reply to post by masterp
1) The IRS is the gyroscopic 'platform' that provides attitude information (pitch, roll) and also position. Earlier versions were called INS for
'Inertial Navigation System'...basically same thing, but the "reference" adds another dimension, as the platforms provide pitch and roll that an
INS does not. Used separate instruments.
Modern platforms actually have no moving parts, unlike the older bulky rotating gyroscopes. The technology today uses devices called "laser ring
gyros", (so did some more modern INS, too)...where a laser light beam is 'bounced' around a series of mirrors (One 'gyro' for each of the three
axis') and acceleration is detected by the incredible tiny deviations of the light beams. This, as they detect the accelerations in three axes,
allow them to infer and compute new position information, due to the inferred movement...very complex machines, and hard to visualize. I'm sure more
info can be found online, no need to repeat at great length here.
Just to add, though...the IRS has no idea
where it is: it has to be "told", and that process is called 'initialization'. The known
current latitude/longitude coordinates are input by the crew (even before this happens, it CAN know its approximate latitude, because it can sense the
Earth's rotation...but can't tell North from South, only distance from equator). Not equipped on American 77 at the time, but most of today's
fleets have GPS, which is used for the more accurate initial position entry.
Otherwise, (no GPS) we just use the info from our charts for lat/long...AND, the units 'remember' where they were last shutdown and will alert if
the input coordinates are well outside a certain 'logical' range.
Anyway, once intialized, they (there are three, typically) are independant, and each calculate from then on, as they sense movement, (laterally only,
they have no altitude reference) to determine where they are, in relation to the starting point. UNTIL they are given outside updating info (whether
GPS, if installed...or, like with AA 77, VOR/VOR or VOR/DME position updates) they will each merrily crunch the numbers....and have no other
reference, except for what I mentioned.
There is inevitably a bit of error that accumulates...but since they aren't used, solely, for precise navigation, it isn't an issue. They are
simply not designed for precision....couple tenths of a mile is "tight", usually....
FYI....after every trip, as part of the post-flight, we go to a page in the computer database that has kept a record of the 'drift' rate, divided by
the time they were in operation. We log that in the aircraft Logbook, andif it is greater than --- I have to look it up, but greater than 3 miles per
hour of operation comes from memory, that has to be written up for maintenance action. Usually, we log something like 0.3 to 0.6....rarely over
Now, to follow with the rest...the IRS ONLY accepts initialization inputs, and radio/GPS updating...actually, the base IRS position where it
'thinks' it is never changes...the radio/GPS updating is averaged, and then presented as the 'computed' position (and, when all three are
operating, they are additionally "mixed" to make another average...it's called "triple mix").
This 'computed' position is now the 'official' postion, and is fed to the Flight Management Computer (FMC).
Within the FMC is also included the navigation database, for airports, waypoints, intersections, etc. AND, it's the FMC that is directly accessible
by pilots, to program the autoflight and other guidance functions.
a feature, via ACARS uplink, when tied into the FMC, for flight plan data to be uploaded (most regularly programmed flight
plans are sent that way when at the gate, nowadays...not sure about AA 77, back then...other, older way was from a stored database in memory...).
BUT, this only sends the data...it must be accepted and "executed" by physical button pushes by a human. THEN, it will just be sitting there, until
it is utilized by other buttons, to make it "active" for the autoflight guidance systems. In any case, as I've noted, the computed IRS POS isn't
nearly accurate enough to hit a building a few hundred feet wide.
2) IS the simplest....the A/P programming, in order to change it, would require a COMPLETE code rewrite --- and, the servos that operate the controls,
in response to A/P commands, do not move the controls as rapidly, or to the extent that a human can. Further, in terms of pitch control (and any
pilots will understand this better) changing the pitch attitude requires a force on the elevator controls...but that is NOT a 'neutral' operation.
Each change in pitch (or in power settings, which also alter the 'balance') results in the force, and requires a sustained force be maintained...OR,
we use the "TRIM" function, to re-align the (in the case of big jets) the horizontal stabilizer, and relieve the need for the sustained pressure.
(Small private airplanes usually have small adjustable 'tabs' on the elevators themselves to accomplish this).
The TRIM on the B-757/767 has two speeds...and the A/P can ONLY access it in the slower, about one-half normal speed of trim change. This would mean
that, in terms of pitch attitude adjusting, the A/P is very limited, as it doesn't have the control authority, nor the 'strength', that a human can
3) Not clear on this one...would not need a "big" network, of course. But, point is, there just is no provision built in .... except, as I
mentioned above, which still needs human activity.
[edit on 29 June 2010 by weedwhacker]