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Tailboom strike by main rotor
Hi guys, I have a question?
How a 4 blades system (like the S76,Bell 429, Agusta 109) can it the tailboom, what are the conditions? Thanks FH |
1. Gusty wind conditions with low Nr during start and shutdown.
2. Droop stop failure during shutdown 3. Heavy landing Inflight it would be rare but probably not impossible with extreme handling. |
Slightly more than a strike for this poor EC120...
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212: can add swashplate bearing failure to your list.
Also, worth noting that some manufacturers include, in their control mixing units, aft cyclic limiting as a function of low collective. Adds a margin of safety when practicing OEI landings with reduced Nr ( which, I suppose is an item on this list ). |
That was not a Tail Boom Strike by a Main Rotor Blade.....that was a Tail Boom SEVERED by the the Main Rotor.....a significant difference in my book.
As to the. cause....something major to cause that amount of downward flexing or tilting of the Main Rotor.. A small clue for you pprune accident investigators.....the close proximity of the severed bits would suggest it happened pretty close to where the rest of the aircraft fetched up. |
Originally Posted by JohnDixson
(Post 11310097)
212: can add swashplate bearing failure to your list.
Also, worth noting that some manufacturers include, in their control mixing units, aft cyclic limiting as a function of low collective. Adds a margin of safety when practicing OEI landings with reduced Nr ( which, I suppose is an item on this list ). The other instance was following a sticky situation upon performing negative g manouevres, again not recommended. Not from the types listed in question but i imagine still relevant. |
Originally Posted by SASless
(Post 11310102)
That was not a Tail Boom Strike by a Main Rotor Blade.....that was a Tail Boom SEVERED by the the Main Rotor.....a significant difference in my book.
As to the. cause....something major to cause that amount of downward flexing or tilting of the Main Rotor.. A small clue for you pprune accident investigators.....the close proximity of the severed bits would suggest it happened pretty close to where the rest of the aircraft fetched up. Often when forced to make an EOL to unsuitable terrain or small landing area - big flare to wash off the speed, spank in the tail which rebounds into the disc coming aft with the cyclic |
Crab,don’t know whether this was passed along thru Westland, but the initial SH3A ( first 61 model ) did not have collective vs longitudinal limiting and the first USN ships had progressed into training the first USN pilots, when after one of our guys, having just been briefed by Bob Decker, the Ch Pilot, on being careful about this ( EOL ), went out and “showed” a USN pilot how to clip the top of the tail drive shaft cover and shaft. That little event resulted in the mixer/limiter, and it worked OK and all since benefitted. Before I signed on.
You may have seen pictures of the original UH-60 prototype with the low main rotor. That ship had both a forward and aft limiter. After we raised the main rotor six months into the test program, we left those limiters in, and were able to do the 12 degree and 15 degree slopes per the Army requirement, so they remain in place today. |
Often when forced to make an EOL to unsuitable terrain or small landing area - big flare to wash off the speed, spank in the tail which rebounds into the disc coming aft with the cyclic I don’t know the word “spank” and its meaning. I am not sure if here it is a verb (“to spank”) or a noun… |
Originally Posted by SASless
(Post 11310102)
That was not a Tail Boom Strike by a Main Rotor Blade.....that was a Tail Boom SEVERED by the the Main Rotor.....a significant difference in my book.
As to the. cause....something major to cause that amount of downward flexing or tilting of the Main Rotor.. A small clue for you pprune accident investigators.....the close proximity of the severed bits would suggest it happened pretty close to where the rest of the aircraft fetched up. Good result in my book if was able engine failure. Machine egress available by opening doors and walking out! I would say they are still making EC120s but they are not… |
Originally Posted by Hot and Hi
(Post 11310355)
Crab, could you kindly rephrase that sentence?
I don’t know the word “spank” and its meaning. I am not sure if here it is a verb (“to spank”) or a noun… When the rear of the skids hit the ground, the tail starts to come up and meets the rotor disc coming back - the result is the sort of picture in the above post. To spank in is to crash - very colloquial use of the word spank, not sure of it's origins but was widely used in the military when I was serving. |
Isn't that why we taught folks to put the aircraft into a landing attitude by leveling the Skids prior to touch down?
The Accident Scene does not seem to fit the narrative of a confined unsuitable landing spot. It is interesting that only a single Rotor Blade shows catastrophic damage....which suggests the engine may not have been driving the Rotor system and the Rotor RPM was quite low. Which would begin to explain how with lots of aft cyclic input....and the aircraft striking the ground with a strong Rate of Descent would follow Crab's spanking exercise. Gentleman's wager is that it was an EOL that went all wrong in a Low Inertia Rotor System helicopter. |
Yes, if you manage to land in a level attitude with no skid or drift, you can survive most things.
The area in the video clearly isn't confined but the pilot may not have liked the look of the surface for a running landing. The fact that only one blade was damaged would imply blade sailing at low rom but without know more about the incident it is difficult to know. The EC 120 isn't a low inertia rotor system unless you compare it to something like a Huey. |
where I come from spank was to slap a young child, normally, on the bottom. Not sure the origin of brand spanking new is though
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Running Landing during an EOL....our standard was one skid length maximum to a designated spot....but then we got lots of practice.
Yes....the Huey is a high inertia rotor system and is a sweetheart to do touch down autorotations with, as are Bell 206's, and 212's. BO's, BK's MD-500's, 76's, Alouette III's..are fun but lose rotor rpm quickly. Chinooks were landed up to 60 KTS but could be coaxed to a near zero ground roll if you did it right.....and was a rewarding feeling when you did. |
EC120 report from Aviation Safety
The helicopter with two certified pilots was involved in a training flight under Visual Flight Rules (VFR) by day. Both pilots performed a comprehensive pre-flight inspection and the aircraft was uplifted with aviation gasoline Jet A1. Take-off was normal and the aircraft headed towards Cape Town International general flying (GF) area for training purpose. It was a 6th 'monthly operator's proficiency check (OPC) for the co-pilot (A trainee at the time of the accident). They started with the simulation of hydraulic failure followed by three simulated engine failure in hover. They climbed to 1000 feet above ground level (AGL) for autorotation practices. The first autorotation went well with the instructor closing the twist grip to ground idle and the trainee going into autorotation. The instructor again opened the twist grip to flight position during flare. The second autorotation was from a downwind position at 1000 feet AGL. The instructor closed the twist grip to ground idle and the trainee went into autorotation. The instructor looked at the Main rotor and Engine N2 (NR/NF) gauge and checked if the main rotor Revolution per Minute (RPM) was under control. The instructor then noticed that the engine RPM was winding down below normal idle indication. The instructor informed the trainee that they have lost an engine and the trainee continued with the normal procedures for autorotation till touchdown. On touchdown the aircraft still had a forward motion, skidded for approximately 3 meters. The speed reduced quite quickly as the skids dug into the soft terrain and the tail boom was chopped off by the main rotor. The aircraft was substantially damaged and no one was injured. Probable Cause Unsuccessful autorotation resulting into hard landing due to undetermined loss of engine power. |
So instead of bouncing it, the nose of the skids dug in - you still get tail coming up and cyclic coming back at low Nr.
I used to put one hand behind the cyclic as a block when teaching new instructors EOLs - if the run on is fast or bouncy, the natural reaction is to pull the cyclic back. |
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the aircraft was uplifted with aviation gasoline Jet A1. |
I've seen 2 S76s where the main rotor has hit the fin, making quite a dent but missing the drive shaft. In both cases the crew were doing single engine landings to a runway. The flare was too much & the r rpm too low causing the blades to flex down. All 4 tips were damaged.
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