IRS Drift

[Dusthog]

I had a couple of flights on the same A320 with IR1 IR2 and IR3 drifting way more than normal, all at the same time and with the same amount.
Position check was OK.

I can understand if one IR is drifting away that it is faulty, but all three at the same time doesn't make sense. Can it be an error from the GPS maybe ?
What if you are on a long flight and they all drift away past 10 or 15 NM ?

Thanks for any information.

[spannersatcx]

did you write it up in the log? if so what was the answer? there are limits and how many times it can happen.

[Capt Fathom]

Dusthog

Google Schuler's Pendulum

I'm not even going to attempt an ad-lib description!

[underfire]

Dust,

The IRS needs to be updated or it does drift. Units are cert to drift less than 0.6nm/hour between GPS updates.
Depending on the unit, they are updated several different ways. Some units even have their own GPS built in, but those are not common.

The IRS has several inputs, such as the TAS abd baro, but for all to be drifting at same time, those systems are not likely the cause.

You location is affects the GPS coverage. The RAIM check is important so one can see where and if there is degredation of the GPS signal. In other threads, the polar route has been discussed.

When the drift was noticed, what was ANP telling you?

Since the IRU is based on GPS position, one has to remember that the GPS tells the ac where it was, not where it is. The Kalman filter estimates actual position. Changes in direction, such as high bank angle turns, affect the Kalman ability to estimate position as accurately, thus the IRS can be affected.

BTW, there is a mechanism for the pilot to update the IRU manually with a GPS coordinate.

[greybeard]

The A-310 did not have GPS update and would drift up to 3 nm between SIN-SGN.
This was on nearly every time I went there, interesting to see the ??? when avoiding storms using the FMS lines from the F/Os who had not seen the situation.
The map updated when on the ILS
Also on SIN-DAC on some occasions.

Cheers :ok:

[Chris Scott]

Quotes from underfire:
"The IRS needs to be updated or it does drift. Units are cert to drift less than 0.6nm/hour between GPS updates.
"Depending on the unit, they are updated several different ways. Some units even have their own GPS built in, but those are not common.
"The IRS has several inputs, such as the TAS abd baro, but for all to be drifting at same time, those systems are not likely the cause."

Really? Would you care to be more specific?

Hej Dusthog,

One posibility for excessive drift of serviceable IRSs is that the position entered at IRS alignment (only possible when stationary, i.e., on the ground) was not very accurate, but not sufficiently inaccurate to be rejected by the three individual IRUs.

BTW, unless things have changed recently (in which case underfire will be correcting me shortly), an IRS position cannot be updated once the vehicle is in motion. What can be updated, of course, is the FMS position. On an A320, the first time that happens on a flight is when the throttle levers are advanced at the beginning of the T/O run, at which point it is updated to the Lat/Long of the runway threshold. After that, the FMS normally uses GPS for its present position (PPOS), but uses a Radio Position (when a suitable pair of DMEs is available) as a back-up.

In the absence or loss of both GPS position and Radio Position, the FMS initially maintains the bias between the FMS position and the mix-IRS position to calculate PPOS from the mix-IRS position. On an A320, the two FMSs gradually reduce that bias to zero. Later in the flight, if GPS positioning is restored, and/or a pair of DMEs generates a Radio Position, the FMS will revert to normal positioning protocols.

[Breakthesilence]

Are you sure it's the IRS and not the FMC position drifting?

You could have all the IRS working properly but a GPS updating that is not doing its job very well (or unable to do it) thus affecting your FMC position and therefore showing the same IRS suspected drift on all the IRS systems.

[Chris Scott]

Hi greybeard,

Yeah, people talk about the "magenta line", but rarely mention that it's a green one on Airbuses. In the 1980s, when I was on your A310 (but usually over Africa), GPS was not yet available for FMS updating, and we rarely had a usable pair of DMEs to generate a Radio Position for the FMS. But that green line can be very compelling, and I once nearly overflew the presidential palace (verboten) at LUN, while self-positioning on left base for the ILS.

[Dusthog]

All of the IRs showed a drift that was well above the normal levels according to FCOM.
Never did a RAIM check but had GPS Primary all the time ( flying in Europe )

The coordinates for departure airport came from the database. Never had a problem with this airport before and the gate was close to the ARP.

Asked an engineer on arrival and he talked about a faulty IRS :ugh: We have two GPS onboard the A320 and the accuracy check with VOR/DME was good.

[Chris Scott]

Dusthog,

Forgive me, but I don't understand the relevance of you saying you had GPS Primary "and the accuracy check with VOR/DME was good". (BTW, a VOR/DME check is only a crude one unless you are looking at gross inaccuracy - the only accurate Radio Positions for an FMS Nav Accuracy check are obtained by DME/DME or LLZ/DME.)

If there is a need to specify excessive IRS drift for an entry in the Tech Log, you merely have to wait until you arrive on the stand at your destination. At that stage the FMS PPOS accuracy is easily verified, and (IIRC) the drift for each IRS is available on the FMS MCDU. (You can divide the drift by the flight time in hours to calculate the drift speed for each IRS in knots.) In the unlikely event that the FMS PPOS was rubbish, you could still establish the drift for each IRS using the IRS MCDU.

Hope this clarifies one or two points. Where I cannot shed any light, I'm afraid, is why all 3 IRSs would show similar unsatisfactory drift characteristics if the full alignment process was carried out correctly. Do you have any figures for flight time and total drift in each case?

[underfire]

CS,

Really? Would you care to be more specific?

IRU fast/regular ALIGN....

Dust,

On the Airbus, the IRU units are connected thru the ADIRS, this is a Honeywell configuration, perhaps this is what the engineer was referring to.

I would look at ANP reading next time this happens

[Dusthog]

The message on the MCDU in cruise was " CHECK IR1(2)(3)/ FM POS" All of the IRS on the MCDU showed 5+ and increasing.
The closest referens in the QRH is "If the message occurs in climb, cruise, or descent: CHECK navigation accuracy, using raw data". After landing all of the IRS on the MCDU had 5+ after the first flight and 9+ after the second flight. Both with a duration of about 1.5 hours.

Before flight a full alignment was carried out.

Cheers

[BOAC]

I know nothing about the way the AB runs its IRUs, but like Chris I cannot come up with an explanation for errors on all 3 UNLESS some common factor was involved. As I say, the only 'common factor' I can think of is alignment position, where perhaps an incorrect position was fed by the system to each IRS despite being correctly entered by either GPS or the crew. Maybe AB has some other common factor in the alignment system, but if the error was 'real' - ie not just an FMC disagreement but an actual position error on arrival - that is all I can come up with.

Dusthog - did you happen to notice if the initial map pos looked right on stand? To get those errors after 1.5 hours would, I think, require quite a relatively large alignment position error. Was the '5+' and 9 in n. miles or what?

[No Fly Zone]

If one believes that his/her airplane is 'lost,' or one or more significant GPS functions are apparently lost, do the same as smart pilots do when approaching questionable weather: LOTFW. Even at night and with no moon, it will probably give you more and better information than some of those instrument systems. You do have a chart and a whiskey compass, and you know how to use them, right?:ugh:

[Dusthog]

The aircraft position on the Navigation Display was correct before, after and throughout the whole flight. On the the A320 the GPS has priority. And the IRS error shows on the MCDU is in Nautical miles.

[BOAC]

Hmm! Even a 5 mile error is most unusual these days - 9 on all three I cannot understand. As you say, one 'rogue' IRS is not unknown - I had one on a 737 which motored off at 20kts to the east on parking on stand - it was on a 'returned' wet lease and the fitted IRS unit was the incorrect model! Anyone know how the AB platforms are fed position during alignment? Dusthog - are you saying the GPS is the priority for position entry on the 320? We certainly used it on the 737NG.

[underfire]

The GPS will always have priority.

[FE Hoppy]

The GPS will always have priority.

That's quite a statement!

Are you sure there are no performance caveats that go with it?

[latetonite]

Thee IRSes going astray? You were navigating in the Bermuda Triangle?

[BOAC]

Reviewing the OP, it does seem that since the errors were "all at the same time and with the same amount." as if we have been looking at an FMC error all along and nothing to do with the IRUs?

I am also puzzled by underfire post#4 "BTW, there is a mechanism for the pilot to update the IRU manually with a GPS coordinate." - this is new to me, since in my day, once aligned, the IRU did not have a 'position' to update, and 'position' was purely an FMC function. Have things changed?

[FE Hoppy]

Reviewing the OP, it does seem that since the errors were "all at the same time and with the same amount." as if we have been looking at an FMC error all along and nothing to do with the IRUs?

I am also puzzled by underfire post#4 "BTW, there is a mechanism for the pilot to update the IRU manually with a GPS coordinate." - this is new to me, since in my day, once aligned, the IRU did not have a 'position' to update, and 'position' was purely an FMC function. Have things changed?

Have they changed?

Very much yes!

Take a look at the spec for a 10 year old IRS: http://www51.honeywell.com/aero/common/documents/Laseref_V_Micro_IRS.pdf

[BOAC]

Nice bit of kit. I can see this unit has effectively a 'mini FMC' built in. I still maintain that whatever 'updates' are available, only the derived positions are updated (via the Kalman filter) and not the platform alignment from which the root accuracy derives. Again, I suspect the OP problem was alignment positional data error or some sort of FMC computational malfunction to produce a common triple system error. It would be nice to see what engineers did as spanners asked

[Chris Scott]

Quote from underfire:
"The IRS needs to be updated or it does drift..."

Response from me:
"Really? Would you care to be more specific?"

Reply from underfire:
"IRU fast/regular ALIGN...."

underfire (again!),
You simply cannot realign an INS or IRS when the vehicle is in motion, which is what we are discussing here. :ugh:

Quote from BOAC:
I still maintain that whatever 'updates' are available, only the derived positions are updated (via the Kalman filter) and not the platform alignment from which the root accuracy derives.

Agreed. Old hands will remember that, before the days of FMSs, we had dual or triple INS systems with an associated computer, into which you entered up to about 9 waypoints manually using the Lat/Long. IIRC, you could display track and distance between selected waypoints (or to the next one), the current TRK(T) and GS, the current HDG(T) and Drift, the current W/V (if you were TAS-equipped), or PPOS (present position).

If you were curious (or desperate), there was a facility to update the displayed PPOS. Unless the INSs were all really in trouble, in those pre-GPS days you could never hope to get a fix of position accurate enough in the air to improve on the (dead-reckoning) position that an INS was giving you. (The least-poor method was when you were passing about 30 - 60 nm north or south of a VOR/DME station.)

Latest IRSs are much more accurate than the INSs we had in the 1970s, but AFAIK they remain a DR (dead-reckoning) form of position estimation based on accelerations in azimuth from stationary at the start point. They are incapable of providing a FIX of position. If they get a bad start (mis-alignment) from the point of origin, particularly if the alignment latitude is slightly wrong, the track and GS values will be inaccurate, and therefore the position error builds up proportional to time. And they cannot be realigned when the vehicle is in motion.

[gums]

I take issue with Chris' idea of in-flight alignment.

As far back as 1968 - 1969, the A-7D had both an inertial and a doppler. When coupled, we could get a great airborne alignment given initial launch point for the flight. Then we could select an update fix and refine persent position and all was well. Sometimes we took off and practiced ACM without a full alignment, but the nav system came thru and we were golden after 15 or 20 minutes.

Our prime mode for nav was doppler-inertial, but we had another four modes below that. For example, I flew from Hickam to Guam in the "airmass" mose - TAS and estimated winds plus heading. Had a tanker to fly with, plus 5 other SLUFs. After ten hours I was about 15 miles off. Wish Amelia Earhart would have been that close.

More about the cosmic stuff on the SLUF at the F-16 net site.

[galaxy flyer]

The Honeywell LaserRef VI can in-flight align off the GPS. Takes 10-20 minutes.

[Mr Optimistic]

If the wrong level datum was input into all 3 systems then I would think the error would be the same as 'g' is incorrectly coupled into the accelerometer channel. Schuler tuning should give this an 84 minute or so period (from memory) but perhaps the observation wasn't long enough to observe. In any case, I would have thought that the system will be correcting against real time GPS with the Kalman filter estimating both position & dynamics, and also estimating the error characteristics of the accelerometers and gyros. So all should have been good but with some unusally high errors in whatever channel was compromised (eg accelerometer bias).