How did the terrorists know where to fly?

reply to post by JohnnyR


Johnny,

the difference between near Sea Level and 35,000 is indeed different....but airspeed relates to not only altitude, but temperature.

When we calculate 'true airspeed' (TAS) we take into account the indicated airspeed (IAS), the temperature, and in the case of jets, the comprssibility effects due to speed....

NOW, we also use Knots, not MPH in aviation. A Statute mile is 5,280 feet, a Nautical mile is 6,060 feet....this calculates to a difference of 115 percent....so to convert Knots to MPH you multiply by 1.15. I know, seems backwards at first....simple example, 100 Knots equals 115 MPH.

You don't have to take my word for it, it is readily verifiable online.

Charts produced by Boeing are trying to show how the airplane will perform at normal cruise altitudes, and trying to sell the airplane to potential customers. We all know that modern commercial jets do not typically fly at 1,000 feet above Sea Level at 500 MPH (same as about 435 Knots). The MAXIMUM airspeed, or VMO is typically 340KIAS....but as I said before, indicated airspeed and true airspeed are not always the same...TAS is typically higher than IAS

Of course, we're not going to get a 95 Knot difference near Sea Level, but when I mentioned the 340KIAS Maximum, I was referring to a 'legal' limit....the airplanes could certainly be flown faster....especially if a dive was involved, thus using Gravity to assist the 'OverSpeed' condition.

Finally, at near Sea Level altitudes, even 435 Knots isn't approaching 'mach' speed. Yes, the alarms in the cockpit will be going off, but the airplane will still be controllable....

Of course, fighter jets can exceed Mach 1, even at Sea Level....but they are designed to do so, and remain controllable.

If a commercial jet approached Mach....there would be a phenomenom called 'Mach Tuck' developing....along with a lot of airframe shaking...if left uncorrected, there could be airframe damage.

But a short, very few minute diving run at a target? Very possible, knowing the aerodynamics of a B757 or B767......



[edit on 4/4/0808 by weedwhacker]

from what i understand the aircraft in question did not have GPS capabilitys this cut that out of the argument.
however i think its mor than safe to say that the flight decks would have had a compass and/or heading indicator.
now if the government are telling the truth in that the hijackers had, had fligth experiance it is more than likely that they would have been able to navigate manualy using these, after all any idiot would know the general direction of a major city like New York espetialy if its been planned, if they knew the general direction they would have known the general heading to fly on. and unless somthings went majorly wrong with thier flying you not going to miss a city of New Yorks size once over the city they would just be looking for thier targets.
not even a need for any GPS or even autopilot.

reply to post by shmo5


NO! the airplanes DID NOT have GPS capability.

shmo5....do you own a car-based navigation system?

If you do, then this is where you have a problem, in understanding how airplane navigation systems worked, before GPS was incorporated.

See, in 2001....the FAA had not approved the GPS Updating for commercial airliners, yet.

The tech existed, but wasn't approved....although business jets had it.

The early days of the platforms, regarding what was then called 'INS', for 'Inertial Refercence System' involved actual, physical gyroscopes....three, in three different axial directions, along with electronics that would measure the precession of each gyro, as it was moved in direction, thus imputing the direction of the airplane. Based on the 'initialization' of the system, telling it, by Pilot inputs, the Latitude and Longitude, the system could then calculate, via the gyroscopes, where it was...up to a point.

Problem is, there is this nagging problem called 'friction'...and, also, 'precession'.

So the early INS's had errors, that could accumulate. That is why for overwater Operations THREE were required. They could check each other, for errors....and if one went foul, it would get 'thrown out', and pilots notified.

Then, we had the 'IRS's....'Inertial Reference Systems'

These used 'laser gyros'. So, instead of mechanically spinning gyros, they used light instead....far more accurate....PLUS now they incorporated something caled 'Radio Updating'....into the software. This means, you still initialize the IRS at your gate, on the ground....(and it remembers where it was last shut down, so you can't fool it, or it will ALERT!)

BUT, now it receives other inputs, when within VHF range of a VOR or DME, to help it verify, or even update its position (where it thinks it is)...(again, 3 of them, to check each other, and throw out one that goes wrong, then notifying the Flight Crew)

NOW, most US jets also have GPS updating.....far, far more accurate.

Same triple redundancy still applies....We can access the positions, of each IRS, on a screen....the three displayed, along with the 'computed' position, as a compilation of all three, if they are still being accessed....that is what is called the 'FMS' position.

BUT, if on a 'precision' approach, we still use standard VHF and UHF radio signals. There is a process in the works for more 'GPS' appraoches, but progress is slow. Safety is paramount.

I've given a lot of detail, but only to summarize.

The onboard computers had a lot of 'waypoints' in the database....think of the GPS in your car, for instance. It is not difficult to access the database, and display it in a way that any pilot, even with minimal skills, could navigate to. Once HE or SHE knew how to access the data.

Then, turn and steer, follow the purple (in Boeings) path, as displayed on the screen......(or white, until you press the 'EXEC' button.....)

reply to post by shmo5


shmo5....I wrote a lot....but, ya know, it is an FAA requirement that EVERY airplane has a magnetic compass!!!!

So, hope I added to your knowledge. Don't go out and hijack an airplane, OK????

EDIT....because, you will fail!!!!

[edit on 8/12/0808 by weedwhacker]

Don't forget that the aircraft will have already been set up with a route, that is displayed on at least one ND, if not both. At the top of the display is a heading indicator.

The hijhackers will at least have a good idea of the flight path of the aircraft prior to them hijacking them, so they would have already worked out that by flying a certain heading, they acn just wait until they see a certain landmark.

Whilst the guys actually carrying out an attack might have issues, the planners are very intelligent, and would have thought about it.

From 35,000 ft major landmarks are evident. Rivers, major roads, railways can still be seen. Airports are also visible.

What you've got to remember is the angle at which you're looking. You aren't looking straight down. The higher you are, the further out is the bit of land you're looking at (it could be as far as 40 miles away, laterally, even though you think you're looking very close).

I've often wondered what they would have done had it been cloudy, and they couldn't see.

[edit on 12-8-2008 by mirageofdeceit]

reply to post by mirageofdeceit


Oh, man!!! I hit 'reply to', and lost your points.

mirage, you just used an acronym....'ND'

I assume you meant that as the 'Navigational Display'??

OK....on the EFIS in the B757/767 the lower display is normally the ND....but it can be selected to various display settings.

Heading UP, Track UP, PLAN, etc. AND, there are various range settings, as well. UP TO 160 NM. (320 NM on the B767-400)

It can also be selected between TRUE and Magnetic....guess you may not have known that.

AND it can be selected to 'Expanded HSI' or 'NORM HSI'

Now, here's where it gets interesting.

I could put you into a simulator, and in a few hours, if you have just a bit of aviation experience with HSIs, I could show you how to navigate a modern jet to where you want to go, as long as you know the waypoint to enter....or, you could just scroll in the already loaded flightplan, and bring the origination waypoint up, and enter it as the new destination. The A/P, if engaged, will now turn to the new waypoint. IF the A/p is not engaged, you can re-engage it, or just steer based on the EFIS image, on the lower screen....which is the one I think you referred to, in your post.

Any questions????

EDIT to make sure my spelling was correct....







[edit on 8/12/0808 by weedwhacker]

weedwhacker,
please read my post agian i said that they did NOT have GPS.
i mearly stated that i was possible for the hijackers (even with minimal experiance) to navigate to the WTC and the Pentagon. Thank you for the info tho it was a good read.

reply to post by shmo5


shmo5....

sorry, lost your post, but reponding now.

Yeah, I know they did NOT have GPS, back then. Hope I didn't imply they did.

I think I'm clear here.....an AutoPilot could not have flown into the targets.

A PERSON could....but not the machine.

Writing, isn't the best way to communicate, every time. Doing my best!

weedwhacker,

just too say thhat I was generaly agreeing with you in my first post, i didnt say they used auto pilot either. I just said that they could have used the compass to fly towards the general location of the WTC.


sorry if im confusing you