View Full Version : CPDLC access problem when using SATCOM datalink

15th Aug 2012, 11:20
G'day Gurus,

I have one out of left field here.

Background. We operate two 737 BBJ aircraft that have the following equipment on board:

FANS 1/A capable running Smiths FMS with FMC software 10.8A installed

INMARSAT and IRIDIUM SATCOM systems running Swift Broadband (via a HSD440 unit), cockpit and cabin voice etc. I-4 and I-3 SAT capable.

ACARS (provider is ARINC Direct). SATCOM datalink provider is INMARSAT/STRATOS

We are currently experiencing a rather strange problem. When our jets are communication purely via our SATCOM (outside VHF datalink coverage) we are seeing intermittent CDU messages: SAT NOCOMM and DATALINK NOCOMM. We seem to be able to use our ACARS ok (to retrieve text weather, D-ATIS etc) but as soon as we attempt a log on to CPDLC the system basically becomes unusable. CPDLC when connected via VHF datalink works fine. Our SATCOM systems were upgraded mid last year and we've been experiencing these issues ever since. Our internet and voice services are working perfectly.

As a side note, we are acutely aware of the known FMC software issues regarding the use of SATCOM datalink and CPDLC. This appears to be a separate issue. (Incidentally, the Boeing FCOM bulletin that addresses the issue of reduced FMC performance works well. Used it in anger the other day)

Any thoughts on this one?

Thanks in advance.

18th Aug 2012, 13:00
Both IRIDIUM satcom and Inmarsat satcom can not operate simultaneously, whenever you use either one kind of satcom, you shall power of another one.

18th Aug 2012, 13:38
You'll have to give your Iridium equip a break while entertaining CPDLC...

19th Aug 2012, 20:32
Both IRIDIUM satcom and Inmarsat satcom can not operate simultaneously, whenever you use either one kind of satcom, you shall power of another one.

if you have a decent installation, then INMARSAT filters should be installed, if also Iridium is installed...then it will work without a problem..

if those installation basics have not been followed on the bird you fly, I would, if I were you, not even trust, whether the oil in the engines or the toilets have been serviced correctly...

20th Aug 2012, 08:03
Interest comments about the INMARSAT and IRIDIUM systems running concurrently. So are you suggesting that the IRIDIUM system be powered down completely when attempting to use CPDLC? Or just 'don't pick up a phone in the cabin'?

I find it a little strange that all of our other INMARSAT services (voice, swift broadband, etc) work perfectly except the one.

24th Aug 2012, 13:45
Both Systems operate in same frequency band, may interfer each other,
it is better to operate one system and power off another.

27th Sep 2012, 19:15
I would like to clear up a few things.. Iridium and Inmarsat actually work on different frequencies. Inmarsat actually works on a range of frequencies and the system chooses the frequency based on several factors. When the Inmarsat system chooses the lower frequencies it causes interference with the Iridium system. Typically the BBJ will have a high gain Inmarsat Antenna which directs the SatCom signal directly at the satellite when the signal is crossing over the Iridium antenna due to the location of the satellite it is connected to and the heading of the aircraft the interference will be present. There are a few filters available one goes on the Iridium RF cables and blocks Inmarsat from getting to the Iridium Transceiver, this filter type has shown to not work well. The other filter type goes in the Inmarsat System and blocks the lower Inmarsat Frequencies, this filter works well but I do not believe it is approved by Inmarsat.
Your Inmarsat HSD440 system uses SwiftBroadband for cabin internet and uses AERO H, H+ for ACARS this is a separate channel from SwiftBroadband. If you are running ACARS away from VHF coverage and it works it means your connection through the Inmarsat Aero H, H+ is good. CPDLC is a part of FANS 1/A which includes both CPDLC and ADS-C and travels inside the ACARS message set; once it reaches the ground CPDLC and ADS-C messages are routed via another path to ATC. That path to ATC could be broke. But since you say the entire system becomes inoperative I would suspect something in the aircraft sounds like software lockup. Your service provider should also be able to check the CPDLC login and see if it is reaching the ground and if there is a break in that network somewhere. :ugh:

Jo Kremsreiter

28th Sep 2012, 19:20
Boeing have issued a write-up of the different aspects of the datalink system that contributed to the problem observed in 737NG's FMC software version U10.8A, the problem is observed when one FMS message is in the ACARS MU queued and an ADS and a CPDLC are in the FMC queue, with the ADS message queued first. After 5 minutes of attempting to send the ADS downlink, the FMC will stop sending the ADS and then will attempt to send the CPDLC. The reset occurs when the CPDLC is finally sent with the FMC queue corrupted due to un-send ADS. the next FMC software version U11 will fix this issue.