The information that the program uses on how each parameter is stored in the subframes is mostly from the partial data frame layout in this file. There is no guarantee that this partial data frame layout is correct since airlines can and do use modifications of the standard data frame layouts. I am not as certain that I have been able to correctly decode the parameters as I was for the UAL93 FDR Decoder program, since I have little data to correlate against.

For the numeric values, I used the same formulas to convert the binary values to engineering units as used in the UAL93 FDR Decoder program where these appeared to work.

There are many other parameters listed in the NTSB's American Airlines Flight 77 FDR Report (available here) that the program currently doesn't handle. If there are any that you are interested in tell me (Warren Stutt) about them and I'll consider including them.

ALTITUDE (1013.25mB)
Although the partial data frame layout referred to above indicates that bits 3 and 4 of the coarse data word are used, they appeared to be the copies of bits 10 and 11 of the fine data word and when I used them, produced values that were far too high unless both bits were zero. Therefore, I changed the program to not use them. Bear in mind that these values are uncorrected pressure altitude values that presume that the air pressure is 1013.25mB and will be different from the true altitude if the air pressure was not that value when the altitude values were recorded.

COMPUTED AIRSPEED
This shows some values of 384 whereas all the values should be 0 or close to it since the plane should be stationery when the engines and the FDR are started. Perhaps the most significant bits in the partial data frame layout referred to above should not be used or perhaps this parameter is just plain wrong.

GPS
I guessed that a value of 1 for this single bit parameter meant the GPS was working and a value of 0 meant that the GPS was not working.

PRES POSN
Although the partial data frame layout referred to above indicates that bit 12 of the Least Significant (LS) data is used, when the NTSB used this bit to generate their CSV files, it produced PRES POSN LONG (DEG) values that were out by about 0.35 degrees. This program and Readout2 provided by UnderTow ignore this bit and produce more accurate values.

RADIO HEIGHT F/O
I couldn't produce sensible values for this parameter so I removed it from the program. The other RADIO HEIGHT parameter values looked good.

Block Position
Bit Offset
Flight Stream Number
These values are calculated from the Bit Position value and can be used to check the order in which the subframes appear in the output file. The fact that there are two flight streams of data that are recorded simultaneously makes it difficult to be certain of the order in which the subframes were recorded. As far as I know, frames of data are alternately recorded in flight streams 0 and 1. If two subframes have the same flight stream number then they will appear in the output file in the order they were recorded. If two subframes have different flight stream numbers and different Block Position values then I am fairly certain that the subframe with the higher Block Position value will have been the later one to be recorded and will appear later in the output file. If two subframes have the different flight stream numbers and the same Block Position values, then the one with the higher Bit Offset value probably but not definitely will have been the later one to be recorded and will appear later in the output file.

Go back to the AAL77 FDR Partial Decoder page.