Posted: Mon Nov 28, 2011 7:40 am Post subject: BUMP IN THE SKY - WHAT WAS IT?
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?
Joined: Feb 14, 2008 Posts: 888 Location: Stavanger, Norway
Posted: Mon Nov 28, 2011 8:39 am Post subject:
Could be a huge number of things. In the R44 we used to get a bump noise regularly as the engine heated the fairing around it and it would expand slightly and make a noise. Having not flown the 480 I am not in a place to say if it would perhaps have a similar issue.
Compressor stall will make a noise, usually a large thumping one, and will have an effect on Tq and therefor yaw, and if unexpected, the colour of your underwear. If you have not heard compressor stall before think of someone hitting the cockpit with a sledge hammer repeatedly.
It could be something simple like a bit of local turbulence from the hangar you flew over, and it perhaps shifted something in the baggage compartment which made the noise.
If in doubt ask your engineering organisation to put their thinking caps on, thatīs one reason why you pay them. And if still unhappy, have the aircraft inspected.
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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