Wednesday, December 03, 2008

A Discussion of the Flight Data Recorder Information for Flight 77

As has been requested several times by Bill Giltner, here's the thread at JREF going over the information retrieved from the FDR of Flight 77. IIRC the PfT Truthers like to say that the FDR ends with the plane still a couple hundred feet in the sky and this proves that Flight 77 did not hit the Pentagon. Anti-Sophist and several other people explain some of the potential problems:

1) Instrument Error
First, and foremost, if the sensor or instrument is giving the recorder a bad number, it’s obviously not going to record the right one. This type of error must be dealt with on an instrument by instrument basis. Any reconstruction should justify the precision used for each value attained. Please keep in mind that all other errors in this document are due to the recording system, and the lost information in the processed version of its data. The uncertainty caused by the data scheduling into a frame, plus the digital buffering, is in addition to any instrument errors.

False Claim: I already debunked the lagging altimeter nonsense!
(The recording-system errors discussed in this document are in addition to, and independent of, any instrument errors.)

2) Intra-frame Time Error
Since we do not have the frame descriptor, all we know is that N samples are taken during a 1 second period. This means that 1/N of a second is the possible error range for a particular data point. With the frame descriptor, this error would be completely removed if using the raw FDR data.

False Claim: The aircraft’s speed at 09:37:14.00 was 305.5 knots!

3) Digital Buffering Latency
One of the most important purposes of the DAU is to buffer digital outputs from things like the ADC (Air Data Computer). It is a reasonably safe assumption that the Air Data Computer updates the DAU at least once per sampling, and more than likely twice. This means that for a 1-Hz sample, recorded into the data stream, the actual measured time could have been anywhere in the entire previous second. This means, combined, a digital reading in the CSV file, like Computed Airspeed, which comes from the Air Data Computer, has an enormous error range, in the vicinity of 2 seconds, although 1.5 seconds is probably a safe estimate (0.5s for the buffering latency, and 1s for the uncertainty of when the sample was actually recorded). More then likely, the raw data stream has embedded the actual measurement times, so this error might be completely removed using the raw FDR data.

False Claim: The worst case scenario for the 9:14:14 frame’s airspeed is 9:14:14.00, then!
(Yes, that is the worst case time it could be recorded… not measured).

There's lots more, but it's even more technical. This is one of those cases where I bow to the experts and say "Have at it," because I'm clearly not going to understand the answer at any kind of a detailed level.

On the non-expert level, I just kind of roll my eyes at the thought that the Troofers have that the government faked FDR information for Flight 77 but screwed up and left the plane a couple hundred feet up in midair. What are we supposed to think, that the guy who was assigned to do it is secretly a troofer but doesn't want to lose his job, so he faked the data but left clues that it was fake. Now that PfT has exposed it, wouldn't he lose his job anyway?

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