3rd IRS

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3rd IRS

Postby danil » 19 Mar 2009, 10:46

We just got a couple of /800 having fail operational certification. Somewhere in the FCOM it says that it needs 3 IRS’s for a LAND 3. I have asked few technicians but no one seems to know anything about the 3rd IRS. Can you help? Where is it? How it operates (alignment, on/off, etc)?
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Re: 3rd IRS

Postby sero90 » 19 Mar 2009, 19:50

i havent seen any 737-800 with 3 adiru. may be it is optional. if cat3 landing required...
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Re: 3rd IRS

Postby esmile » 20 Mar 2009, 09:06

this is somthing new to me. i'll dig and let you know
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Re: 3rd IRS

Postby Dunny » 20 Mar 2009, 18:37

I was under the assumption the Cat 3B aircraft required 3 sources of attitude information, with the ISFD being the third source ?
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Re: 3rd IRS

Postby sjap » 20 Mar 2009, 21:00

i've looked up our FCOM 737NG, this is all it says about cat 3 landing req. (2 adirus)
(its a picture because pdf was locked for copy paste)
Attachments
adirus.JPG
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Re: 3rd IRS

Postby Dunny » 21 Mar 2009, 06:45

Cat IIIa is not fail operational though....
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Re: 3rd IRS

Postby danil » 23 Mar 2009, 00:14

Thanks a lot for the inputs. Our planes are fail operational so they could be certified up to cat IIIb although we use them for IIIa only. The point is that in the FCOM at sys description tit says that it needs 3 IRS sources for LAND e annunciation. It makes sense since other planes need 3 IRS's for a fail operational operation.
Dunny - I think the IFSD takes the inertial data from left ADIRU

below is the text from Autoamatic flight sys description
"Fail-Operational Autoland Status Annunciations
The following annunciations provide the flight crew with autoland system mode and status:
•LAND 3 – two autopilots, three inertial sources, and the associated sensors are operating normally for an automatic landing and rollout
.•LAND 2 – a failure has occurred above Alert Height and redundancy is reduced; but the autoland system is still capable of making an automatic landing and rollout
.•NO AUTOLAND – the system is unable to make an automatic landing."]
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Re: 3rd IRS

Postby Dunny » 23 Mar 2009, 11:17

our latest 737-800 are cat3B and theyt certainly dont have three IRS systems.
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Re: 3rd IRS

Postby sander » 27 Mar 2009, 13:29

Hi guys, I've seen the option for 737 cat 3B advertised a while ago but the company I work for does not have any of them.

And Sjap, Land 3 is not the same as a Cat 3 because we work on Cat 3A 737's (Land, 2 decision height of less than 100ft), these guys have Cat 3B 737's (Land3, decision height of less than 50ft, in the event of a single system failure, the autopilot downgrades to a Cat 3A and is still able to carry out landing with flare).
I've worked on fokker 100's that were Cat 3B. Now my memory is a bit foggy but my colleague managed to remember that on the Fokker 100, the third attitude information came from gyro's in the Yaw Damper.
Also a autoland inverter was installed to supply AC electrical power to the third FCC.

From what I've read on the manufacturers website that tailors for the 737cat3A to cat3B modification (can't remember the link) I saw that the cat3B modification kit consists of a rudder servo (for rollout guidance on the runway) and two new FCC's wich have 2 seperate systems inside, therefore still have 2 seperate systems when a single power failure occurs (wich leaves them with the same redundancy as a fokker 100 with three FCC's and a seperate power source). Hence the fail operational.

Unfortunately I have no data on Cat 3B 737's (I wouldn't mind getting some info on it if you have some to spare).
:o
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Re: 3rd IRS

Postby danil » 29 Mar 2009, 12:31

The rate gyro could be the 3rd IRS source. Anyway I would better shortly describe the fail operational:
With 2 autopilots coupled, APP mode engaged and below 1500 ft RA the LAND 3 annunciation came on – the rudder is couple with autopilot;
At 450 ft - runway alignment - the crab is removed;
200 ft – alert hight – the LAND2 or NO AUTOLAND is inhibited;
Till 50 feet the behavior is similar with fail passive system;
2 feet the ROLLOUT mode is engaged;
An engine out ruder compensation is provided for approach and landing;
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Re: 3rd IRS

Postby danil » 31 Mar 2009, 22:46

I found it. The autopilots take the 3rd inertial data from IFSD :shock: . I thought the IFSD takes all the data from left ADIRU but it actually uses the left adiru only for Heading info. IFSD has its own inertial system.

Thanks for the inputs.
.
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Re: 3rd IRS

Postby sander » 04 Apr 2009, 08:54

danil wrote:I found it. The autopilots take the 3rd inertial data from IFSD :shock: . I thought the IFSD takes all the data from left ADIRU but it actually uses the left adiru only for Heading info. IFSD has its own inertial system.

Thanks for the inputs.
.


Thank you for keeping us updated. This is nice information. :o
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Re: 3rd IRS

Postby TMA737 » 30 Aug 2009, 13:59

Hi all, I was working for the Boeing 717, it's equipped with 2 ADIRU's and we performed CAT III AUTOLAND
Mora I.
B737 Engineer
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Re: 3rd IRS

Postby sander » 01 Sep 2009, 19:24

Hi TMA737,

Yes, aircraft equipped with 2 seperate systems that can distinghuish a failure between the two are called fail passive. because when a failure occurs the system can not decide wich one is at fault.
So it disconnects and gives control to the flight crew.

This is Cat 3a (also called Land 2, don't confuse cat 3 with Land 3)

When you have 3 different sources for the important inputs (IRS, RA, FCC's to name a few) you have a fail operational system, when a single failure occurs, the autopilot system can 'vote' the failing system out and still carry out a autoland including the flare on the remaining two systems. i.e. it is then downgraded to cat 3a ('Land 2').

This fail operational system is Cat 3b (this one is called Land 3).

Regards, Sander :o
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