A question for you jet engine experts

[ZeamerB17]

I have been reading the Miracle on the Hudson books and they refresh a question I have had for a long time.

Why don't jet engines have some type of screen on the intake to keep birds and other debris out of them? Seems like a no-brainer to me, but I vaguely recall there is a good reason, just can't remember it.

Thanks a lot!

[cg51]

What if the screen came loose? That would be worse than eating a bird.

[Pat]

It's not really practical for a large cowl inlet on a high bypass turbofan. There's no place to put a stowable one without destroying the aerodynamics of the engine cowl. A fixed screen would definitely inhibit airflow through the engine, thus making it less efficient. But, I may be wrong.

Last edited by Pat on Tue Mar 02, 2010 7:26 pm, edited 1 time in total.

[RickH]

Then there is the potential icing issue,....

[warbird1]

There are a lot of reasons why, and some have already been covered:

1) It's not practical

2) It would inhibit the airflow somewhat - a definite detriment for high-bypass fan engines which all modern airliners have.

3) You would have to heat it up to prevent the ingestion of ice while going through freezing conditions. Even if you heated it up, the chunks of ice that would break off could be catastrophic and cause engine failure.

4) Probably the most important reason is that it would be have to be so heavy and durable to prevent even the smallest bird from preventing damage. One thing you have to consider is the EXTREME amount of energy a bird imparts on an aircraft when it hits. The amount of energy it imparted would be directly related to the square of the velocity. Kinetic energy = 1/2 the mass times velocity squared. This means that each time you double the velocity of the incoming bird, you impart 4 times the energy in damage. That's huge.

Even something as small as a 1 or 2 pound bird causes MAJOR catastrophic damage, not because of it's weight, but primarily because of it's relative velocity with respect to the engine. If you have ever watched the FAA birdstrikes videos in any recurrent training class,an FAA symposium or your pilot training, you will know how much even small birds can cause huge damage to jet engines. In order to develop a screen that would be effective enough to deflect a bird strike to the engine, it would probably have to be many inches thick and be very, very obtrusive to the airflow of the engine. It's just not practical at all.

One reason that modern airliners have more than one engine is for this very scenario. The statistical odds of FOD'ing out both engines are astronomically rare. Besides the US Air flight on the Hudson, I'm not aware of any time in history that birds have caused a double flameout like that to a commercial airliner. It happens infrequently in the military, but a lot of the time miliary jets have ejection seats - something that airliners don't.

Also, you would not want to make a "light" or non-heavy, durable screen, because if one of those larger birds took it out, the damage from the screen being ingested into the engine, might cause worse damage than the bird itself. At least birds are organic and are "slightly" easier for an engine to "digest" than heavy duty metal, as you would have to use in a screen. It's really an "all-or-nothing" affair. You would either have to build an EXTREMELY thick (think many, many inches) screen or nothing at all. It would have to be built like a tank and probably out of titanium or something along the lines of the A-10. The increase in weight, obstruction to airflow and performance, along with the unaesthetic lines of it make it very impractical, especially considering the odds of that happening barely remotely possible.

I probably left out some other reasons, but those are the ones off the top of my head that I can think of.

[cadet77]

Screens are used on the bellmouth inlet of jet engines in test cells to lessen the chance of fod ingestion. The added weight, drag and loss of airodynamics makes them impractical for most applications in flight. An advantage of the high bypass turbofan engine is that most birds/fod are passed through the fan and the bypass duct and not the compressor section of the engine. But birds/fod can damage any jet engine when ingested, including the high bypass turbofan engine.

cadet77
Larry

[Cvairwerks]

Check around on youtube for some of the test videos of bird strikes. Every engine design is certified to handle a certain amount of birdstrike damage, but in the case of the United flight, they took numerous strikes in both engines. It's like drinking from a firehose, you can only take so much before it overwhelms you.

[warbird1]

Check around on youtube for some of the test videos of bird strikes. Every engine design is certified to handle a certain amount of birdstrike damage, but in the case of the United flight, they took numerous strikes in both engines. It's like drinking from a firehose, you can only take so much before it overwhelms you.

Small point, but I think you meant the "US Air" flight not United. In any case, yes, there are some fantastic test videos of bird strikes. I can't remember if they are U.S.Air Force or FAA videos, but I've seen some in the past where they have "chicken cannons" where they actually fire these huge 8 to 10 pound chickens and turkeys at operating jet engines on the ground. It's pretty amazing that anything as intricate as a jet engine can take ANY bird strike whatsoever, but they can up to a certain size and speed. Does anybody know what the FAA certification numbers are for commercial jet engines in relation to size of the birds and at what velocity?

[John Cotter]

Besides the US Air flight on the Hudson, I'm not aware of any time in history that birds have caused a double flameout like that to a commercial airliner. .

Even though both engines did not flame out like the Airways airplane, DAL had a 767 taking off in Europe and take birds in both engines. They limped the airplane back around and got it on the ground. Very-Very lucky.

[warbird1]

Besides the US Air flight on the Hudson, I'm not aware of any time in history that birds have caused a double flameout like that to a commercial airliner. .

Even though both engines did not flame out like the Airways airplane, DAL had a 767 taking off in Europe and take birds in both engines. They limped the airplane back around and got it on the ground. Very-Very lucky.

I hadn't heard of that one, when did it occur?

[Enemy Ace]

My understanding is that the birds did not fail the engine on the US Air airbus, but ingesting them caused the engine management software to register such an anomaly that it rolled power back to idle on both engines. The engines did not fail per se.

[The Inspector]

My laptop teaching computer has as a screensaver, a picture I took back in the early 80's down the inlet of a P&W 2040 @ #1 on a UPS 757 APF that ate a nine pound blue Heron on rotation out of KPDX. The inlet blades are jagged and badly shattered, one 6 inch piece of blade got out of the inlet cowl and stuck in the fuselage of the airplane. The Feds, Boeing, UPS, and Pratt were all doing the slow panning take from engine to fuselage and saying in unison 'this ain't supposed to happen'. A team of us went to KPDX to change the engine as PDX was (is) sort of an out station for UPS and mostly has lightbulb changers and no facilities for heavy maintenance work, and, since BADWRENCH did do all the checks for UPS we did a road trip down I-5.

The reason you hear so much more about ingestion's these days is because of the engine size, lots of birds have disappeared down the inlets of JT-8's but caused little to no damage because the birds were pretty small, bigger inlets, bigger birds are now capable of going down the inlet to the 'mechanomatic'. The inlets on the 787 are so smooth that they are molded in one smooth piece so there are no seams for airflow disturbance. The orange flames are core stalls caused by airflow interruption through the engine and air flow reversals as the high pressure back end tries to push hot air out the lower pressure front end of the engine so the engine 'chugs' and runs the equivalent of extra lean for a split second. I had a CF-6 tape when @ BADWRENCH that was the engine certification program. They fed the engine everything from ice chunks to seagull sized frozen birds, blew off fan blades and poured 100000 gallons of water in the inlet on a spray rail type shower ring while running the engine @ takeoff settings for 5 minutes to make sure it would continue to run @ Rated power the volume of water is beyond impressive!

The core stall film was really impressive, they ran a camera behind the engine on the test stand and filmed @ about one fifth normal speed of the camera, you can clearly see the engine belch out yellow and orange flame in long, sinuous tongues and see the cowls and engine distort dramatically with each of about 10 'chugs'. then they show the same piece of film @ normal speeds and all you can witness is one tremor and a small flare of yellow flame and it's over in 2 seconds. Keep in mind that a jet engine is basically a long hollow compression ignition (diesel) engine.

And lets not forget volcanic ash, there was a pretty new KLM 747 that had all four choked out by Mt. Redoubt burping several years ago in Alaska, they were lucky to get a couple restarted and made ANC with badly frosted windshields and ribbons for a radome from the abrasive ash plume.

As Mudbone might have said 'stay away from that boy, that's machinery'

[The Inspector]

Sorry, first statement should read 'in the late 80's' not early 80's

[JDK]

To drag in a warbird reference to an otherwise interesting but irrelevant thread, some of the first ground-running anti-ingestion protection were known as 'Daunt Stoppers':

http://books.google.com/books?id=znlqEw ... rs&f=false

[jtramo]

My understanding is that the birds did not fail the engine on the US Air airbus, but ingesting them caused the engine management software to register such an anomaly that it rolled power back to idle on both engines. The engines did not fail per se.

Thats pretty much what every Airways Airbus guy at LGA tells me. They also tell me that the crew didnt get anywhere near stall so that anti stall mush flight control system Airbus brags about had nothing to do with the crews excellent flying. Sully has come out against the book (Fly By Wire) that seems to be written as Airbus propaganda by the son of the classic "Stick and Rudder" author.

One Airways guy told me a story about an MD80 flight he had where he ingested quite a few "heavy" birds in both engines, but since it was a "throttle by cable" system the engines didnt shut themselves down and he made it around the pattern to land. He said there are quite a few guys that refuse to fly the Bus.

FADEC technology is great for fuel economy but blockage to certain sensors and certain overtemp situations etc etc will cause an engine to shut down to prevent damage.