Re: 757 flight recorder located

From: (Sethu R Rathinam)
Organization: INS Info Servcies, Des Moines, IA, USA
Date:         30 Mar 96 16:01:09 
References:   1 2 3 4
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John Witherspoon ( wrote:
: In article <airliners.1996.303@ohare.Chicago.COM>
: (John Bay) writes:

[about flight recorders]

: >To carry the thought a step further (and perhaps to extremes), does current
: >technology make it feasible for airliners to transmit their operating
: >parameters to the ground in real time?

: Sure it would, just as 18 wheel trucks are often equipped with senders
: that transmit the driver's speed, etc. back to the office via satellite.

: The only problem would probably be that the pilots' unions would probably
: howl.  Both cockpit voice and data are collected during flights, and
: I have read that some pilots are so concerned about somebody playing
: back the last 30 minutes of their otherwise private flightdeck
: conversations (i.e. when not talking to ATC) that they go to the
: black (really orange) boxes in the back of the plane (where they're more
: likely to survive a crach) and press the "purge" button, erasing the
: tape so nobody can come aboard and listen to it.  Don't ask me if it's
: even possible to get at the audio without removing the box and taking it
: to a lab...

The trucks are equipped to send/receive a small amount of information
to/from the dispatcher using Satellites (or other RF links). This
information is usually GPS position, dispatch related and emergency

The following is general FDR/CVR information and the  detail will differ
somewhat from airplane to airplane, so your milage may vary.

You are talking about the Cockpit Voice Recorder in your next paragraph.
The modern CVRs record about 30 min to 2 hours on Solid State memories (so
the last 30 minutes-2 hours is always in memory when the airplane is out
of the gate).  There is no interface ON THE AIRPLANE to play back this
data - the unit must be removed to the lab before this data can be
extracted/played back.  [There is a way to monitor the audio as it is
being recorded (upstream from the recorder) for testing the unit, but it
does not have the capability to play back.] It is standard procedure for
pilots to erase this data after landing and the parking brake is applied.
(The erase button is in the cockpit and is interlocked with the parking
brake or engine-off logic).  (Most people should have no problem with
that procedure - there is some disagreement in case of Incidents but that
is another topic.)

(You can make your own calculation for real time transmitting of CVR data
- telephone quality is about 8kBytes/sec uncompressed - as it happens, 4
channels are recorded....).

The Flight Data Recorder records operating parameters - hundreds of them
at various rates (typically 4 Hz to less than 1 Hz). All pitch, roll, yaw,
accelerations, velocities, positions are recorded as well as primary
control inputs and surface positions, (what the pilot "sees on the
instruments"), along with several other pieces of information that would
be of interest after an accident. A modern recorder can record (in solid
state memory) typically 25 hours at 128 words per second (12 bit words).
Most of the information is not of any use unless there has been an
accident/incident. So about 1600 bps of information is produced and it is
possible (purely technically speaking) to downlink this using ACARS.  (But
consider the hundreds of airplanes in the air at any given time and the
bandwidth required simultaneously increases significantly (typically using
VHF links - Satcom links are more expensive presently) and consider the
very small accident rate and you'd realize why a VERY large percentage of
this information is useless.  Now, are we willing to finance this
increased bandwidth and transmit capability for the very small amount of
additional benefit (remember the CVR and FDR are recovered in most
accidents - such as failure of critical systems).  You can extract the
data on the airplane by using the data downloading capability.  Sometimes
this data is recorded on an Optical Quick Access Recorder and the disk is
changed, so there is more than one way of getting this data without
removing the recorder. (Again, YMMV, depending on the type of airplane).

Data transmission errors are another problem if you want to transmit this
data using VHF/Satcom links and not retain it on board.  Note this data
is usually not stored in a buffer for any length of time to re-transmit
in case of an error.  (Usually on board recording is required to have an
error rate is less than 1 bit in 100,000).

ACARS is used primarily for producing reports on certain conditions (for
example Engine Trend monitoring), arrival time information, maintenance
required if any etc. - so these are primarily short messages sent in a burst
of a few seconds and not sustained long messages like you will need for
downlinking FDR/CVR information.  (And of course the ACARS providers,
ARINC/SITA charge the airlines for gathering this information from their
far flung VHF receivers, processing them and sending it to the respective
airline's computers.)

Another related tidbit from FDR data protection requirements (remember it
is just a protection requirement - nothing is REQUIRED to OPERATE after a
crash) (Quoting from TSO-C51a) "Impact: The intelligence on the record
medium shall be capable of being analyzed after the recorder has been
subjected to the following shock: Types I and II - Half sine wave impact
shocks applied to each of the three main orthogonal axes and having a peak
acceleration magnitude of 1,000 g with a time duration of at least 5

Look up the above TSO, also, TSO-C124, and EUROCAE ED-55 (Minimum
Operational Performance Specification for Flight Recorder Systems) if you
want more information.

Sethu Rathinam