1945 a B-25 bomber crashed into the empire state building, page 4

reply to post by thedman

Good compilation of sources, thedman.

Some may eventually wonder how ATC was able to see UAL 175's altitude....and, I suppose some 9/11 "denier" will pop up eventually, so let's nip that in the bud, now.

First, I wall assume everyone knows what the transponder in modern aviation usage. The link is there, for reference and clarity.

Unlike the other three hijackers, the terrorist/pilot of UAL 175 apparently just changed the discrete transponder code, rather than selecting it to STBY.

In STBY it is the same as OFF, essentially...they just aren't designed with a position labeled OFF.

So, because the transponder was still active, and in Mode C, it DID still 'respond' to every radar sweep 'hit', and transmit altitude information.

Switching the originally-assigned code, though, did cause the original 'data block' information that is displayed on the controllers' screens to drop out...this data shows the airline name & flight number, (OR, if not operating under FAR 121 or 135, the airplane 'N' number), model aircraft, departure and destination airports, and groundspeed, in addition to altitude.

In the case of UAL 175, it was somewhat easier for ATC to re-acquire than with AAL 77 and UAL 93, both had transponders in STBY, so reliance was on 'primary' radar skin paint acquisition.

Once they identified it, they could assign any 'name' they wished, as mentioned in the linked articles referenced. It's just a matter of telling the computer, by typing it in...

There is considerable lag time in the ability of the Mode C, as to accuracy..depends on WHEN the antenna sweep 'hits', and the time between each 'hit', etc. The information only updates with the 'hit', and occasionally will even skip once or twice....the system doesn't interpolate.

Groundspeed information may or may not appear, also. It isn't an instaneous value, since the radar computers have to calculate it, it isn't 'transmitted' downrange by the transponder. Only the altitude is.

Found this info, in timeline. I've emphasized bits, hopefully to make it more readable, is all:

8:37- Boston Center asked United 175 and other aircraft if they had seen "American 767" (American Airlines Flight 11). United 175's pilots said they had seen it. The controller directed United 175 away from it as a safety precaution.

8:41- United 175 entered New York Center's airspace. The New York Center controller responsible for United 175 was the same controller assigned the job of tracking hijacked American 11.

8:47- At almost the same time American 11 crashed into the North Tower, United 175's transponder code changed several times. The changes were not noticed for several minutes as the controller tried to locate American 11.

8:48- New York Center manager gave the following report to an FAA Command Center teleconference about American 11:

Manager, New York Center: "Okay. This is New York Center. We’re watching the airplane. I also had conversation with American Airlines, and they’ve told us that they believe that one of their stewardesses was stabbed and that there are people in the cockpit that have control of the aircraft, and that’s all the information they have right now".

The New York Center controller and manager were unaware that American 11 had already crashed.

8:51- The controller noticed the transponder reading from United 175, and asked the plane to go back to the proper code. There was no response.

8:52- The controller began to make repeated attempts to reach United 175, without getting a response.

8:53- The controller contacted another controller, worried that "we may have a hijack." Another commercial aircraft in the vicinity radioed with "reports over the radio of a commuter plane hitting the World Trade Center." The controller started to hand off other flights on his scope and moving aircraft out of the path of United 175.

8:55- The controller-in-charge notified a New York Center manager that she believed United 175 had also been hijacked. The manager tried to notify the regional managers, but was told they were discussing a hijacked aircraft (presumably American 11) and refused to be disturbed.

9:01- A manager from New York Center contacted the Command Center in Herndon, Va.:

Manager, New York Center: "We have several situations going on here. It's escalating big, big time. We need to get the military involved with us ...

We're, we're involved with something else, we have other aircraft that may have a similar situation going on here ..."

The "other aircraft" New York Center referred to was United 175. Evidence indicates that this conversation was the only notice received prior to the second crash by either FAA headquarters or the Herndon Command Center that there was a second hijack.

New York Center also contacted New York terminal approach control and asked for help in locating United 175:

Terminal: "I got somebody who keeps coasting (**)but it looks like he's going into one of the small airports down there."

Center: "Hold on a second. I’m trying to bring him up here and get you— There he is right there. Hold on."

Terminal: "Got him just out of 9,500-9,000 now."

Center: "Do you know who he is?"

Terminal: "We're just, we just we don't know who he is. We're just picking him up now."

Center (at 9:02): "Alright. Heads up man, it looks like another one coming in."

9:02- The controllers observed the plane in a rapid descent; the radar data terminated over lower Manhattan.

9:03:02- United 175 crashed into the World Trade Center's South Tower.

9:03- NORAD air defenders learned of a second hijacked plane about the same time United 175 crashed into the tower.

www.npr.org...

(**) Explanation for 'coast mode':

As soon as the transponder code is interrupted or lost, the radar system displays a target with appropriate data block information at a computer predicted next target position. In this situation, the target and data block flash on the RSiT to indicate that the target is 'coasting'. In the Toronto Area Control Centre (ACC), a target coasts (flashes) for approximately 18 seconds or six radar update cycles following which the radar target disappears from the RSiT. After this time, provided no target is detected by the radar with the correct transponder code, the aircraft data is entered into a coast list, which is also displayed on the RSiT. Entries are placed in the coast list in order of occurrence. If the aircraft's transponder is re-acquired, the data is then again attached to the radar target.

Above was snipped from THIS article.

[edit on 23 June 2010 by weedwhacker]