IRS alignment on Airbus

[Austin_Powers]

Our company SOP's say, for a complete IRS realign we have to switch off the IRS for at least 10 minitutes (this for A330 and A320). As I know that new generation airplanes like the Airbus use laser gyros with no moving parts, this seems a bit long for me. I also could not find any limitation or recommendation neighter in the pilots, nor in the maintance manuals. May be this limitation was on older designs like Boeing?
Does anybody have better information about this?
TKS
Austin

[145qrh]

As far as I know the IRS on Airbus, 330/40 perform a full align if switched of for more than 5sec, less than that they do a quick align..

If you have GPS primary it really doesn't need to be a full align as it updates itself on application of take-off thrust.

145qrh

[FlapsOne]

A full align can take up to 10 minutes but you certainly do not have to switch them off for 10 mins.

Off and on within 5 secs will give you quick align so longer than 5 will give a ON BAT plus full align.

See FCOM 3.04.34.

[Austin_Powers]

Gentelman,
I know all this. But our boss says if we do a full allignment (which is procedure for long range flights) we should switch off the IRS for 10 minutes to save livetime or whatever. But I can not see the point, as in todays IRS there is no moving parts and they do not need a certain time to spool down and come to a full stop like old gyros did need. I also could not find any recommendations in the maintanance manuals nor the flight manulas (manuls for pilots usually don't tell much more than how to switch on and off).
So I hope my question could be answered by an avionics engineer/expert, as there is probably some more know how behind this issue.
For example all Airbuses are designed, to take off with bleed air on. But most airlines require their pilots to T/O with bleed off in order to save live time. It is
true that Airbus has established procedures for T/O with packs off, but the to not force you in that direction as they whant to sell you new engines after a couple of years.
Cheerz

[145qrh]

As you say, doesn't make any sense, don't you just love fleet bosses.

As a rule we leave our IRS on all the time, never get switched off, unless the plane is going to be de-powered, to give them longer life, and for flight plan/wind downloads, helps if plane knows where it is.

As for packs on/off. I think all Trent 330's must take off with packs on to help prevent stall/surge at low speeds with high thrust..

[lyo]

Originally Posted by 145qrh

If you have GPS primary it really doesn't need to be a full align as it updates itself on application of take-off thrust.
145qrh

A general misunderstanding IMHO about the IRS on the airbus. The GPS updates the FMGCs position (hybrid mix of IRS/GPS) not the IRS...
I'm used to see a 5 NM IRS drift after 2 sectors, short haul though. The reason is the first alignement of the IRS at the ARP while the aircraft stands 1NM away... Wrong SOP to my understanding.

[gearpins]

ok let me see if I can say this straight :Assume a configuration of 3 IR's and 2 GPS recvrs. add to that FMGS1 and FMGS2. this is what happens,each IR comes up with a position( measures gravity on alignment subsequently figures out the lattitude compares this to the last memorised posn and the present lat long entered through the mcdu init page which is either bay co-ords or arp and all that on full align only. quick align simply resets g/s to zero) meanwhile each GPS displays its own derived posn(on gps posn monitor pg).Now things start to warm up. Each IR will independently choose GPS1 or GPS2 data and come up with a GPIRS posn.lets pause and review what we have: 3 GPIRS POSN and 3 IRS POSN.
The three IRS agree upon a common posn called MIXIRS POSN(if one of the IRS posn is too far out from the other two that IRS is left out of the calculation). Now each FMGC is presented with 1 MIXIRS POSN and 3 GPIRS POSNs.( take a break!! I need one) At this point the FMGC picks anyone of the three GPIRS POSN( in the decreasing priority sequence of a)on side-GPIRS1 for FMGC1.b)GPIRS3 c) opposite side GPIRS) and does the followingamoungst many others )
-Correct the posn bias of the mixirs posn and thereby contain IR drift(not displayed directly)
-Display a calculated FM POSN
-When T/O pwr set update a/c posn on the rwy to minimise posn errors in the r-nav sid.
when you loose gps then posn is calculated with mixirs posn, updated with radio posn(DME-DME,VOR-DME, etc...)
And when you loose radio updates aswell ...............
NOW is when you are well and truly cast a drift.............
airbus recommends on pwr up like first flt of the day....full alignment.
at all other times if residual g/s<5kts...........no action
if residual g/s on any one IRS>5kts............quick align all three.
full align takes ten min at higher lats 45+. at equator full align takes about 6 min

[PRNAV1]

Don't mean to take over the thread but i have an IRS question to add.

In my company after full alignment our SOP's state we should put the ISDU setting to show IRS 3 with wind, this is with no explination.

Could someone tell me if they also do this where they work and why? if the ND is showing wind form IRS1+2 on the capt and fo screens, why would we need it on the over head for IRS 3....

Thanking you!

[lyo]

Originally Posted by gearpins

airbus recommends on pwr up like first flt of the day....full alignment.

All right, first flight of the day, you align the IRSs with the ARP or the stand coordinates? My point is to align them to their actual psn, because they won't receive any updates after the alignement, not at the runway take off psn, the FMGCs will receive this update.

[gearpins]

"Could someone tell me if they also do this where they work and why? if the ND is showing wind form IRS1+2 on the capt and fo screens, why would we need it on the over head for IRS 3...."

after a/c parking at the gate residual g/s of all irs must be checked for the 5kts threshold....in order to do a quick align...

[PRNAV1]

after a/c parking at the gate residual g/s of all irs must be checked for the 5kts threshold....in order to do a quick align...

...but that can't help because you lose wind readings on the ND's and overhead panel after touch down. You sure you got that right? Also it's your ground speed you should be looking at not wind speed/direction for a quick re-alignment.

correct me if i'm wrong...

[gearpins]

oops I read your question wrong.....sure you loose wind reading on touchdown. but you dont loose your g/s inication on the top left corner of your nd until after a few min after shutdown. nd 1 displays g/s as recorded by irs1 nd 2 by irs2 and to check on irs3....I was refering to g/s all along.....sorry once again.....as for your qest...'wind"....I am clueless at the moment....will get back if i find something........

[Max Angle]

If you have GPS primary it really doesn't need to be a full align as it updates itself on application of take-off thrust.

One aspect which is often overlooked is the effect that IRS drift can have on track information (green diamond and flight path vector) and the wind vector computation (and hence VAPP computation for G/S mini). I guess it depends on the direction and amount of drift but an IRS that is 5-6 miles out can have a huge effect on these computations. It's the reason that the track diamond doesn't seem to work as well in the a/c as the simulator. It is also the reason why the two wind vector displays are always a little bit different, if you have a lot of drift they can vary by a huge amount and cause the VAPP computed by each FMGS to be way out of sync with other. Like all computers it's garbage in, garbage out.

[Swedish Steve]

If you watch the IRS when you switch them off, you will stay that they stay powered for about 30secs. The computors are collecting information. Then they turn themselves off. They are now dead and can be switched on again.

Some computors do need longer before switching on again. The FMGCS is one. It has capacitors inside it to allow for power transfer and will retain information for ages. The MM says 7 mins before it has reset.

But the IRS relies on the batteries and as soon as the lights are out you can turn it on again.

The reason for doing this before a long flight is that on the older IRS there was alimited memory (the old BA A320 have this type) and you could run out of waypoints!! Bit embarrassing if you are in the middle of nowhere! But new IRS should last a lot longer and a Quick Align is enough during the day.

[TomConard]

IRS

I really can't remember...and I suggest you e-mail Airbus to get the correct answer.

Yes, it's true, the FCOM states that anything over five seconds puts the IRS alignment into the "Full" mode. Yes, it's true there are no moving parts in the IRS system (except for maybe the OFF-NAV-ATT switch :> :> :>).

But, the IRU system, when turned off, goes through a self-test process, and a position memorization process that takes some time...I don't know how much time...like maybe five seconds, five minutes...I simply don't know. Again, ask Airbus.

At my old carrier, we were told to, when executing the Securing Checklist (where you turn off the IRUs and shut down the airplane for the night) to delay cutting power to the aircraft for at least two minutes after you turn off the IRUs. In other words, you're done flying for the night...putting the aircraft to bed...thinking about that cold beer or two...don't be too quick to shut down the APU right after turning off the IRUs. Give them a few minutes to 'do their thing' after turning them off, before removing power to the aircraft and heading for the bar.

I realize I haven't answered you question, except top suggest that you contact Airbus for a correct response.


Tom

[captain_jeeves]

Originally Posted by 145qrh

If you have GPS primary it really doesn't need to be a full align as it updates itself on application of take-off thrust.

145qrh

I think I know what you are trying to say, but your response is slightly misleading (no disrespect intended).


True, with FMS1, the FM position is automatically updated at the runway threshold. However, with FMS2, this automatic position update is inhibited when GPS Primary is available.

Next, remember what GPS Primary actually does to the IRS system. When the GPIRS position is available, it overrides the RADIO position, if available. Therefore, the FMS position tends toward the GPIRS position.

However, without GPS Primary – but with GPS supplemental – the GPS position is considered to be an additional form of NAVAID, and can be accepted, if it falls within the radio position or MIX IRS position. In this case, the FM position is updated when thrust is advanced TOWARDS TOGA during the take-off.

Switch the IR’s off for at least 10 minutes to obtain a full alignment?

No idea why this is any companies SOP. You only need them to be off for about 5 seconds to get a full alignment (Maybe you company meant to print 10 seconds in the SOP, and you’re dealing with a typo).


On the other hand, they (IRS's) do continue to work for about 30 to 60 seconds after you select them to off, so maybe your company has found something in this that Airbus has not recognised.

Regardless, if you like your job and present company, DO AS THEIR SOP SAYS until such time as they change it, and then, DO AS THEIR SOP SAYS.

Cheers /cj

[vapilot2004]

I recall the INS units of our recent past required a logging of position accuracy by way of recording and zeroing residual ground speed among other data. This was accomplished at the gate and took a few minutes.

Perhaps the modern laser ring IRS uses this same period to verify its calculated data - thereby quantifying system performance.

Procedures do still have them off only once on stand.

[gearpins]

True, with FMS1, the FM position is automatically updated at the runway threshold. However, with FMS2, this automatic position update is inhibited when GPS Primary is available.

I have not seen that happen on the a/c i have flown,have not come across any such thing in the manuals......moreover dont see the logic in that.........no offence meant

[TomConard]

IRU Operation

Captain Jeeves is, again, correct. Please refer to Airbus FCOM 1.22.20 P5 (Auto Flight).

Now, this applies to systems equipped with the GPS system...where GPS Primary exists.

"Note: The FM position update at takeoff is inhibited when GPS PRIMARY is active."

With GPS PRIMARY NAV ACCURACY HIGH, the FM always knows where the aircraft is, regardless of what you input into the MCDU. There is no runway update necessary, since the aircraft already knows where it is.

As long as the aircraft knows where it is, and it knows where it is supposed to be going, there is no navigation error...no need to do a runway update upon application of takeoff thrust, etc., no "Map Shift" after takeoff...none of this.

You can see all of this on the "POSITION MONITOR" page.

Regards,

Tom

[gearpins]

with FMS2, this automatic position update is inhibited when GPS Primary is available. I have not seen that happen on the a/c i have flown,have not come across any such thing in the manuals......moreover dont see the logic in that.........no offence meant

I seek clarification with respect to the above statement......why not with FMS2 ?????