Bristow S-76. AAIB Report Findings

[baldspot]

76 crash report now available at AAIB website.

Excerpt:

During the metallurgical examination, evidence was found of an anomaly in the scarf joint between the two titanium leading edge erosion strips. The tip of the tang on the inboard end of the outboard erosion strip was bent and folded under the outboard end of the inboard erosion strip (Figure 1). This resulted in a doubling of the thickness of the erosion strip material in that area which in turn resulted in virtual contact between the erosion strip and the blade's titanium spar especially in the areas at either end of the tang's fold line. This anomaly had occurred during the blade's manufacturing process some 21 years prior to the accident.



http://www.aaib.dft.gov.uk/special/gbjvx/gbjvx.htm

[SASless]

Quoting the AAIB report.....

"The AAIB and the helicopter's manufacturer are of the opinion that the electrical energy imparted by the lightning strike in 1999 exploited the anomaly that was built into the blade at manufacture and damaged the spar.

It has not yet been established whether the anomaly seen in the fractured blade went unnoticed or whether it was permitted by concession. If the anomaly was not detected during manufacture, there may be other blades where the anomaly is slightly different in nature and/or severity. The anomaly now exposed on the fractured blade would have been hidden from view during routine in-service blade inspections. Conceivably there could be another damage mechanism such as abrasion or fretting between the titanium spar and the titanium erosion strip that might induce fatigue. Consequently, there may be other S-76 blades that have a similar hidden anomaly that may be relevant to the continued airworthiness of those blades. "

The Federal Aviation Administration mandates appropriate action to ensure the continued airworthiness of Sikorsky S-76 main rotor blades which have either:

A two-piece leading edge titanium sheath (erosion strip).

or

Have suffered a lightning strike.

This safety recommendation was copied to the UK CAA."

Felt these excerpts were worth copying to the thread here.

[widgeon]

The selling point of composite blades has always been the "soft" failure mode . Structural damage will most of the time result in noticeable change in stiffness of the blade that should be detected ( by vibration or change in tracking) long before catastrophic failure of the blade.

I do not think a composite blade would have been returned to service after a lightning strike as in most cases the blades suffer structural ( though non catastrophic ) damage in a lightning strike.

This appears to be one of those one in a million chances ( judging by total fleet hours) but if it can happen it will eventually.

[S76Heavy]

So exactly what constitutes "appropriate action" then? Discard the blades, have them X-rayed, strip and rebuild?

[hobie]

I remember during an earlier discussion on the construction details of the main Rotor that I referred to a lightning strike report on G-BHBF in 1999 which gave a very good description on the rotor structure .... I asked if G-BHBF was in fact the same aircraft that was lost in the current incident but was told correctly that it was not! ....... just noticed in the AAIB report that in fact the failed rotor component was indeed the same item hit in the 1999 incident ...... its a small world I guess!

[SASless]

Also....if my alcohol ravaged memory doesn't fail me....did not the lightning strike report detail at length all the components that were scrapped following the incident? Seems I recall a long list of repairs and component removal and scrapping after that.

Begs the question....why was the blade thought to be exempt from concern....and why was the "spot weld" site not found upon examination of the blade by the manufacturer during its return for inspection/repair/overhaul ?

[hobie]

SASless .... this is the report on G-BHBF ......

http://www.aaib.dft.gov.uk/bulletin/mar01/gbhbf.htm

[SASless]

Read the AAIB report.....question now should be ...."Where are the other three main rotor blades involved in the lightning strike?" Are they still in service...have they been involved in any other "incidents" ?

[S76Heavy]

No, all blades that have EVER suffered a lightning strike or are suspect have been removed. Apparently an ASB will come out in the very near future.

[magbreak]

The ASB has been out for a while (25-7-02). ASB 76-65-55 and ASB 76-65-55A are the two to look out for.

8 blades are 'named' in the ASB (which would suggest two aircraft have been hit), and a warning to remove all blades that have had lightning damage either in the air OR on the ground. Log cards had to be checked and if there was any Uncertainty the blade had to be removed.

Apparently another ASB is due out shortly regarding an inspection on blades with the two piece leading edge to check the condition of the tang as mentioned in the AAIB report.

[S76Heavy]

Ah, it's the latter one that I referred to. I've seen the other ones concerning lightning in print already.
Single strip blades seem to be very much in demand, lately..