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B747-400 L IRU and SATCOM

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B747-400 L IRU and SATCOM

Old 20th Nov 2021, 00:08
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B747-400 L IRU and SATCOM

Hi All,

Quick question. So I've always been told that ACARS messages can only be sent on the ground in the B747-400 with the IRS's aligned (more specifically this related to sending Flight Summary data back to Ops via ACARS after the flight). Was told this was because the SATCOM needed the IRSs to be aligned and in NAV mode.

Only place I can find it written is the QRH for IRS CENTER, LEFT, RIGHT which states: "1 If a LEFT IRU fault occurs, the SATCOM system is inoperative."

Just wondering why there is no mention of this anywhere in the FCOM? (random question I know, just curious).

Thanks in advance if anyone can answer that one.
Richard_Richard is offline  
Old 20th Nov 2021, 09:51
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I’ll start with a massive health warning that I’m well out of date on the 744..so what follows is a vaguely educated guess - I’m not sure what you have been told, effectively on the 744 no satcom means no ACARS on the ground, is totally correct…

As I recall it ACARS would happily communicate via VHF whilst it was on the ground if coverage was available, so in that case satcom not needed.

I do however remember there were some parts of the world/some airports where ACARS via VHF was not available, especially for some reason in parts of the Far East, and if that was the case you did need satcom to use ACARS on the ramp.

Maybe the advice you have been given is a procedure used/recommended by the operator because of the airports it operates into and out of and if that’s the case I wouldn’t expect it to be referenced in the FCOM….

Last edited by wiggy; 20th Nov 2021 at 10:52.
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Old 20th Nov 2021, 13:03
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I donít have the FCOM any more but on our aircraft the satcom was reliant on the L IRU. We always used to send the flight summary before switching its IRUís off although as Wiggy says it would only make a difference if there was no VHF ACARS available.
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Old 20th Nov 2021, 13:45
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Inertial reference is required for the satellite antenna to track the satellite.

Not sure why the FCOM doesnít give much detail except to say that Boeing seems to prefer to give less information rather than more information.
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Old 20th Nov 2021, 16:04
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I think as Locked Door says if you have ACARS via VHF then whether the satcom antenna is aligned or not is irrelevant to most post shut down activities, including sending a post flight report.

Whether you have ACARS via VHF or not at that stage in proceedings is possibly a contractural issue, and possibly dependent on the airline- I know where I worked used to see a mention in company notes of countries over which ACARS via VHF was not available or had been disabled for various reasons…

Given it’s a characteristic of the system that is subject to flux as for example contracts and VHF coverage changes I guess it’s a ? as to whether it’s something that should be in the FCOM.
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Old 20th Nov 2021, 16:21
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Thanks for the replies so far! Agree that Boeing seems to skip some of the finer details in the FCOM - just strange it made it as far as the QRH but not stated elsewhere.
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Old 20th Nov 2021, 16:23
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The ACARS system operates on a priority basis, it will always try to communicate firstly over VHF (or more rarely HF data link if available), then falls back to SATCOM if unable.

Now as mentioned, positional information is required to steer the SATCOM antenna, which is provided via the SDU (Satellite Data Unit). There are two configurations of SDU that I know of in the -400; it can be driven by either the L IRU alone or both L and R IRU's.

Hence the requirement to keep the IRS aligned until the flight summary has been sent if you are unsure if VHF datalink is available.
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