IRS alignment question

[rustbucket732]

What are your SOP's regarding IRS alignment?
i.e. when do you perform a quick vs full?

[ 20 September 2001: Message edited by: rustbucket732 ]

[Checkboard]

Full every turn around.

[magnum]

very interesting point. on the 777 we never do a full alignment. have been told that it doesnt have any affect on navigation accuracy and that this was coordinated with boeing. dont quite believe it though. any experts on the 777 ADIRU out there?

[mcdhu]

A320/1 - full ADIRS align (10mins) with a crew change and prior to a sector involving LRN or MNPS procedures, fast align (3mins) on all other turn arounds.
Cheers,
mcdhu

[therosterman]

I agree with the 777 procedure for a quick alignment. Most new aircraft receive GPS updates from atleast 2 units. Although GPS is not certified for IFR approaches it is alot more accurate than IRS combined with FMS.

[magnum]

sure, i do agree. but who can guarantee gps reliability as long as the military has control over it. but on the subject, does the 777 ADIRU fast realignment cancel out all accumulated errors? of course all drift and position errors are reset, but what about the gyro errors. if a fast alignment is as good as a full one, why should we do one after all. i was tracking the irs-fmc deviation on the POS REF page, and could not proof any accuracy improofments with full alignments, even though i wanted to proof just that.

[magelan]

A fast re-alignment will correct your position to the one entered, but the drift rate of error will remain since it is a function of time. In other words, your helping the system temporarily, but your position accuracy is deteriorating at the same rate. This can only be fixed by a completely new alignment. If your system shows you the ANP-value, you can observe the trend. I always did a complete alignment whenever the groundspeed on the stand showed more than 3 kts.
Cheers, Magelan

[HighSpeed]

on our b777, we do a full alignment when the last full alignment was done more then 24 hours ie. full alignment every 24 hours.

as mention by magelan, this is to rid the adiru of accumulated drift rate error. (which incidently comes from the accelerometers not the laser gyros).

HS

[ 21 September 2001: Message edited by: HighSpeed ]

[CaptA320]

Although I agree with most of the above I would like to add the following. A full alignment will make the IR's seek True north to provide heading information (as well as map information after full alignment) and a Level platform to provide attitude information. If at the gate after landing a drift is noticed (ie sustained groundspeed) a quick re-alignment is recommended which gives the ability of 1) Zeroing all groundspeed and 2) Entering a new present position.

[magnum]

thanks for the replies regarding the 777. we also have the 24hr alignment requirement, nobody is doing it though for some strange reason. i was told that when the ac were new there were problems with the full alignment and so there is some apprehension to do it. tried to find the alignment requirement in the AFM but no luck. any suggestions?

[decato]

This is my scheme:

- Full alignment: every flight if possible or, at least, every 18hrs. All bias update and internal velocities will be reset to zero.

- High latitude alignment, when departing in polar regions.

- Fast alignment. Rest.

[Hot End]

Depends if you have triple IRS at your FMS' disposal, or like the old 73-3 or 73-4's, you have only 2. The first instance gives the FMS the electronic equivalent of the "cocked hat", averageing out drift errors, while the latter only allows a single IRU input to the FMS at any one time.

If your in a part of the world that provides you with little opportunity for rho/rho updates, and, like most early glass flight decks, you don't have GPS imput, a full align seemd sensible, even on a twenty minute turn around. I do 'em all the time.

That help????????

[strobes_on]

With GPS updating, the comments above are very valid.

However, for aircraft relying on DME/DME updating, there is merit in a full alignment, time permitting, on every turnaround.

[QAVION]

"With GPS updating, the comments above are very valid. However, for aircraft relying on DME/DME updating, there is merit in a full alignment, time permitting, on every turnaround."

Although mentioned, I don't think enough emphasis has been placed on things other simple "position" when it comes to IRS alignment. If pilots start believing that lat/long (position) is the only thing to consider, you will eventually get the situation where pilots of GPS-equipped aircraft consider it safe to take off with the IRSs still in the _middle_ of a quick alignment... thinking that the IRS's will align themselves in the air c/o the GPS (It has already happened).

The reason why many manuals state that a full alignment must be made if the IRS's have not had a full alignment for 18hours (irrespective of whether the aircraft has GPS or not) is because not all types of IRS errors are removed during fast alignment.

Remember also that if triple mix IRS position varies by more than a certain amount from GPS position (due to IRS alignment errors), that many aircraft are designed to default to IRS position (irrespective of whether the IRS's are correct or not).

Rgds.
Q.