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# True north and IRS

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# True north and IRS

13th Sep 2010, 22:23

Join Date: Oct 2009
Location: UK
Posts: 1,271
Hi Dick,

Your PFD gets it's info from the IRS. Unless it's mounted correctly, you'd have the wrong pitch attitude showing or one wing low indication when you were straight & level etc.

I agree - it would make no difference to the IRS with it's ability to do the Nav.
13th Sep 2010, 22:46

Join Date: Sep 1998
Location: wherever
Age: 50
Posts: 1,609
Honeywell micro IRU in the Ejets have align in motion capability. GPS required and takes between 15 and 30 minutes. On ground stationary align like most is between 6 and 17 minutes depending on Latitude.

[email protected] gyros drift just as spinning gyros did. In the case of the old spinning gyros the major source of drift was the bearings. With [email protected] its the imperfections in the mirrors that causes noise.

A strap-down system has to compensate for gravity, earth rotation and the earths spherical shape to work effectively.

Vertical velocity and altitude are calculated using the acceleration that is measured perpendicular to the earths surface. However, an inertial accelerometer cannot distinguish between gravitational force and actual aircraft acceleration. So any accelerometer that isn't perfectly parallel to the earth's surface will measure a component of gravity in addition the the aircrafts true acceleration so the irs has to subtract local gravity from the measured vertical acceleration.

As the gyros measure accelerations in inertial space we need to correct for the earths rotation in inertial space too. Thats once per 24hrs and once per year (people forget about that one) that works out at 15.04 degrees per hour. This correction is subtracted from the measured eastward acceleration. This is known as earth rate compensation. Without it for example, if you were on the equator and flew for 12 hours the gyro would think it was upside down.

Transport rate is the correction for navigating in curved paths over the earths surface as remember the gyros work in inertial space and so the accelerations need correcting or again the gyros would think we were upside down if we flew half way around the earth's sphere.

Now for the trivia:
It takes 3000 volts across the anodes to the cathode to start the lasing action and Honeywell ring [email protected] gyros are triangular. The glass is made from Cervit due to its stability over wide temperature ranges.
The [email protected] does a nice quantum trick of changing frequency to exactly match the path length as the gyro is rotated and its the change in frequency that is used to calculate the rotation rate.
13th Sep 2010, 23:23

Join Date: Oct 2009
Location: UK
Posts: 1,271
Trivia

Hi FE Hoppy,

The [email protected] does a nice quantum trick of changing frequency to exactly match the path length
I thought that at rest, a stable interference pattern was formed by the recombined [email protected] light. The pattern changes when it is accelerated and the shifting interference pattern is measured.
13th Sep 2010, 23:46

Join Date: Sep 1998
Location: wherever
Age: 50
Posts: 1,609
I thought that at rest, a stable interference pattern was formed by the recombined [email protected] light. The pattern changes when it is accelerated and the shifting interference pattern is measured.
Yep, I dumbed it down a bit. The [email protected] travels in opposite directions around the triangular path so when the gyro is rotated the light wave in one direction gets a increase in frequency shift where as the opposite path gets a reduction in frequency. The two light waves are combined on a photo detecter and cause a fringe interference pattern. The number of fringes is proportional to the frequency difference and the fringes are detected as pulses on the photocell so appear at the output in digital form ready for processing.
14th Sep 2010, 14:14

Join Date: Sep 2010
Location: Budapest
Age: 56
Posts: 18
I checked today the heading drifting in "ATTITUDE" on airplane with conventional IRS panel. The heading decreased on the panel and on ND as it was expected. I still can't understand why the true heading was not changed on MCDU. Next I will switch an IRS from OFF directly to ATTITUDE. Then we will se...
14th Sep 2010, 18:48

Join Date: Jan 2008
Location: FL 600. West of Mongolia
Posts: 462
One thing that I recently discovered, is that Ring [email protected] Gyros actually WEAR OUT with time, within about 15-20 years. It's not a joke, being fundimentally a gas 'discharge' tube, apparently eventually the lasing surfaces degrade with time.
I was gobsmacked when I discovered that one. (It seems that fibre optic LGs do not suffer the same fate, at least so far ).

Dude

Last edited by M2dude; 15th Sep 2010 at 06:47.
14th Sep 2010, 21:09

Join Date: Oct 2009
Location: UK
Posts: 1,271
Hi Lotetu,

Once you've selected ATT, then you've stopped the IRS from processing data. Did the MCDU have the message, "IRS Frozen at"?

See FCOM 4.3.25. IRS monitor page.

Displays the selected IRS in large white font.
When data is frozen, IRS is replaced by "IRS FROZEN AT", followed by the time at which the pilot has frozen the display.
15th Sep 2010, 10:55

Join Date: Apr 2001
Location: EGPH
Posts: 121
Regarding re alignment on the EJet.

I did one two weeks ago in flight and it took 33minutes.

We tried flying straight and level but eventually had to turn and descend to our destinations , and was impressed that it could cope with turns, descents and increases in speed.
15th Sep 2010, 11:30

Join Date: Jul 2006
Location: London
Posts: 71

We tried flying straight and level but eventually had to turn and descend to our destinations , and was impressed that it could cope with turns, descents and increases in speed.

Last edited by Obi Offiah; 15th Sep 2010 at 11:42.
15th Sep 2010, 13:02

Join Date: Apr 2001
Location: EGPH
Posts: 121
During re alignment.
15th Sep 2010, 14:58

Join Date: Sep 1998
Location: wherever
Age: 50
Posts: 1,609
Faster re-alignment with large heading changes I'm afraid. If you have to do it again go for a couple of 1 minute legs at 90 degrees to each other and it will speed up the alignment significantly. It's to do with the algorithm used to separate the total accelerations felt by the IRS. I'm assuming the GPS signal was good throughout.

This is straight from my friends at Honeywell but I'll try and find the references in the books.
15th Sep 2010, 19:27

Join Date: Sep 2010
Location: Budapest
Age: 56
Posts: 18
Thanks Rudderrudderrat,

it was an option to freez the IRS. I didn't do that, but maybe it did itself. There was no message. The coordinates were lost (dashes).
There are still things to be checked.
15th Sep 2010, 22:54

Join Date: Oct 2009
Location: UK
Posts: 1,271
Hi,

It looks like I won't need to bring my 1944 brass binnacle with Sperry Gyrocompass (to find True North) on the flight deck any more.

This Honeywell link (see page 1.9)

"Rapid Dispatch Option
If extremely rapid dispatch is required, the operator may also elect to use the Align-in- Motion function to complete the alignment in flight. When the IRS is powered-up, the attitudes, accelerations, and rates are available within 5 seconds. If ARINC label 043 (Set Mag Heading) is received from the FMS once at power-up, then all TSO outputs will be available for dispatch. When the IRS completes the align-in-motion, all parameters will be available at Full Performance as specified in section 6.0."

Last edited by rudderrudderrat; 16th Sep 2010 at 14:07. Reason: spelling

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