-oxygen issue.”
The type of incident often depends on the version of aircraft. “It looks like the legacy Hornet is seeing more [ECS] problems, such as pressurization,” says Capt. David Kindley, F/A-18 and EA-18G program manager, referring to F/18 A-D aircraft. “Would you expect that from an aging airframe? You might.”
With that in mind, he says, the Navy has decided to inspect and replace certain parts in those systems more frequently on a predetermined schedule instead of following the previous “fly to failure” policy.
“We’ve seen a marked decrease in ECS issues associated with legacy Hornets,” says Rear Adm. G. Dean Peters, commander of the NAWC Aircraft Division and assistant commander for research and engineering Naval Air Systems Command. “I believe this is due to the influence of improved pressurization tests. We’ve increased the testing cycle.”
The Navy has empirical data and anecdotal examples of the decrease, he says—especially at inspection sites.
“We seemed to hit the twilight a number of months ago, at the end of last year,” says John Krohn, the thermal systems branch head for NAWC and the Navy’s ECS expert. “We are making the jets healthier and the delivery systems healthier. We go through the checks—the diagnostics are more complete. We look at cabin pressurization integrity every 400 hr.”
For the F/A-18E/F Super Hornets, there seem to be more incidents related to the onboard oxygen generating system (Obogs). “I don’t know if I can point to a reason why,” he says.
The EA-18G Growler electronic warfare aircraft—essentially modified F-18s—are another story, Krohn says. “With a Growler, a couple of things are different. They are typically flown in a different profile than Super Hornets—a stable condition at high altitudes for long periods of time.”
Navy officials have noticed that a particular valve that is part of the cockpit’s breathing system, and also associated with cooling avionics, tends to ice up.
Naval Air Systems Command is running laboratory tests of new components for the environmental control system and onboard oxygen generating system to find and fix problems with cockpit breathing issues in the F-18 family of aircraft. Credit: Michael Fabey/AW&ST
“The demand on the cooling of avionics is greater in this environment—it is different from that of the Super Hornet,” Krohn says.
Navy officials say they have made major changes that are helping to cut down on hypoxia or other similar episodes. The service has developed and started to field a new molecular sieve and a catalyst scrubber to better filter carbon monoxide as well as other potential contaminants for the F-18.
The Navy is putting the new sieves and scrubbers into fleet squadron aircraft first, Kindley says.
Given how the Obogs works, updating the sieve and inserting a catalyst are important improvements. “There are two canisters of molecular sieve,” Gordge says. “It’s like really fine kitty litter or sand—the magic part that allows the nitrogen and oxygen to pass. You have the canister, you smash air into it, and the oxygen flows out. All kinds of coolants—anything that goes into the intake will wind up there.”
But the type of sieve used had not been changed in decades. “It was the finest molecular sieve you could get in 1982,” he says.
To make matters more challenging, the sieve used by F-18 aircraft became obsolete 6-7 years ago, Gordge says. “They started repacking them with reprocessed molecular sieves,” he says. “You can [only] do that a couple of times.”
At the time, he says, the choice was to either use reprocessed sieves or ground the aircraft. But a reprocessed sieve has a limited shelf life, especially as it starts to absorb more moisture and clump. Eventually, the Navy settled on a new hybrid sieve that could accommodate the necessary flight conditions. At the same time, the service started to add a carbon monoxide catalyst or scrubber.
Catalyst-maker TDA Research sends its material to contractor Cobham to be packed into its new sieve, Gordge says. The Navy has used the material in 300 aircraft since August 2015. Of those, 14 of the F-18 air crews experienced possible breathing-related events. Half of those appeared to be related to the ECS. Of the remaining seven, no sieve issues were found in four; the other three are still being investigated.
Carrier operations have placed extra demands on breathing systems in the cockpits of F-18 aircraft, leading to dizziness, confusion and other hypoxia-related symptoms in the U.S. Navy and U.S. Marine pilots and air crews. Credit: Michael Fabey/AW&ST
Routine Navy operations and pilots put the aircraft in all types of scenarios that could introduce contaminants to stress the Obogs, Gordge notes, such as the close-quarters jet exhaust of a carrier deck.
“When the plane does aerial refueling and it hits the basket or the basket leaks, it leaks fuel right down to the right intake and right into Obogs,” Gordge says. “Nowhere in Obogs design specification does it say: ‘Pour raw fuel into Obogs.’ But it happens.”
And pilots or air crewmembers may be exacerbating the effects of any contamination when they use their masks to fan their faces. “They pull the oxygen masks away from their faces to allow the oxygen to blow,” Gordge says. “While that’s good for the eyes and it has been a standard practice, the system was not designed to do this.”
When the oxygen is allowed to flow freely in that way, it automatically pushes out a much greater volume of oxygen per minute, pushing any contamination deeper into the system, Gordge notes.
While Navy engineers know of certain operations, procedures and equipment that exacerbate the Obogs and ECS problems, they are still looking for root causes and long-term fixes.
“We’re treating this issue as if someone died yesterday,” Peters says.
Kindley echoes that point. “We treat every single episode as if it were a mishap. There’s a great deal of rigor that goes into that reporting.”
The Navy has a hazard reporting system dedicated to the events and a protocol for investigating them. “We are swarming this from a resource standpoint, near-term perspective and investment standpoint,” Kindley says.
It is still difficult work, officials note. As Manazir puts it: “It’s like chasing a ghost.”
