Flight data recorders

[jaydeehess]

How does the NTSB get positional data from the DFDR? Here's (http://www.ntsb.gov/info/AAL77_fdr.pdf) the pdf of the DFDR report for AA flt 77 that hit the Pentagon



The NTSB also has an animation of the flight from the time it took off to the last recorded data on the DFDR. However I was wondering what data points on the DFDR would be used to plot the position over the Earth of the plane. It has pressure altitude so the height I can see being derived from that but as far as I can ascertain the only data points recorded that would aid in setting the position of the aircraft would be the acellerometers' data. That would make the position no more accurate than the old Inertial Navigation System used before GPS.

[Fazor]

I'm sure they can use flight data collected from air traffic monitoring on the ground as well. They don't *only* use the info from the data recorder.

[jaydeehess]

The point that those who believe its all a conspiracy make is that the animation shows the plane too high and too far north to have hit the lampposts before hitting the Pentagon and I have to admit it seems they are correct.

Now the altitude could be a mistake in deriving true altitude from the recorded pressure altitude but the location over the surface would either be recorded GPS data or acellerometer data or as you say radar tracking data. The first one would be accurate to within feet thus the later two would be unneccessary. If GPS was not part of the data recorded then it would have to be a combination of the later two data sources. This would have an inherent error and if the animation shows the plane's position strictly as the median of those two data sources or even just from inertia calculations from the acellerometers then that would explain why the plane is too far north in the animation.

I can't make much from the FDR data myself, I don't even recognize many of the abbreviations so I can't say how the position of the craft is deduced.

[Larry Jacks]

I'll have to do some research on the navigation equipment available on AA 77. Not all airliners built in the 1980s and 1990s had GPS. They used the inertial navigation system and even old VORTAC technology. For coast-to-coast flights, either one of those sources is more than accurate enough for routine operations. After all, inertial navigation systems are used for things like space boosters and ballistic missiles. They can be plenty accurate and it would be a straight-forward matter to pipe positional data from the INS to the FDR.

[Nicolas]

The first one would be accurate to within feet

Non augmented GPS doesn't have that vertical resolution as far as I know. (if you meant with "within feet" something like "within 3 feet" )

[Larry Jacks]

IIRC, WAAS (Wide Area Augmention System) GPS augmentation wasn't in operation in 2001. Standard GPS vertical resolution isn't all that good, somewhere on the order of 30 meters. WAAS is good enough vertical accuracy (a few meters) to make precision approaches with near ILS accuracy. Standard GPS is not nearly that accurate in the vertical dimension.

[jaydeehess]

Non augmented GPS doesn't have that vertical resolution as far as I know. (if you meant with "within feet" something like "within 3 feet" )

I meant in the lat/long position though, not the vertical. For vertical position it is most likely the pressure altitude recoding that is used.

[Nicolas]

Ah ok :)