ACARS data path?

[poorjohn]

For an aircraft not fitted with satcom, how is ACARS data delivered, e.g. dedicated VHF/HF channel? Is engine-manufacturer's monitoring data a separate data stream, or multiplexed onto ACARS? How is data delivered to two interested entities on the ground?

[BOAC]

dedicated VHF/HF channel? = from 737 days, yes, Com 3 VHF and I believe on larger a/c, HF fallback

is engine-manufacturer's monitoring data a separate data stream, or multiplexed onto ACARS? How is data delivered to two interested entities on the ground?

Cannot help with all that. In my day I was told all ACARS comms were ultimately routed via Singapore, but that may well have changed.

[noughtsnones]

poorjohn,
I don't know about the ground support magic, but on-wing the engine data is collated via an ACMF prior to transmission (i.e. shared) via ACARS ...

Click on "Transfer" at Monitoring systems - Rolls-Royce (http://www.rolls-royce.com/about/technology/systems_tech/monitoring_systems.jsp)
or read a copy at http://www.ingenia.org.uk/ingenia/issues/issue39/waters.pdf
Also extrarcted off the web is the following though the site owner wants $31 for a copy of the proceedings

IEEE Xplore Abstract - The Honeywell on-board diagnostic and maintenance system for the Boeing 777 (http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=282113&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3F arnumber%3D282113)

Avionics have exhibited a phenomenal increase in complexity over the past twenty years. This increase in complexity has brought with it the unfortunate side effect of a significant increase in difficulty of diagnosing the real reasons for a multitude of raw symptoms originating from these complex subsystems. The integrated Avionics Architecture for the Boeing 777 is being designed with an unprecedented attention to fault detection and isolation capability to address this problem. A key element in this effort is an On-Board Diagnostic and Maintenance System (OMS) that integrates two diagnostic subsystems into the Airplane Information Management System (AIMS) cabinets: a Central Maintenance Function (CMF) and an Airplane Condition Monitoring Function (ACMF). The CMF diagnoses faults responsible for flight deck effects that are logged by the crew and facilitates rapid turn-around of the airplane at the gate. The ACMF captures parameters based on predefined trigger conditions for long-term analysis of trends in aircraft systems and the flight crew. It is noted that the OMS constitutes a major evolution of diagnostic systems directed at reducing the operating costs for the airlines.

[BOAC]

Seeking refuge here from the inanity of R&N - can anyone help explain to me the transmission system for EHM? It is suggested on the R&N thread that the transmission system is 'independent' of the aircraft ACARS, which does not make much sense to me since the need for data is promoted by the engine being on a functioning aircraft, so why not just use the ACARS system as per nougtsnones' post?

This query is prompted by the report that ACARS was 'disabled' but that some sort of handshake was being initiated by the aircraft for several hours.

I remain highly dubious of some of the 'feeds' to the public, my views on the reliability of the WSJ are well aired on this site too. We could be in danger of circular self-generating unsubstantiated 'facts' set off by mis-reporting.

[WASALOADIE]

BOAC has the answer, Data is sent via VHF/UHF and HF link.

The 2 providers are ARINC (American company) and SITA (Asian company) but they share some resources to give almost global coverage. If SATCOM is not used by the airline, then coverage is more limited.

Data tends to be periodic unless requested for increased frequency on demand. Data is also send data burst so very short transmission time required. All manner of data can be sent from flight plan info, company info air traffic info in the form of preset messages or an open text based format. Data for aircraft performance and engine performance is virtually continual and direct to company engineering, manufactures etc.

[awblain]

Satellite operators are rather innovative. And the increased internet protocol-ness of all communications is likely to be getting involved. I would not be surprised if VHF radio to ground to internet was available for monitoring information, along with satellite company's phone system: Inmarsat and Iridium. With limited numbers of calls, aircraft systems could even make a deal with ground-based cell providers, or piggyback on satellite links set up for IFE.

I wouldn't worry about misinforming. Since it has been a week, and the communications providers and at least Rolls-Royce would have this knowledge live, so this is all very much after the fact.

If there is any archiving of signals from SIGINT radio stations, then this might also have been known to non-commercial users live. There are space-based assets for the interception of communications, to listen to such a high-value object as a stolen/hijacked airliner with more sensitive ears, although you probably won't be getting any press releases about it.

[island_airphoto]

Here you go:
131.550 Primary Channel worldwide
129.125 Additional channel for USA & Canada
130.025 Secondary channel for USA and Canada
130.425 Additional channel for USA
130.450 Additional channel for USA & Canada
131.125 Additional channel for USA
131.450 Primary channel for Japan
131.475 Air Canada company channel
131.525 European secondary
131.725 Primary channel in Europe
136.700 Additional channel for USA
136.750 Additional channel for USA
136.800 Additional channel for USA
136.900 European secondary
136.850 SITA North American Frequency
136.750 New European frequency
131.850 New European frequency

[SalNichols94807]

Here's a bit of light reading for ya'll:

http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/enroute/oceanic/documents/cross_polar/CPWG11/CPWG11_IP03_PAN_ICD_Att%20A_v0.4.doc

[vapilot2004]

This query is prompted by the report that ACARS was 'disabled' but that some sort of handshake was being initiated by the aircraft for several hours.


BOAC: The ACARS on the 777 is an integral part of the AIMS system. The data is collected and channeled out of the aircraft in 3 ways, VHF, HF, and SATCOM. The system allows one to choose the primary and secondary data paths, and it is here that one can disable it though the FMS CDU.

The handshake continued thanks to the SATCOM transceivers having been left on. They do not have a power switch, only circuit breakers. In addition, the radio pack is located above the ceiling fascia, aft, and not in the EE bay - so no easy direct access to the hardware.

[BOAC]

Thanks for the explanation, VA - I assume therefore that the system will 'auto acquire' whichever TX is available?

Sounds as if the crew missed a trick.

[vapilot2004]

No worries BOAC and yes, ACARS data path auto acquisition is entirely programmable on the ground with uploaded mapping data telling the system where to use VHF and where not to. The data path options are driven, not surprisingly, by cost, with VHF being the cheapest.

[Lazerdog]

The -777 uses the ARINC 629 buss and much of the equipment and LRU's are linked by twisted pair cables to carry the data. I think the failure of those cables is worth considering in terms of getting data to the satellite transceiver which keeps its connection whether or not it is getting data from the aircraft systems. Remember also that the cockpit VHF and HF control heads use the buss to connect to their LRU in the avionics bay.

[gums]

Good to see some "technical" discussion of ACARS.

It is my understanding from Wiki ACAR article and other sources that FMS data is usually "one way" - from ground to the aircraft. And then the crew can use a change in route or ignore it. Going air-to-ground, seems the data is up to the carrier to decide what is reported.

Am I close?

I also note that the aircraft we are looking at sent ACARS at 30 minute intervals. Is that the norm?

[vapilot2004]

Gums: it is a 2-way system by design and you are right on with your understanding. Data, including FLT PLAN and PERF DATA can be uplinked and downlinked to/from the FMS, however uplinked data will not become active until reviewed and EXEC'd by the flight crew.

Every 30 mins: On the 777 and most modern Boeing aircraft, downlinked reporting frequency is entirely controllable via the FMS CDU. I am unaware of a normal timing standard, but 30 minutes seems reasonable. Typically the minimum reporting points for a block would be OFF STAND, AIR/GROUND=AIR, AIR/GROUND=GROUND, ON STAND.

[gums]

Thanks, VA.

I ask about ACARS because it seems that AF447 ACARS transmitted over 20 messages within the last 5 minutes of flight. The Final Report also mentions transmitting position every ten minutes or so. Granted, 447 had good electrical systems all the way.

So I guess it's up to the carrier to determined what goes out and what comes in and timing, huh?

[DaveReidUK]

I also note that the aircraft we are looking at sent ACARS at 30 minute intervals. Is that the norm?

I believe that 30 minutes is the interval at which data is sent to Boeing by aircraft fitted with its Airplane Health Monitoring (AHM) system, for those carriers that subscribe to it.

[vapilot2004]

Thanks, VA.

I ask about ACARS because it seems that AF447 ACARS transmitted over 20 messages within the last 5 minutes of flight. The Final Report also mentions transmitting position every ten minutes or so. Granted, 447 had good electrical systems all the way.

So I guess it's up to the carrier to determined what goes out and what comes in and timing, huh?

It is. According to our guy in coveralls, you can set the ACARS transmit time from almost every minute to any period of time longer. The whole thing is highly customizable including what maintenance/NAV/flight parameter information will be sent. With messages bound for maintenance, there is a burst mode on the Boeing.

Not privy to the details on how Airbus handles the data, but it seems they have a different arrangement, or perhaps Air France doesn't mind paying for near real-time reporting.

[gums]

Good update, VA.

From the AF447 interim report, it is not clear if the two dozen messages were in separate "packets" , but it seems they were grouped into several. Most were ECAM alerts, with a few indicating other data.

It also seems AF had the ACARS programmed to transmit if some problem occurred versus a fixed time interval. Also from the interim report, ACARS transmitted position every ten minutes, including one seconds before the "upset" happened.

The last AF447 ACARS transmission was about 2 seconds before impact. One of the last "faults" was related to cabin pressure. Not surprising as the jet was descending at 10,000 feet per minute.

BTW, AF used satellite comm when available for their ACAR. The ACAR/ECAM data was available well before the wreckage was finally located and the FDR/CVR retrieved.


So my conclusion is that the employment of ACAR data is up to the carrier, and maybe some influence by the manufacturer and/or motor folks that have a vested interest in their hardware and future sales, ya think?.

Secondly, seems to this old pilot that the satellite is the way to go, considering the Inmarsat data we have seen the last few days.

[Yancey Slide]

It also seems AF had the ACARS programmed to transmit if some problem occurred versus a fixed time interval. Also from the interim report, ACARS transmitted position every ten minutes, including one seconds before the "upset" happened.
Those might not be mutually exclusive settings.

[gums]

That's right Slide.

My point was that AF447 transmitted a "routine" position update on a fixed schedule, then a few seconds later started transmitting ECAM alert stuff and aircraft system stuff for the next four and a half minutes. The data was invaluable when the FDR and CVR was available to correlate.

This is not what happened with 370, and it's time interval was 30 minutes, apparently. It also did not start up when "something" happened, if there was a severe problem versus conspiracy theory stuff. I wonder if the carriers preserved ACAR data from the 9/11 planes?

Sheesh, if the flight goes smoothly than there ain't much to look at on the tapes/disks. If there's minor mechanical problems, you can do required trouble shooting and repairs. If the end result is a crash, then the data is invaluable for preventing a future episode.

BTW, I flew the first USAF fighter with a solid state recorder to keep you from lying to the accident board. Sucker was in our seat, and we didn't know it was there until after a crash or two. Was not nearly as comprehensive as the FDR on the big jets, but it sure helped re-construct what happened and keep it from happening again. Also had a big time FDR in one outta six of the first 40 - 50 jets. If it was recovered, we had even more data. Also had a video recording of our HUD, which had some gruesome footage on a few crashes.

[aveng]

I have never heard of using HF for data transmission all the aircraft I've worked on use VHF when in range and satcom when out of VHF range. HF is too noisy for data.

[Lazerdog]

Can't say I know what companies might be using HF ACARS, but the equipment can support it. Ground links are out there too.

HF ACARS (http://www.blackcatsystems.com/radio/hfACARS.html)

http://www.udxf.nl/What%20Is%20HFDL.pdf

[Lazerdog]

It has been around a lot of years. The Collins Rockwell HF Transceiver brochure has a basic block diagram of the data transport choices and overall interconnect.

https://www.rockwellcollins.com/~/media/Files/Unsecure/Products/Product%20Brochures/Communcation%20and%20Networks/Communication%20Radios/HFS-900D.aspx

By the way, if there are any old-hand RF types out there familiar with air transport HF antenna installations, and airborne antenna tuners like the 490T and later, please send me a PM.

[island_airphoto]

HF data is certainly possible and is done every day. It is slower and less reliable than VHF or Sat Comms, but it works. Sailors all over the world use it for email.
SailMail (http://sailmail.com/) ;)

[CW5301]

High Frequency Data Link - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/High_Frequency_Data_Link)