FAA alert requires remedy to wiring/fuel risk

[Peristatos]

The aircraft manufacturer discovered that its 777 liners have poor electrical insulation near its fuel tank, according to a proposed rule the Federal Aviation Administration posted in March. “This condition, if not addressed, could result in an ignition source inside the fuel tank and subsequent fire or explosion,” the Airworthiness Directives note states.

https://nypost.com/2024/05/22/us-news/boeing-777-plane-fuel-tanks-could-explode-due-to-electrical-flaw/

[ATC Watcher]

NY post is always a bit sensational I believe . Because in reality :
The FAA reported the issue in March and ordered Boeing to respond by May 9. If adopted, Boeing would have as long as 60 months to make the repairs — a timeline that indicates the vulnerability is not a pressing concern.

[Expatrick]

Reviving memories of TWA 800 - unfortunate if exaggerated as suggested.

[Peter Fanelli]

So what is the relevance of the A320 electrical issue video in that article?

[tdracer]

No knowledge pf this particular issue outside of what I've seen in the media, however an educated guess at what's going on:
The HIRF/Lightning people do periodic inspections of random in-service aircraft to test how well the HIRF/lighting protections are holding up. Grounding is critical to the protection, so one of the checks is to measure the electrical resistance of the various grounding systems. The allowable resistances are quite low - as in a few milliohms - and it's pretty normal for the resistances to increase as an aircraft ages (corrosion and such on the electrical connections). If they go up too much, corrective action must be taken to insure continued adequate HIRF/Lightning protection.
So, I'm guessing when they did recent tests on older 777s, the found this particular electrical ground was degrading more than allowed, so they need to correct it.
Assuming I'm correct, this sort of thing goes on all the time (tests and inspections of aging aircraft to see how they are doing), and sometimes corrective action is required. SOP is Boeing (or Airbus) issues a Service Bulletin providing instructions for cleaning up the item in question such that it passes muster - and since many operators routinely ignore SBs, the Feds issue and AD to mandate the fixes. In the current feeding frenzy of all things Boeing, the media saw the FAA NPRM and jumped on it as further evidence that Boeing can't build aircraft...
In other words, Tempest in a Teacup...

[Speed_Trim_Fail]

No knowledge pf this particular issue outside of what I've seen in the media, however an educated guess at what's going on:
The HIRF/Lightning people do periodic inspections of random in-service aircraft to test how well the HIRF/lighting protections are holding up. Grounding is critical to the protection, so one of the checks is to measure the electrical resistance of the various grounding systems. The allowable resistances are quite low - as in a few milliohms - and it's pretty normal for the resistances to increase as an aircraft ages (corrosion and such on the electrical connections). If they go up too much, corrective action must be taken to insure continued adequate HIRF/Lightning protection.
So, I'm guessing when they did recent tests on older 777s, the found this particular electrical ground was degrading more than allowed, so they need to correct it.
Assuming I'm correct, this sort of thing goes on all the time (tests and inspections of aging aircraft to see how they are doing), and sometimes corrective action is required. SOP is Boeing (or Airbus) issues a Service Bulletin providing instructions for cleaning up the item in question such that it passes muster - and since many operators routinely ignore SBs, the Feds issue and AD to mandate the fixes. In the current feeding frenzy of all things Boeing, the media saw the FAA NPRM and jumped on it as further evidence that Boeing can't build aircraft...
In other words, Tempest in a Teacup...

As ever sir you add a great deal to this forum - I’ve learned something new today!

[tdracer]

As ever sir you add a great deal to this forum - I’ve learned something new today!
In the last ~15 years of my career, I learned a great deal about HIRF and Lightning since I was responsible for the FADEC installation on several models of Boeing aircraft (FADEC is considered flight critical, and is treated as such for HIRF/Lightning protection).
That being said, I basically learned enough to be dangerous - I knew all too well when I needed to defer to experts in the HIRF/Lightning group.

[Lascaille]

We don't need to guess - clicking the link in the article (https://www.regulations.gov/document/FAA-2024-0761-0001) takes you to the 'Notice of proposed rulemaking' which says:

This proposed AD was prompted by a determination that the nitrogen enriched air distribution system (NEADS) cover plate assembly attached to a certain vent stringer in the center wing tank was installed without a designed electrical bond. This proposed AD would require installing electrical bonding and grounding, installing the cover plate assembly with new fasteners, and revising the existing maintenance or inspection program, as applicable, to incorporate new airworthiness limitations.

Now I would guess that that a cover plate assembly, by nature, is fixed in place so it'll be grounded by incident. But things have to be grounded by design - there's a general rule to not combine electrical security and mechanical security i.e. you can't ground something with the same bolt that holds it in place.

[Big Pistons Forever]

tdracer

Would I be correct in assuming that there is a longstanding FAA mandate (FAR ?) that Boeing has to have a program to ensure the continuing integrity of the grounding protection in vulnerable areas of the airframe, and that Boeing recently discovered this particular issue and reported it ? So the system is working exactly as intended.

[island_airphoto]

We don't need to guess - clicking the link in the article (https://www.regulations.gov/document/FAA-2024-0761-0001) takes you to the 'Notice of proposed rulemaking' which says:



Now I would guess that that a cover plate assembly, by nature, is fixed in place so it'll be grounded by incident. But things have to be grounded by design - there's a general rule to not combine electrical security and mechanical security i.e. you can't ground something with the same bolt that holds it in place.
This is grounding 101. The bolts/screws have some kind of sealant involved and a cover plate has a gasket. There may be good metal-metal contact and there may not be. There needs to be a jumper wire to be sure.

[tdracer]

tdracer

Would I be correct in assuming that there is a longstanding FAA mandate (FAR ?) that Boeing has to have a program to ensure the continuing integrity of the grounding protection in vulnerable areas of the airframe, and that Boeing recently discovered this particular issue and reported it ? So the system is working exactly as intended.
That would be a reasonable assumption (a lot of the stuff coming out on the 787 has been self-reported), but I've not seen anything one way or the other regarding how this particular issue came up.
So in other words, I don't really know :rolleyes:

[remi]

How often are nitrogen inerting or ignition mitigation devices used now?

And a suspected contributor to the TWA 800 explosion was heating of the fuel from PACS adjacent to the tank during unusually warm conditions on the ground, to impressively high temperatures ("the highest ullage temperature measured within the CWT was 145∞ F"), which I assume is not a thing here.

[tdracer]

How often are nitrogen inerting or ignition mitigation devices used now?

And a suspected contributor to the TWA 800 explosion was heating of the fuel from PACS adjacent to the tank during unusually warm conditions on the ground, to impressively high temperatures ("the highest ullage temperature measured within the CWT was 145∞ F"), which I assume is not a thing here.
The NGS as installed on the 777 is designed to run pretty much anytime the engines are running - dumping nitrogen into the center fuel tank. I suppose there is a way to turn it off from the flight deck, but the design intent is that it's always running when there is engine bleed air available. 747 is set up the same way (all 747-8 have the NGS, and I think most 747-400s still operating had it retrofit). The center tank fumes can be combustible at much lower temps than what TWA 800 experienced, and nearly all Boeings with center wing tanks have the packs located such that they can heat residual fuel in the center tank.
As I understand it - on the 787 - the NGS feeds nitrogen into all the fuel tanks, not just the center wing tank (IIRC, with aluminum wings, the fuel tank temp drops quick enough as the aircraft climbs such that the tanks are rarely in a combustible state, but the plastic wings act a bit like a Thermos bottle - keeping the fuel temps higher for a longer time - leading to a much greater risk of the fuel tanks being in a combustible state).

[Mechta]

Do any airliners have a way of measuring the oxygen percentage in the ullage yet? It was an issue when inerting first reared its head in the early 2000s as the only suitable oxygen sensors at the time required very frequent servicing.

This (https://www.aviationpros.com/home/press-release/10432717/fuel-tank-safety-system-comes-standard-on-new-boeing-787) article from 2009 supports what TDRacer wrote about the 787 system. Is it still the case that the inerting system can be unservicable for up to 10 days without affecting the aircraft's use?