Helijet S76 loses 2 tail rotor blades in lightning strike, and lands safely
wouldn't imagine it'd be any fun having both AP's drop at cruise speed.
Edited to add that I just heard from a friend that works there and he said: "They initially lost all EFIS and both AP while IMC. UA recovery"
I would say that the account from the pax was entirely accurate! That would have been a horrible few moments.....
Last edited by 212man; 25th Oct 2023 at 16:14.
If one was sat their feet back, doing anything but being close to the controls and both AP's went on strike together at the same time.....yes there would be a diversion from straight and level no doubt.
If you were feet and hands on the controls when that happened.....yes....the diversion would occur.
How big the diversion would depend upon the reaction time of the Pilot "flying" the machine be it by buttons or by human appendages.
Add a complete and instantaneous loss of AP's and everything but standby/emergency instruments.....it would get interesting.
To be IMC when it happened....would be really interesting.
Add night time to it and Holy Mackerel Batman!
I recall the good old days when Air Log sent their brand new 76A's offshore to earn revenue until they could be sent for installation of the Sperry Kit....and those were not good ol' days.
I also recall a Bell 212 Operator that did not think SAS/AP were needed for night offshore flying citing "But, Gee....it is VMC weather minimums we are using.". Never mind the absence of surface lighting or horizon.
This 76 Crew rose to the occasion and did an excellent job and deserve accolades for their performance under what must have been very stressful conditions.
If you were feet and hands on the controls when that happened.....yes....the diversion would occur.
How big the diversion would depend upon the reaction time of the Pilot "flying" the machine be it by buttons or by human appendages.
Add a complete and instantaneous loss of AP's and everything but standby/emergency instruments.....it would get interesting.
To be IMC when it happened....would be really interesting.
Add night time to it and Holy Mackerel Batman!
I recall the good old days when Air Log sent their brand new 76A's offshore to earn revenue until they could be sent for installation of the Sperry Kit....and those were not good ol' days.
I also recall a Bell 212 Operator that did not think SAS/AP were needed for night offshore flying citing "But, Gee....it is VMC weather minimums we are using.". Never mind the absence of surface lighting or horizon.
This 76 Crew rose to the occasion and did an excellent job and deserve accolades for their performance under what must have been very stressful conditions.
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The 212 I was just flying got hit by lightning a few years ago, also in a tail rotor blade. The current traveled through the aircraft and down the attached 150ft long line, into the pond he was dipping out of.
The tail rotor stayed on with a tiny burn hole, but according to the pilot, pretty much every circuit breaker popped on the overhead panel, and he got a false engine fire light. Interesting how completely different the effects of lightning can be.
The tail rotor stayed on with a tiny burn hole, but according to the pilot, pretty much every circuit breaker popped on the overhead panel, and he got a false engine fire light. Interesting how completely different the effects of lightning can be.
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Presumably the strike just happened to travel down one blade and out along the diametrically opposite blade,?, "releasing" both, and leaving a more or less balanced two blade rotor.
Very lucky, and very well done the pilots. Helicopters are scary things, mechanically speaking.
Very lucky, and very well done the pilots. Helicopters are scary things, mechanically speaking.
The S76 tail rotor assembly consists of two paddles attached to the Tail Gearbox, each paddle assembly consisting of a common spar with two blades attached to it - none of which can be disassembled in the course of normal operation and maintenance. It will be very interesting to understand the failure mode in this incident.
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If one was sat their feet back, doing anything but being close to the controls and both AP's went on strike together at the same time.....yes there would be a diversion from straight and level no doubt.
If you were feet and hands on the controls when that happened.....yes....the diversion would occur.
How big the diversion would depend upon the reaction time of the Pilot "flying" the machine be it by buttons or by human appendages.
Add a complete and instantaneous loss of AP's and everything but standby/emergency instruments.....it would get interesting.
To be IMC when it happened....would be really interesting.
If you were feet and hands on the controls when that happened.....yes....the diversion would occur.
How big the diversion would depend upon the reaction time of the Pilot "flying" the machine be it by buttons or by human appendages.
Add a complete and instantaneous loss of AP's and everything but standby/emergency instruments.....it would get interesting.
To be IMC when it happened....would be really interesting.
Wow, I had no idea that graphite was so strong in tension or resilient to vibration - I thought it was a soft material and not very mechanically strong at all.
But graphite, being made of carbon, might behave as an electrical resistor if a very large current from a lightening strike travelled through it. If so; its resistance to current flow would have made it get very very hot and disintegrate.
But graphite, being made of carbon, might behave as an electrical resistor if a very large current from a lightening strike travelled through it. If so; its resistance to current flow would have made it get very very hot and disintegrate.
Wow, I had no idea that graphite was so strong in tension or resilient to vibration - I thought it was a soft material and not very mechanically strong at all.
But graphite, being made of carbon, might behave as an electrical resistor if a very large current from a lightening strike travelled through it. If so; its resistance to current flow would have made it get very very hot and disintegrate.
But graphite, being made of carbon, might behave as an electrical resistor if a very large current from a lightening strike travelled through it. If so; its resistance to current flow would have made it get very very hot and disintegrate.
Last edited by albatross; 26th Oct 2023 at 23:34.
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Graphite does some interesting things when struck by lightning. When I worked on AH-64D Apaches years ago a thunderstorm came across the field one night and an aircraft in parking suffered a strike to the VHF whip antenna at the top of the tail pylon.
This antenna is half aluminum and half graphite, the lower aluminum half exploded with enough force to cause puncture damage to the skin on the two tail rotor blades that happened to be aligned with the antenna.
The graphite portion, normally approximately 2 feet long and maybe 5/16” diameter, resembled a horses tail, all the fibers had separated into their individual strands.
The stabilator and tail pylon had to be removed for repair because of extensive additional structural damage(including sheared rivets and distortion in secondary structures), and the tail landing gear drag beam bushings were also damaged as the lightning traveled through to the ground wire on the tail gear.
Both the tail rotor and intermediate gearboxes were replaced as well.
Quite impressive.
FltMech
This antenna is half aluminum and half graphite, the lower aluminum half exploded with enough force to cause puncture damage to the skin on the two tail rotor blades that happened to be aligned with the antenna.
The graphite portion, normally approximately 2 feet long and maybe 5/16” diameter, resembled a horses tail, all the fibers had separated into their individual strands.
The stabilator and tail pylon had to be removed for repair because of extensive additional structural damage(including sheared rivets and distortion in secondary structures), and the tail landing gear drag beam bushings were also damaged as the lightning traveled through to the ground wire on the tail gear.
Both the tail rotor and intermediate gearboxes were replaced as well.
Quite impressive.
FltMech
Thanks for the illustration Cyclic Hotline. Hard to otherwise understand the 'innards' when seen from the outside.
Below are close-ups of the damaged tail rotor as seen in the TV footage. The 'paddle' with pair of blades that was on the side closer to the tail boom was the one that failed. There is no obvious sign of the graphite spar left sticking out of either side of the 'rubber boots' still on the inboard side of where the blades had been. So perhaps their graphite spar failed at the point where the pitch control rod passes through it? Either that or the spar failed in two locations either side of the pitch control rod but inboard of those 'boots' (which seems less probable?). If the spar or rotor attachment had fail further out, then I might have expected the unbalanced remaining blade and spar would shear off the pitch control link when they parted. Clearly, that didn't happen and the pilots presumably still had control of tail rotor pitch (for the remaining blades).
From the TV footage, it seems one of the remaining blades was also damage at leading edge near its attachment lugs(?):
Here is a photo of an intact S-76 tail rotor including cropped to just its hub assembly:
Below are close-ups of the damaged tail rotor as seen in the TV footage. The 'paddle' with pair of blades that was on the side closer to the tail boom was the one that failed. There is no obvious sign of the graphite spar left sticking out of either side of the 'rubber boots' still on the inboard side of where the blades had been. So perhaps their graphite spar failed at the point where the pitch control rod passes through it? Either that or the spar failed in two locations either side of the pitch control rod but inboard of those 'boots' (which seems less probable?). If the spar or rotor attachment had fail further out, then I might have expected the unbalanced remaining blade and spar would shear off the pitch control link when they parted. Clearly, that didn't happen and the pilots presumably still had control of tail rotor pitch (for the remaining blades).
From the TV footage, it seems one of the remaining blades was also damage at leading edge near its attachment lugs(?):
Here is a photo of an intact S-76 tail rotor including cropped to just its hub assembly:
Last edited by helispotter; 27th Oct 2023 at 07:42.
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After seeing these pictures, I’ll throw out my theory of what happened, as I think it would best explain, considering the construction of the tail rotor system, how both blades could depart the aircraft simultaneously.
The s-76 tail rotor appears to be a smaller version of the UH-60, which uses an inboard and outboard retention plate to hold (and drive) the tail rotor paddles. There is an amount of clamping force that is checked with shims when installing the paddles and torquing the plates together, but that clamping force alone wouldn’t be sufficient to contain the blades from departing the aircraft.
As you can see from the schematic, the (graphite? Fiberglass? I forget) paddle spar has a hole in it that contains an oval shaped aluminum plug, with a hole in it for the pitch change shaft to go through, bonded into the spar(conveniently called a spar plug). The inner paddle is slid over the pitch change shaft and set into a recess in the inboard retention plate, then the outer paddle is placed over the inner one and recesses into the outboard retention plate.
I suspect the paddle failed at the spar plug area, as the spar is thinnest at the point either side of the plug. As the clamping force alone isn’t sufficient to contain the paddles, the now separated spar with blades attached overcame the clamping force and simply “slid” out from between the retention plate because of the massive rotational force. Pitch links probably didn’t even slow them down.
The only thing that makes me somewhat puzzled and could throw a wrench in my theory is the rubber boots shown still attached. On UH-60 these boots are bonded with Proseal to a piece of composite material that is in turn bonded to the spar at the inboard(retention plate side).
These are not structural parts of the paddle, just something to seal up the end of the blade root area to keep water
and debris out. I have seen these become disbonded from the spar before and they tend to tend to try to slide outboard towards the blade tip but they can only go a couple of inches.
The other end(blade side) is secured with a zip tie. So if the spar failed like I described the boots should have departed with the severed halves, unless the S-76 has a different attachment method?
I’d LOVE to watch the tear down of this assembly!🤣 As usual, Guess we will all have to wait for the investigation to see what really happened. ☹️
FltMech
The s-76 tail rotor appears to be a smaller version of the UH-60, which uses an inboard and outboard retention plate to hold (and drive) the tail rotor paddles. There is an amount of clamping force that is checked with shims when installing the paddles and torquing the plates together, but that clamping force alone wouldn’t be sufficient to contain the blades from departing the aircraft.
As you can see from the schematic, the (graphite? Fiberglass? I forget) paddle spar has a hole in it that contains an oval shaped aluminum plug, with a hole in it for the pitch change shaft to go through, bonded into the spar(conveniently called a spar plug). The inner paddle is slid over the pitch change shaft and set into a recess in the inboard retention plate, then the outer paddle is placed over the inner one and recesses into the outboard retention plate.
I suspect the paddle failed at the spar plug area, as the spar is thinnest at the point either side of the plug. As the clamping force alone isn’t sufficient to contain the paddles, the now separated spar with blades attached overcame the clamping force and simply “slid” out from between the retention plate because of the massive rotational force. Pitch links probably didn’t even slow them down.
The only thing that makes me somewhat puzzled and could throw a wrench in my theory is the rubber boots shown still attached. On UH-60 these boots are bonded with Proseal to a piece of composite material that is in turn bonded to the spar at the inboard(retention plate side).
These are not structural parts of the paddle, just something to seal up the end of the blade root area to keep water
and debris out. I have seen these become disbonded from the spar before and they tend to tend to try to slide outboard towards the blade tip but they can only go a couple of inches.
The other end(blade side) is secured with a zip tie. So if the spar failed like I described the boots should have departed with the severed halves, unless the S-76 has a different attachment method?
I’d LOVE to watch the tear down of this assembly!🤣 As usual, Guess we will all have to wait for the investigation to see what really happened. ☹️
FltMech
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Here’s what the H-60 paddle spar plug area looks like for anyone who’s interested:
And the outboard retention plate(pictured from underneath):
And in my last post I mentioned the zip ties, they are on both ends on UH-60, but they are also Prosealed on the side closest to the retention plate.
FltMech
And the outboard retention plate(pictured from underneath):
And in my last post I mentioned the zip ties, they are on both ends on UH-60, but they are also Prosealed on the side closest to the retention plate.
FltMech
Last edited by 60FltMech; 27th Oct 2023 at 18:11.
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Having suffered through an idiotic, self induced total loss of AFCS in IMC cruise at night in a 76 I can say that, even with a full panel in front of us, it was a interesting and scary minute until we fully regained control. Far and away one of the stupidest things I ever did in a helicopter. Many lessons relearned that night. Came close to having an accident report with “Albatross was an idiot” as the only cause.
The crew in this event, made much worse by the loss of primary flight instruments, did an excellent job! Kudos to them.
Some folks on another site started criticizing their handling of events in the first comments before any details came to light.
It must be nice to be a “perfect pilot” and judge others from afar.
The crew in this event, made much worse by the loss of primary flight instruments, did an excellent job! Kudos to them.
Some folks on another site started criticizing their handling of events in the first comments before any details came to light.
It must be nice to be a “perfect pilot” and judge others from afar.
Last edited by albatross; 27th Oct 2023 at 18:14.
Also think the spar fractured somewhere in the middle to get this result. Anywhere else and only one blade would have left plus the gearbox.
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