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Dangerous Oscillations on R44


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#1 r44 in Chile

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Posted 08 August 2010 - 15:12

Yesterday I experienced one of the most fearful sensations as a pilot when flying my recently overhauled R 44 Astro (85 hrs since last OVH done in the Factory, 1.700 TT- built Sept 1996).

10 minutes after Take off, (2.000 feet ASL field, 10 knots head wind, 50 degrees Farh., 25 inches manifold for less than one minute, then 23 inches, 4 passengers 165 to 187 pounds, 150 liters (40 gallons) of fuel) - I felt a sudden “bump” or head up, head down movement, similar to bucking on a horse (just one) giving me the sensation that one of my rear passengers had jumped on his seat. I asked my passengers if anyone had moved, but they have not moved.

I continued flying assuming that the abrupt fore – aft movement would have been caused by turbulence. During the flight I noticed vibration on my pedals that felt a bit stronger than I was used to. I felt it as a soft tickle sensation on my feet. I did not pay much attention to that as the helicopter had been tracked just before the flight and the report from the service station was that they found the fan vibration near its maximum limit (they corrected that) and all other parameters, main rotor and tail rotor were normal. (The reason I had this Vibrex done at 80 hours SLOVH was because on my previous flight I began feeling a rougher vibration on my pedals and a rougher ride related to vertical vibration like main rotor.)

10 minutes after the first “bucking” , flying at 2.500 feet, , 100 knots, I found increasing clouds and reduction of visibility so I began a quick descent by lowering the collective to reach 1.000 fpm maintaining my speed of 100 knots making a right turn towards a clearer area. During the descent the helicopter began oscillating fore and aft, nose up, nose down at a heart rate frequency, one per second. First with an angle of approximately 10 degrees increasing rapidly to more than 20 or more perhaps. I had the cyclic frozen, meaning that I did not try to compensate the oscillation as it was to fast to try to attempt to do anything to stop the oscillation. I felt for a moment that the helicopter was completely out of control, I was sure that the main rotor would soon struck the tail cone or the cabin and we would all kill as we were 1.000 feet above the ground. I did not panic, but instead I concentrated in what I could do to stop this. So I reduced my rate of descent to 500 fpm by pulling back slowly the cyclic reducing speed from 90 to 70 knots approximately.

I continued the right turn as I found clear and flat space to land. The oscillation increased to an estimation of nearly 30 degrees head up and down during most of the descend, even when I reduced vertical speed to 500 fpm). When approaching the ground for normal headwind landing (5 knots steady wind) , I reduced to 60 knots my speed, applied collective, and the oscillation finished and I could land safely.

After one hour recovering my senses and checking that everything was normal, called my service center and other experienced R44 pilots and they all thought it could be turbulence. I was sure, that was not the case, as I would have experienced a rough ride, but my descent slope was steady and completely normal except for the fore aft oscillation. So I tried a test flight with only one passenger and the helicopter was completely normal. I then decided to continue the flight to my destination, as I could not leave the helicopter and my passengers in the middle of the country. The rest of he flight was at a maximum ground altitude of 500 feet and a maximum speed of 90 knots. I did not feel the oscillation again but I continued feeling the itchy vibration on my feet and a relatively rougher ride than when just out of the OVH.

I have read that this oscillation has been given the names of “bucking” or “chugging” and that pilots in panic have destroyed their helicopters trying to land immediately. I have read that the factory does not have an answer for this situation, and it does not appear on the pilots manual or on a Safety Tip or alert.

Can any body comment on this extremely frightening experience I just survived? Until I do not find a reasonable explanation, and my mechanics discover any logical cause, I will not fly again as I lost all feeling of security that I have build after 13 years flying my R44.


Some information:

Pilot experience 56 years -1.100 hours as PIC and more that 3.000 on helicopters, more than 800 on R44, plenty of recent flying experience –
Loads: Pilot 75 kilos (165 pounds)
Front left seat passenger 85 kilos (187 pounds)
Rear right side passenger 80 kilos (176 pounds)
Rear left side passenger 80 kilos (176 pounds)
No bags or cargo under seats

Fuel : 40 gallons, ¾ on main – ¾ on aux

Outside temperature 10 Celsius – 50 Fahrenheit
Cloudy, 2 mile visibility in part of the route, (CAVOK on destination), 5 to 10 knot head wind, 3.000 feet hills on side of very wide valley.
2.500 feet ASL

Sergio from Chile
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#2 DanceswithCyclic

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Posted 08 August 2010 - 23:31

Trust your instincts, what you experienced was not turbulence. It is something mechanical, but I don't know what.

I also know someone from this forum will have good advice on where to start looking for a cause. Check back in a few days, for as I write this it is late on a Sunday nite.

Please post when the cause has been found.

Kevin M
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#3 Eric Hunt

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Posted 08 August 2010 - 23:49

Not an R44 pilot so I don't have the figures, but was your CG in limits?

Sometimes an oscillation like that is due to CG being outside the envelope.

#4 Goldy

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Posted 09 August 2010 - 00:45

Not an R44 pilot so I don't have the figures, but was your CG in limits?

Sometimes an oscillation like that is due to CG being outside the envelope.


I have never experienced anything like that in a 44. However, you would be at the upper limit of weight and although you would be nose heavy you would be within CG, depending on what the dry weight of your ship is. With a basic weight of 1468 (which is the ship I fly) you would be 12 pounds over max. However, that is based on a Raven, I dont have any approximate Astro weights.

So yes, you were probably pushing limits, but that oscillation is not anything I have ever heard of when overweight.

Goldy

Fly Safe !!

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#5 r44 in Chile

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Posted 09 August 2010 - 16:46

Hello
You are close to right as I was 1 pound over. 2401 TO weight. My CG was at ist limit but I have flwon many times max weght and similar CG and never happened before. I have contacted the Factory and they are giving it a thought.
Sergio from Chile


I have never experienced anything like that in a 44. However, you would be at the upper limit of weight and although you would be nose heavy you would be within CG, depending on what the dry weight of your ship is. With a basic weight of 1468 (which is the ship I fly) you would be 12 pounds over max. However, that is based on a Raven, I dont have any approximate Astro weights.

So yes, you were probably pushing limits, but that oscillation is not anything I have ever heard of when overweight.

Goldy



#6 r22butters

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Posted 09 August 2010 - 18:10

I cut an R44 flight short once due to a lateral oscillation. It definitely felt mechanical to me (like a bad mag) as I suggested, however everyone there (a group of about 6 Cfis) insisted that it was just LTE. I decided not to fly again until the mechanic could check it out, however the next group decided to still fly. I got a call later that it stalled over the taxiway, just before takeoff. The next day they replaced a mag. <_<

Definitely trust your instincts, no matter what anyone else says!

I understand how you can be under gross weight, and still have the CG too far forward, but I don't know how that would cause the nose to "buck"? :huh:

I will definitely "keep my eyes open" the next time I fly an R44.

Edited by r22butters, 09 August 2010 - 18:14.

The only dream I have left is to live long enough to see the pilot shortage. Its been about fifteen years since they first told me it was coming, so,...

Aaaaaaaany day now! :D

#7 Eric Hunt

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Posted 09 August 2010 - 19:04

LTE???

Frank Robinson would tear your throat out, and so would Nick Lappos, formerly the chief test pilot for Sikorsky.

LTE is almost exclusive to the B206 with the small tail rotor, but their Spin Team has got everybody convinced that every helicopter can get it. Keeps them out of the spotlight.

And Frank Robinson is a tail rotor specialist, so to accuse one of his babies of being subject to LTE is a mortal sin.

#8 Goldy

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Posted 09 August 2010 - 23:02

LTE???

Frank Robinson would tear your throat out, and so would Nick Lappos, formerly the chief test pilot for Sikorsky.

LTE is almost exclusive to the B206 with the small tail rotor, but their Spin Team has got everybody convinced that every helicopter can get it. Keeps them out of the spotlight.

And Frank Robinson is a tail rotor specialist, so to accuse one of his babies of being subject to LTE is a mortal sin.


You said it before I could. You won't run out of tail rotor authority on a 44 or even an R22. The wind might move your tail around, and some pedal will move it right back.

Fly Safe !!

Goldy-CPL(H),R22A, HP, B, BII, R44 Astro, R1,RII,R44ClipperII, R66, B47G2, S300C, S333, B206B3, DG500, RV10, E480B, AS350BA, S-58T, what next?

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#9 MileHi480B

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Posted 09 August 2010 - 23:51

This is very troubling. And THIS is exactly why aviation forums can save lives. With a few strokes of a keyboard and clicks of a mouse, we can get the views of pilots across the world.

Thanks for posting this. I own and fly an R44. I am relatively new to the Robbie (just under 200 hours) and have not experienced anything like that.

I applaud you for your decisive action.

I anxiously await other responses from pilots and mechanics more knowledgeable than me. Meanwhile ... if you find the answer please POST IT!

Regards,

Tom

Edited by MileHiR44, 09 August 2010 - 23:51.


#10 Mungo5

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Posted 10 August 2010 - 12:17

A yaw oscillation is easily explainable in terms of the tail rotor, but a fore and aft oscillation isn't. I can't offer a solution, but it has got me thinking about what was actually going on at the time.

If you think about the mechanics of what would actually cause the hull of the aircraft to oscillate fore and aft, that's quite an aerodynamic or mechanical force.

If the force to move the hull that way is coming from the rotor head, it's being transferred down the control rods, which means the disc is doing some dramatic movement itself. Remember gyroscopic precession means the action is applied 90 degrees after the force, so you'll be hunting for something that is acting on the sides of the disk.

Interesting that it started during the descent, which would imply a lower blade pitch and low power setting - sounds like a blade could at a different pitch to the other possibly.

Did you notice anything happening to the RRPM during all of this?

#11 Marc D

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Posted 10 August 2010 - 12:26

LTE???

Frank Robinson would tear your throat out, and so would Nick Lappos, formerly the chief test pilot for Sikorsky.

LTE is almost exclusive to the B206 with the small tail rotor, but their Spin Team has got everybody convinced that every helicopter can get it. Keeps them out of the spotlight.

And Frank Robinson is a tail rotor specialist, so to accuse one of his babies of being subject to LTE is a mortal sin.



All helicopters with a tail rotor can and do get LTE-Loss of Tailrotor Effectiveness. Most just overcome it successfully.

#12 Eric Hunt

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Posted 10 August 2010 - 17:00

Marc D, don't confuse Effectiveness with Authority.

Any helicopter can run out of pedal Authority, when the pilot demands more from it than it can provide. A thoughtful pilot will always be aware of how much left pedal is being used, and when he approaches the stops, he will gain some forward speed and/or reduce power. Common sense.

Losing Effectiveness means that the tail snaps away when you are nowhere near a pedal limit and is unexpected. This one has now been taught to so many generations of 150-hour instructors who pass it on to the students 150 hours behind them, that it has become part of the helicopter psyche, and that is what Bell wants. Even the FAA says that LTE is possible in any helicopter. Urban myth, propagated and believed without question.

#13 IFLY

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Posted 10 August 2010 - 17:34

Thanks, makes more sense now and I'll change how I teach it.

Jerry
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#14 r22butters

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Posted 10 August 2010 - 17:44

LTE???

Frank Robinson would tear your throat out, and so would Nick Lappos, formerly the chief test pilot for Sikorsky.

LTE is almost exclusive to the B206 with the small tail rotor, but their Spin Team has got everybody convinced that every helicopter can get it. Keeps them out of the spotlight.

And Frank Robinson is a tail rotor specialist, so to accuse one of his babies of being subject to LTE is a mortal sin.


I had 6 Cfi's who were positive it was LTE (yes effectiveness, not authority) due to mechanical turbulance, but I guess they haven't been to your blog? :blink:

Its ok though, I didn't belive them either. B)
The only dream I have left is to live long enough to see the pilot shortage. Its been about fifteen years since they first told me it was coming, so,...

Aaaaaaaany day now! :D

#15 Eric Hunt

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Posted 10 August 2010 - 21:07

Not my blog - just Google "helicopter urban myths" and it will direct you to Another Helicopter Forum which sounds like pea proon and read the whole story.

Nick blows the rest of the myths out of the water, makes great reference material.

#16 captkirkyota

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Posted 11 August 2010 - 08:30

With my limited experience I will ask this to provoke another line of thought toward possibly solving your problem. Someone made a comment about the fore and aft "bucking" problem your having that makes me doubt my direction here but it could not hurt.

After doing whatever overhaul work, did the mechs. re-weigh and balance your A/C ? It is possible that the CG is now different and the weight limits may also have been effected. Whether or not that could cause your problem or not could be in question, but checking the weight and CG is something you may want to re-check anyway.
I am also intrigued about the possible diff. pitch between blades suggestion, do you know if just one blade was changed, or if both were replaced were they in fact a matched set?
Glad to hear you retained your wits and got out as a successful story, please keep us all posted.

#17 Marc D

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Posted 12 August 2010 - 11:06

Marc D, don't confuse Effectiveness with Authority.

Any helicopter can run out of pedal Authority, when the pilot demands more from it than it can provide. A thoughtful pilot will always be aware of how much left pedal is being used, and when he approaches the stops, he will gain some forward speed and/or reduce power. Common sense.

Losing Effectiveness means that the tail snaps away when you are nowhere near a pedal limit and is unexpected. This one has now been taught to so many generations of 150-hour instructors who pass it on to the students 150 hours behind them, that it has become part of the helicopter psyche, and that is what Bell wants. Even the FAA says that LTE is possible in any helicopter. Urban myth, propagated and believed without question.


I just read all the recommended reading. Change the names as you will---but we're talking about the same thing(I mean that in a good way). As I have always said, "All single rotor helicopters get in to LTE, but only some have a problem with it because their T/R is inadequate to compensate." According to yours and Mr. Lappos definition, you would simply say, "well, then you didn't get LTE---if you were able to compensate for it with enough t/r authority." Fair enough. I see what you are saying. I have done lots of explaining over the years myself. There is a "fear" of LTE out in the ranks because of this lack of understanding that Mr. Lappos talks about. He is right, I've help dispel the fear myself. I have just jumbled the words a little differently, with the same end result. I guess it could be better stated---All single rotor helicopters(excluding notar) get into the conditions favorable to LTE, but most are able to overcome the potential problem and simple fly away. They can do this because they have adaquate T/R autority to do the job required.
Thinking of the FAA AC about LTE---All the conditions presented in the AC would be considered "normal" to all flight operations, and therefore will not be deemed a problem unless your T/R is inadequate to over come the situation.
How's that. I think we are on the same page?

Marc D.

#18 dooly3006

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Posted 14 August 2010 - 01:11

Sergio

What you experienced sounds like a perfect example of "chugging" which can occur in the R44 at high gross wieghts. Do a search on the NTSB accident website for Robinson helicopters in Alaska, there was an R44 that had the same problem and damaged the helicopter while making a precautionary landing in Iliamna, Alaska. It seems that "chugging" usually occurs at high gross wieght, and most commonly forward CG, while reducing power in my experience. The most important thing to remember with chugging is to resist the natural reaction to reduce power, you actually need to apply power and level out. Try to make small inputs with the collective and make a flatter approach path if your helicopter is loaded so that it is susceptible to chugging.

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20090513X60228&key=1

This link will give you a more detailed description of the flight condition and why it happens. The NTSB database can be a great source of information. If you do an accident query for the specific helicopter that you fly you can see a pattern unfold for how most accidents happen in that helicopter. I fly an R44 myself and learned about "chugging" here before I experienced it, which prepared me to make the correct decision about how to handle the situation when it did happen. I have also noticed a lot of accident attributed to power management in the R44, which has caused me to be acutely aware of my power at all times and make my approaches accordingly.

I hope this experience will not discourage you from flying more in the future. The most important thing is that you did not panic, you thought through the problem, found a solution, and made a safe landing.

Safe Flying :)
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#19 Trans Lift

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Posted 14 August 2010 - 14:30

Thanks for that Dooly, you could have just saved someones ass by putting up that info!

#20 Marc D

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Posted 14 August 2010 - 17:07

Sergio

What you experienced sounds like a perfect example of "chugging" which can occur in the R44 at high gross wieghts. Do a search on the NTSB accident website for Robinson helicopters in Alaska, there was an R44 that had the same problem and damaged the helicopter while making a precautionary landing in Iliamna, Alaska. It seems that "chugging" usually occurs at high gross wieght, and most commonly forward CG, while reducing power in my experience. The most important thing to remember with chugging is to resist the natural reaction to reduce power, you actually need to apply power and level out. Try to make small inputs with the collective and make a flatter approach path if your helicopter is loaded so that it is susceptible to chugging.

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20090513X60228&key=1

This link will give you a more detailed description of the flight condition and why it happens. The NTSB database can be a great source of information. If you do an accident query for the specific helicopter that you fly you can see a pattern unfold for how most accidents happen in that helicopter. I fly an R44 myself and learned about "chugging" here before I experienced it, which prepared me to make the correct decision about how to handle the situation when it did happen. I have also noticed a lot of accident attributed to power management in the R44, which has caused me to be acutely aware of my power at all times and make my approaches accordingly.

I hope this experience will not discourage you from flying more in the future. The most important thing is that you did not panic, you thought through the problem, found a solution, and made a safe landing.

Safe Flying :)


Boy, you really think that that info would be more widespread? Is it in any service letters yet? Or in the POH? Mentioned at the RHC class?

MD




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