Plane Down in Hudson River - NYC
I don't own this space under my name. I should have leased it while I still could
Just to let it stand on its own:
Any bird control issue should be directed to solving the problem that caused the crash - a mid-air collision at 3000 feet with birds probably in transit.
We should not be looking at how to keep birds away from airfields - that is a quite different issue.
Any bird control issue should be directed to solving the problem that caused the crash - a mid-air collision at 3000 feet with birds probably in transit.
We should not be looking at how to keep birds away from airfields - that is a quite different issue.
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New footage of the landing
BBC has just been showing new footage of the impact.
Easily the best yet.
Now online
BBC NEWS | World | Americas | New footage of Hudson plane
Easily the best yet.
Now online
BBC NEWS | World | Americas | New footage of Hudson plane
Last edited by beamender99; 22nd Jan 2009 at 21:05.
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I can hardly believe some of the posts on here....
Do we really have to wait for a few more crashes.. not "miraculously" without loss of life like this one, but wiping out 150-odd people (another 320 or 737) or more than 300 people (7*7, A330... ) and maybe another hundred or so on the ground, before anything is done?
Removing one of the "clear and present dangers" in the form of bird "sanctuaries" within a couple of miles of LGA is fairly straightforward. Moving LGA, JFK and Newark, plus the New York metropolis, is not.
The birds have plenty of other places to go. New York does not.
To "protect" a few thousands birds (those in the area) we now daily put how many people at risk?
CJ
Do we really have to wait for a few more crashes.. not "miraculously" without loss of life like this one, but wiping out 150-odd people (another 320 or 737) or more than 300 people (7*7, A330... ) and maybe another hundred or so on the ground, before anything is done?
Removing one of the "clear and present dangers" in the form of bird "sanctuaries" within a couple of miles of LGA is fairly straightforward. Moving LGA, JFK and Newark, plus the New York metropolis, is not.
The birds have plenty of other places to go. New York does not.
To "protect" a few thousands birds (those in the area) we now daily put how many people at risk?
CJ
Do we really have to wait for a few more crashes.. not "miraculously" without loss of life like this one, but wiping out 150-odd people (another 320 or 737) or more than 300 people (7*7, A330... ) and maybe another hundred or so on the ground, before anything is done?
Removing one of the "clear and present dangers" in the form of bird "sanctuaries" within a couple of miles of LGA is fairly straightforward. Moving LGA, JFK and Newark, plus the New York metropolis, is not.
The birds have plenty of other places to go. New York does not.
To "protect" a few thousands birds (those in the area) we now daily put how many people at risk?
CJ
Removing one of the "clear and present dangers" in the form of bird "sanctuaries" within a couple of miles of LGA is fairly straightforward. Moving LGA, JFK and Newark, plus the New York metropolis, is not.
The birds have plenty of other places to go. New York does not.
To "protect" a few thousands birds (those in the area) we now daily put how many people at risk?
CJ
Your suggestion sounds as simplistic as putting screens in front of engines. The practicality and method vs effect is what always makes life difficult for us humans who try to fool with mother nature.
I'm all for some surgical intrusion for a small effect overall (we reduce 1 out of 500 bird strikes in the new york area), but let's not fool ourselves that it will amount to hill of beans in the worldwide risk.
We still don't know where in hell those birds came from and whether or not they were on the visa waiver program or not. If we can identify the birds and their likely takeoff and landing points then we might have a chance of doing something as practical as closing down a runway at certain times of the year. You try fooling with mother nature and she will beat you every time by simply moving in a new species of bird and more of them.
I for one was pretty happy about JFK, after the DC10 accident in 1975 trading off a 5-7 lb great black back gull problem for a 1. lb smaller gull and more of them. Of course they still damage a lot of planes but at least we don't take out multiple engines at the same time.
With the accident on this thread we still don't know the particulars of how big or how many birds and how much damage to the engines. When all of this becomes known then we can start searching for practical recommendations.
I don't own this space under my name. I should have leased it while I still could
As Lompaso says, it does not seem to be the resident bird population in the adjacent sanctuaries.
Me neither.
A quick fag packet calculation shows that geese would need to fly for some 3 miles before they could reach a height of 3000 feet. Why would they do a circle climb to height?
Look at all the websites both quoted here and elsewhere, where is Cheaspeake Bay? Maybe it needs concreting over or netting?
Who designated the Atlantic Flyway for geese? Can the FAA realign the flyway?
Me neither.
Removing one of the "clear and present dangers" in the form of bird "sanctuaries" within a couple of miles of LGA is fairly straightforward. Moving LGA, JFK and Newark, plus the New York metropolis, is not.
The birds have plenty of other places to go. New York does not.
The birds have plenty of other places to go. New York does not.
Look at all the websites both quoted here and elsewhere, where is Cheaspeake Bay? Maybe it needs concreting over or netting?
Who designated the Atlantic Flyway for geese? Can the FAA realign the flyway?
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lomapaseo and Pontius, thanks.
The suggestion isn't mine....
But getting rid as much of possible of the "locals" and hopefully changing their flightpaths (even for some of the residents) doesn't seem beyond the wit of man. I don't consider that as 'too simplistic'.
It won't eliminate the high-altitude strikes and it might not even have avoided this one either (which was at about 3000 ft). But it would reduce the total number of strikes, if pursued sensibly and agressively.
On a lighter note, if they continue to "go forth and multiply", as they seem to be doing, how long will it take for them to overtake us humans? Isn't it high time we started eating them, rather than the few remaining fishes in the sea?
CJ
The suggestion isn't mine....
But getting rid as much of possible of the "locals" and hopefully changing their flightpaths (even for some of the residents) doesn't seem beyond the wit of man. I don't consider that as 'too simplistic'.
It won't eliminate the high-altitude strikes and it might not even have avoided this one either (which was at about 3000 ft). But it would reduce the total number of strikes, if pursued sensibly and agressively.
On a lighter note, if they continue to "go forth and multiply", as they seem to be doing, how long will it take for them to overtake us humans? Isn't it high time we started eating them, rather than the few remaining fishes in the sea?
CJ
airfoilmod;
Re, "As Humans we overestimate our importance to the Planet, vis-a-vis survival. We can't kill it, it can kill us."
No kidding! Another thread of course, but "profound indifference" comes to mind when notions such as "human progress" and "human survival" are raised in the same breath as "the", (not "our"), planet. We could disappear due to our activities or through other natural occurence and there wouldn't be so much as the slightest quiver on earth, so indifferent is it to our continued and incidental existence. It will carry on, perhaps changed due to what we have done to the accidentally congenial environment in which we evolved, without intention, without direction, and without us, as it did before we arrived 3 billion years ago.
protectthehornet;
Re, "Contrary to the other answers you received, I believe the answer is yes, you can maneuver an airliner and avoid birds. I've even reminded my colleagues of the maneuver and they seem to agree. I've decided not to publish my maneuver here, but should the need arise, I would certainly use it. I think it would be quite easy if you had a true 3 seconds."
On takeoff, in general, the possibility of avoiding birds if one sees them in time, is there. I know of several takeoffs in which the pitch attitude has either been flattened or increased sufficiently to decrease/increase rates of climb right after liftoff, with good success and safety, and well within the performance capabilities of the aircraft in the 2nd segment climb phase.
That said, let us examine the question of bird avoidance using known facts about eyes, speed and mass:
From a discussion on depth of field, (image sharpness, not resolution), and why it works in photography, by Ansel Adams in his Basic Photo Series, the human eye, he states, generally cannot resolve any image sensed by the retinal structure (rods, cones) that is less than 100th of an inch in length (as projected on the retina). Obviously distance and object size determine the angle subtended and the consequent image size projected onto the retina but if the distance between light-sensing rods/cones is less than the size of the image as focussed by the lens/iris, it literally cannot be seen where there are no "sensors".
An imperfection in a photographic negative known as "out of focus" is known as a "circle of confusion" and will appear as a tiny blob instead of as a sharp pinpoint. An image, (retinal or photographic - same principles) is made up of points of light - our eyes are able to focus only very narrowly so our depth of field is small, and sharpness of the image only occurs in a very tiny area in the center of our field of view. Peripheral vision can sense motion and color but cannot focus sharply.
So, if that tiny blob on the photographic film, the CCD of a digital camera or our retinas is less than 1/100th of an inch in diameter, the image is perceived as "sharp" by the eye and therefore the mind.
A large goose 1000ft ahead of the aircraft will not be seen by the human eye because it's size that far away subtends an angle which is too narrow to project an image onto the retina larger than 1/100th of an inch - the eye is blind to the "dots" that are large geese 1000ft away. All we have to do to consider this is to draw, in our mind's eye, two lines from the body of a large goose 1000ft away and imaging, through an intuitive comprehension of trigonometry, the angle subtended by these two lines and image the size of the image thereby generated on the retina.
If we instead choose the size of a flock of geese, we may see it as a whole in which case the chances of avoidance are increased slightly.
At the initial climb speed of the A320, accelerating through about 180kts (above 1500ft AGL at the time of the collision), the aircraft is doing about 300fps, (feet per second).
I know you know this but for reference for others, 250kts is 420fps, 300kts is 505fps and 350kts is 590fps.
Now, that's all "Calibrated Airspeed" - TAS's will be higher, again as you know. For example, a pilot who chooses to fly at 250kts IAS below 10,000ft in descent may actually be doing close to 300kts, "physically" through the air depending upon temperature and pressure of the air, (wind and therefore groundspeed are obviously not factors!) and, given the per-squared law, will be meeting any object at 500fps and the energy commensurate with the mass so encountered - again, you've menioned this so I know you know this.
You have correctly pointed out that a pilot who has spotted birds ahead may have, at "lower speeds", (flaps extended speeds), up to 3 seconds for "avoidance" but I would disagree that this can be done at 420fps (250kts) only because of the above reason...the birds cannot be seen in time at that speed and one does not have 3 seconds at 500fps but perhaps 1 to 1.5 second(s) at best.
Airliners do not maneuver quickly not only because of mass/weight but because they are travelling so fast and require, as any aircraft does regardless of mass, very high bank angles at such speeds to achieve a high rate of turn. Thus, for passenger comfort, unless it is an emergency survival maneuver, an increase and especially a (negative 'g') decrease in rate of climb or altitude is attended by significant 'g' loads which can present risk to those in the back, the only maneuver left that is not attended by such forces is roll, and of course even that would not be for turning out of the way (given the physics of turning at high speed described) but would be for momentarily presenting a smaller (banked frontal profile to what we assume would be a mainly horizontal formation of larger birds. (That said, seagulls don't flock or fly like that).
These notions and one solution are not unusual but are not present in any SOPs, AOMs or unofficial techniques I have read anywhere. One is in test pilot territory beyond 45deg of bank in airline work, notwithstanding the captain's authority to take any action he or she deems necessary to preserve life and property.
The Airbus 320/340 series will not exceed a 45deg bank angle without holding the stick in the roll-command position. But it will do just over 60deg of bank if held there manually. C* laws will not permit further bank angles.
The conclusions I would like to draw here are:
1. The human eye is incapable of resolving images smaller than 100th of an inch such that size, vector (speed and direction) and altitude can be determined with accuracy. Birds that can do substantial damage to an aircraft are still not large enough for the human eye to see from 1500ft or 3 seconds away at 300kts. Even at 300fps, that only leaves 5 seconds for reaction.
2. That at typical jet transport speeds even below 10,000ft, avoidance of birds when such are spotted, "even in time", is not a certain exercise and will always have an unpredictable outcome.
3. That the per-squared law guarantees that at higher speeds damage is likely to be exponentially worse depending upon where on the airframe the strike occurs.
4. I think the term "playing with fire" is appropriate where a crew chooses to exceed the legal 250kt speed limit below 10,000ft when descending.
As discussed earlier in this thread, the "elasticity" of flesh rapidly reduces with higher speeds so the "splat" factor is higher at low speeds and spreads the impact loads over a wider area. At high speeds the flesh mass is concentrated in a smaller area because it does not have time to splat, or spread out, so the impact is just like taking a very heavy, very fast sledge hammer to aircraft skin, engines or windshield - the equivalent of over 100,000lbs of force which has to be absorbed by the airframe, etc.
As far as I know, there is no waiver or relief in descent except if a higher speed is required to remain clean. I know that for the B777-300, the max clean speed at MTOW is approximately 264kts so it is highly unlikely that any heavy transport needs to exceed the speed limit to remain clean while descending into the terminal area.
All this said, birdstrikes are part of the inevitability of aviation, mitigated by knowledge, regulatory compliance and airmanship.
Re, "As Humans we overestimate our importance to the Planet, vis-a-vis survival. We can't kill it, it can kill us."
No kidding! Another thread of course, but "profound indifference" comes to mind when notions such as "human progress" and "human survival" are raised in the same breath as "the", (not "our"), planet. We could disappear due to our activities or through other natural occurence and there wouldn't be so much as the slightest quiver on earth, so indifferent is it to our continued and incidental existence. It will carry on, perhaps changed due to what we have done to the accidentally congenial environment in which we evolved, without intention, without direction, and without us, as it did before we arrived 3 billion years ago.
protectthehornet;
Re, "Contrary to the other answers you received, I believe the answer is yes, you can maneuver an airliner and avoid birds. I've even reminded my colleagues of the maneuver and they seem to agree. I've decided not to publish my maneuver here, but should the need arise, I would certainly use it. I think it would be quite easy if you had a true 3 seconds."
On takeoff, in general, the possibility of avoiding birds if one sees them in time, is there. I know of several takeoffs in which the pitch attitude has either been flattened or increased sufficiently to decrease/increase rates of climb right after liftoff, with good success and safety, and well within the performance capabilities of the aircraft in the 2nd segment climb phase.
That said, let us examine the question of bird avoidance using known facts about eyes, speed and mass:
From a discussion on depth of field, (image sharpness, not resolution), and why it works in photography, by Ansel Adams in his Basic Photo Series, the human eye, he states, generally cannot resolve any image sensed by the retinal structure (rods, cones) that is less than 100th of an inch in length (as projected on the retina). Obviously distance and object size determine the angle subtended and the consequent image size projected onto the retina but if the distance between light-sensing rods/cones is less than the size of the image as focussed by the lens/iris, it literally cannot be seen where there are no "sensors".
An imperfection in a photographic negative known as "out of focus" is known as a "circle of confusion" and will appear as a tiny blob instead of as a sharp pinpoint. An image, (retinal or photographic - same principles) is made up of points of light - our eyes are able to focus only very narrowly so our depth of field is small, and sharpness of the image only occurs in a very tiny area in the center of our field of view. Peripheral vision can sense motion and color but cannot focus sharply.
So, if that tiny blob on the photographic film, the CCD of a digital camera or our retinas is less than 1/100th of an inch in diameter, the image is perceived as "sharp" by the eye and therefore the mind.
A large goose 1000ft ahead of the aircraft will not be seen by the human eye because it's size that far away subtends an angle which is too narrow to project an image onto the retina larger than 1/100th of an inch - the eye is blind to the "dots" that are large geese 1000ft away. All we have to do to consider this is to draw, in our mind's eye, two lines from the body of a large goose 1000ft away and imaging, through an intuitive comprehension of trigonometry, the angle subtended by these two lines and image the size of the image thereby generated on the retina.
If we instead choose the size of a flock of geese, we may see it as a whole in which case the chances of avoidance are increased slightly.
At the initial climb speed of the A320, accelerating through about 180kts (above 1500ft AGL at the time of the collision), the aircraft is doing about 300fps, (feet per second).
I know you know this but for reference for others, 250kts is 420fps, 300kts is 505fps and 350kts is 590fps.
Now, that's all "Calibrated Airspeed" - TAS's will be higher, again as you know. For example, a pilot who chooses to fly at 250kts IAS below 10,000ft in descent may actually be doing close to 300kts, "physically" through the air depending upon temperature and pressure of the air, (wind and therefore groundspeed are obviously not factors!) and, given the per-squared law, will be meeting any object at 500fps and the energy commensurate with the mass so encountered - again, you've menioned this so I know you know this.
You have correctly pointed out that a pilot who has spotted birds ahead may have, at "lower speeds", (flaps extended speeds), up to 3 seconds for "avoidance" but I would disagree that this can be done at 420fps (250kts) only because of the above reason...the birds cannot be seen in time at that speed and one does not have 3 seconds at 500fps but perhaps 1 to 1.5 second(s) at best.
Airliners do not maneuver quickly not only because of mass/weight but because they are travelling so fast and require, as any aircraft does regardless of mass, very high bank angles at such speeds to achieve a high rate of turn. Thus, for passenger comfort, unless it is an emergency survival maneuver, an increase and especially a (negative 'g') decrease in rate of climb or altitude is attended by significant 'g' loads which can present risk to those in the back, the only maneuver left that is not attended by such forces is roll, and of course even that would not be for turning out of the way (given the physics of turning at high speed described) but would be for momentarily presenting a smaller (banked frontal profile to what we assume would be a mainly horizontal formation of larger birds. (That said, seagulls don't flock or fly like that).
These notions and one solution are not unusual but are not present in any SOPs, AOMs or unofficial techniques I have read anywhere. One is in test pilot territory beyond 45deg of bank in airline work, notwithstanding the captain's authority to take any action he or she deems necessary to preserve life and property.
The Airbus 320/340 series will not exceed a 45deg bank angle without holding the stick in the roll-command position. But it will do just over 60deg of bank if held there manually. C* laws will not permit further bank angles.
The conclusions I would like to draw here are:
1. The human eye is incapable of resolving images smaller than 100th of an inch such that size, vector (speed and direction) and altitude can be determined with accuracy. Birds that can do substantial damage to an aircraft are still not large enough for the human eye to see from 1500ft or 3 seconds away at 300kts. Even at 300fps, that only leaves 5 seconds for reaction.
2. That at typical jet transport speeds even below 10,000ft, avoidance of birds when such are spotted, "even in time", is not a certain exercise and will always have an unpredictable outcome.
3. That the per-squared law guarantees that at higher speeds damage is likely to be exponentially worse depending upon where on the airframe the strike occurs.
4. I think the term "playing with fire" is appropriate where a crew chooses to exceed the legal 250kt speed limit below 10,000ft when descending.
As discussed earlier in this thread, the "elasticity" of flesh rapidly reduces with higher speeds so the "splat" factor is higher at low speeds and spreads the impact loads over a wider area. At high speeds the flesh mass is concentrated in a smaller area because it does not have time to splat, or spread out, so the impact is just like taking a very heavy, very fast sledge hammer to aircraft skin, engines or windshield - the equivalent of over 100,000lbs of force which has to be absorbed by the airframe, etc.
As far as I know, there is no waiver or relief in descent except if a higher speed is required to remain clean. I know that for the B777-300, the max clean speed at MTOW is approximately 264kts so it is highly unlikely that any heavy transport needs to exceed the speed limit to remain clean while descending into the terminal area.
All this said, birdstrikes are part of the inevitability of aviation, mitigated by knowledge, regulatory compliance and airmanship.
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I recall a report, must be 40 years ago, of an Electra that took a goose through the centre windscreen. It fetched up in the toilet at the back. The aisle, fortuitously was clear and the toiler empty. The aircraft was at 180 and the goose iro 60lb. The figure that sticks in my mind was the force - 300,000 ft lbs.
Even 20lbs would be a large goose, no way a goose hits 60lbs.
From wiki:
Canada Goose - Wikipedia, the free encyclopedia
An exceptionally large male of the race B. c. maxima, the "giant Canada goose" (which rarely exceed 8 kg/18 lb), weighed 10.9 kg (24 pounds) and had a wingspan of 2.24 m (88 inches). This specimen is the largest wild goose ever recorded of any species.
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PJ2
I believ much has been learned about the eye since ansel. It was said that it would be impossible to see certain things from orbit for example...indeed anything manmade.
But, ships could be seen on the ocean, found by their wake.
Other interesting things could be seen and the eye books were rewritten.
One must be vigilante while being in the pointy end of the plane...birds, planes, toy balloons...and everything else one could imagine.
But, ships could be seen on the ocean, found by their wake.
Other interesting things could be seen and the eye books were rewritten.
One must be vigilante while being in the pointy end of the plane...birds, planes, toy balloons...and everything else one could imagine.
protectthehornet;
No doubt, but not sure of your point. That the eye is better than that? Perhaps, but you don't say how so.
The wake-of-a-ship example is the same as the birds-on-a-wire example which was always the case and not the result of new learning in the physiology of the eye and doesn't change the 100th-of-an-inch resolution or so, capability.
Anyway, all fwiw.
I believ much has been learned about the eye since ansel
The wake-of-a-ship example is the same as the birds-on-a-wire example which was always the case and not the result of new learning in the physiology of the eye and doesn't change the 100th-of-an-inch resolution or so, capability.
Anyway, all fwiw.
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Do we really have to wait for a few more crashes.. not "miraculously" without loss of life like this one, but wiping out 150-odd people (another 320 or 737) or more than 300 people (7*7, A330... ) and maybe another hundred or so on the ground, before anything is done?
Removing one of the "clear and present dangers" in the form of bird "sanctuaries" within a couple of miles of LGA is fairly straightforward. Moving LGA, JFK and Newark, plus the New York metropolis, is not.
Removing one of the "clear and present dangers" in the form of bird "sanctuaries" within a couple of miles of LGA is fairly straightforward. Moving LGA, JFK and Newark, plus the New York metropolis, is not.
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I beg all your pardon, but why at 3:29 are we turning down the Hudson and not back to LGA?
I chose JFK, (if only because I was familiar with it) and was very pleasantly surprised to see how far we managed to stretch the glide - i.e.,the aeroplane (a 773) went much further than I would have hoped, and we got in, but only with a VERY late selection of gear and a very late turn onto finals.
But had it been the real world outside my windscreen and I had just suffered a real double engine failure, would I have even considered stretching the glide, unsure I would make it, in a dead stick approach over the very built up southern Long Island, when I had a 'sure thing' - the Hudson - as an alternative? Absolutely not.
The Monday Morning Quarterbacks armed with computer models may prove the the aircraft would have made it back to LGA from the 3:29 position. However, that will not have been totally (note that word) obvious to the captain in the split second (and note that vitally important phrase as well) that he had to make a decision.
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Turning back to LGA?
Absolutly no way on the earth would you want to turn an aircraft at 1200 feet more than 90 degrees (actually loooks like 120 ish) back to LGA ACROSS NEW YORK. I'm no expert on the glide distance of an A320 but it would have been less than the actual glide distance he actually achived just due to the loss of lift in the turn. In my view that would have been a massive risk and almost certainly ended up with HUGE loss of life. Go's against every basic instinct taught at an elementary level, loose engines and turn towards one of the biggest citys on earth Hmmmmmlet me think, maybe not. I'm sure his logic would have been (in that split second) possibly land in a New York street and definatly die, or maybe make it in the Hudson. I'm certain he made 100% the correct call becuase they all made it.
Looking at the map above it looks like he made the decision at 3:28 and 1600 feet to ditch the LGA idea, and frankly i don't blame him. Just look at his position, where he had to glide to and over what!
Absolutly no way on the earth would you want to turn an aircraft at 1200 feet more than 90 degrees (actually loooks like 120 ish) back to LGA ACROSS NEW YORK. I'm no expert on the glide distance of an A320 but it would have been less than the actual glide distance he actually achived just due to the loss of lift in the turn. In my view that would have been a massive risk and almost certainly ended up with HUGE loss of life. Go's against every basic instinct taught at an elementary level, loose engines and turn towards one of the biggest citys on earth Hmmmmmlet me think, maybe not. I'm sure his logic would have been (in that split second) possibly land in a New York street and definatly die, or maybe make it in the Hudson. I'm certain he made 100% the correct call becuase they all made it.
Looking at the map above it looks like he made the decision at 3:28 and 1600 feet to ditch the LGA idea, and frankly i don't blame him. Just look at his position, where he had to glide to and over what!
E.Z. Flyer: I think the answer to your question is that up until the 3:28 mark (1600 ft) the crew were planning a return-to-airport at LGA. At that point they had a clear view of the airport and decided it was too risky. A 2-D map does not show vertical obstructions, which were, of course, not a factor on the big wet runway called the Hudson.
The pilots looked at all the factors and decided the Hudson was the preferable option.
One other point that was brought home to me in simulating this event (and the simulation actually tracked the real world better than I expected) was that even if one could make either LGA or TEB, one did so with no reverse thrust available.
I'll leave it to the real A-320 drivers to calculate how much runway they'd need with no reversers and a touchdown in maximum-glide configuration/speed.
The pilots looked at all the factors and decided the Hudson was the preferable option.
One other point that was brought home to me in simulating this event (and the simulation actually tracked the real world better than I expected) was that even if one could make either LGA or TEB, one did so with no reverse thrust available.
I'll leave it to the real A-320 drivers to calculate how much runway they'd need with no reversers and a touchdown in maximum-glide configuration/speed.
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Resolution limit of the eye
I'm not sure it's the number of megapixels in the eye that determines its optical performance. The detectors are a bit flakey, but the processor is very fast and well tuned.
An absolute limit to performance should be diffraction from the pupil, which could be up to 5mm behind sunglasses. This corresponds to a best possible resolution ability of order (wavelength/diameter) ~ 1/10,000 radians, 20 arcsec, 1ft at 10,000ft, or 50ft from low earth orbit. (1 arcsec is about an inch from 30,000ft.)
However, to see a bird you don't have to resolve it as such, as long as it's dark enough to see as a black dot on the sky background. I suspect this is the factor that determines when they could be spotted, rather than the angle subtended. Of course, even if they can be seen, seeing is one thing and avoiding is another.
To confirm little things with high contrast are visible: stars are tiny (~1 arcsec with atmospheric refraction, ~0.1 milliarcsec without). Your eye can't perceive their extent, but they can certainly be seen.
An absolute limit to performance should be diffraction from the pupil, which could be up to 5mm behind sunglasses. This corresponds to a best possible resolution ability of order (wavelength/diameter) ~ 1/10,000 radians, 20 arcsec, 1ft at 10,000ft, or 50ft from low earth orbit. (1 arcsec is about an inch from 30,000ft.)
However, to see a bird you don't have to resolve it as such, as long as it's dark enough to see as a black dot on the sky background. I suspect this is the factor that determines when they could be spotted, rather than the angle subtended. Of course, even if they can be seen, seeing is one thing and avoiding is another.
To confirm little things with high contrast are visible: stars are tiny (~1 arcsec with atmospheric refraction, ~0.1 milliarcsec without). Your eye can't perceive their extent, but they can certainly be seen.