This is a post from -PLB- (page 5), he did check the raw FDR data too, he said :
When you check the rest of the data from the flight recorder, alignment with the gate does not happen that often. So it looks like that in
practice it is not very strict. Or maybe this is just specific to the AA77 data, but I do not have any other data as reference.
When you remove odd movement in the flight path (where the movement is not straight), you end up with a perfect match to gate 26. It looks like drift
occurred at those positions. Those positions coincide with the runway holding position markings, which suggests that the plane was not moving when the
Balsamo (TigerTracks) answered him (partly wrong) :
"UA93" data is also showing the same type of offset and then alignment in flight. Neither "AA77" nor "UA93" had this capability. But
military aircraft do.
Note that he meant with "offset" the same type of "3000+ feet" positional shift/drift.
But he stumbles over his own feet when he tries to implement the following "alignment in flight" as coming via GPS updating like in "military
No Balsamo, it came through the standard radio updating. No conspiracy here.
But in the last seconds of AAL77 its flight?
See this post of mine in the other thread cross-linked to by me already, for an answer :
Source : www.pprune.org...
Since inertial systems accumulate position errors as a function of time, the position information being used by the FMC is slowly accumulating
errors. These position errors can be detected by observing the position of the airplane on the HSI map. If an extended ground delay occurs and a
significant map error is noticed the IRS should be realigned and present position re-entered [on the ground].
-- those things are so accurate in the 75, 76, 73, and FK100, that we always, always put in the exact gate coordinates on each gate at each airport
when preparing to go --
I expect the problem that Balsamo has, is him not willing to understand and accept that the positional information the pilots see on their
instruments, is not the same as what is obviously registered (in different format) in the FDR, for later checks.
It looks to me that the FDR slowly accumulates and adds together, all former flights their arrival and departure Lat and Lon positions. Without
correcting them for the difference in arrival and departure gate positions, or drift during taxiing and waiting at the holding positions.
Gate position difference and positional drift is a consequence of the usual towing overnight, and happened every time the pilots performed their early
morning full align procedure.
The FDR does not register the difference in arrival and departure gate position, only the gate position typed in by the pilots, at every morning's
full align and adds that to its data stream from the day before.....that's why you see the jump in position every morning before the full align,
since the overnight tow is not registered in the FDR. That jump occurs after electrical power has been applied. And after the full align is ended and
This is obvious, since we can control that behavior in all former flights from flight 77 and flight 93 their FDR's, they were all still in those two
recovered FDR's. And both showed for all preceding flights, the same accumulating positional drift during all those former flights.
And a for example 3000+ feet / 914 meters discrepancy with the real gate and runway positions, retained inside the FDR (which data is not shown and
used at all by the pilots.! ) does not have any influence on the behavior of the pilots, since they can't see those retained figures, they are not
meant to assist the pilots.
Not at all, they are for later reference, if needed to be checked by others than the pilots.
And an experienced FDR specialist will know that, and implement that knowledge in his real time positional plot based on his extracted FDR raw
The pilots see nothing at all from this FDR its accumulation of raw data.
They perform a full or fast align, and see as a result their Lat and Lon position updated on their IRS screens when moving around after that, until
they eventually perform a next fast align when standing too long in a holding area. Fast aligns can only be performed on the ground. It was not
possible for a 2001 civilian owned Boeing 757 to align in flight.
Only to use radio signals from all around it, to update its plane's position, quite fast by the way.
To correct an obvious misconception by Balsamo:
The early morning full align is indeed registered in these two FDR's, for every flight it holds data from. It's not always the gate position input,
since the tow tractor who pulls the plane from the passenger tube position, sometimes tow them all the way to the rim of the parking and taxiing area,
uncouple, and then the pilots performed their full align at that spot tenth of meters away from the gate position.
Only Balsamo's eyes do not seem to register that fact. He keeps insisting that there is no sign of a full align. He's wrong.
Look at the tenth of meters/yards, 90° angled two legged position jump every morning.
See my corrected Zelman map with his not fully corrected green line drawn in it. It can be seen at the very top. As the first 90° green two legged
angled line. The diagonal between the end of each Latitudinal and Longitudinal leg depicts the factual positional jump distance, and that distance you
have to retract from all the positional data points for the gate departure and taxiing distances up to the runway 30 holding position.
In the bottom of that Zelman map.
And then you arrive every time again, at the proper departure gate, the proper taxi lanes and the proper runway holding position, just before
take-off. In case of 9/11, Gate D26 and Runway 30 at KIAD ( International Airport Dulles).
Those FDR-data are getting registered for airline security and post-crash flight data recovery, and can be checked after any flight, if the airline,
the NTSB or any other security institution wants to check them.
That's why the pilots do not need to see or check those FDR data on the ground or in flight.
They check the ground-position of their airplane on their HSI map, and if it drifted too much, they perform a fast align. They know that their systems
at wheels off automatically begin to quickly align with their real exact position through radio updating signals.
Thus, there's no real need to be exact on the meter while still taxiing. Small positional data errors will be updated fast, after take off.
The FDR is a system for use by the NTSB, or the airline maintenance personnel.
Not for the pilots eyes during taxiing nor in flight. It is locked up in a box in the tail section.
It is connected to all the plane's systems, all signals stream however towards its box, to record the whole flight, and a lot of preceding flights.
No interaction needed with the pilots.
They have other means in their cockpit to check and/or input their real position.