Tailboom strike by main rotor

[FlyingHead]

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

[212man]

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.

[Bomber ARIS]

Slightly more than a strike for this poor EC120... video

[JohnDixson]

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 ).

[SASless]

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.

[Ammo Boiler]

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 ).

Agreed, nearly all instances i have personally seen the aftermath of were a result of excessive flapping following the above highlighted conditions, despite the mixer reducing availble aft cyclic and publications warning to avoid doing so.
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.

[[email protected]]

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.
The one not specifically mentioned so far - the EOL that doesn't quite get the Green Endorsement.

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

[JohnDixson]

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.

[Hot and Hi]

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, 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…

[EMS R22]

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…

[[email protected]]

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…

Sorry Hot and Hi - the tail hits the ground because of the large nose-up attitude and the fuselage then pitches forward - the natural reaction to this from the pilot is to pull back on the cyclic further.

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.

[SASless]

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.

[[email protected]]

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.

[widgeon]

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

[SASless]

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.

[megan]

EC120 report from Aviation SafetyThe 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.

[[email protected]]

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.

[JimEli]

EC-135 Cave Creek, AZ (https://aviation-safety.net/wikibase/72681)

[601]

the aircraft was uplifted with aviation gasoline Jet A1.

That fuel would give any aircraft engine a failure.

[Nigel Osborn]

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.

[skadi]

The old BK-117 was equipped with a small foam insert at the vertical stabilizers to minimize the damage in case of a rotorblade strike ( SB-MBB-BK-30-103 from 1990 ) which could occure in heavy gusts during startup.

skadi

[FlyingHead]

Thanks gents, all good info.
FH

[LRP]

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
Another possibility...

[brett s]

Inflight it would be rare but probably not impossible with extreme handling.

Here's an example of that.

[Dai Whirlybird]

Slight creep off topic, ie this is a three bladed machine, (Cabri G2) but nonetheless it's a powerful image.

I believe anecdotally that this was the result of a night EOL, but I am quite happy to be corrected.

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/650x366/cabri_f3b587d2e6d7558829ac6a75c4da150325620661.jpg

[albatross]

Nice clean cut!
Kudos to whomever sharpened the blades.

Slight creep off topic, ie this is a three bladed machine, (Cabri G2) but nonetheless it's a powerful image.

I believe anecdotally that this was the result of a night EOL, but I am quite happy to be corrected.

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/650x366/cabri_f3b587d2e6d7558829ac6a75c4da150325620661.jpg

[ShyTorque]

Judging by the registration, I think it probably did!

[Blackhawk9]

UH-60M has a problem with tail boom strikes doing tactical landings the UH-60A/L didn't have , the A/L had metal blades with the sweep at the tip of the blade flat , when the Black Hawk was thrown on the ground the stiffer metal blade still clears the tail, the newer UH-60M with new wide cord composite blades has a swept tip that angles down and are a softer blade , if you throw a UH-60M onto the ground with the same vigour as a UH-60A/L you will have a blade strike on the tail , pilots coming from older metal blade versions have to learn a new less vigorous tactical landing in the UH-60M.

[SASless]

BH9.....perhaps John Dixson can offer some perspective on that as he has provided very useful information re the Blackhawk in the past and is certainly quite knowledgeable about the Aircraft and its design and flight characteristics.

I will send him a Jingle and tell him of your post.

[JohnDixson]

The new composite rotor with anhedral tip was tested in the 1990’s, and yes the word “ anhedral “ means that an outboard tip was tilted downward a few inches. The hover performance improvement was fairly substantial. So sure, if you hamfisted the landing, and have the stick near or on the aft stick stop while dropping the collective to hit the ground with some vertical rate, you’d have less clearance and are asking for trouble. Not to mention that if you are hot coming in, the right way to handle it is to put the tail wheel on the ground and use the rotor to kill your speed*. I should note here that for those reading that have no UH-60 background, there is a mechanical control mixing unit installed. One of several functions relates to limiting aft cyclic range with increasingly lower collective position. Can one defeat the built in protection? There were a couple of instances with the original blades in the US Army Usually related to something else going on ( my personal but limited knowledge ), but the normal ( incl.maybe a bit hotter than normal ) tactical landing isn’t a problem.
*in fact, this is the reason why there is a tail wheel on the ship in the first place.

[albatross]

John:
Any truth to the old tale that. the engineer who designed the mixing unit for the 76 ended up in deep therapy?
US after looking at the mixing unit on display at FSI WPB: “Can you explain how this all works?”
FSI Instructor of great knowledge and experience: “NO!”
We all laughed at that one.

The new composite rotor with anhedral tip was tested in the 1990’s, and yes the word “ anhedral “ means that an outboard tip was tilted downward a few inches. The hover performance improvement was fairly substantial. So sure, if you hamfisted the landing, and have the stick near or on the aft stick stop while dropping the collective to hit the ground with some vertical rate, you’d have less clearance and are asking for trouble. Not to mention that if you are hot coming in, the right way to handle it is to put the tail wheel on the ground and use the rotor to kill your speed*. I should note here that for those reading that have no UH-60 background, there is a mechanical control mixing unit installed. One of several functions relates to limiting aft cyclic range with increasingly lower collective position. Can one defeat the built in protection? There were a couple of instances with the original blades in the US Army Usually related to something else going on ( my personal but limited knowledge ), but the normal ( incl.maybe a bit hotter than normal ) tactical landing isn’t a problem.
*in fact, this is the reason why there is a tail wheel on the ship in the first place.

[JohnDixson]

Hah-I haven’t heard that one.
A related anecdote from the UH-60 basic controls design: the lead designer for the UTTAS ( original name for that competition ) was a relatively new, quite intelligent young engineer, who forced thru a design that had fixed length control rods throughout. Thats the way to save on cost and maintenance man-hours said he. Chief Engineer on the program bought in ( although the old head chief of that area in engineering was dead against ).
Scene shifts to the experimental build-up area, where the initial three prototypes were in the final stages of assembly and pre-flight test procedures. BUT…..hold on,…..we couldn’t get any of the three ships controls rigged to specification and all those fixed rods were chucked in place of adjustable rods.
As to mechanical mixing units in general: necessary in the “ old days “ in order to get decent control harmony and the top deck of the 92 is but one example of why fly by wire brings both flying qualities and safety improvements thru getting rid of all that clap-trap and the opportunities for human beings to make rigging and/or maintenance errors.

[212man]

Any truth to the old tale that. the engineer who designed the mixing unit for the 76 ended up in deep therapy?

That’s what I was told when I did my conversion in 1994, so I guess it has some basis!

[JohnDixson]

Getting old and had to ask for a reminder from Nick, 212. The main rotor servos on the 76 were placed in a not normal position for other reasons, and the solution to obtain control orthoganality ( choice of words questionable ) was to add another bar in the mixer to account for that. So yes, the 76 mixer has one more function than the usual. Messrs Bursteiner and Irons, the designers, did not require a shrink, but gained some new respect for the accomplishment.

[Al M]

The static stiffness of the blades is likely irrelevant. Once centrifugal force is applied due to rotor rpm the blades become really stiff.

[JohnDixson]

The static stiffness of the blades is likely irrelevant. Once centrifugal force is applied due to rotor rpm the blades become really stiff.
Yes to that. Soon after signing in at SA in 1966, one of the senior handling qualities engineers, Dick McCutcheon was talking about this subject and related some findings in a test done on an S-58, to wit, that the tip response to a collective input would result in a transient overshoot of the tip path, relative to the new tip path trajectory, whereas a cyclic input did not. That sounds counter-intuitive doesn't it and when I remonstrated with Dick, he gave me the legendary Igor Sikorsky advice about what to do when the facts and theory disagree!