FMS & IRS & Time

[EGMC]

FMS
Database is installed by ground crew in preparation for the implementation/switch-over by the flight deck crew in the air at the appropriate time.
Q1: is the activation done on a second system as i would expect a risk exists in messing around with the software whilst live. Does the activation HAVE to occur at midnight UTC, or is it whenever the pilot does a checklist... and when IS this 28th day, i mean does ICAO standardise a 28day calendar or does every operator/system use its own independant date?

IRS
I understand 3 gyroscopes with accelerometers positioned orthogonally but where are they...In the wing/nose?...and how big? not counting any backup systems like IRS(2/3) etc, would there be any other instruments which require a gyro in a big new jet

Time
I know this might sound weird or plain stupid but...how is time measured in a big new jet. I mean surely it must be something connected to isotope half-life, i wouldnt expect a M.84 a/c routing trans-atlantic on a pendulous/crystal mechanism.

sorry if anything's been covered b4 but i keep searching and then reading for hours and having a hundred more questions pop into my mind:ugh:

[dv8]

FMS (GPS)
Only once have I been in-flight when a MSG popped up to say the data base is switching over It was at midnight (cant remember LMT or GMT)
IRS
Are in the avionics bay on the F100 No idea how big
Time
Err an LCD clock on the glare shield
If I want an accurate time (Q400) on the Universal FMS its on the DATA page

[phillpot]

EGMC

FMS databases im am fairly sure would run UTC as do most things (cross the date line though then who knows) strap down irs with rlg are fairly small and I believe on some types are integral with the adc( boeing call them adciru )and as already mentioned will be located in the avionics bay,they supply all the Primary attitude data as well as nav but can supply just attitude data if a fault occurs, the standby attitude indicator will have its own gyro so that it remains autominous and will be electrically driven ICAO states the battery must be able to run it for a minimum of 30 mins in the advent of a failiure of electrical power.
As for clocks im sure they are of good quality but nothing real fancy, ATC will supply the departing a/c with a time check which has to be accurate to 30 seconds so you check/set onboard timepieces to that time.
Hope that helps.

[esreverlluf]

Database runs to a 28 day cycle - always changes on a Thursday as per the Aviation Information Regulation and Control (AIRAC) cycle.

Can get a bit interesting, especially when a long sector will straddle the activation time. In this instance one can only be cleared via the current SID and air route, but by the time you get to destination a different database may be in use. Not that much of an issue as you should have the appropriate, updated charts on hand to make any manual ammendments to the FMS route. I haven't seen an FMS that is smart enough to update in flight, though they may be available.

According to the Australian AIP, "Documents and charts issued in accordance with the AIRAC cycle become effective at 1600 hours UTC on the day prior to the nominated date unless otherwise notified; eg, a document with an AIRAC date 26 Jun becomes effective 06251600UTC (ie, during Australian Eastern Standard Time, becomes effective at 0200 hours local on 26 Jun)."
I am aware that New Zealand uses a time of 1200 UTC the day prior to the new Effective Date Range. Elsewhere, and in the absence of other information, I would use 0000Z. Some charts additionally have an exact UTC time of activation.

There does continue to be a little confusion where there is a major change to a procedure or air route. I was once cleared for a SID that was not yet effective

[Dan Winterland]

The changeover depends on the aircraft type. In the Airbus, the flight is initialised with the database current at the time. If the flight straddles the changeover, no matter. But the return flight, if you try to use the old database, you get a warning. I operte mostly from the GMT+8 timezone. So this is quite common. You initialise before 8am local on one database and have to swap for the return. If you swap databases once initialised, you then have to reprogram the route.

IRSs used ring laser gyros. There are three per IRU, (one for each axis) and the crystal blocks in which the lasers shine anre about 2 inches across. They will often be integrated with other systems such as an Air data Computer or radio nav system. Airbus calls them GNADIRS (Global Navigation Air Data Inertial Reference System) which gives you a clue to their functions. They will invariably be found in the radio electric bay.

Clocks vary, but the tendancy is now for clocks to be updated by the GPS. A fully integrated system nowadays will have the Flight Management Computer receiving a GPS signal and sending the FMC time to the display clock on the panel. Of course this is not always the case and we do have a couple of aircraft in our fleet which have manually set LCD clocks.

Incidently, although the FMC will normally dispaly the time as UTC (I still like to think of it as GMT) it's nearly always GPS time which is not the same. GPS time is set as a constant and can't be changed, but since the first GPS satellite was launched in the late 70s, GMT has been altered several times to correct for variences in the Earth's rotation rate. The difference is now up to about two seconds. However, some FMCs have the difference in their database and will adjust the display time corrected to GMT if required. One aircraft I flew had the correction facility. This difference won't affect a 747 crossing the Pacific, but this facilirty was in a military aircraft where a couple of seconds can make a big difference.

[EGMC]

Thanks all!

dv8: did the message tell you it was going to change or ask you, and can I assume that was on a Fokker 100? [do they use the same systems as Boeing or Airbus or another?]

I should have said not the RLG [so maybe i meant the old INS...i dunno!:}], what kind of diameter are we talking about with the traditional spinny thing in a big a/c- the main ones, not the backups.... size of a steering wheel perhaps?

are they not spun by little air jets like we have on SEP's?

can anybody from JAR land confirm it is a Thursday for the switch-over, if so when was the last one?- surely you can just match it up with the lunar cycle and have a constant reference... or is that a bit sad

...i think we can put a full stop on the Time questions, although it did get me thinking about GPS time... which I hadn't previously considered!

EGMC

[dv8]

EGMC
Sorry cant recall it was several years ago on the ATR night mail flight (We only flew sensible hours on the F100) with the KLN 90
I would put money on it being an INFO type MSG