reply to post by eyestotheskies
I've ..... read reports that the data retrieved was partial and unreliable at best, and non existent at worst.
Because, there is AMPLE evidence that the SSFDR is quite accurate. The ONLY "argument" or complaint was with the very final few seconds. An
Australian computer coding /software expert, Warren Stutt, was able to properly decipher what the NTSB couldn't (or, didn't bother to do). There is
a recent thread on it, trying to find now....
And...probably the best one, for PILOTS familiar with the B-757/767 to understand, and it nails the reality for me:
AAL 77 Navigation System Activity
Figure 3 shows the VOR stations tuned to by the two VOR receivers on American Flight 77.
The EFIS mode determines the type of display shown on the EHSI. During the initial part of
the flight, the EFIS is in “MAP” mode. In this mode, the EHSI displays an airplane symbol
pointed towards the top of the display, with the magnetic heading and track shown in a
partial compass rose at the top of the display. Various points of interest - VOR stations,
fixes, airports, and so on - can be displayed in their correct position relative to the airplane.
The planned and projected route of flight can also be displayed in the MAP mode. The
range of the MAP mode can be adjusted from 5 miles to 160 nautical miles, depending on
the detail or scale of map required.
That was a brief, and general background description....now, note here, next:
At about 09:08:20, the display switched to VOR mode;....
There is only ONE reason for this. The pilot, on board, selected a mode of display that he was more familiar with, from flying other airplanes. The
VOR mode changes the EFIS screen from the MAP mode, to an electronic depiction of a typical Horizontal Situation Indicator type instrument display.
It was more "comfortable" for him, as he was more used to it.
This is a typical Electronic HSI (from a different airplane, not a 757, but similar enough):
Here's the EFIS screen in MAP mode (this, from a Boeing 737, but again, very similar display):
Here, the EFIS control panel from the 757/767 (at the top, unrelated, is the Radar Altimeter controls):
There are two "VOR" position selections...."expanded" and "full". "Full" gives you the entire HSI...."expanded" gives you just the upper portion of
the HSI compass rose...similar to the compass rose arc in the MAP display.
ALSO, when taken out of MAP mode,it activates the manual tuning panel, for the VOR. They are in "auto-tune" for the inertial navigation system (IRS)
to auto-update and refine its calculated position, based on ground sources for reference. The EFIS controls, and the VOR tuning panels are all on the
center pedestal, between the two pilots' seats positions.
Continuing from the NTSB narrative:
The points during the flight at which the VOR receivers were tuned to new frequencies are
shown on the map in Figure 2 as yellow diamonds. The points shown occur after the
hijackers took control of the cockpit. Lines from the airplane flight path to the stations
indicate the VOR stations tuned by the left and right VOR receivers. The point on the flight
path from which the lines originate are the points at which the station was first tuned, i.e., the
points at which the VOR station frequency selected by each receiver changed.
Note that while the EFIS was initially in MAP mode, the left and right VOR receivers were
tuned to stations whose bearings from the airplane differed by about 90 degrees, at the time
at which the VOR station pairs were changed. This illustrates the method the system uses
for obtaining VOR position fixes to update the INS.
During the turn back to the east, the frequency of the right VOR receiver was set to 111.0
MHz, corresponding to the VOR station located at Washington Reagan National Airport
(DCA). At the time the DCA frequency was selected, the station was too far away for its
signals to be received by the receiver.
edit on 9 February 2011 by weedwhacker because: (no reason given)