ACARS messages to UA175 and UA93

[CptColumbo]

I've sent and received many ACARS messages and they never had any data regarding how the message was routed. It only included the flight number. However, I never worked in ATC and they may have a different system.

 

[femr2]

I've sent and received many ACARS messages and they never had any data regarding how the message was routed. It only included the flight number. However, I never worked in ATC and they may have a different system.

The messages referred to do include additional data...

(on my site. hotlinking fine.)

Do you have any specific knowledge of the acknowledgement mechanism ?
Do you have any knowledge of the mechanism which determines the transmission location ?

 

[apathoid]

1. Do transmitted ACARS messages indicate the automated acknowledgement within the message text ?

I've already told you that ACARS is a one-way VHF radio data transmission either from the ground, or the aircraft. There is no reasonable way for the sender to know if the message was received. If there was, I wouldn't need to call my company's MCC to set up a test message, which I have to be in the cockpit to receive and then call back the MCC to tell them if it was successful. If there was an acknowledgment, it could be done solely by the MCC with no need for a middleman.

In normal flight ops, the MCC could always pick up the radio and call the aircraft on a company frequency to determine if a message was received.
ACARS even has voice capability, so the pilot could simply punch the VHF 3 mic select button, switch the ACARS transmit from data to voice and say "got it, thanks".

2. Are ACARS messages routed to be transmitted based on expected flight-plan, or live position ?

I'd have to assume planned position, as there is no reasonable way for the ground station know where the aircraft's live position is. They may know from an earlier downlink that includes lat/long where the plane was when it sent it's message, but what if the next uplink was to be sent two hours later with no recent downlinked lat/long? I think you are taking this notion that ACARS is a data-link system a bit to literally.

As I said before, when I call or radio for a test message, I need to tell them where the plane is presently(what airport it's at), as that's the only way to know since theres no flight plan data for the provider. You could also go into ACARS and setup a manual downlink which includes lat/long if you wanted to.

3. What is the behaviour of the printed ACARS message record in the event of a transmitted message not being received by the aircraft it's sent to ?

No difference as far as the sender is concerned. There is no acknowledgment.

 

[CptColumbo]

The messages referred to do include additional data...
[qimg]http://femr2.ucoz.com/_ph/2/331321151.png[/qimg]
(on my site. hotlinking fine.)

Do you have any specific knowledge of the acknowledgement mechanism ?
Do you have any knowledge of the mechanism which determines the transmission location ?

I am not familiar enough with UA's ACARS system to say what the third and fourth lines represent with 100% certainty. If it is the similar to the system I've used, I can say that the top two lines are most likely who you are sending the message to and where the message is coming from. Usually you will find the address of where the message is coming from on both lines, so the sender has a copy of what was sent. So the "CH1AK" is most likely where the message is being sent from.

In the system I've used the first code would usually include the tail number of the aircraft, so maybe that is their general address for all aircraft, since there was more then one flight that you are trying to warn. As ACARS (again in the system I am familiar with) can only have 5 codes in the top line (two of which are your address and the admin) it would seem more likely that this is a general address, as filling out a telex for each individual aircraft would be very time consuming (and waste a lot of paper and jam up the dot matrix printer or whatever is being used to receive telexes). The third code would usually be a administrative telex to show the chief pilot or anyone else who needed to know what messages have been sent. That is all a guess on my part.

The only way I am aware to get acknowledgement is for the receiver to reply back. Which is why I would end my messages with "please inform [my address] when received" if it was important that they got the message.

 

[femr2]

I've already told you that ACARS is a one-way VHF radio data transmission either from the ground, or the aircraft. There is no reasonable way for the sender to know if the message was received.

All ACARS documentation and specification details I've found include acknowledgement retry descriptions...

Incoming voice communications can occur at any time--even during times when the ACARS VHF transceiver is in the middle of a transmission. In such instances, amplifier 58 will drive relay contacts 58 open and the ACARS radio transmission will be terminated before it has completed its current transmission. Because of the ACARS guaranteed reliable positive acknowledge protocol in use, the ACARS CMU will in this case attempt to retry transmission when it does not receive a timely acknowledgement from the ground station network. Thus, once received voice communication terminates, the presence of communications interlock circuit 50 will once again allow the ACARS CMU to re-key the ACARS transceiver to retransmit the previously interrupted transmission. Meanwhile, clear priority is given to allowing the flight crew to receive voice communications.

...and ACARS message transmission really does get described as a packet relay-type transmission protocol.

I'd have to assume planned position, as there is no reasonable way for the ground station know where the aircraft's live position is.

Will cross-reference where the aircraft should have been with transmission location.

 

[CptColumbo]

Keep in mind that I worked in scheduling, and rarely was anything I sent important enough to require immediate response.

 

[femr2]

I am not familiar enough with UA's ACARS system to say what the third and fourth lines represent with 100% certainty.

It looks like various message types are *printed* in differing formats, but the underlying block format is available and seems fixed...

Label Message Title
_j No information to transmit, Polled mode
_DEL General response, Demand mode, no information to transmit
0 HJK Emergency situation report
2S Weather request
2U Weather
4M Cargo information
51 Ground GMT request response
52 AGM Ground UTC request
54 Aircrew initiated voice contact request
57 AEP Alternate aircrew initiated position report
5D TIS ATIS request
5P Temporary suspension of ACARS
5R AEP Aircraft initiated position report
5U WXR Weather request
5Y ETA Revision to previous ETA
5Z AGM Airline designated downlink
7A ENG Aircraft initiated engine data
7B ABM Aircraft initiated miscellaneous message
80-9 Aircraft addressed downlinks
A1 CLX Deliver oceanic clearance
A2 CLD Deliver departure clearance
A4 RCA Acknowledge PDC
A5 RPR Request position report
A6 RAR Request ADS report
A7 FTU Forward free text to aircraft
A8 DDS Deliver departure slot
A9 DAI Deliver ATIS information
A0 AFN ATIS Facilities notification
B1 RCL Request oceanic clearance
B2 CLA Request oceanic readback
B3 RCD Request departure clearance
B4 Acknowledge departure clearance
B5 PPR Provide position report
B6 PAR Provide ADS report
B7 FTD Forward free text to ATS
B8 RDS Request departure slot
B9 RAI Request ATIS information
C0 Uplink message to all cockpit printers
C1 Uplink message to cockpit printer #1
C2 Uplink message to cockpit printer #2
C3 Uplink message to cockpit printer #3
CA Printer status = error
CB Printer status = busy
CC Printer status = local
CD Printer status = no paper
CE Printer status = buffer overrun
CF Printer status = reserved
F3 Dedicated transceiver advisory
H1 Message to/from terminal
HX REJ Undelivered uplink report
M1 MVA IATA Departure message
M2 MVA IATA Arrival message
M3 MVA IATA Return to ramp message
M4 MVA IATA Return from airborne message
Q0 ACARS link test
Q1 ETA Departure/arrival reports
Q2 ETA ETA reports
Q3 CLK Clock update
Q4 Voice circuit busy (response to 54)
Q5 Unable to process uplinked messages
Q6 Voice-to-ACARS change-over.
Q7 DLA Delay message
QA DEP Out/fuel report
QB DEP Off report
QC ARR On report
QD ARR In/fuel report
QE DEP Out/fuel destination report
QF DEP Off/destination report
QG RTN Out/return in report
QH DEP Out report
QK ARR Landing report
QL ARR Arrival report
QM ARR Arrival information report
QN DIV Diversion report
QX Intercept
RA RPR Command aircraft terminal to transmit data
RB Response of aircraft terminal to RA message
:; Command aircraft transceiver to change freq.

#characters Purpose Comments
16 Pre-key Xmitter warm-up/Rx AGC adjustment
2 Bit sync establish bit synchronisation
2 character sync establish character synch
1 SOH indicate start of message
* 1 Mode ground system interface configuration
* 7 Address aircraft resgistration number
1 Ack/Nak acknowledge/non-acknowledge marker
* 2 Label type of message
* 1 Block ID message block number
1 STX indicates start of message text
* 4 Sequence# message sequence number**
* 6 Flight number airline flight number**
* 210 Text message text
1 ETX indicates end of text
16 Block Check Seq error dedection polynominal value
1 BCS suffix last character

*)highlighted text
**)air-ground message only

 

[CptColumbo]

The first series of letters and numbers on the fourth line, to me, are the tail number of UA175. The "GL PIT" could mean where the aircraft is based (possible) or where the Pilots are based (I don't see why it would be included in an ACARS message) or be unrelated to the first part of the line. It could be the Ground Location of where it is being sent from, or where the Chief Pilot or the Administrators who need to receive the message are.

Basically, this looks like the copy that either went to the telex where the message was sent from or the CC sent to admin.

 

[femr2]

The "GL PIT" could mean where the aircraft is based (possible) or where the Pilots are based

The link in the OP (which I assume you have read) highlights the changes in the element referenced PIT...

(on my site. hotlinking fine.)

How would you explain the change from MDT to PIT ?

I suggest the GL references Ground Link/Location.

 

[Obviousman]

I can't believe the length Truthers will go to to support their fantasies.

Did anyone check they hydraulic fluid found at the scene? Maybe it wasn't the recommend grade but a 'may be substituted'...

What about the inflights? Maybe the catering service normally doesn't offer that menu.

Sheesh..

 

[CptColumbo]

The link in the OP (which I assume you have read) highlights the changes in the element referenced PIT...
[qimg]http://femr2.ucoz.com/_ph/2/331321151.png[/qimg]
[qimg]http://femr2.ucoz.com/_ph/2/2/513740653.png[/qimg]
[qimg]http://femr2.ucoz.com/_ph/2/2/849396678.png[/qimg]
[qimg]http://femr2.ucoz.com/_ph/2/648161484.png[/qimg]
(on my site. hotlinking fine.)

How would you explain the change from MDT to PIT ?

I suggest the GL references Ground Link/Location.

If you look at the first two lines of each you will see that what I believe to be the senders and (on some) the CC address have changed. Assuming that the CH stands for Chicago, where IIRC UA has some of its offices (we use CHI for our Chicago base and NYC for the New York Base rather than the individual airport codes) and perhaps the people sending them are not actually at their office in Chicago or all ACARS from UA have an address out of the home office regardless of their origin. This could be true of admins on inspection or Pilots at a ground stop. Since IIRC MDT is Harrisburg, PA and PIT is Pittsburgh, PA, I find it unlikely that a transmission to a plane would need to be switched between airports so relatively close together, especially when it would more likely switch between PHL and PIT (or even BUF DTW or CLE).

What do you base your conclusion that GL means "Ground Link?" Just that it fits the initials?

I spent my first year thinking ORD was Orlando. I had to keep reminding myself that it was O'Hare.

 

[apathoid]

All ACARS documentation and specification details I've found include acknowledgement retry descriptions...

I've taken the liberty to bold the important bit.

"Incoming voice communications can occur at any time--even during times when the ACARS VHF transceiver is in the middle of a transmission. In such instances, amplifier 58 will drive relay contacts 58 open and the ACARS radio transmission will be terminated before it has completed its current transmission. Because of the ACARS guaranteed reliable positive acknowledge protocol in use, the ACARS CMU will in this case attempt to retry transmission when it does not receive a timely acknowledgement from the ground station network. Thus, once received voice communication terminates, the presence of communications interlock circuit 50 will once again allow the ACARS CMU to re-key the ACARS transceiver to retransmit the previously interrupted transmission. Meanwhile, clear priority is given to allowing the flight crew to receive voice communications."

You referenced how the system actually works to establish connectivity via the ground station network. The ground station network, as was already explained, does not equate to an airline ops center, or the aircraft. It's the service provider and it makes sense that if there's no connectivity or an interruption, the message will try to resend. And just to be absolutely clear, this acknowledgment isn't something the message originator would see. It's not talking about some sort of flight deck chime or something appended in the printout of a message.

on edit: Also just wanted to point out that what you posted pertains to downlinks. What does your source say about acknowledgment for uplinks?

...and ACARS message transmission really does get described as a packet relay-type transmission protocol.

Only when a message is enroute, or when the mic is keyed. When no uplinks or downlinks are in progress, no data is being transmitted.

 

[apathoid]

The link in the OP (which I assume you have read) highlights the changes in the element referenced PIT...
http://femr2.ucoz.com/_ph/2/331321151.png
http://femr2.ucoz.com/_ph/2/2/513740653.png
http://femr2.ucoz.com/_ph/2/2/849396678.png
http://femr2.ucoz.com/_ph/2/648161484.png
(on my site. hotlinking fine.)

How would you explain the change from MDT to PIT ?

I suggest the GL references Ground Link/Location.

Do you really need this explained? Look at the time stamps. The PIT(Pittsburgh) uplink attempt happened 20 minutes after the last attempt, which was from the MDT(Harrisburg) ground station. Now look at a map. This fits our theory perfectly that the service provider selects the ground stations based on the flight plan.

 

[carlitos]

femr2, last time I asked, you said that your "hypothesis" about 9/11 involved the apparent lack of "definitive explanation" of collapse initiation in the towers. (something about "column creep" in the collapse initiation, if I remember correctly)

How in the hell do these ACARS messages have anything to do with the apparent lack of "column creep" or the initiation of the building collapse?

What is your hypothesis about the day's events that tie these two things together?

 

[femr2]

Do you really need this explained?

Yes, clearly.

Look at the time stamps.

Yes, I have. The primary reason for seeking clear and definitive detail on the behaviour of the ACARS messaging system is to determine a clear and definitive explanation of the final message, several minutes after Flight 175 impacted WTC 2.

The PIT(Pittsburgh) uplink attempt happened 20 minutes after the last attempt, which was from the MDT(Harrisburg) ground station. Now look at a map. This fits our theory perfectly that the service provider selects the ground stations based on the flight plan.

I'll cross-check, but that's fine, and from the additional details I've looked at, yes, would appear to be the case.

If I can gather clear and definitive information on message acknowledgement mechanisms (we're not there yet) then the *anomoly* will be explained and my interest in the thread will be done with.

 

[CptColumbo]

Do you really need this explained? Look at the time stamps. The PIT(Pittsburgh) uplink attempt happened 20 minutes after the last attempt, which was from the MDT(Harrisburg) ground station. Now look at a map. This fits our theory perfectly that the service provider selects the ground stations based on the flight plan.

I don't know about you, I'm having trouble reading these (probably done on a dot matrix printer). Looking at the other printouts I now see that what I believe is the senders address on the first one displayed is "CHIAK." That fits into what I've used. My address used to start with MSP, followed by a code for the building we were in, a code for the floor and one for our specific printer (there were four on our floor).

 

[beachnut]

just reread woodybox:

"Now these position informations reveal shocking news: Winter explicitly confirms that United 93 received the last ACARS messages when it was near Fort Wayne (Indiana) and, some minutes later, near Champaign (Illinois):

"...Messages #18 and #19 were sent to the aircraft from CHIDD using the RGS near Champaign, IL CMI as designated in the line "AN N591UA/GL CMI...". Both messages were sent to the printer and Message #19 also activated an audible signal in the aircraft..."

"

WoodyBox has no clue what it means and some lost CD believers are exposing their ignorance on flying topics that equal that on structures.

You seem to be gullible on all nut case conspiracy theories based on pure nonsense like the woodbox claptrap. Why?

 

[Simplicissimus]

reread Woodybox

"Messages #18 and #19 were sent to the aircraft from CHIDD using the RGS near Champaign, IL CMI... Message #19 also activated an audible signal in the aircraft."

This was at 10.10, seven minutes after UA93´s official crash near Shanksville

 

[apathoid]

Yes, clearly.

Sorry. I wasn't paying attention to the discussion between you and Colombo.

Yes, I have. The primary reason for seeking clear and definitive detail on the behaviour of the ACARS messaging system is to determine a clear and definitive explanation of the final message, several minutes after Flight 175 impacted WTC 2.

I'm sorry, I still don't see the problem. The message you are talking about is an uplink. There's nothing to stop the airline from attempting to message a downed plane. If you had a downlink, that'd be interesting.

If I can gather clear and definitive information on message acknowledgement mechanisms (we're not there yet) then the *anomoly* will be explained and my interest in the thread will be done with.

You won't unless we could show what we're talking about. No amount of googling will provide you with acknowledgment confirmation with ACARS messages. Perhaps you should just try to call UA's Ops Center; get it from the horses mouth.

 

[apathoid]

I don't know about you, I'm having trouble reading these (probably done on a dot matrix printer). Looking at the other printouts I now see that what I believe is the senders address on the first one displayed is "CHIAK." That fits into what I've used. My address used to start with MSP, followed by a code for the building we were in, a code for the floor and one for our specific printer (there were four on our floor).

I can only decipher bits and pieces of it, as our format is quite different too. Its reasonable to assume that GL means the ground station, though I'm not sure. If it is, it supports the idea that the flight plan is using for message routing.