Ka8 Flyer

Hi folks,

we all know the position in the IRU's drift over time as the accelerometers aren't 100% accurate. (Just as the heading drift on old DGs)
But what about the laser/ring gyros? I've never read that you need to check their accuracy but wouldn't the attitude be just as affected by gyro drift?
Granted, airliners are usually limited to small pitch / bank values (30-40°) and the rotation rate is also quite low but still, wouldn't turbulence cause errors?

Or is there a way to re-calibrate the gyros in straight and level flight?

Thanks,
Mark

BOAC

No 'drift' as you know it in a laser-ring gyro, but other errors. No real need to 're-calibrate' either.

Ka8 Flyer

Hi BOAC,

thanks for the reply. May I ask why?
I mean, the gyro measures a relative value (namely the rotation around the 3 axes) and this value is then added to the current value to get the absolute orientation in 3D space.
Or am I missing something?

BOAC

All done electronically - it knows where it is - usually (INS) and the 'earth rate' is applied appropriately. That is 'earth rate. You seem to be trying to cover all sorts of 'spinning gyro' errors (which just do not exist) in your post? Friction - take it as nil. 'Accelerometers' - pretty well perfect. Profile rate - INS driven. Add Schuler Loop tuning and you have a pretty good system. Obviously no software is perfect, but platform errors are pretty well a forgotten item.

FlightPathOBN

There are also the GPS update...

BOAC

There is no GPS update to the platform! We are discussing IRUs not FMCs.

FlightPathOBN

The standard ADIRU on Airbus platforms is provided by Honeywell....

Honeywell (NYSE: HON) announced today that its Air Data Inertial Reference System (ADIRS), which provides navigational data, has become the standard inertial reference system for the Airbus A320, A330/A340 and A380 family of aircraft.

"Three Air Data Inertial Reference Units (ADIRUs) each comprise three ring laser gyros and three quartz accelerometers to accurately sense aircraft position and attitude. By integrating data from the inertial sensors and Global Positioning System (GPS), ADIRS provides automatic initialization, faster align times and 100 percent availability of Required Navigation Performance approaches."
Honeywell's Inertial Navigation System Becomes Standard Equipment On Airbus


Evolve dude...

Ka8 Flyer

So if I understand you correctly, the laser gyros are practically error free and the modern accelerometers, while being very accurate, aren't 100% accurate (causing the eventual map shift if not updated by an external source via VOR/LOC/DME or GPS through the FMC).

So what about the mechanical counterparts found in GA aircraft, the ones spinning at 30k RPMs? Can the attitude drift there?

@OBN:
No, the external sources will never update the actual internal position provided by the ADIRS. That's done in the FMGC in the Airbus. And definitely no GPS update will be used for attitude data (and that's what this post is mainly about).

BOAC

Basically yes - all systems have errors. Any error, however small, in the achievement of a perfect alignment of a platform will induce a false acceleration/s which will integrate to position errors over time. These are corrected in the FMC by some sort of update as you list. I don't know how good your 'GA' ones are, but if they are like the 'old' gymballed platforms we began with, you would be pushed to see any attitude error - we are talking fractions of a degree normally. It will be 'map shift' you will see plus possibly a 'residual' groundspeed on stand. Map shift is becoming a rarity in airline usage as GPS updating becomes the norm. In the 'old days' a shift of 30 nm or more was not unknown and I had one of those at the end of the Black Sea after an hour without a DME update. Didn't half screw up my descent planning into Baku

FPOBN has got confused during his 'evolution', I think, between using GPS for alignment and updating, and is under the impression that a GPS can produce attitude information of the accuracy required for a platform re-alignment. That'll be the day! That WILL be evolution, dude.

Many people get confused between the 'gyrosopic' attitude functions of a platform and the position derived from accelerations to that platform and then displayed to the pilot.

Ka8 Flyer

Thanks a lot BOAC for your explanations.
I still find it fascinating that a pure mechanical device would drift as little as 30 nm after an oceanic crossing of say 3000nm, so a 1% error.

Fortunately (or unfortunately?), EFIS is becoming more and more straight-forward for GA aircraft. Heck the last prop I flew (a C172) had a G1000 cockpit - compare that to a B733/B757/B767 cockpit and you'll get pretty envious if you're into EFIS

FlightPathOBN

"The IR component of an ADIRU gives attitude, flight path vector, ground speed and positional data. The ring laser gyroscope is a core enabling technology in the system, and is used together with accelerometers, GPS and other sensors to provide raw data. "

The FMC may be supplied by other manufacturers than Honeywell. The ADIRU data is available, whether it is used or not, is dependent on the box. You can have a Smiths, or Thales box with the Honeywell ADIRU, they may integrate the raw data or not.
In designing RNP and GBAS procedures, the update and drift rates are a significant part of the calculations.

This ADIRU IS GPS updated.

Denti

If only "raw" data is provided it is one box that provides unfiltered and unupdated attitude, accelerometer and GPS data. The GPS does not need to update the IRS portion for that.

On Boeings there is no internal GPS updating in the ADIRUs and they come right of the box GLS and RNP 0.1 approved and equipped. Our A320/A330s come right out of the box only RNP 0.3 and not GLS approved and equipped. We could equip them for both, GLS costs are over a million $ per unit and the upgrade to RNP 0.1 on A320s cost around 300k to 500k $ per unit.

nitpicker330

Sorry but the ADIRU/IRS itself is NOT updated by GPS or any other source during flight.

The FMC or FMGC is, but not the ADIRU/IRS

Hence the Airbus requirement to do an "IRS Performance check" at the end of each flight.

– IRS PERFORMANCE...........................................CHECK
•
Residual ground speed check:
— If the residual ground speed indication on any IRS MONITOR page is greater than 15 kt, make a Maintenance Log entry identifying the associated IRS. Perform this check within 2 min of aircraft stop.

Position error check:
•
— If any radial distance error on the POSITION MONITOR page is greater than the limits in the table below, make a Maintenance Log entry identifying the associated IRS and stating the BLOCK/NAV time and error in nautical miles.

FlightPathOBN

FlightlessParrot

On the relative accuracy of mechanical and laser gyros, as of the 1980s, I found this book interesting: _Inventing Accuracy_, Donald MacKenzie, MIT Press, 1990. More history of technology than state of the art, though.

aterpster

TO: Flight Path and BOAC.

It takes a team to forumate operations procedures such as what we (in this case "we" being a team of industry experts with different backgrounds, and the FAA) who cobbled together the criteria and general obstacle clearance and performance parameters for RNP AR. Two of our experts were FMS engineers. We left the details about how IRUs perform the position calculation and where in the "black box" stream the GPS sensors updated the final position calculation; the FMS position.

Others of us had flight operations backgrounds and a few of us had TERPs experience, either formal (FAA) or in my case informal.

Throughout all of this all I needed to know is that the IRU postions were massaged by the FMS (sort of like triple mix in the L1011s I flew) and GPS, when available, indeed performed a postion update at some point. Whether it actually tweaked the IRUs or the FMS positon was immaterial to us operations types unless our FMS engineers told us it matter. They spent a career being trained, then working on that stuff.

Personally, I often see operations folks such as pilots trying to be avionics engineers when they aren't. Then again, they fly airplanes a whole lot better than FMS engineers and they, the pilots, have invaluable input as to how the airplane will behave in the performance based nav world.

I don't know which of you is right, and it is unimportant to me for my purposes.

nitpicker330

All very well but if the actual IR position is updated by the GPS in flight then why
1/ require a IRS performance check post flight
2/ why do I always see residual groudspeed and position error from the 3 IRS's

3holelover

Maybe you're flying something without gps updating. Just because it exists, doesn't mean you should expect all birds to have it. I'm certain the 320's I know don't have it.

FlightPathOBN

Exactly, there are many ADIRU systems, and not all have a GPS update. The Boeing variants that are certified down to 0.1RNP would have to have this, or the drift rates would push it to a 0.3 RNP...
Honeywell recently announced their ADIRU for the Airbus 300 family, so it will take a bit for the certification to catch up. The A380 is already certified.

mickjoebill

In the land of airborn stabilised platforms (gimbals) for use by TV police and military, in general two categories of gyros have been in use. ITAR and non ITAR restricted.

In general ITAR restricted gyros are significantly better than others, one can see the difference in picture stability when using the equipment.

(ITAR restricts the export of particular components or equipment with components installed to countries that are not friendly with the western world)


Which type of gyros are in use in Airbus and Boeing ADIRUs?



Mickjoebill