Thursday, January 15, 2009

“The reports of bird strikes come from eyewitnesses on the ground.”

It seems that a plane from LaGuardia Airport en route to Charlotte, N.C. crashed into the Hudson river this afternoon. I was relieved to see that (thus far) there were no fatalities or injuries reported. The reason given for the crash? Birds cut into the engine. I am slightly afraid of birds and have a flying phobia--so to see that somehow all the passengers made it through the crash is very reassuring. I can't imagine what an ordeal this must have been. However, I have to wonder, why can't an engine stand up to a few birds flying into it?


Blogger Shane said...

Engines are tested against bird strikes, and are designed to survive. However, they are tested against a small number of birds. If the plane ingested the better part of a flock through both engines, that'd cause problems.

4:57 PM, January 15, 2009  
Blogger Dave Cornutt said...

How can one bird strike put a modern jet engine out? It can't. You've all seen the chicken-gun tests. However, make it a big enough flock of birds, and all bets are off -- no engine can be designed to withstand everything. And apparently this was more than a few birds, since the strike put out not one but both engines.

Decades ago, there was a bad crash at Logan (Boston) caused by hitting a flock of birds on takeoff. And that was a four-engine plane.

4:57 PM, January 15, 2009  
Blogger Sad_Dad said...

It depends on what type of engine was on the plane, from my USAF experience when birds fly into a jet engines it's called F. O. D. (Foreign Object Damage). And when this happens it cuts down dramtically on the efficiency of the engines capacity to compress the air going into it thus reducing thrust. If you get too many bird strikes it can be very bad. When you have a plane with four engines it is not felt as much as a plane with two engines.

To fix this is a pain staking process of climbing in the inlet of the engine and filing down all the rough spots on the compressor blades until smooth then applying blue dye to it so that future strikes will not be confused with the old ones. Just thought I'd toss that out there.

5:08 PM, January 15, 2009  
Blogger Nathan Mates said...

Take a desk fan, and put a weight on one of the blades. Notice how it shakes as it gets up to speed. If you've got lots of bird parts all over it, it'd shake more, especially because unlike a desk fan, it's harder for solid but sticky parts to leave an engine w/ the cowl on.

Looking at these specs, the A320 is powered by a pair of CFM56-5 engines, which run at about 5200 rpm peak, and have diameter of 67". That's 83 revolutions/second, and the outsides of the blades travel about 1450 feet/sec.

Basically, if the engine's running really unbalanced-- like the desk fan above-- then it could shake itself apart, throwing lots of heavy metal chunks at the rest of the plane at high speed. It's safer to kill the engines rather than have them disintegrate.

All engineering is a study in tradeoffs. You could make an engine so resilient that it can handle a flock of birds. Or, put 16 engines on a plane. But, it'd probably weigh 4-5x what current engines weigh. For 99.99% of the time, that's unnecessary.

I think that the FAA recently announced that 2008 was the 2nd year in a row w/ no deaths in commercial aircraft flight. To me, that's really darn good, and means that most of the engineering & maintenance standards are doing their job.

5:22 PM, January 15, 2009  
Anonymous Anonymous said...

From what I've heard, it wasn't just birds but a flock of Canada Geese, which weigh up to 18 lbs and can have wingspans up to seven feet, depending on the subspecies.

Multiple big birds into each engine will cause significant damage. It is difficult to design a light, energy efficient, economical to operate engine that can withstand that kind of impact.

5:53 PM, January 15, 2009  
Blogger Dave Cornutt said...

Dogwood, I was suspecting Canada geese. They are the only birds I know of who are stupid enough to be hanging out in New York in January. For some reason, Canada geese have pretty much stopped migrating over the last 20 years or so. All other birds have more sense than that. And, as you say, Canada geese are big. We aren't talking starlings here.

6:14 PM, January 15, 2009  
Blogger Ern said...

For some reason, Canada geese have pretty much stopped migrating over the last 20 years or so.

I believe that the reason is that far more people leave food out for birds during the winter than did so forty or fifty years ago. I've read that birds migrate because of the lack of food supply where they are, not because of colder temperatures per se .

6:32 PM, January 15, 2009  
Anonymous Anonymous said...

I suspect geese stay in northern Indiana because our mild winters have been leaving corn and soybean fields exposed rather than buried by several feet of snow.

We'll probably need a few consecutive years of harsh winters before their migration patterns change, or the stay behinds starve to death.

7:01 PM, January 15, 2009  
Anonymous Anonymous said...

Here is an interesting article from Air & Space Magazine discussing the operational envelope of jet engines.

8:03 PM, January 15, 2009  
Blogger Cham said...

Canadian Geese are very large birds.

11:59 PM, January 15, 2009  
Blogger Locksmith Mesa said...

Sarawak bird even bigger I think.. =)

locksmith mesa

8:33 AM, January 16, 2009  
Blogger DADvocate said...

I heard on the radio it was geese. Geese are "very large birds." Around here flocks in ponds and small lakes can be 40-50 or so.

Helen, if you want to keep birds away, apparently border collies do an excellent job. A caller on the local talk radio station said he made a living using his border collies to chase geese away from golf courses, parks, apartment complexes, etc.

9:04 AM, January 16, 2009  
Blogger Cham said...

Here is an article about the of that US Airways plane. Right place right time.

9:42 AM, January 16, 2009  
Blogger SGT Ted said...

The non-migrating geese are what you get when the golf courses, parks etc allow local flocks to establish themselves. There is no place *for* then to migrate to and no reason to do so, as the weather isn't foul enough (yukyuk) to force migration. The migrating geese are from Canada(duh) on north with lesser Canada geese (cacklers) from Alaska latitudes. The North East is like Palm Springs to them.

11:33 AM, January 16, 2009  
Blogger DADvocate said...

The geese we get in northern Kentucky and southern Ohio are Canada Geese or look just like them. Yes, they migrate through but large gaggles of them will be found at local ponds, lakes, streams (uncluding the creek behind my house), golf courses, etc. for at least several weeks during migrations.

I don't know if these are geese that stay here for a period of time and continue to migrate or what. I saw a bunch sitting contently on a frozen pond just a couple of days ago. But, the geese in this area are largely absent in the summer months.

1:31 PM, January 16, 2009  
Anonymous Anonymous said...

Having had the wind knocked out of me from getting hit in the solar plexis by a robin at about 30 mph on a motorcycle, it's not much of a wonder what a few canadian geese can do entering the inlet of a jet engine at take off speed +.

m x a = f.

One heck of a pilot, I'd say.

4:23 PM, January 16, 2009  
Blogger Cham said...

The more I read about this story the more it is uplifting. Naturally, a Canadian Goose in your jet engine is no fun, but it's really heartwarming to see a pilot who was prepared for a crash, a group of passengers who didn't panic and were appreciative, a flight crew that seemed to care, a flotilla of boat pilots that were able to quickly realize they needed to stop what they were doing and attempt a successful rescue and police officers who were able to get into their SCUBA gear and save lives.

The whole story is an upper.

5:14 PM, January 16, 2009  
Blogger Timothy Power said...

My dad was on several accident investigation teams when he was stationed in Germany in the mid-80s. A few of those were bird strikes (including one where the eagle was still carrying the rabbit it had just caught when it went through the canopy). Yes, a single bird can wreck an engine, or even bring down an airplane, if it's big enough and the plane is going fast enough--and it hits in just the right spot.

Think of it like this: suppose I were to throw a 90-mph fastball at the side of your car. That's likely to leave a good sized dent, right? Well, the baseball has a mass of 0.142 kg, and a (metric) velocity of 40 m/s. Its total kinetic energy is 1/2 m v^2, or about 114 Joules, when it hits your car.

In comparison, a Canada goose is about 9 kg (20 lbs), and the takeoff speed of an Airbus A320 is about 75 m/s (170 mph). If the Airbus hits the goose right on takeoff, it has to absorb 0.5*9*75*75, or more than 25000, Joules.

That is, the aircraft has to absorb well over 200 times the energy that the baseball dented your car with. Yes, that can knock a turbine blade clean through your engine, if it hits just right.

And that's just at takeoff speed. Jetliners cruise at roughly 500 mph, give or take. And the energy of impact varies with the square of the speed, meaning that when your speed doubles, the impact energy goes up four times. I don't know exactly how fast the Airbus was going when it hit the flock, but it was likely going moderately faster than the example I gave above.

I do have to agree with Cham's assessment: the whole story is an upper. It could have turned out really badly, but everyone from the pilots to the attendants to the passengers to the ferry pilots on the river did exactly the right thing. It's stories like these that give me hope for the future of this country.

10:51 PM, January 16, 2009  
Blogger Ken Mitchell said...

Hi, Dr. H. There is a whole generation of aircraft engines that are practically immune to bird strikes, and indeed, FOD of most types. They are called "propellers".

A radial piston engine with a four-bladed prop laughs at entire SPECIES of avian predators and comes out victorious.

Turbine engines? Not so much. A little rock, a nut or screw, a bird; any of these can shred the tiny, delicate, precisely engineered vanes of a turbine, causing a snowball effect so that in each stage of the turbine more and more FOD comes through. First it's the bird breaking off vanes from the first stage of the turbine; then it's bird guts and shards of the first-stage turbine blades causing a cascade of tiny steel turbine fragments blowing out the second, third fourth.... stages until the entire engine falls out. Oh, yes; and throws turbine parts into the wing and fuselage and fuel tanks, and....

It isn't a pretty sight.

1:53 PM, January 17, 2009  
Blogger Skyler said...

Just to be more precise, usually the damage is heaviest in the compressor section, not the turbines. Large birds can make it back to the turbines, but if the bird is that big, it's more likely that pieces of the compressor are what causes the damage that far back.

3:18 AM, January 18, 2009  
Blogger Michael Lee said...

I love you, Dr. Helen, but, man, what a chick thing to say.

10:31 PM, January 18, 2009  
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