Sikorsky S-76 fleet - 7.5 million hours

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CharlieOneSix
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Sikorsky S-76 fleet - 7.5 million hours

#1 Post by CharlieOneSix » Thu Aug 27, 2020 4:21 pm

A joy to fly and an enduring and still attractive design some 41 years after the first commercial delivery. 7.5 million hours of safe, reliable flight? Sadly like almost every aircraft type not all of those hours have been safe and reliable. In particular remembering those lost as a result of the main rotor failures on G-BGXY (1981) and G-BJVX (2002).
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Re: Sikorsky S-76 fleet - 7.5 million hours

#2 Post by TheGreenGoblin » Thu Aug 27, 2020 6:33 pm

There is something particularly aesthetically (to this neophyte anyway) pleasing about the Sikorsky S-76 and this interesting thread got me to looking at this site...

https://www.sikorskyarchives.com/S-76.php

The very complex report into the loss of G-BGXY led me to have a look at the S-76's rotor hub, which is in itself a piece of functional beauty...

S76RotorHub.JPG
S76RotorHub.JPG (36.36 KiB) Viewed 165 times
S-76A.JPG
S-76A.JPG (65.46 KiB) Viewed 165 times

The design and precision engineering behind such a machine is inspiring.
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Re: Sikorsky S-76 fleet - 7.5 million hours

#3 Post by FD2 » Thu Aug 27, 2020 10:03 pm

Thanks C16 - I agree about the S76 - what a great machine and I think a long service life ahead still. Good offshore but limited to 9 in comfort with goonsuits and good onshore for 9 or 10 pax, or less as a 'VIP' version.

The G-XY failure was particularly poignant and G-VX's blade problem might have shown up under X-ray back in the States but I don't suppose any aircraft can be 100% 'safe' from failure if made, maintained and flown by humans.

Do you agree that the main rotor head in GG's photo is a bit different from the version we flew? It appears to have an extra layer of anti-vibration thingies (excuse technical terminology) at the top. I'll hunt my old manual out. If so, it should be very smooth in flight.
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Re: Sikorsky S-76 fleet - 7.5 million hours

#4 Post by FD2 » Thu Aug 27, 2020 10:14 pm

The MRH looks different in the early models but it's not surprising if it was modified as the machine was developed and improved through the various versions.
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Re: Sikorsky S-76 fleet - 7.5 million hours

#5 Post by TheGreenGoblin » Fri Aug 28, 2020 3:41 am

FD2 wrote:
Thu Aug 27, 2020 10:03 pm

Do you agree that the main rotor head in GG's photo is a bit different from the version we flew? It appears to have an extra layer of anti-vibration thingies (excuse technical terminology) at the top. I'll hunt my old manual out. If so, it should be very smooth in flight.
I wondered about those "thingies"and noted the term Bifilar vibration absorbers in the G-BGXY accident report. That report is very interesting, if complex, because it appears that there had been a similar accident in Brazil previously and if the manufacturer had not been so tardy in responding to the accident investigators in that case the issue of fatigue cracking in the blade spindle could have been highlighted and it is possible that the G-BGXY accident might have been averted.

https://assets.publishing.service.gov.u ... G-BGXY.pdf

S76Summary.JPG
S76Summary.JPG (54.02 KiB) Viewed 134 times
PT-HKB - Brazil

Nonetheless to misquote the bard, "I come to praise Sikorsky, not to bury it" and the ongoing improvement of an essentially excellent design means it will be around for many years yet and is a credit to all the people who were ever involved in it, including the pilots who flew it so safely over the years...
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Re: Sikorsky S-76 fleet - 7.5 million hours

#6 Post by Seenenough » Fri Aug 28, 2020 4:12 am

I would guess that the BO 105 Commercial Fleets will take quite bit to catch.

The Goog gave them over 8 million hours back in 2014.

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Re: Sikorsky S-76 fleet - 7.5 million hours

#7 Post by FD2 » Fri Aug 28, 2020 4:32 am

Bifilar vibration absorbers - yes it came back to me as I was laying in bed this morning wondering whether it was time to get up.

Here is a photo of G-SZ from 1983 at Redhill - the second set of bifilars are evident:

SZ.png

And one of VX after an overhaul at Redhill, in an unauthorised paint scheme (someone had a mighty bollocking for doing it), the year before it crashed:

G-VX scheme.png
G-VX scheme.png (220.39 KiB) Viewed 122 times

And the MRH of KLM S76B G-UKLU at Norwich about 1997:

G-UKLU head.jpg
G-UKLU head.jpg (14.2 KiB) Viewed 122 times

I suspect the second set was removed as it made little difference to vibration levels and saved a little weight. Anyway not really a significant matter GG - just something that caught my eye.
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Re: Sikorsky S-76 fleet - 7.5 million hours

#8 Post by TheGreenGoblin » Fri Aug 28, 2020 4:47 am

FD2 wrote:
Fri Aug 28, 2020 4:32 am
Bifilar vibration absorbers - yes it came back to me as I was laying in bed this morning wondering whether it was time to get up.
;)))
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Re: Sikorsky S-76 fleet - 7.5 million hours

#9 Post by CharlieOneSix » Fri Aug 28, 2020 9:03 am

FD2 wrote:
Thu Aug 27, 2020 10:03 pm
....Do you agree that the main rotor head in GG's photo is a bit different from the version we flew? .......
The only S-76 I flew to any great extent was the BCAL G-BIBG and then for only a year. Memory dulls with age but I was surprised when I looked for a photo of it on the ground to check on the "thingies" and came across this one. Poor quality scan of a photo in my logbook, it seems to show an additional set even on this early model - I took the photo during a fuel stop on the delivery flight from Conroe, Houston to New York.
BG-Delivery-1.jpg
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Re: Sikorsky S-76 fleet - 7.5 million hours

#10 Post by FD2 » Fri Aug 28, 2020 10:22 am

Thanks C16 - I remember BG at Norwich and the need for collective input on the ILS. It foxed some of the guys who'd been on the Bristow A+ until they got used to it.
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Re: Sikorsky S-76 fleet - 7.5 million hours

#11 Post by FD2 » Fri Aug 28, 2020 10:56 pm

From an engineer friend:

I seem to remember the lower bifilar took out a 3p and the upper a 5p rev, there was also a vta in the nose that worked in harmony with the airframe. Bristow and others removed the vta and upper bifilar and focused on 1 2 and 4 p which meant it could be lumpy but some aircraft had that cobblestone effect anyway.

I guess they want VIP machines to be as smooth as possible so retain all the anti-vibration features.
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Re: Sikorsky S-76 fleet - 7.5 million hours

#12 Post by CharlieOneSix » Fri Aug 28, 2020 11:30 pm

I’m trying to think what vta stands for - vibration transmission absorber? I seem to remember it was just a tuneable lump in the nose. It’s so long ago but I think we must have removed the upper bifilar on BG once it arrived in ABZ as I don’t really remember it.
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Re: Sikorsky S-76 fleet - 7.5 million hours

#13 Post by FD2 » Fri Aug 28, 2020 11:40 pm

I think a ‘large tuneable lump’ rings a bell with me too!
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Re: Sikorsky S-76 fleet - 7.5 million hours

#14 Post by TheGreenGoblin » Sat Aug 29, 2020 5:52 am

Bit more on Sikorsky and the vibration absorber theme here.. passive bifilars and hub-mounted vibration suppressor (HMVS).

viewtopic.php?p=251203#p251203
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Re: Sikorsky S-76 fleet - 7.5 million hours

#15 Post by fareastdriver » Mon Sep 14, 2020 3:10 pm

I always understood that the second bifilar was to absorb the interaction between the Allison engines. When they trialled the first one with Arriel engines they found that the top bifilar was unnecessary so it was removed.

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