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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).

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...


The design and precision engineering behind such a machine is inspiring.

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.

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.

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

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...

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.

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:


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:


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


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.

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.

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.

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.

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.

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.

I think a ‘large tuneable lump’ rings a bell with me too!

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

viewtopic.php?p=251203#p251203

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.

GBXY's accident was unfortunate. Jerry Hardy was doing instrument training for a new pilot and an engineer had arranged for a friend of his to come as a passenger. I was detached to Yarmouth and I flew back that day and it was the taxi driver who told me about it.

The next morning all 76 pilots were assembled and given the details. The yellow blade spindle bearing had worn and migrated out of the spindle housing followed by a failure of the spindle. We started flying that day because the spindle assemblies had been modified: There were two tie-wraps around the outer section of each spindle to stop the bearing migrating.

Later we had new spindles and we could carry passengers again.

Great that it was fixable, but like G-BJVX in the SNS it's not one of those things one likes to dwell on too much, falling apart in flight. Down the road at BAH we were all shocked too. We only had one and that had been blown over by strong winds when it was parked overnight on a Forties platform deck. The Radio Op didn't wake the crew when the wind shifted and increased.

The S76 went on to be such a good machine for offshore work.

FD2 wrote: ↑

Sat Feb 06, 2021 7:10 pm

....... We only had one and that had been blown over by strong winds when it was parked overnight on a Forties platform deck. The Radio Op didn't wake the crew when the wind shifted and increased. ......

One of the crew was hico-p who posted a few times here in the early days of the forum. If I recall correctly it wasn't the crew's fault - a weld on a tiedown point on the helideck failed in the strong winds.

VTA stood for Variable Tuning Attenuater. or 'or' in English. The idea was when you settled in the cruise one fiddled with the knob so as to get the smoothest ride. It was effectively a hydraulic hammer that hit the aircraft in the opposite direction to which it wished to shake. You would notice the vibration get worse if you wound the knob to one end or the other but the middle bit seemed to cover everything. In the end we ignored it and it was only checked if some vibration or other came up.

The big difference came when we were advised to crank the Rrpm up to 107% when at 3,000 plus. This improved the ride no end as well as the fuel consumption

The first summer showed up the pathetic helipad performance offshore. Our AUW had just gone from 10,000 lbs to 10,300 and one would find at that weight one just staggered around the deck. After one very close tail strike on the safety net on a departure we had to go to the books and reduce our departure weight according to the Flight Manual. It was even more evident in China where I flew the plus with the Arreils. There we would crank the Rrpm to 107 and overpitch it down to about 98% in a towering type take off with a plunge over the side of the deck relying on gravity to get you up to VTOSS.

The Super Pumas there used to route landside unit east pf Hong Kong before heading south. We couldn't do that because our single engined safety height was less that a 1,000 ft so we had to route over the Pearl River almost to Macau and then southeast to the platform.

Thanks for a clear explanation fed.

The A+ was pretty gutless on bad days, also with added weight from additional avionics and offshore kit plus fat pax at standard weights! One take off from the Clipper was so alarming for the HLO that he pressed the crash alarm - it was just after Bristow took the whole of Shell North Sea contracts from KLM. The B was outstanding by comparison, with the reassurance of a certain additional power available in case of 'emergency'. Pity about the fuel consumption but always nice to get off the deck easily with the required load.

On standard weights, I flew in with 19 pax from the Piper in an S61 and much to the annoyance of management the captain weighed all the passengers and found we'd been about 1,000 lbs over MTOW. The CAA eventually increased the 'standard' weight to something more sensible but still not enough in some cases!