# question regarding airline engines (USAir bird stall) **URL:** \ **Category:** Factual Questions\ **Created:** [January 21, 2009, 3:25am UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255 "2009-01-21T03:25:18Z")\ **Posts on this page:** 17\ **Page:** 2
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [January 22, 2009, 1:27pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/21 "2009-01-22T13:27:47Z")
> [@Stink Fish Pot](#): > > so something like a titanium screen, strong and light, would cause significant drag and significantly higher fuel usage? > > I’m thinking as long as the engine is sucking in the right amount of air, the safety aspect outweighs the extra fuel consumption. Which to me would be minimal if made of a light enough product. The other problem is that in icing conditions such a screen would tend to accumulate ice which would then choke off air flow to the engine resulting in the very failure you tried to prevent. Icing conditions are far more common than bird strikes. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [January 22, 2009, 1:45pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/22 "2009-01-22T13:45:40Z")
> [@Stink Fish Pot](#): > > This is interesting to me. Is there any data that supports your claim? I can understand the cost aspect of your argument. However, based on your answer, they have already tested something like this. I’m curious to read anything that shows that a screen will cause turbulent air flow that would disrupt engine performance, as well as the screen keeping the right amount of air from flowing into the engine. Have a read of some of this stuff from the [NASA site.](http://www.grc.nasa.gov/WWW/K-12/airplane/inlet.html) > [@](#): > > Because the inlet is so important to overall aircraft operation, it is usually designed and tested by the airframe company, not the engine manufacturer. But because inlet operation is so important to engine performance, all engine manufacturers also employ inlet aerodynamicists. The amount of disruption of the flow is characterized by a numerical inlet distortion index. Different airframers use different indices, but all of the indices are based on ratios of the local variation of pressure to the average pressure at the compressor face. It’s not easy to find something that says “jet engines don’t work well with mesh covering their intake,” however it’s readily apparent that jet engine design in general including inlet design is very critical. You can’t just go and slap a thick mesh in front of the engine and still expect it to perform. I suspect it’s difficult to find research on this sort of thing because it is so evident to those involved in engine design that it is not considered worthy of discussion, also the Googlefoo is not strong in me. Consider how strong your screen would have to be. It needs to be strong enough to withstand a large bird like a goose or eagle traveling at up to 250 knots. A large bird will easily puncture the wing leading edge and in the following case actually damaged the wing spar. > [@](#): > > Occurrence Number 9601590 > > Birdstrike > > The Dash 8, with a crew of three and 14 passengers, was passing 4,800ft on descent to Broome at 243kts when it struck a wedge-tailed eagle. The bird impacted the leading edge at the root of the left wing. The wing-to-fuselage fairing was punctured and the forward wing spar and the electrical components attached to the spar were damaged. That excerpt was take from [this article on bird hazards](http://www.atsb.gov.au/publications/2003/pdf/Hazard_aircraft_by_birds.pdf) by the Australian Transport Safety Bureau (PDF.) As an aside I found this bit amusing, > [@](#): > > Many studies that have attempted to investigate the impact force of birdstrikes, have made the assumption that the force of impact is equal to half of the mass of the bird, multiplied by the square of the speed of the aircraft (E=1⁄2 x Mass x Velocity2). However, this formula does not take into account the shape of the bird, the speed and direction which the bird is flying, nor the extent to which the birds’ body deforms during the collision. > [@indian](#): > > How many sets of vanes ( for compressing air intake )are there in a jet engine for A320 ?? or for similar engines ?? > > An aircraft engine repair guy once told me something like 24 . ( from my memory) is this correct ? Here’s a cutaway of a [CFM56-3](http://attach.high-g.net/attachments/cfm56-3.gif), it’s hard to distinguish all of the compression stages but it looks like 13 including the bypass fan at the front. If you were to include the stator vanes, which are stationary vanes that sit between each set of rotating vanes, you’d get a number close to 24. ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [January 22, 2009, 2:04pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/23 "2009-01-22T14:04:10Z")
Even if the mesh was indestructible and didn’t harm the engine performance, I’m still not convinced it would do any good. A bird hitting the mesh isn’t going to be deflected. It’s going to be sliced into pieces, which are then going to go into the engine. And from the engine’s point of view there’s not much difference between ingesting an intact bird or an equal weight of bird puree. To protect the engine from birds, you’d need to somehow push the bird out of the way, which given the speeds involved is going to be pretty much impossible. ---
**Author:** ![GusNSpot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gusnspot/32/436_2.png) [@GusNSpot](https://boards.straightdope.com/u/GusNSpot)\ **Post date:** [January 22, 2009, 2:43pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/24 "2009-01-22T14:43:36Z")
> [@AndrewL](#): > > Even if the mesh was indestructible and didn’t harm the engine performance, I’m still not convinced it would do any good. A bird hitting the mesh isn’t going to be deflected. It’s going to be sliced into pieces, which are then going to go into the engine. And from the engine’s point of view there’s not much difference between ingesting an intact bird or an equal weight of bird puree. To protect the engine from birds, you’d need to somehow push the bird out of the way, which given the speeds involved is going to be pretty much impossible. Bird puree is the same as heavy rain as far as the engine is concerned. They used to shovel walnut hulls into running engines to clean the compressor stages. (No idea if they still do this) They eat bugs and all sorts of itty bitty critters. Locust swarm might do one in but… Have just one itty bitty blade break off a running engine and if it goes through the wrong place the engine explodes. Other times they take a huge amount of damage and just slow down and stop. One size does not fit all when it comes to jet engine damage… Still and all, screens are not the answer to the hypothetical. ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [January 22, 2009, 2:56pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/25 "2009-01-22T14:56:55Z")
[quote=“1920s Style “Death Ray”, post:22, topic:482255”] It’s not easy to find something that says “jet engines don’t work well with mesh covering their intake,” however it’s readily apparent that jet engine design in general including inlet design is very critical. You can’t just go and slap a thick mesh in front of the engine and still expect it to perform. **I suspect it’s difficult to find research on this sort of thing because it is so evident to those involved in engine design that it is not considered worthy of discussion** , also the Googlefoo is not strong in me. [/QUOTE] (bolding mine) This is basically what I was going to say when I saw **Stink Fish Pot** ’s question. Asking for a cite on this is a bit like asking a physicist for a cite that what goes up must come down. It’s fully in the realm of common sense that putting a blockage upstream of an engine will impede flow, so not many modern researchers are making that the focus of their efforts. And even for papers that discuss it as an incidental thing, there aren’t many out there that make good references and are also available to the general public. Usually, you need a membership to the particular journal or whatnot to get access. But many fluid dynamics books will have a discussion on how to calculate the pressure drop through a screen. Keep in mind that it’s proportional to the square of velocity. The air flowing into an aircraft engine is fairly high speed, so the pressure drop can be more extreme than you might think. So a screen that seems quite free-flowing if you imagine trying to breathe through it might be literally suffocating for a jet engine at full power. Also consider this: When they want to test how sensitive jet engines are to non-uniformities in the flow (what we call “distortion” in the biz), they have to intentionally create turbulence and pressure losses in the incoming air. How do they do this? Screens. Engine testers use distortion screens that provide particular patterns of disrupted flow to the enigne. So not only _can_ a screen impede engine performance, testers often count on the fact that it _will._ And as for those “inlet aerodynamicists” that the NASA site refers to…that’s me. I don’t work for an engine company, but inlet design and analysis is my work. I’m not trying to use that to say “my post is my cite”, but I can certainly vouch for how irritatingly picky turbofans are to the quality of incoming flow. And I know exactly what would happen if I suggested that a FOD screen was a good choice for a commercial engine. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [January 22, 2009, 8:35pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/26 "2009-01-22T20:35:34Z")
You seem to know your stuff, Dave. Suppose instead of a screen you just had a few lengths of cheesewire over the intake. The point being, not to stop a goose from entering, but to chop it into small chunks before it hits the blades. a) would this idea even actually reduce the damage at all? b) would a few bits of wire act like a screen causing pressure drop? c) would a few bits of wire be a risk of icing up? ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [January 22, 2009, 9:55pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/27 "2009-01-22T21:55:51Z")
Good question. I think in the end, the answer still end up being the same: Somewhere between “not worth it” and “dangerous” As for your individual questions… a) Probably not. Some widely-spaced wires may dice up the goose into chunks, but wouldn’t effectively distribute the mass enough to make it much different than a solid bird. You’ll still have a whole bird’s worth of material hitting the fan in mostly one spot. And I doubt that uniformly distributing the goose across the whole engine face would do much good anyway. You’re still going to be giving the engine a lot of solids to cope with. b) With very loosely spaced wire, the pressure drop may well be negligible. There may be some distortion caused by the individual wakes of the wires. However, I’m willing to agree that you could put a mesh in front of the engine that is coarse enough that the engine wouldn’t notice. c) Yes. If conditions are right for icing, than a set of wires in front of the engine becomes an excellent substrate for ice accumulation. And then it becomes quite likely that dangerous chunks of ice break off and enter the engine. This may be no big deal depending on a lot of factors, but it’s still not something anyone would design intentionally. Also important is that the wires themselves become significant FOD hazards themselves. It’s the kind of risk you just don’t want to invite. ---
**Author:** ![crypto](https://avatars.discourse-cdn.com/v4/letter/c/b3f665/32.png) [@crypto](https://boards.straightdope.com/u/crypto)\ **Post date:** [January 22, 2009, 10:41pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/28 "2009-01-22T22:41:15Z")
> [@aerodave](#): > > (bolding mine) > > This is basically what I was going to say when I saw **Stink Fish Pot** ’s question. Asking for a cite on this is a bit like asking a physicist for a cite that what goes up must come down. It’s fully in the realm of common sense that putting a blockage upstream of an engine will impede flow, so not many modern researchers are making that the focus of their efforts. And even for papers that discuss it as an incidental thing, there aren’t many out there that make good references and are also available to the general public. Usually, you need a membership to the particular journal or whatnot to get access. FTR, just because a cite isn’t readily available, I can still accept the word of someone who works in the field. And **aerodave** , you certainly seem to have the experience and hands-on knowledge to speak to this topic. I suspected that the idea of a screen of some sort has been explored and rejected. I think the interesting part for me is that there doesn’t seem to be a real solution to the “bird” problem. With the massive cost of an airliner, the additional cost of a solution to stop birds from killing engines in-flight would be minimal… assuming such a solution exists. So the next logical question to me is, if money wasn’t an issue (i.e you could spend whatever was necessary), is a solution out there? ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [January 23, 2009, 12:44am UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/29 "2009-01-23T00:44:20Z")
I think that, ignoring penalties that ultimately boil down to cost, a clever solution could certainly be found. For example, just use a combination of image processing algorithms and airborne laser technology to create a system that incinerates birds that are on their way toward the aircraft. 😉 This issue is not about what’s technically possible. It’s about what is affordable. And the problem of bird strikes is on one side of that dividing line of affordability. It’s similar, but not quite as extreme, as the problem of meteor strikes on aircraft. If a meteor of any appreciable size hits an aircraft, the plane is toast. But since the odds are so low, and the penalties for protecting against that so severe, you just fly accepting the risk that if one hits you, you’re dead. Sure, the risk of the meteor problem far lower than that of the goose problem. But they exist on the same side of the trade between risk mitigation and affordability. You fly knowing that there is a one in a million chance that birds take out your aircraft. And the other 999,999 times, you don’t spend a lot of extra money being over-prepared for it. In the end, the Hudson river incident won’t change anything about the way airline safety is managed (at least in regards to actively protecting against bird strikes). Nor should it change anything. The right answer is to just accept the risk. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [January 23, 2009, 1:29am UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/30 "2009-01-23T01:29:36Z")
One thing that doesn’t get brougth up much is that each solution poses its own risks, too. After vapor in a nearly-empty fuel tank brought down a 747, there was talk about adding a system to the aircraft to fill the empty tanks with nitrogen. 747s have been flying for decades, and more than 1,000 have been built. Can we install a pressurized nitrogen system that will fail only once in that kind of service, or with less catastrophic consequences when it does? Not that all new ideas are bad, but they have to be truly safer than what we already have. The bar is set pretty damn high already. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [January 23, 2009, 1:54am UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/31 "2009-01-23T01:54:14Z")
> [@aerodave](#): > > In the end, the Hudson river incident won’t change anything about the way airline safety is managed (at least in regards to actively protecting against bird strikes). Nor should it change anything. The right answer is to just accept the risk. I vaguely recall that authorities posted falcons around runways at Heathrow or Gatwick a few years ago, in order to scare off gulls or somesuch that were potential birdstrike victims/threats. Googling doesn’t seem to help though. ---
**Author:** ![BorgHunter](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@BorgHunter](https://boards.straightdope.com/u/BorgHunter)\ **Post date:** [January 23, 2009, 8:05am UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/32 "2009-01-23T08:05:05Z")
> [@Really\_Not\_All\_That\_Bright](#): > > I vaguely recall that authorities posted falcons around runways at Heathrow or Gatwick a few years ago, in order to scare off gulls or somesuch that were potential birdstrike victims/threats. > > Googling doesn’t seem to help though. There’s a [border collie](http://www.usatoday.com/news/offbeat/2008-11-06-birddogs_N.htm) at the airport in Fort Myers. I do somewhat question the idea of introducing birds at airports to reduce birds at airports, though. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [January 23, 2009, 10:37am UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/33 "2009-01-23T10:37:10Z")
The problem with putting a screen in a turbofan engine is the nature of the design. It’s one compressor blade in front of another and any screen would cut down on airflow. there is no place to pull air efficiently except in a straight through flow. I’m reminded of an air-filter box n a sports car I had that included a “bug screen”. I measured the area loss in front of the inlet and it turned out to be a loss of 34%. Since there is a much larger air filter around it I was able to remove the screen. In the case of an jet engine there is no place to pull additional air except from the compressor blade in front. The final blade on a turbofan is creating engine driven thrust (as if it was a ducted turbo-prop. Any change in the flow to the engine would come at the expense of by-pass thrust. So if a screen was added it would take away from the performance of the engine. As for the idea of adding more engines it’s a function of cost. Engines are not linear in respect to cost and thrust. 2 engines can be constructed to make the same thrust of 4 smaller engines at a smaller total cost. This is magnified over the life of the engine when they are overhauled. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [January 23, 2009, 10:39am UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/34 "2009-01-23T10:39:22Z")
> [@BorgHunter](#): > > There’s a [border collie](http://www.usatoday.com/news/offbeat/2008-11-06-birddogs_N.htm) at the airport in Fort Myers. I do somewhat question the idea of introducing birds at airports to reduce birds at airports, though. At Meigs they had sound cannons to keep the birds from settling down around the airport. Mayor Dailey fixed that problem.:mad: ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [January 23, 2009, 3:41pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/35 "2009-01-23T15:41:11Z")
Somebody needs to photoshop a [cowcatcher](http://www.istockphoto.com/file_thumbview_approve/4456584/2/istockphoto_4456584_red_cow_catcher_on_a_small_locomotive.jpg) onto an engine pod. 😃 ---
**Author:** ![slaphead](https://avatars.discourse-cdn.com/v4/letter/s/e9bcb4/32.png) [@slaphead](https://boards.straightdope.com/u/slaphead)\ **Post date:** [January 23, 2009, 4:05pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/36 "2009-01-23T16:05:33Z")
> [@BorgHunter](#): > > I do somewhat question the idea of introducing birds at airports to reduce birds at airports, though. The theory is that a single bird that takes off and lands when (and where) a human instructs it to is preferable to hundreds of birds flying around in large flocks whenever and wherever they choose. Seems reasonably logical to me. It’s apparently a fairly widespread practice [USA](http://news.nationalgeographic.com/news/2003/03/0325_030325_falconry.html) [Spain](http://www.iberianature.com/spainblog/2008/10/falconry-at-spanish-airports/) [Scotland](http://thescotsman.scotsman.com/scotland/Airport-wings-it-with-hawk.4044239.jp) ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [January 23, 2009, 11:33pm UTC](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255/37 "2009-01-23T23:33:04Z")
> [@Magiver](#): > > At Meigs they had sound cannons to keep the birds from settling down around the airport. Mayor Dailey fixed that problem.:mad: Where I used to be based, at Broome, the guy in the tower (not an ATC, just gave traffic information) had a shotgun type thing that made lots of noise, he’d pop out and fire it when needed. [Previous page](https://boards.straightdope.com/t/question-regarding-airline-engines-usair-bird-stall/482255.md?page=1)