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Bump in the Sky


MileHi480B
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The strangest thing happened to me when taking off today in my Enstrom 480B.

 

I lifted up, made my way east down a taxi way, then when I had sufficient height to clear light poles, I turned left over a large hangar to head north. As I cleared the north side of the hangar, I heard and felt a slight bump -- very, very quick and very faint. It felt like it came from the rear area of my left skid. Almost like I scraped something in the air.

 

The helicopter's nose veered a few inches to the right and back again (almost instantly). I felt it ever-so-slightly in the seat of my pants but felt nothing in the cyclic.

 

Before and after that "bump" everything looked and sounded normal. The chopper ran smooth as glass.

 

I immediately landed and inspected the helicopter. There was not one dent, ding, scratch or even a rub mark anywhere. I checked high (rotors) to low (skids). I checked every square inch of every surface and inside the engine compartment. I found nothing unusual. The luggage compartment was empty -- so no shifting there.

 

Even though I knew there were no obstacles anywhere near me, I flew back over the hangar. Nothing there. No antennas, no flag poles, no wires, no dead birds. But I would've sworn I hit something.

 

By the way, I was right near the tower when I took off. I asked them if they saw anything unusual. "Negative" was the response.

 

I then went and did a series of touch-n-goes without any problems.

 

So what the heck was it? Here are some possibilities (but I'd also like your input):

 

1. Hydraulic strut may have had an air bubble and released to full extension?

2. Lead/lag piston may have had air in it and adjusted in flight?

3. Engine backfire?

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When i used to fly aerobatics, (in fixed wing) at the end of a loop, (if you did it properly) you would feel a 'bump'. It was flying thru your own wake turbulance. How high were you over the long hangar? and what was your speed? I would't rule this out, or rely on it to be the cause. Loops i used to do were about 500 foot in diameter and took about 14 seconds to complete. Wake turbulance can hang around a while.

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1. Hydraulic strut may have had an air bubble and released to full extension?

2. Lead/lag piston may have had air in it and adjusted in flight?

3. Engine backfire?

 

the nitrogen that is used to charge the strut is "dissolved" in the hydraulic fluid ---so? NO

 

if you had a M/R damper problem? You would have experienced a 'vibration' of some sort ( due to blades becoming out of phase & not in "sync" with the other 2

 

turbine engines dont 'backfire'-----at least not like a piston engine does

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Depending on the wind, etc., it could have been localized turbulence, wake turbulence from another aircraft.

Turbines don't "backfire" but some do have idiosyncrasies.

I've flown some RR/Allison 250s where the bleed valve closing was distinct change in the power. It's not supposed to be obvious enough to cause a noise or induce a yaw change, so you may have an issue building there... or it could just be that that particular power cycle phased in-synch with the valve closing and the pneumatic fuel control was a bit abrupt.

Compressor stalls come in all flavors, could sound like an impact in that airframe(?), but that classically should have been a left yaw. I've been flying French for so long I forgot that in my first post.

And finally, I've hit birds that didn't leave a mark anywhere. A strike significant enough to yaw the aircraft should leave some blood, hair or feathers somewhere.

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Bird remains on a crosstube can be hard to see, but sometimes not. I once came back with a bird, and landed, with the bird still wrapped around the crosstube. I never felt anything, and had no idea I had hit anything until I saw it after shutdown.

 

I don't think it is within the realm of possibility for anyone to say with any certainty at all what caused the bump, if the pilot doesn't even know. We can speculate until the cows come home, and it will all still be just speculation. Maybe an alien force beam that just missed?

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I had almost the same thing happen to me today in Ft Collins while making a northbound approach about 300' AGL with about 10 kt winds and light turb out of the west. Suddenly it felt like I hit a building on my right and immediately stopped all lateral movement! No yaw, no loss of alt, I assumed it was just small wind shear from the east. Never experienced anything like it.

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UPDATE: My wife and 2 kids were with me. My wife said the helicopter's nose did not veer. She said as soon as we felt the bump, I immediately tightened my grip on the cyclic and turned the helicopter immediately to the right and circled to land. Now that I think about it, she's right. There was no movement of the chopper. She also said, the bump was felt but not heard. Kinda like a quick "clunk".

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And how's this for a friend: After hearing my story, they had someone prank call me to say they had an antenna knocked over on that hangar I flew over and they had heard that I experienced a "bump in the sky". They wanted immediate compensation or they were going to the media and authorities.

 

Mmmmm, I got suspicious when he said he would take a credit card, then started laughing.

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turbine engines dont 'backfire'-----at least not like a piston engine does

 

Oh, yes they do! You've never experienced a compressor stall? I had one earlier this year that turned out a nice thirty foot fireball in front of the aircraft. You better believe they backfire.

 

The "backfire" that most call a "backfire" in a piston aircraft, incidentally, isn't a backfire. It's an afterfire. A backfire comes out the induction, and can often separate the carburetor or induction tubes, or set fire to the engine. Afterfires, not so much.

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I would have to say if it was a comp stall you would know it. It may not be as violent with a tiny turbine, but I know when I had a #2 engine on a F-15E stall on me it was bad enough to shake me up. ( of course the jet was tied down and it probably feels different in the air)

 

I still think it was not a compressor stall you experienced.

 

Oh, yes they do! You've never experienced a compressor stall? I had one earlier this year that turned out a nice thirty foot fireball in front of the aircraft. You better believe they backfire.

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"gary-mike" is correct that my guess was wrong in this scenario, a compressor stall would have been a yaw nose left.

But, compressor stalls come in a lot of flavors. I'm only a pilot, but I've had events that were much less dramatic that I've been told were nonetheless compressor stalls by technicians. Compressor stalls in my experience, vary from barely audible "chuffs" (bleed valve issues on 250) while accelerating the engine on the deck, to bang and yaw at cruise power in flight (inlet guide vane failure, also a 250). "Bang and yaw" as the power is suddenly reduced by the compressor stall, causing previous trim anti-torque to become excess pedal, like a power chop.

 

Me, I'm leaning towards the alien suggestion. Or localized turbulence.

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  • 2 weeks later...

Still no clue. But subsequent flights have been perfectly uneventful!

 

I love winter! The power reserve is incredible. I almost feel like I'm at sea level.

 

Mile Hi- I am so jealous, since I have flown the 480B I find myself comparing everything else to it....except maybe an Astar 350. For a big guy, its a very comfortable ship to fly.......check out next months Rotorcraft Pro for a story on it.

 

Goldy

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