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ERROR: 'Pentagon Attack Maneuvers Preclude a 757'

A fact frequently cited as evidence that the aircraft that attacked the Pentagon on 9/11/01 was not Flight 77, a Boeing 757, is that the aircraft tracked by air traffic controllers made a spectacular spiral dive, losing 7000 feet and turning 270 degrees in about 2.5 minutes -- a maneuver alleged to be impossible for a 757. A September 12, 2001 CBS News report described the maneuver:

Radar shows that Flight 77 did a downward spiral, turning almost a complete circle and dropping the last 7000 feet in two-and-a-half minutes.

Air traffic controller Danielle O'Brien told ABC News that the maneuver was not one expected of a jetliner:

The speed, the maneuverability, the way that he turned, we all thought in the radar room, all of us experienced air-traffic controllers, that that was a military plane. 1  

However, the fact that the plane was being flown in a manner not typical for a jetliner does not mean it was not a jetliner. A 757 is capable of rather extreme maneuvers: It is capable of taking off on one engine, and can execute pitch accelerations of over 3.5 Gs (gravities) as demonstrated by the following incident report of an IcelandAir 757-200:

REPORT 7/2003 - Date: 22 January 2003
serious incident to icelandair BOEING 757-200 at oslo airport gardermoen norway 22 january 2002

...
1.1.14.5 At this time the First Officer called out PULL UP! - PULL UP!. The GPWS aural warnings of TERRAIN and then TOO LOW TERRAIN were activated. Both pilots were active at the control columns and a maximum up input was made. A split between left and right elevator was indicated at this time. It appears the split occurred due to both pilots being active at the controls. The pilots did not register the aural warnings. During the dive the airspeed increased to 251 kt and the lowest altitude in the recovery was 321 ft radio altitude with a peaked load factor of +3.59 gs.
2  

How does this apply to the 2.5 minute 270-degree spiral turn? The G forces produced by such a turn can be calculated using the following formula.

RCF = 0.001118 * r * N^2
where
RCF = Relative Centrifugal Force (gravities)
r = rotation radius (meters)
N = rotation speed (revolutions per minute)
If the plane were traveling at 400 miles per hour it would travel 16.666 miles, or 26,821 meters, in 2.5 minutes. Assuming it was traveling in a circular arc, it would trace out 3/4ths of a circle with a 35,761-meter circumference, giving a rotation radius of 5,691 meters and rotation speed of 0.3 rotations per minute. Plugging those values into the above equation, we obtain a centrifugal force of 0.5726 Gs -- hardly a problem for a 757 whose rated G limits are over two.

Final Approach

Also cited as evidence against 757 involvement in the attack is the shallow descent angle of the aircraft as it made its final approach of the Pentagon. Photographs show no signs of gouging of the lawn by a 757's low-hanging engines, even though direct impact damage was limited to the first and second floors of the building. How could such a large aircraft be flown so close to the ground, and with such precision?

Two distinct questions are implicit in the previous one.

  • Were alleged hijackers capable of piloting the airliner through the maneuvers?
  • Could a 757-200 perform the maneuvers?
Hani Hanjour may not have been up to the task, but a 757's flight control computer seems sufficient. It's equipped with radar altimeters and accurate GPS monitors for precise altitude and position tracking. It can analyze and respond to conditions hundreds of times per second. Examples of the extreme capabilities of fly-by-wire systems are reverse swept-wing aircraft, which are inherently unstable and require rapid adjustment of the plane's control surfaces.


References

1. Air Traffic Controllers Recall Sept. 11, ABCNews, 10/24/01 [cached]
2. Aircraft Incident Report, Norwegian Accident Investigation Board, 1/22/02

page last modified: 2009-04-21
Copyright 2004 - 2011,911Review.com / revision 1.08 site last modified: 12/21/2012
The final portion of the flight path of Flight 77 as reported by the NTSB