Uplink for wind and performance data

[Will]

This is a beginner's question... how does the FMC connect to the ground station for the wind and performance data uplinks? The sound station would need to know what aircraft it's communicating with, and the aircraft would beed to reach out to the proper station. Where is that handled? Is it specified within the FMC (via the CDO) on pages that the pilots don't have access to? Or does it go through something like its SELCAL registration? Some hard-wired unique frequency that the dispatch office knows about (but the pilots don't care about)?

[Jeroen Hoppenbrouwers]

The ACARS network is not based on frequencies to address a station, it is a real network. Radios use whatever frequency they happen to be on to get to a tower, then from there on everything is routed to central exchanges (SITA, ARINC), and these then connect the aircraft (by tail ID) to their company. As far as I know, winds and routes are provided by the operator's backend, not any central "FAA service" (unlike D-ATIS, METAR, FANS, ...). So the tail ID already identifies who is the far end receiver of the message and the aircraft has no say in it.


Hoppie

[DougSnow]

Yep - the aircraft sends a request to the ground system with the aircraft registration. The AOC Uplink sends the data back to that aircraft registration as a specific ACARS SITA message. AOC Uplink is coming from the airline's flight planning system.  Here is what a random AOC Uplink looks like - note the arrival and departure is not uplinked just the section between TOC and TOD.

The CE78 is a checksum so the FMC can validate the whole message was received.

QU xxxxxxx
.xxxxxxx 091819
M99
FI XYZ    /AN NXXXZZ
-  /UPLINK /UID FFPSAOC  /FTX
FPN/RI:DA:KMEM:AA:EGSS:F:HUMMS,N36214W088591..PXV,N37557W087457..ROD,N
40173W084026..SLT,N41308W077582.J190..RKA,N42280W075144..MIILS,N46524W
067029.N279A..TUDEP,N51100W053140..N52000W050000..N53000W040000..N5400
0W030000..N54000W020000..DOGAL,N54000W015000..BEXET,N54000W014000..BAK
UR,N52145W005408.UN546..STU,N51597W005024.UP2..NUMPO,N51366W003170.Y3.
.BEDEK,N51223W001335CE78

Wind is similar but only a few FLs get uplinked for the fixes along the flight planned route - everything else the FMC is taking an educated guess of what the winds aloft are...

There is also a standard for uplinking takeoff data - but we havent yet incorporated it (its in progress now). With the request the crew can request up to three runways, the system calculates the data, then the system uplinks it all the crew has to do is to uplink it directly into the FMC.

[Will]

So if Lufthansa buys an old United aircraft, does Lufthansa start listening on their frequencies for the old United aircraft's registration?

Or do some maintenance guys go in with a screwdriver and change the way the former United plane communicates with the world?

[DougSnow]

That is part of the conformity inspection required - there is a file called the AMI file which the airline will load that defines all of those variables.

Loadable Software gets a part number just like a fuel control unit, and the airline creates documentation with engineering approvals to insure that only approved loadable software part numbers are installed on an aircraft.

So yeah, if DLH buys an old UA airplane, they will load all of the DLH-centric Loadable Software Airplane Parts (LSAP) so their new aircraft can talk to the Systems at DLH, AOC Uplinks, Airplane Health Monitoring (AHM), AMI data, literally hundreds of configurable items in the electronic airplane.

[Jeroen Hoppenbrouwers]

The ACARS frequencies are mainly determined by the communications provider (Collins (ex-ARINC) or SITA) exactly as they are determined by the provider of your cellular plan (AT&T, Verizon, T-Mobile, ...). Each of these providers has licensed frequencies from the government of the state, and they equip their towers with transceivers for those frequencies only. So if your aircraft is configured (by LSAP) for SITA, it won't talk to Collins towers.

Just as with cellular, once your radio has made contact with a tower, the rest of the traffic goes "underground" from the tower to the central message exchange at Collins or SITA. This is usually not over radio links, but if it is, then those links are not part of the aircraft radio frequency plan. And from the exchange it goes via ground lines to the operator's system, or to the local ATC authority, depending on the message type. Pilots basically have no say at all in where their messages go... the message type determines that, and it is all decided in the exchange.

If an aircraft changes owner or the owner changes "plan" from Collins to SITA, a few things need to be adjusted aboard. All of these are LSAPs and none are pilot-changeable or even tech-changeable, unless you start fumbling with what should have been a certified part (the LSAP). Data comm is part of operations and thus airworthiness comes into play.

Just overheard here that a specific software update for the 787 takes 22 hours to roll out per aircraft... that indicates the number of bits and pieces that need to be loaded into all those hundreds of boxes and subcomponents with software...

Hoppie