Big THUMP in flight - What was it??
 

I was on an AA MD-80 a few weeks ago, and we experienced a big thump during cruise. It appeared to come from an engine, maybe one, maybe both, but I think it was just the starboard engine. Along with the thump came maybe a half-second power loss (IIRC) and a definite momentary deceleration - it felt somewhat like what you might experience in a car if you hit a big puddle. Everything was normal after that, and the pilot said it was some kind of turbulence, if I understood him correctly. Can anyone elaborate?

Might have been Clear Air Turbulence.

Could be a range of things. The fact that everything carried on as normal shows how resilient aircraft are to this sort of thing. Could be:

Lightning strike (noise but no real change to the motion).
Compressor stall/surge on the engine. They just do this occasionally and normally recover.
Clear Air Turbulence. This just comes up on you suddenly, rather than anything sustained, often with a momentary loss of aircraft attitude. And as that's what the skipper said, I'd believe him.

Don't worry, the crew are trained for it all. And if they don't, then the autopilot will handle it.

Thanks for the replies. It was definitely unlike any turbulence I'd ever experienced, but I don't think I've ever experienced clear air turbulence before. The weather was clear that day, so definitely not lightning. I guess the closest parallel I can draw is what might happen if you were in a powerboat and hit a small sandbar on the bottom which was small enough to keep the boat on-course, but big enough to make a good thump, and to make your heart skip a beat. It was there and gone, just like that. My wife was a little panicked, but I was mainly just curious as to what it was.

I didn't do it! Honest.

"Uh, folks, we're experiencing some moderate Godzilla-related turbulence at
this time, so I'm going to go ahead and ask you to put your seatbelts back on.
When we get to 35 thousand feet, he usually does let go, so from there on out,
all we have to worry about is Mothra, and, uh, we do have reports he's tied up
with Gamera and Rodan at the present time. Thank you very much."

On a DL flight out of Salt Lake City a few years ago we started hearing regular thumps (like a bowling ball rolling around in the trunk of your car) right after takeoff. Then we noticed that we weren't climbing as fast as usual. Then we noticed that we were circling back. Then we noticed that we were landing. By the time we figured out that we ought to be scared, we didn't have to be any longer.

Turned out that someone hadn't secured the baggage compartment door properly. It opened during flight and cut off airflow into the (rear-mounted) engine on that side, which flamed out. This took away enough pull on the door for it to move away from the engine intake. The automatic restart gizmos worked, after which the engine inflow pulled the door right onto the intake again. And so it went, until the pilot shut down that engine and landed with the other one.

Needless to say, Delta was very nice about reaccommodating everyone.

So that's where my tax refund check went!

AndrewC75,

Your description of a "half-second power loss" sounds exactly like what happens when a power source drops off line in a DC9/MD80. You'll see the same thing during engine start as each engine's generator takes over it's busses from the APU or GPU.

The question is WHY you were getting the power source dropping off line? I really can't tell from your description what might have been happening but it would happen if an engine failed and it would also happen if a generator failed. (Obviously, when an engine fails the associated generator also fails due to lack of rotation) Was the "bump" from tubulance? It would have to be a really big bump to cause problems with the engine or generator.

Efrem,

What kind of airplane were you on? The DC9/MD80 has cargo doors which open inward so they couldn't have been getting in front of the engines. Can't think of an airplane with tail mounted engines and outward opening belly doors.

way back when [late 70s] the 727 aborted because of a loud bang on takeoff. pilot said it was a compressor stall.

LarryJ,
What I perceived as a power loss occured simultaneously with the bump and slight deceleration. The whole thing lasted maybe 1/2 second. The deceleration wasn't enough that bodies lurched - just enough to notice. If indeed this was turbulence, it may not have been a power loss at all... I may be remembering a power loss which was really just a slight change in velocity or attitude. I don't clearly remember the cabin lights or A/C going out. May have - not sure. The pilot said turbulence, so I suppose I should trust that - I was just curious as I'd never experienced such here-one-second-gone-the-next and loud turbulence.

So how long was the line at baggage services to fill out the form for lost luggage? http://www.flyertalk.com/travel/fttr...um/biggrin.gif

One less bird heading south for the winter??

Ever see that Twilight Zone episode with William Shatner (pre-Kirk) who hears a thump and looks thru his window to see a monster picking at the rivets out on the wing?

That's what it was.

If so, then we may have killed the only known specimen of a bird capable of flying at 35,000 ft. http://www.flyertalk.com/travel/fttr...um/biggrin.gif