Collective Bounce

[snowedin]

Interested in knowing more about this...have heard of it happening to a few guys, interested in hearing from others who have experienced it, what caused it, and recovery...

 

cheers

[VTA]

Happened to me many moons ago, just started flying Sea Kings single pilot IFR. Happened at 4000ft in cruise 90kts (Brit' Sea Kings AEW). I thought (at the time) that you had to have all the collective friction off before engaging the Barometric Height Hold, and accidently hit the collective as I reached down. The ensuing "accelerating" oscilations of full collective travel, scared the s--t out of me before my hand finally caught up with the collective!!

[cap]

Most common cause------releasing the load (usually a heavy one) with tension still on the cables or lanyard. What happens?........you pull up on the collective and the a/c goes down.......you push down on the collective and the aircraft goes up. The next sound you hear is the panels "oil canning". By the time you hear that, get your 40 second rosary beads out because thats all you got time for if you are anywhere close to the terre firma. CURE: get that nose down quick and get airspeed NOW! GUARANTEE: if it happens to you once, it will never happen to you again....alive or dead. ONLY THING REMOTELY SIMILIAR: real bad ground resonance on a log pad or bad plywood pad while flying an Allouette or Lama.....also carry some 40 second rosary beads for that occasion also, if you ever fly one that gets you into that situation. The former is usually caused buy the "loose nut between the collective and the cyclic" and will usually happen on a Bell medium of some description. The latter is usually caused by bad dampners on the blades and/or bad shock struts on the latter two types of aircraft.

 

 

Don't wish to have the experience?........then slow down and quit jerkin' that collectrive all over the place.

[snowedin]

was curious to know whether it was pilot induced...and by the sounds of it cap it most definately is. :shock:

 

thanks for the input...

 

cheers

[skullcap]

I have heard that it can happen relatively easy on Bell medium if the built in friction on collective is not set correctly, ~7lbs I believe, and can be induced as CAP says by loose link in seat.

 

At canadian 10 12 years ago there was an aircraft 350B that got into a similar situation whereby the pilot input a sharp drop in collective which caused the blades to go into an out of track situation. The pilot put out a mayday call was so violent. The could reproduce the occurance but no matter what maintenence was done could not eliminate it. They talked to eurocopter and they said that a similar incident occurred on ONE other aircraft and that it was PIO or pilot induced occilation, and the correct method to get out of it was to let the collective go as the geometry of pilot's arm, seat, together with the sharp input caused the collective to be continually moved to a point the blades went out of track. To prevent the occurence, and this made the pilot wild, was to change pilots. I almost pissed my pants when I read that.

 

The allouette 2 used to have a collective bounce but only when overloaded so they tell me.

 

As CAP says be careful with collective and not move it around like a monkey and should be no problem.

 

sc

[VERTICAL REF]

Skully is on the right track with regards to the mediums. For the 205 and 212 the min "built in" collective friction is set at 14 lbs. Then the pilot has the option of adding more friction with the friction knob! It is pretty much impossible to experience collective bounce if the mechanical friction is set properly! If you experience it get the engineer to check/reset it right away. They are a clamp friction and do experience wear after time.

 

I disagree with caps most common cause! I release hundreds of logs a day all under tension and have never experienced collective bounce under those conditions! Different strokes for different folks I guess!

[rainman]

Hey Vertical Ref, by chance are you a 214 driver, or 205/212? Just curious?

[Skids Up]

In my limited experience with the Bell mediums, I have always understood the collective bounce is a result of low collective friction, plus the aerodymanics that I don't fully understand.

 

In Ontario last year, a 204 got into it while (can't remember all the details) loading /unloading a crew when it started the dance. The driver reported that the collective was going from 'stop to stop' and he was only able to 'follow the machine' around the swamp trying to the best of his ability to avoid the ground crew! When he finally arrived over a place he deemed suitable to get it on the ground he rolled off the power and said the "collective bounce" quit as soon as he did.

 

Good job done, but it was found that the preset collective friction was low, and no other problems with no damage done. :up: :up:

 

That is tucked into the back of the mind as a possible solution should it ever happen.

 

I had in my first year, an experience that I thought was a collective bounce, but the collective wasn't moving! The machine (205), with a sling load of diesel on the long line, started bouncing up and down quite rapidily. To the point of thinking seriously about punching the load. Hung on for the last 15 feet till the net hit the ground and it stopped. :shock:

 

Found it to be a combo of things: poor tranny mounts, loose cyclic friction, and a pilot induced fore and aft motion on the cyclic which just kept getting worse as it started to shake and my arm got into the "motion". (That doesn't sound very good ) The solution, after talking to few other pilots was to increase the cyclic friction to help prevent it from occuring again which it did. You could sometimes feel it starting, but just wouldn't build to the extent it did.

[cap]

VR ------then you haven't operated with a lanyard(s) that are like rubber bands. My preference are web or wire rope lanyards for that reason. Also, if you are logging, you ain't "hot-diggin' into some landing spot with the load, dumping the collective or making large, quick downward movements and picklin' the load NOW. Being swift and "hot-doggin" ain't the same and you know it. Additionally, you're on a long line and if/when that day should come, you have a built in "out". If you are drivin a 214, then with those big blades, she'll be bouncin' like "beJesus". The fact you haven't had one says more about you as a pilot than it does the aircraft you fly or the equipment you use.

 

 

Mine occurred while slinging about 8 drums of fuel and was caused by a "major brain cramp" because I knew better. I was also in a large hole and tall timber and had some room and height to get some airspeed QUICK. Bear in mine also, that the load had not completely settled on the ground.. The nose went down, I got some airspeed and by the time I ran out of room and ideas, I was able to pull up out of the hole with some speed on and the collective had gone "friendly" by then.

Anything that was not screwed or lashed down on the inside was all over the place and if my seatbelt and inertia belt would not have been fairly taut, I wouldn't be telling you this now because I'd have lost control. It's a strictly up and down motion, but feels as though you have hit the ground very hard, bounced up into the air and struck something overhead very hard and then down again before you can blink, to do the same thing all over again. You don't have time to say or think "what the Jesus" or figure out what you are going to do. You have seconds to react and if the panels start "oil-canning", then that means she's starting to come apart. I witnessed the same thing from the outside one time and the upward and downwards cone of the blades I will never forget....everything gets exaggererated as far as the violence or movements are concerned. In that particular case a brand new 3/4 T truck was pickled from about 50'. Bottom line: load up the disc well back, let'er talk to you as you're goin' in and put the load on the ground or wherever the way you were taught to do. Such wasn't the case with me, but if some newly converted driver that used to "hot-dog" on occasion with Lights or Intermediates tries the same with a medium then he'll also "get to dance with the Black Dog" sooner or later. It's like sex gentlemen....you can talk about it all you want, but until you experience the act, you don't REALLY know. Don't get the idea that you got too much experience for this to happen to you either. I was no "green horn", knew better, but just tried to get one more load in before dark which made me move a "tad" quicker than I would have normally. There's the "set-up" for a whole host of things that can happen....the rest you know.

[VERTICAL REF]

You are correct Skids up,

 

One of the first thing I do when taking over an aircraft, during the power check and performance check, is to check the minimum friction! After that is complete, I personally fly with almost full friction on the cyclic and collective - especially in the single hydraulics machines. Just ask the boys that crew me out and do the first take off without adjusting the friction to their own preference!

 

Tiring transmission mounts and a "jittery" cyclic can also lead to "pylon rock" which feels like a **** of a combination of vertical and laterial going on. When the mounts are really bad, just an authoratitive pull of collective alone will get things rocking! Time to change some parts...