B777adiru/lrn
 

Our company is about to start Trans Atlantic Operations. However according to the literature I have read. We need a minimum of 2 operational Long Range Navigation systems. Apart from the ADIRU we also have 2 GPS receivers.

However there are a number of senior captains who say because the ADIRU consists of 3 IRS's we in fact have 3 Long Range Navigation (LRN)systems and that is all that is required. Unfortunately there is no literature from Boeing that states that the ADIRU consists of 3 IRS's. Further our MEL states at least 1 GPS receiver must be operational for MNPS ops.

I realise the MEL is the definitive document but could somebody please clarify the ADIRU and the 3 IRS's.

The ADIRU uses six ring laser gyros and six accelerometers to calculate and supply inertial reference and navigation data to user systems.

The air data inertial reference unit (ADIRU) has two functions:

Air data
Inertial reference data.
The ADIRU has internal redundancy and automatically makes allowances for failures to keep its complete function.

The ADIRU has these internal components:

Six laser gyros
Six linear accelerometers
Four processors
Three power supplies
Three dual channel ARINC 629 input/output interfaces.

Secondary Attitude Air Data Reference Unit
The secondary attitude air data reference unit (SAARU) is a back-up source of attitude, heading and air data. The SAARU uses these inputs to calculate and supply air data to user systems:

Pitot pressure
Static pressure
Total air temperature
Angle of attack.
The SAARU uses four fiber optic gyros and four accelerometers to calculate and supply inertial reference data to user systems.

The SAARU supplies attitude data to the standby attitude indicator.

Boeing 777 Maintenance Manual Part 1 - Reference 34-20-00 all you need to know if you have access to it. Don't mean that in a negative way as you said you have access to the MEL. Posted it then Spannersatcx beat me too the paste ! Cheers buddy :)

Hmm, extrapolating from my 738 knowledge here - the IRS itself is not a 'means of navigation' by itself in my opinion, but an FMC using information from a number of IRS is. Thats why single FMC installations don't qualify for navigation outside the range of conventional navaids, but twin FMC setups do... After all you can't enter the number of IRS in your FMC flight plan but you do have to state if you have one or two FMCs (or even more) ;)

STBRUB is right and your captains are wrong (in fact I am amazed at the lack of basic understanding from what you claim are senior captains)

Neither the ADIRU (whether you have a single intergrated unit or 3 individual units) nor the GPS can navigate. they can only tell you where you are (and in the case of ADIRU's only if you accurately tell it where you started)

An FMC however, uses data provide by the ADIRU or GPS to navigate the a/c along a determined route.

Many of the more modern a/c have 2 FMC's (some even a 3rd) which satisfy the long range requirement. Often, those with only 2 FMC's have an MFCDU which has an integral back up nav computer built in so in the event of an FMC failure there are still 2 long range nav computers.

B777adiru/lrn
 

Spanner and mono thanks for the info.

Greetings,
As Far As I know there is only ONE ADIRU on the B777, only one unit, SARU is the back up

This is correct; One ADIRU and one SARU. But as the other chaps have correctly stated, the ADIRU is effectively THREE IRU's and THREE DADC's in one covenient package, with massive amounts of redundancy built in.
(This convenient package weighs a 'ton', and needs a gorilla to replace one...oooh ohh ohh ooooh!!).

MONO
 

Knowing your current position is navigation, no need for FMC's, it's simple math. A whole lot better than relying on the whisky. What has happened to piloting skills today? The ADIRU's/IRU's are a whole lot more accurate than the old Delco units. Even W/O GPS position updating.

Back on topic yes 2 ADIRU's are long range navigation systems, just as INU's are. I believe it is an ICAO standard.

Looking for a quote and found this old doccument (1955) warning of increased traffic in the future 1970-1975, and the standards for what we know now as the RNP of the day.

http://www.icao.int/icao/en/nr/1955/pio195512_e.pdf

Here is a good ref. Badically if an aircraft has inu's/iru's and maintains rnp10 it is another standard for GPS.

http://www.icao.int/nacc/meetings/20...TCARWGip03.pdf

http://www.icao.int/icao/en/ro/nacc/...TCARWGwp06.pdf

This answer..

One rather interesting point here is that while we consider that the Honeywell ADIRU (B777) is the equivilant of 3 DADCs, srictly speaking, the INERTIAL equivilent of this unit is that of only TWO IRUs. We require a RLG and accelerometer for each of the X, Y & Z axis, however the ADIRU houses 'only' SIX RLGs and accelerometers respectively. (Technically of course, we should have nine of each). However, these are extremely reliable individual components, far more reliable and accurate than those found in the Delco C1VA and Litton INSs of yesteryear. (I remember that the C1VAC used to cough on a very regular basis, gyro failures was one of the main culprits here). There is more redundancy however as far as the rest of the modules are concerned; 3 Power Supply Modules, 4 Processor modules and a total of 6 Databus Modules. (There are three 629 bus outputs, each with a dual Databus Module). The gyros/accelerometers are housed in fault containment areas, and the Honeywell blurb is that you can fail any TWO components. Unless I am totally missing something here, if you lost two RLGs or Accelerometers on the SAME axis, you would be in the poo. (Of course we still have the SAARU, and having said all this, I do not personally recall the complete failure of an ADIRU in flight, at least with my airline).
:)

Yeah but I sure miss re-racking an old Delco carousel with a calibrated drop to knock the carbon off the gimbles, used to get them off the gate like that all the time. It used to be normal to be 30 miles off target after an 8 hour flight.

The RLG/accelerometer setup is extremely reliable, just not soo much fun.:E

Ah, yes, the old 3T+3 calculation. I miss handing them up to the guy in the hole and wondering how much the corners would hurt if they were to drop it.

Hi M2dude,

I'm struggling with the maths here - but if the chance of any RLG failing during the flight was say Z, and we consider the 3rd IRS equivalent to the SAARU

Then with 2 separate IRS units, the chance of the first RLG failing would be 6Z, and the chance of the second one failing in the remaining unit would be 3Z. So the chance of being down to just 1 IRS = 18 Z squared.

On the 777 unit, the chance of the first RLG failing = 6Z, and the chance of the second failing = 5Z. The chance of the second RLG being in the same axis = 1/3. So chance of total failure = 6Z * 5Z * 1/3 = 10 Z squared. Then you are down to the single SAARU. So the 777 arrangement appears to be 1.8 times more reliable than 2 separate IRS units.

Is that correct?

Hi Rudderrudderrat

You make your point well; I was really just pointing out a rather interesting anomaly regarding the architecture of the ADIRU, rather than perceived question regarding reliability. (My head is swimming a little with your maths, but I think I see your point :confused:). I agree that the chances of a failure of two same axis RLGs OR accelerometers is extremely remote; I was merely making the point that CERTAIN dual component failures could in theory render the ADIRU useless. BUT, there still maybe something that Honeywell are not telling us, regarding RLG or accelerometer design that blows my argument completely out of the water. But at the moment, as I read it we still have 2 X, 2Y and 2Z axis RLGs/accelerometers; as I said before, I may be missing something here. :)

Refer to the AMM about the condition monitoring of the ADIRU. Yes you can have a dual gyro failure on the same axis, actually that would happen the same day you had a engine failure on take off, got a ticket on the highway and won the lottery.
Note that the ISIS has it own gyro too.

Happened to me yesterday!!, seriously, this is a point of pure INTEREST, not likelyhood. :)
However, if YOU read the 777 MM you will see that you are kind of missing the point here. A SINGLE RLG/Accelerometer failure does not generate a STS MSG, but only an SMT. Depending on the individual airline's policy, SMT's are generally only checked during, yes, SCHEDULED Maintenance, not on the ramp, so it is not inconceivable that a single failure remains semi-dormant here. (Witness the QANTAS incident in August 2005).
I'm waiting for some info' from Honeywell which, as I said before, may blow my arguement out of the water completely. (eg, are the RLGs/Accelerometer triple axis units maybe?). Either way, this is a fascinating debate, but none of us should ridicule the possibility of multiple case failures; much grief and headaches have been caused by such things in the past, nomatter how unlikely they may seem.

OK guys, I'm blown out of the water (or at least my argument is). It seems that inside the Honeywell ADIRU, the RLGs/Accelerometers are NOT arranged in a strict X, Y, Z axis; they are arranged non-orthoganally at obviously known VERY specific angles to one another. (No problem of course with strap down systems). So therefore, the loss of a single RLG/Accelerometer does not necessarlily result in the loss of any axis information, this information is derived from the surviving bits. (Of course it is still vital for a defective unit to be disconnecred from the ADIRU decision making chain; that was the main problem with the QANTAS event in 2005, dealt with an ADIRU Ops Software revision to 3470-HNC-100-03 (version -03).
Honeywell rules OK. :rolleyes:

M2 Dude, Check your email. see attatchment it should be of some use in your discussion.
I'm off to bed,- some of us have to work.:(

Thanks GWH, already got this but thanks anyway. DON'T BREAK ANYTHING!!:ugh:

To be sharp the ISIS has its own Gyros on the 300 series, on the 200 series the SAARU supplies informations to the attitude indicator.