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The recent crash of a Taiwan aircraft, got me thinking, do aircraft have built in siren, which can warn people on the ground of a possible crash to give them some headway.

If that is not practicable or feasible, then what solutions are feasible, or one should abandon the idea altogether and think of making planes safer in the first place?

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  • $\begingroup$ If the aircraft is about to crash, you should expect failures of several systems, including the siren (not usefull to control the airplane, e.i. not as redundant as other critical systems). Moreover, with all the potential alarms triggered in the cockpit of an airplane, the siren's noise could disturb the pilot. $\endgroup$ – Manu H Feb 28 '15 at 9:35
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    $\begingroup$ Planes are already safe. Such siren would be useful on really rare occasions (maybe less than once per year), so unless someone find a cheap solution that introduce no complexity, the cost to develop, certify and install such a solution would be dissuasive $\endgroup$ – Manu H Feb 28 '15 at 19:10
  • $\begingroup$ You should look into the Jericho-Trumpet! The Germans did this; It was a powerful psychological weapon. youtube.com/watch?v=nZZ504TGDpE $\endgroup$ – Bassinator Apr 6 '15 at 21:03
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I understand where you are at with this, I'm going to start off with a bit of math to show how it's not workable. Commercial airliners move quickly, even in a controlled glide one is going close to 3 miles (4-5 km) per minute. To give people on the ground enough useful warning to perhaps find some sort of cover you would need to give at least 20 seconds. This siren would have to be able to penetrate the walls of buildings and houses and cut through the background noise of a built up area at least a mile ahead. This is a very powerful siren and you'll have the following challenges:

  • At close range you would deafen anyone nearby, injuring the people you are trying to warn
  • The mechanism to generate this siren would likely be large, heavy, and challenging to design into an aircraft frame
  • You would need a great deal of mechanical or electrical power to generate that amount of noise. If you have no engine power you won't be able to sound the siren, and if your engines are working you won't need to as the engines make very good sirens already

There are very, very few cases where a siren such as this would save very few lives compared to justify the costs.

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    $\begingroup$ While it's true enough that aircraft engines are extremely loud, just hearing an aircraft engine doesn't by itself alert you to the fact that something is wrong. I'd even go so far as to say not hearing the engines when an aircraft is nearby is a better indicator that something is wrong. :) $\endgroup$ – reirab Feb 27 '15 at 17:10
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    $\begingroup$ +1 for discussion of how loud the siren would have to be, though. In addition to the people on the ground, it would also render the pilots unable to communicate with each other or ATC and quite likely incapacitate them entirely, which will almost certainly lead to a worse outcome than in the absence of the siren. Also, the audible range of the siren in the area that the aircraft is moving towards would be greatly reduced from normal because the aircraft is traveling at a very significant percentage of the speed of sound. $\endgroup$ – reirab Feb 27 '15 at 17:23
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    $\begingroup$ It's true that hearing an engine doesn't by itself mean something is wrong, however hearing one out of place certainly gets your attention. People are generally attuned to their environment, hearing a jet close by when you're not near an airport would be noticed. $\endgroup$ – GdD Feb 27 '15 at 19:47
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    $\begingroup$ @Mark An open 737 cockpit would be inside the pain zone. But 737s don't have open cockpits and are quite well insulated from the sound. For example, pilots and passengers aren't deafened by the engines. $\endgroup$ – David Richerby Feb 28 '15 at 12:44
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    $\begingroup$ @Mark, Actually, it would require even more than that to reach 90 dBA 2 km ahead of the aircraft (which is presumably the place you'd want to alert.) This is because in the time it takes the sound to travel those 2 km, the aircraft itself has traveled a significant percentage of that distance, which means the distance from aircraft to person will be significantly less than the original 2 km when the audible level actually reaches 90 dBA at the person's location. Another thing to note here is that this isn't much of a warning. That's probably around 20 seconds of warning for an airliner. $\endgroup$ – reirab Feb 28 '15 at 23:49
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No they don't.

If you think about it, it is very rare for an aircraft to crash into a built-up area. People on the ground would have very little time to hear a siren from an aircraft approaching at a hundred miles per hour or more. Anyone hearing it might be as likely to run into the path of the aircraft rather than run away from it.

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    $\begingroup$ Of course, it's rare for aircraft to crash at all. But, if an aircraft does crash, it's quite likely to be soon after take-off. Since many airports are in or close-to built-up areas, such crashes are often in built-up areas. Examples include TransAsia 235 (as mentioned in the question), Air France 4590 (the Concorde crash) and El Al 1862. $\endgroup$ – David Richerby Feb 27 '15 at 10:40
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    $\begingroup$ Arguably US Airways 1549, too ("The Miracle on the Hudson"). Also, TransAsia 222 crashed into a built-up area on approach. $\endgroup$ – David Richerby Feb 27 '15 at 10:44
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No, and even if they did you would probably not hear it. Engine noise should be enough to alert someone who is paying attention, but the truth is that the average pedestrian has no cognizance of unusual noises.

Also, the odds of hitting a pedestrian are extremely remote. Most people move around in cars. Unless you are landing on a park or a beach there will generally be noone around.

I did hear of one case a couple of years ago where a guy ran out of fuel and landed on a beach and killed a jogger. The jogger didn't hear him because he was wearing headphones.

When you come down it all happens really fast. A pedestrian, even with a lot of warning would have to be paying attention, be alert and react right away. Most people are absorbed in what they are doing and a siren going off 1000 feet away and 300 feet up is not going to be on their radar.

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    $\begingroup$ I'm not sure why you assume that the only people on the ground who might need to be warned are pedestrians. Also, a plane that has run out of fuel is a glider, and gliders are very quiet. $\endgroup$ – David Richerby Feb 27 '15 at 11:01
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    $\begingroup$ @DavidRicherby Well, if they are in a car, they are definitely not going to hear anything (unless its a convertible LOL). Also, in a car there will be lots of noise from your car plus the cars around plus the radio plus the book you are listening to plus the guy you are talking to on the cell phone, etc. Something thousands of feet away will be completely invisible. When planes land on highways, it is always a complete and total surprise to the motorists to whom the plane "appears out of nowhere". $\endgroup$ – Tyler Durden Feb 27 '15 at 11:04
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    $\begingroup$ The fact that fire trucks, ambulances and police cars carry sirens to alert other road users clearly demonstrates that motorists can hear sirens outside their vehicles. In fact, forget the emergency services: every motor vehicle is equipped with a horn. Audibility to motorists is not the reason that planes don't carry sirens. $\endgroup$ – David Richerby Feb 27 '15 at 11:15
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    $\begingroup$ @DavidRicherby I know that, I am just saying the utility of a siren, even in an emergency situation, may be a lot less than the OP imagines. There is a big difference between a fire truck coming up behind you and a plane landing on you. Most people don't make any changes to their vehicle's heading until they can SEE the threat vehicle. $\endgroup$ – Tyler Durden Feb 27 '15 at 11:23
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    $\begingroup$ @DavidRicherby An airliner makes for a really big glider. The engines are not actually that much louder than everything else. $\endgroup$ – Matt Nordhoff Feb 27 '15 at 13:24
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No they don't, crashes over land where the plane was stable enough to provide enough warning to the people on the ground are very rare.

The only incident that would have benefited from it which comes to mind is the gimli glider.

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    $\begingroup$ except that landed safely. $\endgroup$ – Octopus Feb 27 '15 at 20:26
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    $\begingroup$ and nearly killed 2 kids riding their bike on the (decommissioned) runway $\endgroup$ – ratchet freak Feb 27 '15 at 20:45
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    $\begingroup$ @ratchetfreak: Never mind two kids riding their bike - how about the dozen stock cars and associated spectators holding a drag race. $\endgroup$ – Pieter Geerkens Feb 27 '15 at 23:03
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They did design one plane with a siren to warn people on the ground. The aircraft was called the "Stuka" The pilot would diligently turn on the warning siren right before he nosed down and dove at 300 miles per hour to accurately release his bombs onto the people he was warning.

The warning siren terrorized the general public, so they stopped using it out of consideration for the bomber pilots, who didn't like getting shot down by the military employed to protect the public.

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    $\begingroup$ while most of this is true, the answer would benefit froma rephrasing to be less humoros and more factual, possibly with links to autoritative sources. $\endgroup$ – Federico Apr 11 '15 at 6:26
  • $\begingroup$ See also this question. $\endgroup$ – Henning Makholm Apr 11 '15 at 10:35

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