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Michael Moe

IRS schematic questions (ngx)

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Hi,

 

Back in 2006 there was some (alot) incidents or pilotlogs regarding IRS issues (737-400). I am not going to go into that discussion but from the FCOM2 chapter 11.20.8 NG (6-900)  schematic i can tell that FCC A (Autopilot A) is getting roll information from IRS 2 FCC B.

 

I am just trying to understand the logic behind it thats all.

 

If someting went wrong with either IRS the Autopilots disconnects (as it should) but it seems like there is no option to use Autopilots anymore with this schematic ?

 

Boing has designed it that way and with very good reasons I am just wondering why and want to learn  :smile:

 

 

There were a case (in bad weather and crosswind) where the Autopilot A was holding a 5 degree left wheel (aileron to wing level the plane (why is rudder not used btw or is this a normal behaviour from the FCC?) and the pilot made a choice and moved the IRS 2 to ATT. That would disconnect the Autopilot A as design. (QRH 11.9 would reguire a minimum of 30 seconds wing level without acceleration for basic heading calibration (which should be calibrated with magnetic heading every 15 minutes or so) 

 

Thanks

 

Michael Moe

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Hi,

 

Back in 2006 there was some (alot) incidents or pilotlogs regarding IRS issues (737-400). I am not going to go into that discussion but from the FCOM2 chapter 11.20.8 NG (6-900) schematic i can tell that FCC A (Autopilot A) is getting roll information from IRS 2 FCC B.

 

I am just trying to understand the logic behind it thats all.

 

If someting went wrong with either IRS the Autopilots disconnects (as it should) but it seems like there is no option to use Autopilots anymore with this schematic ?

 

Boing has designed it that way and with very good reasons I am just wondering why and want to learn :smile:

 

 

There were a case (in bad weather and crosswind) where the Autopilot A was holding a 5 degree left wheel (aileron to wing level the plane (why is rudder not used btw or is this a normal behaviour from the FCC?) and the pilot made a choice and moved the IRS 2 to ATT. That would disconnect the Autopilot A as design. (QRH 11.9 would reguire a minimum of 30 seconds wing level without acceleration for basic heading calibration (which should be calibrated with magnetic heading every 15 minutes or so)

 

Thanks

 

Michael Moe

Do you have a link to more info about your last scenario?

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I haven't read the whole report. The rudder trim, roll isn't probably part of the IRS issue. Neither is the crosswind.

 

Real world 737 often need rudder trim to "level" the yoke. Check out the FCTM on Trim Technique. I was surprised by this, having flown straight-wing aircraft for so many years, but you use rudder trim to neutralize the roll input.

 

So these guys didn't have the plane trimmed up and turned off the autopilot. The roll correction from the A/P went away and the plane started to roll. The pilots didn't catch it and it's bye bye.

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I haven't read the whole report. The rudder trim, roll isn't probably part of the IRS issue. Neither is the crosswind.

 

Real world 737 often need rudder trim to "level" the yoke. Check out the FCTM on Trim Technique. I was surprised by this, having flown straight-wing aircraft for so many years, but you use rudder trim to neutralize the roll input.

 

So these guys didn't have the plane trimmed up and turned off the autopilot. The roll correction from the A/P went away and the plane started to roll. The pilots didn't catch it and it's bye bye.

 

 

Thanks , so i was correct understanding about they needed to use rudder trim i guess so the Autopilot would have "center" the controlwheel to winglevel. Just thought the Auto rudder trim would have done this with the autopilot engaged. Thanks.

 

 

Do you have any ideer why IRS1/FCC A (Autopilot A)  is using roll from IRS2/FCC B and vice versa?

 

I guess offcause its safety related like any thing else with cross sources on the bird.

 

Thanks again

 

Michael Moe

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Thanks , so i was correct understanding about they needed to use rudder trim i guess so the Autopilot would have "center" the controlwheel to winglevel. Just thought the Auto rudder trim would have done this with the autopilot engaged. Thanks.

 

 

Do you have any ideer why IRS1/FCC A (Autopilot A)  is using roll from IRS2/FCC B and vice versa?

 

I guess offcause its safety related like any thing else with cross sources on the bird.

 

Thanks again

 

Michael Moe

The autopilot will command whatever roll is needed to keep the track. It is not level if the plane isn't in trim. There is no rudder auto trim. The yaw damper is pretty weak and will mostly just keep the Dutch roll away and assist with turn coordination.

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The autopilot will command whatever roll is needed to keep the track. It is not level if the plane isn't in trim. There is no rudder auto trim. The yaw damper is pretty weak and will mostly just keep the Dutch roll away and assist with turn coordination.

 

Thanks , that makes sense , just cheked the FCTM . The alternative trim technique is a bit of a nut to crack for me to understand :-)

 

Thanks 

 

Any comments on the schematic ?

 

Michael Moe

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Any comments on the schematic ?

 

Michael Moe

Hi Michael

I’ll try and answer your question!

 

As you well know the -800 is equipped with two IRS’s. If either IRS fails in flight the A/P will disconnect and neither A/P can be engaged as information from the IRS is sent to both Flight Control Computers which control the AFDS.

 

I can’t remember exactly but with the A/P engaged roll signals come from one IRS and pitch signals the other, I don’t know which one! If each IRS independently gave information to just one FCC (eg if IRS R provides both roll and pitch information to FCC B ) in the event of an IRS failure and you were unlucky enough to have the opposite FCC fail you would have one remaining IRS unable to provide any pitch and roll information to the other FCC.

 

This setup allows you to still receive inputs from the remaining operative IRS to either FCC so you can still use the flight directors, you just have to fly the flight directors instead of George!!

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Hi Michael

I’ll try and answer your question!

 

As you well know the -800 is equipped with two IRS’s. If either IRS fails in flight the A/P will disconnect and neither A/P can be engaged as information from the IRS is sent to both Flight Control Computers which control the AFDS.

 

I can’t remember exactly but with the A/P engaged roll signals come from one IRS and pitch signals the other, I don’t know which one! If each IRS independently gave information to just one FCC (eg if IRS R provides both roll and pitch information to FCC B ) in the event of an IRS failure and you were unlucky enough to have the opposite FCC fail you would have one remaining IRS unable to provide any pitch and roll information to the other FCC.

 

This setup allows you to still receive inputs from the remaining operative IRS to either FCC so you can still use the flight directors, you just have to fly the flight directors instead of George!!

 

 

Thanks just what i needed to hear  :smile: Ad the expense of a working Autopilot 

 

FCOM2 chapter 11.20.8 NG

 

 

Michael Moe

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