Now let's take a look at the altimeter and see what's used in a 757 instead of a Cessna 172.

I'm not familiar with flight data recorders, but I assume either digital instruments or analog-converted-to-digital signals are used to record the data.

Here's the NTSB's summary of the data recorded for AA Flight 587 that crashed in 2001:

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Note that as with Flight 77, NTSB does not correct the data for pressure altitude.

Note that one of the instruments used for altitude reporting to the FDR is a radio altimeter - and that the radio altimeter's data points were found to be "erroneous".

One of the digital instruments used in a 757 is a Rockwell-Collins LRA-900 Low-range Radio Altimeter:

LRA-900

Apparently, one of the issues with these altimeters is inaccuracy when a plane is doing anything other than flying straight and level - here's a patent application that describes - and hopes to resolve - the problem:

Quote
0001]In radar altimeter operation, during aircraft roll or pitch maneuvers, it is possible for the altimeter track (range) gate to slide off the true altitude because the signal level is not maintained with sufficient accuracy. When the aircraft banks errors in the altitude can be generated due to coupled-control-loop induced positioning errors between the track gate and the level gate that are positioned relative to each other at a fixed separation. Likewise, variations in the terrain with respect to the attitude of the aircraft cause errors or inaccuracies due to the coupled-control-loop induced positioning errors. In the worst case, these positioning errors can result in coupled control loop oscillations that result in oscillations in the altitude value the radar altimeter reports. Such errors can cause unsafe flying conditions especially for aircraft that bank at large angles or fly over steep terrain, especially if these occur at low altitudes and the size of the altitude oscillations is a large fraction of the actual altitude.


Patent Description

WRT Flight 77, here's one electrical engineer's opinion of the accuracy of the FDR:

Quote
The Final 1 to 2 Seconds

Given that the data was compressed, and synched, itÆs very likely that any frames that were not complete would be difficult to recover, if even recoverable at all. The implication of this is quite simple, and that is the FDR data in the CSV file ôruns outö well before the plane actually hits.

This means that 9:37:44 was the last, complete frame, gathered by the recorder. That puts the likely time of impact in the 9:37:45-6 range, and possibly even into the 9:37:46-7 timeframe. The presence of 9:37:46 in this data suggests that its timestamp may have made it onto the tape. How is that possible if 9:37:45 is not a complete frame? ThatÆs a good question, but a reasonable hypothesis has to do with the storage mechanism used. Solid State Recorders, like all medium, are quite unpredictable if they fail during write operations. The actual area being used to record data can very easily be corrupted if power fails while writing. ItÆs plausible that the crash caused problems in and around this local area of data, causing corruption of the 9:37:45 data frame (again, changing a single bit in a synch word is enough to cause software to completely choke).

The moral of the story here is that the FDR data runs out anywhere from up to 2 seconds before the plane actually crashed into the Pentagon.


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So, here we have a possible combination of:

1) Electronic latency (historesis) that cannot record in exact real-time. If the last 1-2 seconds of flight data are not recorded, then at 500mph the potential error is between 733-1466' - obviously not straight down as a loss of altitude, but certainly some loss of altitude at the end of the flight.

2) Inaccuracy of the reported altitude due to aircraft attitude and control input just prior to the crash.

Face it, Phil..... if reasonable people thought there was any "truth" in the claims that Flight 77 didn't hit the Pentagon, we would have had full exposure long ago.

Truthers looking for that plane remind me of O.J. Simpson looking for Nicole's killer. <img src="/forums/images/graemlins/lol.gif" alt="" />

Where is that plane?

Frank

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