This article, originally published at noon ET Oct. 14, has been updated to include comment from the Joint Program Office, and an Oct. 16 Air Force statement.

WASHINGTON — Weeks after Defense News revealed that the military services had restricted lightweight pilots from flying the F-35 joint strike fighter, the US Air Force officially acknowledged an increased risk of neck damage during ejection to middleweight pilots as well.

In a news release issued Oct. 16, the Air Force confirmed a Defense News report that pilots under 136 pounds are currently barred from flying the fifth-generation aircraft, expected to be the backbone of American airpower for decades to come. It also acknowledged an "elevated level of risk" for pilots between 136 and 165 pounds.

"We expect the manufacturer to find and implement a solution," said Air Force Secretary Deborah Lee James in the statement.

The Air Force, Navy and Marine Corps instated the ban after testers this summer discovered an increased risk of neck damage when pilots are ejecting from the plane. The Joint Program Office blamed the phenomenon on the jet's ejection seat, Martin-Baker's US16E.

But interviews conducted by Defense News in recent weeks indicate the added weight and bulk of the new F-35 helmet complicates the problem. It is still unclear whether the blame rests squarely with the helmet, or the seat, or somewhere in between.

The Helmet Problem

The JPO is trying to improve safety for lightweight pilots during an ejection by reducing the weight of the new helmet, built by Rockwell Collins and Elbit Systems of America, which is on its third iteration due to repeated technical problems. Rockwell Collins is now on contract to build a Generation III "Light" helmet, David Nieuwsma, company vice president of strategy and business development for government systems, told Defense News.

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"All ejections from any fighter aircraft are risky and place extreme amounts of stress upon the body," JPO spokesman Joe DellaVedova told Defense News in a Wednesday email. "The safety of our pilots is paramount and the F-35 Joint Program Office, Lockheed Martin, and Martin-Baker continue to work this issue with the US Services and International partners to reach a solution as quickly as possible."

The tests this summer that revealed the problem used mannequins equipped with the new Gen III helmet, a spokesman for the Pentagon's director of operational test and evaluation confirmed to Defense News in a recent email. Testers found the ejection snapped the necks of lighter-weight test mannequins, according to a source with knowledge of the program.

The potentially fatal problem did not occur during previous tests with the slightly lighter Gen II helmets, according to the source.

Until a permanent fix is found, the US military services have grounded pilots weighing less than 136 pounds, Defense News first reported Oct. 1. The restrictions only impact pilots under this threshold because lightweight individuals generally have lower less neck strength to absorb force, DellaVedova said. The services are not placing any flying restrictions on heavier pilots, he noted.

But the risk does not disappear above 136 pounds, experts stressed. The low-speed ejection problem is worst with the lightest pilots, from 103 to 135 pounds, and gradually lessens as aircrew weight increases. F-35 pilots above that weight could still experience serious and potentially fatal injury during a low-speed ejection.

One solution is designing a lighter helmet, which will weigh about 4.67 pounds, DellaVedova said. The JPO is looking at reducing internal strapping material and removing an external visor to reduce weight and bulk. A preliminary design review on the improved helmet is scheduled for December, with full implementation planned by summer 2017.

However, DellaVedova stressed that helmet weight was not a factor in the Aug. 27 decision to ground lightweight pilots.

"That was an ejection seat issue discovered during the parachute opening phase and was not related to the differences between the Gen II and Gen III helmets," DellaVedova said.

But the JPO may not be able to find an easy solution, one expert warned.

A Martin-Baker F-35 ejection seat test.

Photo Credit: Martin-Baker

The Physics of Ejection

The sequence of an ejection is basically the same across different seat designs, according to a subject matter expert in crew escape systems. After the windscreen canopy is breached, the seat and pilot are launched upward via a rail system at a jarring rate of at least 12-14Gs. The acceleration force is greater for a lightweight person, and can be as much as 18Gs. Back and neck injuries can occur at this point if the pilot is not in the correct position, with his or her head directly centered on the spine.

Once the pilot and seat reach the top of the rails, a rocket under the seat is ignited to lift the pilot-and-seat package free of the plane. At this point, the seat can begin pitching back and forth, a motion much like that of a rocking chair, due to an offset of the rocket thrust with the pilot's center of gravity. The pilot's physical build determines the direction and degree of the pitching motion, according to the expert: a tall, heavy person with a high, forward center of gravity tends to pitch forward; while a short, light person with a low, aft center of gravity tends to pitch backwards.

Wind speed and air drag also aggravate the pitching problem, the expert said. Some seats, like the F-35's, utilize an airbag system stowed in the headrest that deploys on either side of the pilot's head. This is meant to stabilize the pilot's head and neck during ejections. However, the inflated air bags add wind resistance near the top of the seat, causing a lightweight pilot and his or her seat to pitch backwards even further.

These two factors combined cause a potentially dangerous position of the pilot when the main recovery parachute deploys at speeds under 250 mph, the expert said. This can cause a "snapping" of the head and neck backwards, leading to serious and potentially fatal neck injuries.

Ejection seat manufacturers have figured out how to correct the pitching phenomenon at high speeds, before the main parachute deploys. In these conditions, most systems deploy a small drogue chute that slows down the seat, the expert said. But during low-speed ejections, 250 mph or lower, the main parachute must deploy immediately to prevent the pilot crashing into the ground. Drogue chute deployment at that speed would slow deployment of the main chute at a critical moment. Even worse, the two chutes could get entangled.

Some companies have developed pitch-control technologies to solve this problem at low speeds. For instance, UTC Aerospace Systems' ACES II ejection seat — featured in the Air Force A-10s, F-15s, F-16s, F-22s, B-1s and B-2s — and UTC's new ACES 5 seat use what is called a stability package, or "STAPAC" to control pitch motion, according to Jim Patch, senior program manager for ACES 5. STAPAC is a small vernier rocket motor mounted under the seat, which is mechanically redirected by a spinning gyroscope to correct the pitch. In addition, the catapult rocket used on all ACES seats can sense the weight of the pilot and change the ejection thrust to push softer for smaller aircrew and harder for legacy aircrew, Patch said.

Martin-Baker does not appear to have any pitch control at low speeds. The UK-based company has not responded publicly since Defense News broke the story about the dangers to lightweight pilots earlier this month.

A bulkier helmet, like the 5.1-pound Generation III, increases the risk of neck injury during both phases of an ejection, because the forward center of gravity brings the pilot's head down and strains the neck. However, removing helmet weight will not solve the basic problem of misalignment of the pilot when the main parachute deploys, the expert said.

The Rise of Helmet-Mounted Displays

The advent of modern "helmet-mounted displays," which are rapidly displacing traditional helmets, contribute to an increased risk of neck injuries during ejections. HMDs project information similar to that of head-up displays (HUDs) on an aircrew's visor, and are now commonly used in combat aircraft. These devices often incorporate night-vision systems, cuing systems and more. While HMDs greatly enhance pilots' situational awareness, a March DOD Inspector General report found these systems add weight and bulk compared to a traditional helmet, increasing the risk of neck injury — particularly for lightweight pilots.

The F-35 helmet is large even compared to today's average HMD. All the information pilots need to complete their missions is projected on the helmet's visor, rather than on a traditional HUD. The latest iteration of the helmet, Gen III, is wide from side-to-side and front-to-back, meaning that if a pilot's head is even slightly off center, a significant amount of weight is displaced.

If a pilot's head is off-center during the first phase of ejection, acceleration of the pilot and seat up the rail, serious neck and spine injuries can occur.

The F-35's Helmet Mounted Display Systems provide pilots with unprecedented situational awareness.

Photo Credit: Lockheed Martin

Martin-Baker's F-35 seat, it seems, attempts to fix this problem with airbags on either side of the pilot's head — the same airbags that aggravate the pitching phenomenon that occurs in the second phase of the ejection.

Congress recently tried to crack down on ejection seat safety issues due to HMDs. In a report accompanying the f iscal 2015 National Defense Authorization Act, lawmakers wrote that a secretary of the Air Force report on various aspects of the health and safety risks associated with ejection seats "confirmed that, with increased use of helmet-mounted devices, the risks of death or serious injury increases, and increases even more for lighter aircrew." Lawmakers called on the Air Force to review and update a 2010 analysis of alternatives exploring options for a safer ejection system.

In light of the recent revelations about the danger to lightweight pilots, the chairman of the House Armed Services subcommittee on tactical air and land forces, Rep. Mike Turner, R-Ohio, has pledged to hold an oversight hearing on the issue, scheduled for Oct. 21.

The Fix

In addition to designing a lighter helmet, the JPO is looking into two other fixes to the ejection problem, DellaVedova said.

First, the team is working on installing a switch on the seat for lightweight pilots that will delay deployment of the main parachute. The proposed weight switch set will keep the drogue chute attached longer to further reduce the speed of the seat before the main parachute deploys.

In addition, the program will mount a "head support panel," a fabric panel sewn between the parachute risers that will protect the pilot’s head from moving backwards during the parachute opening. This will prevent the potential hyperextension of the neck and protect the head. Martin-Baker is financing an upcoming ejection seat sled test that will test the head support panel, DellaVedova said.

These two fixes will be introduced when the next upgrade of the ejection seat comes online near the end of 2016.

All three fixes will be fully implemented by summer 2017, DellaVedova noted.

If further testing reveals these proposals are not sufficient to fix the problem, the program may look at replacing the Martin Baker seat altogether, a senior Air Force official said Oct. 15. One alternative option is the ACES 5 seat, the official said.

For now, the Air Force is working through the problem to ensure it does not impact initial operational capability, which is planned for late 2016, according to the official. One male pilot at Eglin Air Force Base was grounded by the restriction, but Air Force leadership has decided to move him to a new career path, the official said.