Boeing 747 fuse pins

[stressguyMike]

Writing a presentation for stress corrosion. An illustrative example of a catastrophic event due to stress corrosion was the crash of El Al Flight 1862 in Amsterdam in 1992. Stress corrosion of the fuse pins, m/f Nitronic 60, was determined to be the cause of engine 3 disengaging from the wing & taking out engine 4 & thus the crash.

So, simple question is, what material was used for the new fuse pins after this accident?

If anyone has that specific info & possibly more to share on this topic, I'd appreciate it! Thank you.

 

[Tmoose]

FWIW this report claims the original pin was 4330 M steel, and the replacement would be "stainless steel."
I believe Nitronic 60 is considered a stainless steel.

https://reports.aviation-safety.net/1992/19921004-2_B742_4X-AXG.pdf

 

[WKTaylor]

StressGuy...

NOTE.
The accident report cited 'fatigue cracking' of the 4330 Steel Fuse Pin... NOT stress corrosion cracking [SCC].

The Dutch National Aerospace Laboratory was contracted to carry
out a metallurgic investigation of the fuse pin . The results of this
investigation are contained in report CR 93030 C: "Investigation of the
Outboard Midspar Fuse Pin from the Pylon of Engine #3 of El A l
1862" . The laboratory concluded the following:

1. A large fatigue crack was present at the outboard location of
minimum wall thickness of the fuse pin, which was of the "
bottle bore " configuration . This fatigue crack was up to 4
millimetre in depth and encompassed about 50% of the
inside circumference.

2. The fatigue crack had developed from multiple initiation
sites along poor quality machining grooves. There was no
evidence of corrosion pitting that could have contributed to
fatigue initiation.

3. The material of the fuse pin met the chemistry specification
for 4330 M steel. However, hardness measurements indicated
that the tensile strength was about 117 KSI, which is
lower than the specified range of 126- 139 KSI.

It should be noted that a low hardness does not mean the pin was
understrength, because sometimes the final machine cut is adjusted
based on the testing conducted in the sampling process.

Boeing also carried out a metallurgic investigation of the fuse pin.
The Boeing findings concur with the NLR findings.

Boeing was able to derive a crack growth curve of the fatigue fracture
surface as a function of total airplane cycles (flight cycles) versus
crack depth.

Based on this curve Boeing concludes that at the last inspection of
the fuse pin, 257 flights before the accident flight, the fatigue crack
would have had a depth of .14 inch. As the ultrasonic reference depth
is .085 inch a detectable crack existed at the last inspection.

NOTE.
15-5PH CRES HTxxx is used on the Boeing **-**** re-engine strut Fuse Pins. Similar design and manufacturing methodology as cited in the accident report. This aircraft Fuse Pins have deformed as the result of 'hard-landings' [a FEW very rare cases].... but has NEVER sheared due to overload or fatigue-cracking.

NOTE.
As I understand it, the 'giant aircraft manufacturer's' philosophy for pylon-engine attachment has changed... in-part due to this accident.

In-lieu-of allowing the pylon to fail benignly so the pylon-engine 'separates safely'.... pylons are now designed to resist failure and RETAIN the pylon-engine under foreseeable catastrophic circumstances. This is a real challenge with the monster fan engines... but is considered safer than allowing separation.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

It seems to me that SCC is fairly rare in high strength steel... temper embrittlement, liquid metal embrittlement, pitting corrosion and/or impact damage combined with KIc [notch/fast fracture] sensitivity... are more likely.

ONLY cases of SCC I can clearly remember are...

[1] SCC failure within a **-**** main landing gear 'wheel-truck' barrel at a high stress-point [corrosion pitting]. The 'wheel truck' broke in-half during an aircraft 'washing' and settled suddenly on the end of the shock-strut. a civil employee got banged-hard-on the head due to contact with the lower wing-skin.

[2] F-111 wing pivot/carry-thru D6AC HS steel that failed due to combination of pitting corrosion, SCC on-set and severe notch [stress concentration] sensitivity. Center-section failures made for a 'bad day' for several F-111 crews.

Aluminum alloys on the other hand are notorious for SCC failures. I can cite [have seen] hundreds of examples on old military jets.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[stressguyMike]

Hi WKTaylor, thanks for the info. So you think the new fuse pins are 15-5 Hxxx. Any idea what heat treat?

I read this ASME report that indicated stress corrosion cracking as the initial failure. I know Boeing changed their design so that the engines do not disengage during flight but they do while on the ground. The initial design was based on assumptions as to how the engine would come off the aircraft in flight. They did not anticipate that engine 3 would take out engine 4.

Cheers,
Mike

 

[WKTaylor]

The pins cited in the report you attached are within the engine that led to catastrophic engine failure... that then fratricided the adjacent engine.

The 'fuse pin' I was discussing is the strut-to-wing lug-joints shear-fuse-pins. Fatigue damage facilitated the failure of the pylon + engine from the wing.

And... yes... separation under the wing, and aft, is permitted during a ground crash: but separation in-flight under most conditions is NO LONGER acceptable.

NOTE.
The BA 777 that crashed landed short [due to fuel-icing-induced double-engine failure] at Heathrow airport [several years ago] actually RETAINED both Trent engines... but sheared both main landing gears aft.

NOTE.
Crash landing design mandates that the wing integral tank structure remain as intact as possible to minimize fuel loss/fire.


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[3DDave]

Wil - I recall that crash and recall that on at least one side the MLG punch a hole through the top of the wing.

Ahh yes British Airways Flight 38. It pulverized the wing at the root from the MLG attachment on aft. That has to have been terrifying thing to see. 47 injuries, no fatalities. And just a 300 yards short of the runway out of a 5,000 mile trip.