How easy is it to hijack a plane? Are pilots trained to handle such a situation? ABCNEWS Aviation Analyst John Nance will answer your questions about today's events in a live chat at 7:30 p.m. ET.
    Nance is a decorated Air Force pilot and a veteran of Vietnam and Operation Desert Shield/Desert Storm. He serves as a lieutenant colonel in the Air Force Reserve and has extensive flight experience, having logged more than 13,000 hours of flight time in his commercial airline and Air Force careers.
    Please post your questions for him now in the space below.

Moderator at 7:19pm ET
John Nance answered a few of our questions earlier today. Look below for his responses, and stay tuned for the live chat at 7:30 p.m. ET.

If you have a question for John Nance, please type it in the space below and click on "Post Message."

Moderator at 7:20pm ET
Do you have to be a skilled pilot to fly the 757/767? We're trying to understand how easy it would be for someone to just take control and man the flight.

ABCNEWS' John Nance at 7:25pm ET
Only a skilled pilot can takeoff, fly, and land a Boeing 757 or 767 and do it safely. It would be extremely difficult to imagine a small airplane pilot, untrained in large aircraft, initiating flight in such a big jet.

But is it possible that a pilot of lesser training (smaller aircraft) could take over in flight and guide such a jet into a specific building? Yes, it is, but keep in mind that there is no question here that extensive, meticulous planning was behind this outrage, and if "you" are the leader of the terrorist organization planning such a crime against humanity and expending incredible sums of money and effort, would you want to leave the potential "success" of your operation to the chance that a pilot of lesser training might foul up the operation and not hit the intended target? I would think not. Logical speculation leads directly to the thought that the terrorist-pilots involved here had at least some training in or exposure to large aircraft.

Again the problem is precision and certainty. The enemy here wanted to hit four targets dead on, and to do that requires quite a bit of pilotage skill in the least few seconds. A small aircraft pilot will be very unfamiliar with the aerodynamic responses of a 757 or 767, and will more than likely mishandle it in those critical last seconds.

As I said on the air awhile ago with Peter Jennings, it is also very curious and a potential clue that the two types of aircraft in today’s acts of war against the United States are the only ones which have what's known as a common type rating: i.e., training in a 757 qualifies you completely and legally to fly a 767. That could be coincidence, but it’s probably by design.

Moderator at 7:26pm ET
What defense does a pilot have in the cockpit if someone is trying to hijack the plane?

ABCNEWS' John Nance at 7:27pm ET
Professional pilots have extensive training in how to handle a hijacker who wants something specific (such as transportation or money, or the opportunity to make a statement and survive).

We have long known, however, that there is nothing but grave danger in the idea of either arming aircrews in order to give us the possibility of shooting a hijacker, or in otherwise fomenting an airborne struggle in the cockpit. (And If you’ll recall, we had an unsuccessful airborne struggle in the cockpit of an EgyptAir 767 nearly two years ago, and the captain lost his battle to regain control from a suicidal/homicidal co-pilot.)

In addition, there is no way we can so harden the cockpit door that no one could force or coerce his way in.

Finally, when someone is well armed, and has carefully planned an attack that includes their plan to die in the process, our defenses in a free society with a robust, accessible airline system rest almost exclusively on a combination of good intelligence and intelligent screening.

Moderator at 7:28pm ET
What do you think a pilot would do if confronted at gun point and asked to re-route a plane?

ABCNEWS' John Nance at 7:30pm ET
I can tell you quite clearly what any of some 29-thousand airline pilots would do if confronted with a gun to the head and asked to divert to another airport: we would comply.

BUT, if given any indication whatsoever that the plan was to crash the airplane into anything, there would be absolutely no compliance, because to comply would guarantee our deaths and the potential deaths of many others.

Given that order -- fly into a building -- any airline pilot would fight, or at the very least, attempt to thwart the attempt. These terrorist planners knew this, and therefore they would not have depended on the possibility of intimidating the pilots; they would have killed them or completely incapacitated them immediately.

You can take this to the bank: the human organisms at the controls of each of those aircraft were not the airline pilots assigned to those flights. While I say this with full knowledge that human nature dictates that a turncoat is possible, postulating four turncoats coordinated on four different airliners going in four different directions exceeds rational probability.

Moderator at 7:31pm ET
How common are hijackings?

Moderator at 7:32pm ET
Hijackings are very uncommon in the United States and Canada because of reasonably good security procedures and because of reasonably good intelligence. But such defense systems are obviously not perfect. We stand a very good chance of thwarting any hijacker who is mentally unbalanced or simply wants money or attention.

We stand vulnerable in a free and robust system to the professional suicide terrorist.

ABCNEWS' John Nance at 7:32pm ET
Hijackings are very uncommon in the United States and Canada because of reasonably good security procedures and because of reasonably good intelligence. But such defense systems are obviously not perfect. We stand a very good chance of thwarting any hijacker who is mentally unbalanced or simply wants money or attention.

We stand vulnerable in a free and robust system to the professional suicide terrorist.

Moderator at 7:33pm ET
John Nance will join us shortly to answer your questions. Please post them in the space below.

Moderator at 7:36pm ET
Welcome John Nance. Thanks for joining us.

Milind at 7:37pm ET
Three of the four planes were going to L.A? Why do you think the hijakers chose the flighs going to L.A?

ABCNEWS' John Nance at 7:38pm ET
There's a very solid reason to explain this. We don't know for a fact why these murderous individuals did this for certain, but we do know that if you hijack an airplane that is going to another side of the country, you end up with an aircraft full of fuel and more than likely that was the reason.

ABCNEWS' John Nance at 7:40pm ET
Why would you want an aircraft full of fuel? If your intent was to do the maximum amount of damage with an aircraft without loading additional explosives which might be detected by security, crashing thousands or tens of thousands of pounds of jet fuel into the target would aid your design.

Gretchen C. at 7:40pm ET
Is it reasonable to assume that the pilot of the plane which crashed in Pennsylvania must have done so deliberately in order to save lives by crashing into a sparsely populated area rather than the intended target?

Moderator at 7:43pm ET
John Nance had to step out for a few minutes .... he'll be back shortly. Please keep sending in your questions below.

ABCNEWS' John Nance at 7:54pm ET
No, it is not reasonable to assume that, and I'll explain why. First of all, an air crew member is going to fight for his or her life and the lives of those onboard, and is never going to essentially commit suicide in order to prevent some greater harm.

More than likely, this crash of the fourth aircraft was a result of some hellacious battle in the cockpit, and if that is true, then it is more than likely because the hijackers, unlike their compatriots on the other three airplanes, failed to neutralize both pilots rapidly enough.

ABCNEWS' John Nance at 7:56pm ET
Remember that in the case of Egypt Air's 767 that crashed off New York, the captain was in a battle with a suicidal/homicidal co-pilot and lost that battle, and the aircraft crashed. This is why we do not teach air crews to attempt to fight hijackers because where an otherwise salvagable situation occurs with a hijacker who does not intend to die in the process, having an airborne battle can make an otherwise recoverable situation fatal.

FJ from fcr.arizona.edu at 7:58pm ET
How does a pilot/aircrew signal the plane is being hijacked?

ABCNEWS' John Nance at 8:00pm ET
I know, but I really can't tell you because even though it's not top secret in a military sense, the methods we have to signal air traffic control in a salvageable situation should really not be openly available.

But suffice it to say that we're well trained and equipped to positively indicate to air traffic control that we have been commandeered.

The fact, if it is a fact, that the pilots of at least 3 of these 4 flights were unable to utilize those methods indicates strongly that they were killed very rapidly by the hijackers.

Brian (Dublin Ireland) at 8:01pm ET
Is this going to change aviation in America and indeed around the globe? The whole terrorist operation seems to have happened far too easily?

ABCNEWS' John Nance at 8:01pm ET
Change following this act of war, because that's what it is, is inevitably going to affect the way we process and accept passengers in North America, if not throughout the world.

ABCNEWS' John Nance at 8:03pm ET
But there is one thing that you need to keep clearly in mind: In order to have an open, easily accessible and robust commercial aviation system, we are always going to have some vulnerability to professional terrorists who are willing to die in the process of committing mass murder.

Moderator at 8:03pm ET
Carrie wonders: "How could all of the terrorists make it through airport security? I can understand one, but four almost simultaneously??"

ABCNEWS' John Nance at 8:05pm ET
First of all, we just do not know how they did this for certain, although the cellular phone call from one of the hijacked aircraft strongly suggests the utilization of knives that might have been easily taken through security. If that's true, then further screening crackdowns need to be effected worldwide.

ABCNEWS' John Nance at 8:06pm ET
However, answering the question of how four sets of terrorists could slip through is fairly simple: This was a sufficiently professional and well-studied operation that they knew precisely how to do it, and simply replicated their exploitation of the weaknesses in the system.

Texas at 8:07pm ET
How long will it take to find ALL the black boxes, and what are the odds of finding the black boxes from the two planes that hit the trade center?

ABCNEWS' John Nance at 8:09pm ET
Finding the flight data recorder and the cockpit voice recorder from the aircraft near Pittsburgh and the one that hit the Pentagon will be fairly easy.

Finding the two boxes from each of the aircraft that were rammed into the World Trade Center Towers will be infinitely more difficult because of the sheer volume of the wreckage of those 2 buildings.

Eventually, the World Trade Center boxes will be found, but it literally could be months.

Moderator at 8:10pm ET
We've heard that these planes made significant flight deviations. Do you think ground control knew what was going on? If so, what, if anything, could they do to help?

ABCNEWS' John Nance at 8:14pm ET
Air traffic controllers apparently saw radical changes in the flight path of each affected airplane at the same time they realized that the air crews were no longer responding to normal radio calls.

Even in the absence of the signals that we discussed earlier, indicating a hijack, the fact that the aircraft deviated from their assigned flight path, while they were under "positive control" (all airliners fly under a set of rules called IFR, Instrument Flight Rules, which require continuous obedience by the pilots to their assigned or cleared flight path as coordinated by air traffic controllers) would have indicated a hijacking to most any air traffic controller immediately.

Sebastian at 8:17pm ET
I heard rumors concerning a number of planes unaccounted for. Do we know for certain where all of todays flights ended up?

ABCNEWS' John Nance at 8:19pm ET
At this hour, almost 10 hours after this horror began, it's pretty clear that there are no further "missing" aircraft.

This morning, at least for the duration of time that it took to get affirmative reports from the FAA, there was great concern that one or more aircraft might be in flight with hijackers waiting to strike. In many respects, this was one of the reasons for the mass grounding, which was a very appropriate and wholly unprecedented act.

Corbett Vanoni at 8:23pm ET
When the first report of a hijacking comes in - what is ground control's first priority - and will those procedures change in light of this incident?

ABCNEWS' John Nance at 8:26pm ET
When notification of a hijacking or possible hijacking rumbles through the air traffic control system, it becomes a general alert and is tracked not only at the local facility, such as an air route traffic control center, but is also tracked and coordinated at the Herndon, Virginia facility that is the master control center for the entire air traffic control system (like mission control at NASA).

Whether procedures will change in the future is going to be a hotly questioned subject, to the extent that there may be evidence that some sort of earlier warning could have been useful.

ABCNEWS' John Nance at 8:28pm ET
However, one very important word of caution: Asking the question who is wrong here, i.e. what individuals or agencies failed, is absolutely the wrong methodology. The appropriate question that must be asked and quickly answered is: What were every single one of the systemic failures that contribued to this disaster?

Moderator at 8:28pm ET
John, thanks for your time this evening. Any final remarks on the day's events?

ABCNEWS' John Nance at 8:33pm ET
We have to understand that this was not just an attack on the United States, but a crime against humanity, and insofar as that is true, this should be the beginning of the end of willingness on the part of any nation on earth to stand in the way of swift and vicious crushing of terrorist forces, terrorist training camps, or terrorist financing anywhere on this planet.

In a more nationalistic sense, it is appropriate to remember that on December 8, 1941, President Roosevelt, referring to the heinous attack on Pearl Harbor the day before, defined it as "a day of infamy." So is Sept. 11, 2001.

If you so desire, please visit my Web site over the next week for some periodic updates on the aviation aspect of this JohnJNance.com

ABCNEWS' John Nance at 8:34pm ET
We have to understand that this was not just an attack on the United States, but a crime against humanity, and insofar as that is true, this should be the beginning of the end of willingness on the part of any nation on earth to stand in the way of swift and vicious crushing of terrorist forces, terrorist training camps, or terrorist financing anywhere on this planet.

In a more nationalistic sense, it is appropriate to remember that on December 8, 1941, President Roosevelt, referring to the heinous attack on Pearl Harbor the day before, defined it as "a day of infamy." So is Sept. 11, 2001.

If you so desire, please visit my Web site over the next week for some periodic updates on the aviation aspect of this at JohnJNance.com

Moderator at 8:35pm ET
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