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Dangerous Oscillations on R44


r44 in Chile

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The NTSB says the manufacturer recommends the addition of power to counteract the oscillation. Good to know stuff!

 

 

This issue was addressed when I was at the Robinson Safety course last year. It only occurs in a certain range of serial numbers of the R-44. They found that the fore/aft oscillation was due to a certain type of rubber transmission mounts that were used and only happens at a certain rate of descent when flying over 100 knots. I can't remember all the details, but that's what it is.

 

The 2009 report below issued some information. Transmission mounts as stated above:

NTSB Identification: ANC09GA040

14 CFR Public Use

Accident occurred Tuesday, May 12, 2009 in Iliamna, AK

Probable Cause Approval Date: 04/22/2010

Aircraft: ROBINSON HELICOPTER R44, registration: N7196H

Injuries: 3 Uninjured.

The National Transportation Safety Board determines the probable cause(s) of this ACC as follows:

 

The main rotor transmission mount design, which resulted in an in-flight vibration/oscillation, and damage to the helicopter during an emergency descent and hard landing. Contributing to the accident was the lack of information from the manufacturer regarding this known flight oscillation, and loading the helicopter beyond the forward center of gravity limit by the pilot.

 

The IIC contacted Robinson Helicopter, and asked if they were familiar with the term "chugging" as it related to Robinson helicopters, particularly the model R44.

 

During a telephone conversation with the manufacturer's accident investigator on May 27, the investigator said that he was familiar with the phenomenon known as "chugging," and that the manufacturer had conducted flight tests related to the phenomena. He said the tests determined that an oscillation may develop during operation of the helicopter at high gross weight, about 90-100 knots, and that the oscillation was more of a "bucking" motion due to the fore-and-aft movement of the rotor mast. He said that the tests showed the tendency to enter the oscillation regime was exacerbated by a forward CG (within the CG envelope) and a 30 degree banked turn to the left. He further indicated that the helicopter may also begin to oscillate in a right turn, but it entered the oscillation regime more easily in a left turn. According to the investigator, the tests also showed that chugging could occur within the normal CG range, and most typically at or near a gross weight condition. According to the manufacturer, it was determined that the oscillation is not divergent (destructive to the helicopter), and that the helicopter can be landed safely. He said that the oscillation can be mitigated by the application of power, and the condition flown out of. He said that the only damage he had seen to the test helicopter was on one occasion, when the pilot landed while the helicopter was still "bucking," and that the damage was manifested as dents/marks on the cabin-top, made by the fore-and-aft movement of the main rotor-shaft fairings.

 

The Robinson investigator said that the manufacturer attributes the oscillation to the firmness, or lack of firmness, of the transmission mounts, and that the manufacturer changes the mounts on helicopters that exhibit a tendency toward chugging during post manufacturing test flights. The investigator said he did not know the standard by which mount firmness was measured. He said he believed that approved helicopter service centers were aware of chugging, although currently there are no service alerts/bulletins referencing the phenomena. The investigator said to his knowledge there were no factory provided alerts/bulletins, pilot training, or Pilot Operating Handbook (POH) entries, pertaining to the chugging and the remedies to resolve it.

 

http://www3.ntsb.gov...513X60228&key=1

Edited by iChris
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WOW !

 

I guess I have just been lucky. Until this thread I have never heard a word about this, and was flying an R44 (or two). I was often at max gross, and due to my physique, usually leaning toward being front heavy. The R44 likes to fly around 100-105 knots, so I guess maybe I never made a left turn?

 

I'm glad they mentioned it at the safety course a year ago. Last I attended was about 5 years ago, so maybe when I go again it will come up.

 

Not knowing any of this, I think my instinct would have been to slow the ship down, which usually means a reduction of power..sounds like that may not have ended well.

 

Thanks again to the OP.

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Unbelievable. He builds em, requests a special FAR to fly em, requires a safety course to be in compliance with "purchase contract" and then makes new owners agree not to sell to anyone unless they also take the course, then sticks owners with all kinds of mandatory "upgrades" based on lawsuits filed against him, but somehow in all of this paranoia he manages NOT to inform owners of this potentially lethal "quirk".

 

Nice going Frank. Your company's inconstancies in the market place are only preceded by your arrogance.

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Confirmed my suspicions this morning: current Safety Notice as published by Robinson on their web (for R-44 &-22) are SN-31 & SN-34; neither is regarding the "Chugging" or "Bucking".

 

WTF?

 

Might need to give Mr. Tucker a call.

 

-WATCH FOR THE PATTERNS, WATCH FOR THE WIRES-

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  • 1 year later...

Well, apparently the R44 is not alone in being susceptible to this condition. The B206 can also get into it although it was dubbed "pylon whirl" instead of "chugging". Basically, if you are flying a teetering system, which all have tall masts, then there is a possibility it can happen at high fwd cg and high cruise speeds.

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Honestly, I don't know if we know enough about it to predict exact conditions upon which it can occur. It does not happen very often at all, but from what I have been able to find, it happens at high cruise speeds at or near max gross in the R44. I also know that it should absolutely be avoided in the B206 because when cruise speed is over 80 knots, you are limited to 85% torque (generally near max gross in a 206 you are around 80% anyway) or you will twist the mast, so if you start getting pylon whirl, the corrective action being to pull more power, you might go from one bad condition to a worse one.

 

Going back to the R44, I suppose there could be other scenarios that lead to chugging, considering it is usually due to dry-rotted transmission mount absorbers, my best guess would be that anything that gets the mast going in a pendular fashion might get you into it.

 

Some interesting links to check out regarding the subject:

http://www.pprune.org/rotorheads/461705-r44-mast-rocking-aka-chugging.html

 

http://www.ainonline.com/aviation-news/aviation-international-news/2011-10-04/robinson-working-faa-r44-mast-rocking

 

http://www.robinsonhelicopters.org/index.php?option=com_jfusion&Itemid=53&topic=1053.0

 

http://aircrashed.com/cause/cANC09GA040.shtml

 

There's a lot more out there. Needless to say many many operators have never heard of this, or did not know what was going on if they experienced it. Reports started as isolated, but as the R44 community becomes more aware of the issue, more pilots are stepping forward and reporting incidents where this has happened. Robinson's reaction to this has been less than stellar in my opinion. I would think that this should be blasted at high volume so that pilot's can avoid it, and trained in recovery procedures. I think they are coming around now, but only after the FAA and many concerned operators have made a stink about it. They also just settled a huge lawsuit on a fatal crash that may have been related to this. In typical Robinson fashion, someone needs to die and they need to get sued before they address an issue. Maybe that's not completely fair to single out Robinson for this, as it seems to be the standard for most manufacturers these days, but they seem to have more issues than other manufacturers.

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  • 3 years later...

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