When concerns emerged in late 2015 that a seat designed to save pilots’ lives might suddenly endanger them, it created a crisis for British family-owned ejection-seat manufacturer Martin-Baker.
Now, more than a year later, the company has concluded tests for a modification to the seat that will accommodate and keep safe all the pilots who hopefully will never have to use it.
The F-35’s US16E seat is a very different one from its predecessors, says Steve Richards, head of the US16E program at Martin-Baker. Because of the large number of countries expected to adopt the aircraft, the seat had to be designed for a variety of physiological needs.
Seat Stresses
US16E seat designed for operation from -60-600 kt.
First Western ejection seat capable of being ejected automatically
More than 20 ejection tests needed to requalify
It must accommodate males and females of varying height and weight and must be able to jettison them, fully kitted, at speeds of -60-600 kt. and altitudes of 0-50,000 ft., as well as receive electronic signals from the aircraft to eject the pilot automatically should the F-35B model’s vertical-landing lift-fan system fail. The company has had to develop the US16 with the F-35B’s powered lift capabilities in mind. As the F-35 can fly backward, Martin-Baker had to account for that by designing the seat to operate in non-forward flight, accounting for the negative speed value.
More critically, it also had to factor in the new helmets, which are much larger and heavier now because of the new helmet-mounted display model that has replaced the cockpit-mounted head-up display.
“Although we bought affordability, the US16E had to be better than legacy escape systems, have better head-neck loads, more protection and bring lower risk,” says Richards.“It has been like designing a sports car to deal with lots of extremes.”
The company had already completed qualification of the US16E in 2010 with the initial so-called Gen 2 helmet produced by Vision Systems International, but when issues arose with that helmet, the F-35 Joint Program Office (JPO) decided to introduce the Rockwell Collins and Elbit Systems Gen 3 Lite system, which brought with it a number of changes related to weight and the center of gravity.
Because of this, the replacement helmet had to undergo its own set of ejection-seat tests, which began in October 2013.
“We started getting failures, . . . and it got dramatic in July 2015 when we did a low-speed test at 160 kt.,” Richards tells Aviation Week.
Data showed that as the seat pitched back in the moments after ejection, the mannequin experienced what Martin-Baker calls a neck injury criteria exceedance, where loads on the mannequin’s neck posed a potential or real risk of injury. Exceedances were also recorded in higher-speed tests.
“The last time we did a 160-kt. test, years before, we had passed it . . . . The program didn’t expect us to fail it that much,” Richards says.
When news of the failures became public, the U.S. Air Force banned pilots who weighed less than 136 lb. from flying the aircraft. One Air Force pilot was reassigned to a different aircraft type as a result.
“The failure rate at 160 kt. was one that could not be tolerated, . . . and we had to go back at the design of the helmet and the seat,” explains Richards “We could have put in an interim measure to keep that person flying, but we were told to focus on the solution.”
Lockheed Martin and Martin-Baker began working on a fix in the summer of 2015 and began testing the fixes last October. What was expected to be a delta test program turned into a full-blown requalification of the seat, with 22 ejection tests from rocket sleds in Northern Ireland and New Mexico as well as from Martin-Baker’s test aircraft in France.
The lighter helmet led to a center-of-gravity shift. As for the seat itself, the company has adjusted the software in the seat sequencer, which, as its name suggests, sequences the timings of the various ejection processes, including opening the drogue, releasing the seat and deploying the main parachute.
The timing of these processes is based on velocity, air pressure and altitude information taken by sensors on the seat at the time of ejection; look-up tables (a data process technique that replaces computing with indexed information) provide the correct timings for the conditions.
For pilots in the lower weight range, the company has introduced a graduated delay—around 0.45 sec. for the smallest pilot—into parachute deployment. This change is activated via a switch on the seat.
According to Richards, 99% of pilots will have this switch in the standard position, while the low-weight mode will be activated if the pilot weighs less than 150 lb., although the final decision on the threshold weight will be made by the Air Force and Navy.The other modification is the introduction of the head support panel (HSP), a fabric panel at the base of the harness risers that stops the head from moving backward as the seat pitches back.
This works in conjunction with an inflatable head-and-neck device, which works rather like an airbag on a car, with three fingers inflating around the top and either side of the head, holding it in position.
The HSP is most useful in low-speed ejections after the seat’s rocket motor has burned out and the drogue does not have much control authority. The seat pitches back, and the HSP stops the pilot’s head from doing the same.
Richards says the introduction of both the HSP and new sequencer timings has reduced head-neck loads by a factor of six. “We went from a massive exceedance to a magnitude under the requirement,” says Richards. “The wider [JPO] program liked it . . . . It was a cost-effective, low-mass and quick-change fix and didn’t require software or much complex testing.
“But there were a lot of ‘doubting Thomases,’” Richards adds. “They couldn’t believe it was possible through a simple change.”
The new tests have focused on three key anthropometric groups, Case 1, Case 6 and a hybrid Case 2ST, which covers short torsos. The “cases” are based on a U.S. and European database of consumer body measurements, the Civilian American and European Surface Anthropometry Resource Project, or Caesar. Cases 1 and 6 were tested because they encompass the extremes of the seat. Case 1 represents the smallest adult while Case 6 covers the largest-limbed and tallest pilots.. Case 2ST was tested to determine the mass weight at which the seat switch should be engaged.
Manufacturing of the retrofit for the seats began in January 2016; shipping began last November and modifications are getting underway this month.
The retrofit program is expected to take nearly a year. However, one of the biggest challenges, Richards says, will be making the aircraft available to the Martin-Baker teams. When the retrofit work is completed, the Air Force plans to reopen F-35 training to pilots under 136 lb. by December, the Office of the Director of Operational Test and Evaluation noted in its fiscal 2016 report on the F-35’s progress.
The F-35 JPO tells Aviation Week that it has received the escape system data and documentation from industry and will soon release this to the Air Force and Navy for review, as the services own the airworthiness process. Each will provide their assessment of the escape-system upgrades.
“The JPO has been working hand in hand with the airworthiness officials in the Air Force and Navy to develop the escape fixes and to develop the test plan for the fixes,” the JPO said in a statement. “The services have seen draft reports and are encouraged by the results but are holding their final airworthiness and risk assessment until the final reports are provided. The final reports are scheduled to be delivered soon to facilitate the services’ airworthiness and safety analysis processes.”
However, the tests beg the question of whether, if there are further changes to the helmet or if a new HMD is introduced, a new round of costly ejection-seat testing could be required.
“It all depends on what mass and center-of-gravity changes there were,” Richards says. “If it’s a major change, then it is quite a big program, and if there is a Gen 4 or 5, we will need to do some testing if that came up.”
http://aviationweek.com/combat-aircraft ... new-helmet
11 Jul 2016, 16:40
