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F-35 accident Fort Worth 15/12/22 - pilot ejected ok

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F-35 accident Fort Worth 15/12/22 - pilot ejected ok

Old 16th Dec 2022, 22:51
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Being reported - Lockheed Martin’s F-35 production line is located at Air Force Plant 4, an Air Force-owned facility adjacent to Naval Air Station Joint Reserve Base Fort Worth. The accident aircraft, which Lockheed Martin has not yet transferred to the U.S. government, was reportedly being flown by a government employee on a test flight.
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Old 16th Dec 2022, 23:03
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FkCO8elWIAAAjue.jpeg (1362×923) (usni.org) & F-35B Joint Strike Fighter Crashes in Texas, Pilot Safely Ejects - USNI News
"...The pilot ejected safely, according to the statement. The pilot was a U.S. government employee, Pentagon Press Secretary Brig. Gen. Patrick Ryder said during a Thursday press conference. The plane had not yet been transferred to the U.S. government, Ryder said...."

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Old 16th Dec 2022, 23:36
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Originally Posted by stilton
Looks like he waited until it was upright, I wouldn’t think ejecting sideways on the ground would end well ?
With you on that - wings level and pull. Not always that simple but worked well in this instance. Glad the seat did its job, incredible piece of engineering.
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Old 17th Dec 2022, 07:32
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Originally Posted by Thrust Augmentation
Is it not a bit early to be pointing the finger at the lift fan - for all we know this operated as expected, but the engine did not?
Simple observation. The aeroplane bounces and is in a stable hover when suddenly it pitches violently nose-down, rotating about the rear nozzle. The engine is clearly still producing thrust and the rear nozzle stays at the same height (not rising or descending) until other factors intervene. This is extremely unlikely to be a reaction controls issue. The sudden removal of support from the lift fan is the explanation which fits the observed dynamics. Whether this is due to a transmission failure (clutch, shaft, gearing etc) or some misadventure in the control shuttering is something the investigation will discover. There was no sign of mechanical distress from the fan prior to or during the pitch-down - no thrown debris etc, so it's unlikely to be mechanical break-up in the fan itself. The high pitch rate would (in my view) tend to point to the transmission failure because I don't think even full uncommanded shuttering could achieve that sort of response.

The inability to shut down the engine is possibly a separate, and more concerning, issue. AIUI in the hover the F35B only has digital throttle due to the need to maintain synchronisation between nozzle and fan thrust - there is no "manual fuel control" option. This would tend it suggest the software was still commanding thrust even after we assume the pilot had closed the throttle, right up until the ejection sequence cut the fuel. Why this happened needs to be understood as quickly as possible as it has other ramifications[IMHO].

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Old 17th Dec 2022, 13:06
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Originally Posted by Auxtank
It's one of the first things - if not THE first - to happen in the ejection sequence. The arms and legs are brought in tight to the chair by restraints.
It's been while since I've seen a modern ejection seat, so is it the case that the arms are now retrained upon ejection?
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Old 17th Dec 2022, 13:46
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Originally Posted by Saintsman
It's been while since I've seen a modern ejection seat, so is it the case that the arms are now retrained upon ejection?
Has been for quite a while depending on the type - faster types normally had them previously (Tornado being an example)
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Old 17th Dec 2022, 15:52
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Originally Posted by PDR1
The SHAR had an engine "dump valve" which diverted fuel flow to filling a sump as soon as it got weight on wheels* to ensure that when it touched the deck it stayed firmly there rather than being light or bouncing regardless of what the pilot did on the throttle. From the look of that video F35 doesn't have a similar system, which surprises me. Lots of things in that video look a bit odd. The initial hover is stable, and the reconfiguration for vertical descent seems to be to schedule (tailplane trim change) - thedescent rate is a bit higher than I'd expect, although the attitude is constant suggesting it was under control. The touchdown was definitely on the firm side, but seemingly not beyond the energy dissipation range of the undercarriage. But the thrust seems to remain at the descent setting (ie thrust = weight) allowing the aeroplane to bounce where I would have expected it to be closed to a safe value.

Shortly after the bounce there then appears to be a complete failure of the forward lift fan with ensuing pitch-down, wiping out the nosewheel etc. But the think that REALLY surprises me is that the engine seems to be still running at a high-ish thrust setting right up to the point where the pilot bangs out, suggesting that engine wasn't responding to the throttle commands until the ejection sequence cut the fuel flow.

The lift fan failure is obviously a concern and its cause will need to be established. But for me the bigger concern would be why the engine continued to deliver thrust for so long. I can't believe the pilot didn't try to shut it down, so that could imply uncommanded throttle operation. That's a bit scary.

PDR

* not sure it operated through the WoW switch - I think it was a completely separate system but I've forgotten the details as it's been 20 years since I last thought about it

Well, this is news to me! No one ever mentioned a “dump valve” in the Harrier fuel system in all the 2000+ hours I spent flying it. The only “dump valves” to my knowledge were those used in the fuel jettison system - and one pilot proved that those were not wired through the weight-on-wheels switches - oops!

Perhaps I didn’t pay enough attention in groundschool.

Mog
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Old 17th Dec 2022, 18:22
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Originally Posted by gums
Salute!

We have seen at least two nose gear collapses with the 35, and maybe there's a design flaw that can be fixed.

Gums sends...
Gums...

Great to see you here!

The nose gear sees a big load a a fairly odd angle... perhaps 10 degrees nose down. Could it be that the component of that load is not within the design limits of the gear?

DS
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Old 17th Dec 2022, 18:38
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Originally Posted by Mogwi
Well, this is news to me! No one ever mentioned a “dump valve” in the Harrier fuel system in all the 2000+ hours I spent flying it. The only “dump valves” to my knowledge were those used in the fuel jettison system - and one pilot proved that those were not wired through the weight-on-wheels switches - oops! Perhaps I didn’t pay enough attention in groundschool. Mog
As an ex Harrier engine guy, this also surprises me! I'd be interested to see a schematic if one exists - it doesn't appear here: https://www.dowtyheritage.org.uk/con...pegasus-engine
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Old 18th Dec 2022, 00:32
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Blue Sky OPS 26 Apr 2012 Mark Ayton "Mark Ayton spoke with Peter Wilson, a former Royal Navy Sea Harrier pilot and now STOVL lead test pilot at NAS Patuxent River... ...Some of the [test] vertical landings required extreme nose-down attitudes on the aircraft at various weights and phenomenal descent rates. Recounting the landings, Peter Wilson told AIR International: “I was trimming nose down to make the nose gear hit first rather than the main gear coming down as fast as I could, given the control law of the aeroplane. When the nose gear (underneath the pilot’s seat) hits first at that sort of descent rate it gets your attention because it’s a pretty heavy landing and a remarkable experience in the cockpit.”..." AIR International F-35 Lightning II http://militaryrussia.ru/forum/downl...e.php?id=28256
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Old 18th Dec 2022, 02:44
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Originally Posted by melmothtw
I think the point being made isn't that the nosewheel failures are linked, but that the nosewheel may have an inherent weakness that needs to be addressed.



Indeed, though I don't think the automatic ejection system is what sent the pilot skywards on this occasion, as whatever parameters would have caused the seat to fire automatically were present some seconds before it actually fired. Suspect the pilot made the choice and ejected himself.

One other thing that’s apparent is the aircraft does touch down with some forward speed, not sure if that was intended by the pilot or perhaps a by product of a systems failure
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Old 18th Dec 2022, 06:57
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My bit of pure speculation.

When the F-35B flight control system detects weight on wheels it immediately commands a series of vanes to close off the airflow to the lift fan thus cutting thrust immediately, this can be clearly seen in videos of the lift fan in operation. So there is no “throttling back”, the intake side of the fan (i.e. the top) simply cuts off the airflow.

That bit is established fact, now for the conjecture.

When the aircraft bounced 2 things would have happened, firstly the aforementioned vanes closed on contact, but when the flight control system almost immediately then detected weight being taken off the wheels (the bounce) it reverted to flight mode and throttled up the engine. The vanes on the lift fan would still be closing and couldn’t cycle that quickly. Thrust asymmetry, nose down pitch, wheel collapse, ejection.

That would be my guess, we could have a problem with the FCS being unable to adequately interpret a condition where the aircraft bounces on landing.

Last edited by Fonsini; 18th Dec 2022 at 07:33.
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Old 18th Dec 2022, 08:36
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Maybe PoW loosing a shaft was a blessing in disguise. Reading through this interesting thread, we might be seeing a situation emerge where a number of design issues need to be looked at in order to make the F35b sufficiently robust for CVRL. Perhaps now we will see an investigation that properly sorts issues and CVRL can proceed. Whereas if there had been a catastrophic incident this summer, CVRL might have lost backing.
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Old 18th Dec 2022, 10:02
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On the video the sequence is:
~ descent
~ touch
~ bounce
~ aircraft ascends to about 5'
~ aircraft still in controlled horizontal hover - power clearly feeding to both fans
~ aircraft suddenly pitches nose down
~ nose hits ground

The newscaster states "the nose gear appears to snap off, sending that jet spinning out of control", but it was nothing to do with the nose gear - it obviously broke, but that was well after thrust from the front fan had gone.

Or maybe, thrust from the rear nozzle suddenly increased to cause the pitch down. Could that happen? There are white vapour emissions from the rear nozzle area during the descent - are these normal or possibly a hydraulic leak from the nozzle actuators?

Does the F-35 have an afterburner?
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Old 18th Dec 2022, 10:11
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I’m tending towards a software issue where the bounce has confused the lift-fan shutter system! 😬
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Old 18th Dec 2022, 10:52
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The puff of smoke out of the back end at about 15 feet leads me to conject that it may have ingested (there is a lot of sucking going on) a bird which has cause a partial loss of lift fan thrust, but insufficient loss of thrust to trigger the auto eject which is triggered on a sensing of high rate negative pitch. The pitch rate we see is not as high as I have been led to believe we would see if for example the lift fan drive shaft disintegrated. Maybe a bird did some damage.
In regards to the arm restraint question earlier, the Mk16US seat has arm, leg and head restraint (to protect from neck injuries while wearing the HMD)
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Old 18th Dec 2022, 14:24
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Playing the video in slomo shows that vapour/smoke is emanating from around the exhaust nozzle durng the last several metres of descent and increases considerably during the bounce which initially appears to be fully controlled. But then as the nose drops savagely and the aircraft rolls and yaws the really big burst of vapour/smoke appears instantly from under the rear fuselage roughly in line with the stbd main gear. However it does this before (only just before) that tyre touches the ground so is not rubber smoke, and is surely far too immediate intense to be tyre smoke anyway?
Something was coming apart in a big way in the power train imho, and was doing so some time before the first touchdown.

It would not surprise me in the least if next week we don't see all F35s grounded indefinately pending further investigation of the front fan/gearbox/driveshaft systems.

Last edited by meleagertoo; 18th Dec 2022 at 18:37.
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Old 18th Dec 2022, 14:37
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Salute!

The F-16 net, sub section F-35, has excellent descriptions of the FLCS and the engine/fan logic depending upon 'mode" the pilot selects and then the "mode" that HAL selects, heh heh.

I highly recommend the King's subjects, that dominate posts here, dare to visit the F-16 net, f-35 forum. Some of the other forums are worth exploring, as well. Mog and other Harrier folks will enjoy the description of the vertical landing mode.

A few of us have posited that HAL got confused with the bounce, reverted to "flight" sub-modes, the fan had shut down and finally, the pilot initieated the ejection. A few of us have been looking at the WoW switch logic, but we all felt something did not work as expected.

Gums sends...
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Old 18th Dec 2022, 17:22
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Originally Posted by meleagertoo
......Something was coming apart in a big way in the power train...........
Brilliant phrase !

Could this, (abrupt and uncontrollable pitching down), happen to a Harrier, just out of interest?
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Old 18th Dec 2022, 17:41
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Originally Posted by Uplinker
Brilliant phrase !

Could this, (abrupt and uncontrollable pitching down), happen to a Harrier, just out of interest?
I stand to be corrected, but I doubt it. The Harrier forward nozzles use LP air whereas the F-35 uses a fan driven by a shaft. So anything affecting the thrust of the Harrier forward nozzles would probably be affecting the whole power unit. The F-35 could have front lift fan failure modes that still allow the deflected exhaust to produce significant thrust. My take on a simple understanding of the two power systems.
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