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According to reports, there were 4 helicopters used in the raid on Osama Bin Laden.

Is it conceivable that even one helicopter could be flown so close to the capital of another country without raising alarm bells?

Considering these were military helicopters and there were four of them, I am skeptical that this would be allowed. Surely the airspace in Pakistan is monitored just as well as other countries?

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    And no "By Helicopter" link either :( I guess there is a "seal only" version of google maps with an "on foot with 70kg backpack" icon
    – mplungjan
    Commented May 4, 2011 at 12:33
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    Something to consider is that it's an open secret that the US has been operating in western Pakistan for years now with drones and possibly other military air craft. What are the chances that locals and the military ignored or at least didn't react if/when they spotted the US aircraft?
    – stoj
    Commented May 4, 2011 at 12:58
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    Raising alarm bells? Where? Why wouldn't it be allowed? I'm sure Pakistan knew beforehand, Obama even pointed out that Pakistani intelligence helped find where bin Laden was... They wanted to get rid of him as much as the US wanted.
    – Lennart Regebro
    Commented May 4, 2011 at 13:15
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    @Lennart - that's a pretty sweeping generalization. There are elements of both ISI and the army (and overall population) that are at best rooting for Islamists/against USA and at worst actively supporting them.
    – user5341
    Commented May 4, 2011 at 14:25
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    This article suggests the helicopters used had a range a special radar / IR stealth capabilities and acoustic reduction technology.
    – Skizz
    Commented May 4, 2011 at 15:05

3 Answers 3

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EDIT TO ADD INFORMATION REQUESTED.

If a helicopter is flying low, and not "squawking", there is no way that any civilian radar will pick it up. Especially in a mountainous region such as Pakistan. Simple mechanics of radar as illustrated by this picture from Answers.com. Note that mountains between the radar site and aircraft will further block the ability of the radar to detect an aircraft.

Radar Mechanics

Keep in mind that if they can't be seen, they can't get shot at either. And these are some of the best helicopter pilots on the planet. Most radars are unable to detect anything flying contours, but I would wager that for part of this they were flying nap of the earth. The below image is taken from the Global Security Website (the exact image is from Figure 28 on this page) where they discuss many modes of flight for helicopter safety from enemy fire.

Nap of the earth flying

Even larger aircraft like the FB-111 would use this technique to avoid detection without the need for stealth technology.

So even though the airspace is monitored, if they can't be seen, it doesn't matter. Also, I don't think this was "allowed" or "disallowed" by the Pakistani government. Some covert operations are carried out, and then back-briefed if the target (such as Osama bin Laden) is important enough.

ADDED INFO:

Now, several folks have asked about the helicopters used, and some of their performance characteristics. The most likely aircraft (as reported in a couple of other answers as well) is the MH-60 Pave Hawk (a Blackhawk variant modified for special operations). Again, to quote Global Security, the performance characteristics are:

Primary Function Infiltration, exfiltration and resupply of special operations forces in day, night or marginal weather conditions.

Power Plant Two General Electric T700-GE-01C engines

Thrust 1,630 shaft horsepower, each engine

Length 64 feet, 8 inches (17.1 meters)

Height 16 feet, 8 inches (4.4 meters)

Rotary Diameter 53 feet, 7 inches (14.1 meters)

Speed 184 mph (294.4 kph)

Maximum Takeoff Weight 22,000 pounds (9,900 kilograms)

Range 445 nautical miles; 504 statute miles (unlimited with air refueling)

Armament Two 7.62mm mini-guns

Crew Two pilots, one flight engineer and one gunner

In particular, note that these aircraft are mid-air refulable from a KC-130 (NOT KC-135), thus they have a nearly unlimited range. I highly doubt that they took off from anywhere inside Pakistan (i.e. Ghazi) but rather originated in Afghanistan. I cannot say where exactly though, but no matter where they took off from, if they received refueling prior to entering Pakistan, the range is more than adequate to get to Abbottabad and back on one tank of gas (looking at google maps, it appears that the distance is less than 350 KM from Kabul, or about 200 miles). So a little less than 400 mile round trip, at about 200 MPH would be about 2 hours total (add in the actual assault and there you have your timeline). These figures are approximate though because things change with load-out and other configurations. These aircraft would probably have flown in a formation that would probably helped to disguise their true numbers.

Someone mentioned that ATC must have a squawk to paint these aircraft. That is overstated, however as previously mentioned, they were probably well below the radar, and aided by the mountainous terrain. Add in they were most likely using EMCON 4 procedures, and then it would be even more difficult to pick them up by any means. As the cited article also mentioned, the noise reduction and additional radar absorbent paint just added to the stealthiness of these aircraft (as if SPEC OPS flight patterns were not enough).

And thanks to Kit Sunde, we have further info: Here's Pakistan denying having known about the raid, "Bin Laden: Pakistan intelligence agency admits failures", BBC which also states: "US helicopters entered Pakistani airspace making use of blind spots in the radar coverage due to hilly terrain."

Feel free to leave more questions if you have them.

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    You're a real military pilot. I'm just a civilian student pilot. When I'm below 1000' at 100 knots I'm really focussed because that's the danger zone. It's hard to imagine you guys at half that height, at much higher speeds, at night, without lights, in formation. How on earth do you avoid hitting stuff?
    – Mike Dunlavey
    Commented May 4, 2011 at 2:09
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    Lots and lots of training! I have over 3000 flying hours, and the guys employed for the SOF mission usually have even more. Helicopter flying is much different than fixed wing. Also, we have some of the best equipment that technology can give us. Some night flying is like flying at day.
    – Larian LeQuella
    Commented May 4, 2011 at 2:13
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    I was only a line pilot. Those SOF guys are THE BEST.
    – Larian LeQuella
    Commented May 4, 2011 at 2:28
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    @xiaohouzi79, you are correct. It's INCREDIBLY RISKY. But the return on taking that risk was deemed worth it. Remember, combat flying and combat operations are pretty much totally foreign to anyone who has not been in those situations. And we are "notionally" friendly with the Pakistani government, so AFTER the mission we let them know. Not that they could do anything about it considering the results.
    – Larian LeQuella
    Commented May 4, 2011 at 2:30
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    Okay, I can link the images. As to the types of helicopters, I need to see if any of that information is unclassified, otherwise I'd be speculating (but with a good background since I did work A3 in the CENTCOM AOC).
    – Larian LeQuella
    Commented May 4, 2011 at 10:26
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I recall Mathias Rust landing a plane in the middle of Moscow during the Cold War, completely undetected and unknown to the Soviets. I'm thinking Pakistan's surveillance is probably not as good as the Soviets. In any case, by the time Pakistani authorities would have been notified about it and had time to do anything, they were already out of the country again.

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    You're thinking about this guy: en.wikipedia.org/wiki/Mathias_Rust
    – Nevermind
    Commented May 4, 2011 at 12:22
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    ALL ATC (Air Traffic Control radar) is dependent on a Transponder to identify an aircraft accurately. They are able to do "skin paints" but are generally not powerful enough to do that reliably. The aircraft being tracked needs to activelly broadcast their position, something the SPEC OPS guys probably weren't doing.
    – JasonR
    Commented May 4, 2011 at 14:08
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    According to the link, he was detected, part of the time. They just didn't take him seriously and erred on the side of non-action. This was only 4 years after KAL 007 where they erred on the side of action. A friend's brother was on that plane. That also helped Gorbachev.
    – Mike Dunlavey
    Commented May 4, 2011 at 14:27
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    @Brightblades ATC needs a transponder to IDENTIFY an aircraft, but not to detect it. ATC can pick up an aircraft by radar, and know its position (subject to all the restrictions identified by other answers) they just won't know what it is without a transponder or radio contact.
    – DJClayworth
    Commented May 4, 2011 at 16:26
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    Brightblades overstated the need for a transponder (but what does he know, he was always in back of the airplane flying the boom). Skinpaints are possible, but they don't give a lot of info. Not to mention that a target needs to be out of ground clutter. Something I doubt these helicopters were doing.
    – Larian LeQuella
    Commented May 4, 2011 at 22:44
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Well from this article I understood that there was some cooperation between the Pakistani and US governments. More specifically, it is about this quote:

Obama praised Pakistan for its "close counter-terrorism co-operation". But officials said the US was the only country that knew in advance of the operation.

I think that it is plausible that Pakistan had an agreement to allow the US to execute the raid, although the specifics were not given the US.

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    seems from what I've heard that even the white house and especially congress were kept in the dark because of the very real potential for leaks from there, and only informed after the operation was over (or well underway). This after leakage by those channels of classified information in the recent past.
    – jwenting
    Commented May 31, 2011 at 5:37

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