Tuesday, January 20, 2009

So Mosquito Netting Won't Work, Eh?

I do have some December flying stories to write up and post, but I am still reading and thinking about the US Airways ditching, so they can wait a few days more.

People ask why they couldn't just put screens in front of the engines to keep the birds out. I suppose people are envisioning the sort of things you staple around your eaves to keep the birds from flying in and nesting. One letter to the editor of the New York Times compared the concept to that of cowcatchers. Let's compare. A cow weighs about 500 kg. A locomotive tops out around 80 mph, or 36 m/s. The kinetic energy of a moving object is 1/2 m v2, so for the locomotive-cow collision that's 0.5 x 500 x 36 x 36 = 324 kilojoules, if I have my units right. I'm told the A320 is going to be climbing out at about 250 knots (corrected from a conservative or 210) or 128 m/s. A Canada goose weighs in at around 5 kg. Put those two together and we have a kinetic energy of 41 kJ. So the energy that must be dissipated when a slowly climbing airplane hits a single goose hits is about an eighth that of a speeding train hitting a cow. Could a structure be built over engine intakes so as to absorb that energy?

The structure would have to span what is about a two metre diameter across the intake, be strong enough to withstand bird strikes and never itself shed foreign objects into the engine. If you're having trouble believing that eight birds in the air equals one cow on the tracks, type "birdstrike" into Google's picture search for images proving that birds penetrate windshields, radomes and wings. This giant screen would have to have small enough holes not to let birds pass through, yet large enough holes to not impede airflow to the engine. That combination is an impossibility, as anything in front of the engine impedes airflow, so either the engine would have to be bigger or the performance of the airplane less, in order to compensate. I don't have the flow dynamics knowledge to begin to construct equations for this, but LTV Aerospace Corp. figures it's feasible, and here's a 2001 patent for a retractable bird deflector grille. I do know that tiny changes to an airframe, such as an angled piece of metal the size of half a Ritz cracker, can make huge differences in handling and controllability, so that even if such a structure were built, the whole airplane might have to be redesigned around it.

A reporter tried to answer the screens question live on CNN while I was watching, and he first admitted he didn't know, and then speculated that accumulated debris would block airflow into the engines. Not a bad speculation. The screens would have to have some way of preventing ice formation, probably by being electrically heated, so that chunks of ice neither blocked the intake nor fell through the engines. Heating the intake air would decrease engine performance, however, and heating a screen that size and weight would take a lot of power.

A lot of thought has gone into birds damaging engines. A Wall Street Journal article pointed me to the lengthy FAA rules governing the testing of engines against birdstrikes. Here are some excerpts.

All ingestion tests must be conducted with the engine stabilized at no less than 100-percent takeoff power or thrust, for test day ambient conditions prior to the ingestion. In addition, the demonstration of compliance must account for engine operation at sea level takeoff conditions on the hottest day that a minimum engine can achieve maximum rated takeoff thrust or power.

The impact to the front of the engine from the large single bird, the single largest medium bird which can enter the inlet, and the large flocking bird must be evaluated.

Medium bird engine tests shall be conducted so as to simulate a flock encounter, and will use the bird weights and quantities specified in Table 2. When only one bird is specified, that bird will be aimed at the engine core primary flow path; the other critical locations on the engine face area must be addressed, as necessary, by appropriate tests or analysis, or both. When two or more birds are specified in Table 2, the largest of those birds must be aimed at the engine core primary flow path, and a second bird must be aimed at the most critical exposed location on the first stage rotor blades. Any remaining birds must be evenly distributed over the engine face area.

The specs include tables of bird weights and numbers and expected performance that any engine must meet before certification. And it has to be said, because this is the internet and someone else is going to say it if I don't: "chicken cannon." Yes, they actually fire (already dead) poultry at airplane components to test them. And no, it isn't always necessary to thaw the chicken first.

23 comments:

Sarah said...

I've got it! Just mount a large, spinning metal bird deflector in front of each engine. You'd have multiple "blades", perhaps each having a lifting wing cross-section to get thrust back out of the deal.

Should I patent it?

ZachandHeather said...
This comment has been removed by the author.
Aviatrix said...

I have 210 as flap retraction speed, but wasn't too confident in my source. Seeing as Canada geese do range up to 14,000 feet or so, I'll recalculate with 250 kts.

Colin Summers said...

I saw a flock at 10,500 as I came into Albuquerque and it spooked me. They were flying the wrong way for VFR flight at that altitude and it would have been head on for me.

Chris Taylor said...

Just upgrade the radars to AESA and put some AMRAAMs under the wings.

Aluwings said...

The birds will actually get out of the way if they see us coming. They aren't stupid. That's one reason why we routinely leave "landing" lights on below ten thousand, and if we know the birds are around we can operate at a lower speed. Some pilots also run the radar in the hope that perhaps the bird can sense radar waves and so get out of the way (We cannot see them on our existing radar at least).

I wonder if there could be an airplane equivalent to the deer whistles that we install on our car bumpers in the hope of discouraging animals from crossing the highway in front of us...

Lord Hutton said...

damn! What Sarah said.

N6349C said...

I think we are missing something important here. I heard someone on Fox News state these were CANADA geese, who without provocation, make a suicide attack on a US AIRWAYS plane made in FRANCE. It ended up at the World Trade Center (Ground Zero) in New York.

Enough said. The invasion starts today.

zb said...

n6349c, you are funny. Dude, are we glad that the pilot had the skill to, contrary to the report you mention, put the plane into the river and not into a skyscraper...

The funnies aside, a moskito net (or something alike) in front of the engine intakes will likely, as has already been said, clog with bird parts just like fuel the filters in some 777s' engines allegedly clog with cold fuel that contains some frozen water.

By the way... Bird! Parts! (via a-net). The picture looks less gross than what I imagened after having read the very technical description above with all "the bird weights and quantities specified in Table 2" and how they "must be aimed at the most critical exposed location on the first stage rotor blades."

Sarah said...

Sorry about being rilly flippant in my earlier comment. I suspect that while some protection scheme is possible ( I liked the patent mentioned ) economics and actuarial analysis will result in no change. It's a big sky, and no protection has worked so far (mostly), right? We'll see. There are certainly a lot more big geese around these days, and they stay all winter for the free urban food. I think better attention to warning away the flocks with noise, hunting, what ever it takes, is a better approach at airports with waterfowl cautions.

Aviatrix said...

Hey, I liked your suggestion. I was waiting for someone to pop up with evidence and analysis comparing engine failure inducing bird strikes suffered by prop planes and jets using the same airport, controlled for frequency of service and time of day.

When I started the research for that blog entry I thought I was being stupid. I was surprised to discover that bird grilles were considered feasible, whether or not they are economical.

Sarah said...

Well good. About your previous point though about a 14,000 max height for geese... here is someone's picture of

Canadian geese at 18,000

This was in mountain wave so it wouldn't be common, but there they are.

The thread about it at
rec.aviation.soaring

Apparently high altitude encounters do happen. So a bird catcher would have to be used full-time and better not kill efficiency too much. Not sure that's possible.

Aviatrix said...

The 14,000' number was from memory, from the AIM. I tried to look it up and link it for this comment, but they seem to have taken that section out. At least I can't find it, and there used to be several pages including diagrams of migration routes and altitudes and bird weights. I wonder where it went.

Louis Bourque said...

I just heard about this story on CNN that the same plane as the one that ditched in the Hudson had an engine failure two days earlier.

http://www.cnn.com/2009/US/01/19/hudson.plane.folo/index.html

Thoughts?

Louis Bourque said...

Oh, and aviatrix, regarding the AIM info you're talking about:
In the April-October 2006 edition of the AIM, the migratory routes are in RAC 1.15.2. Figure 1.4(a) states altitudes to 10 000 Feet, Weight to 14 lbs. They can be flying at speeds of 30 to 45KT

Aviatrix said...

I'm not sure "engine failure," is accurate for a compressor stall. Air entering the jet engine is compressed by a series of moving and static fins. Each fin is like a miniature airplane wing, and it has to hit the airflow at the right angle and speed to work. If there is a disruption in the airflow over the blades, the engine hiccups. It's comparable to missing the water with an oar when you're rowing a boat, except that the engine compressor stall makes a REALLY loud noise. I know of an airline captain who assumed a terrorist bomb had gone off in the back when he heard it.

A momentary compressor stall can occur during flight in turbulence, or changes in engine power. Worn engine components or ingestion of foreign objects could cause it too. You can bet the FAA will be looking at whether US Airways maintenance staff took the correct action in response to the first report. Personally I doubt that whatever caused the first compressor stall had any influence on the engine's ability to ingest multiple geese a few days later. But I'm neither an engineer nor an accident investigator.

And apparently I can't find things in books, either. Thanks for the birds. I was thinking it would be in AIR. I looked in RAC briefly, but only looked at the headings, so missed it under general.

Anonymous said...

Don't MD-80s (Delta's, at least) have some kind of protector covering the intakes? It's not a screen so much, but more like a hub with 10-12 "spokes" that seem like it would keep large birds (e.g., geese) out. Maybe it serves some other purpose.

Anonymous said...

No, those are inlet guide vanes designed to present the first stage axial compressor with air at the proper AOA. Some designs are fixed, some are variable in pitch.

N6349C said...

I wonder if aircraft with big, hi-bypass ratio engines hung under the wings (almost all newer a/c such as 737, 757,767, 777, 787, A3xx) have a higher rate of bird ingestion than aircraft with rear mounted engines (Embraer, MD80, almost all biz jets). I would bet that even when adjusted for intake area, rear mounted engines ingest fewer due to the birds bouncing of the fuselage skin.

zb said...

@n6349c: Oh, the pleasures of engineering and design... they are all about finding the right amount of pros and cons for a certain solution to a problem... What you say seems reasonable, but so far, I have only heard people complain about rear-mounted engines because they are allegedly more prone to ingestion of ice falling of the fuselage in front of them...

Sarah said...

@ zb said:
heard people complain about rear-mounted engines because they are allegedly more prone to ingestion of ice falling of the fuselage in front of them...

True enough. I recall hearing about a 727 that "lost an engine", I mean really, it fell off the pylon, when it ingested "blue ice" leaking from a lav. dump. How embarrassing.

Callsign Echo said...

I've got it. Install a golden retriever on each wing.

outriess said...

Maybe there is a way to make the aircraft more visible to the birds. The current strobe lights we see clearly may not be emitting in the best frequency however for birds to see... Here's one smart kid who figured that out for reducing bird strikes on windows:
click here