B777 A/P roll mode failure out of LNAV

[agg_karan]

Hello,

Route flown - KSFO TUFLY 6430N10000W 69N090W 72N080W DAPAK 77N040W 77N020W EXITA 74N020E TOPLO VANOS - DESTINATION

B777-200LR
Nearing 72N080W, I observed the following -
1) ISFD HDG scale 'blank' with no flag
2) TRU annunciated on ND hdg scale
3) I selected HDG SEL (came out of LNAV) and saw few seconds later an amber band across HDG SEL FMA (on PFD) with caution message.
Same cautions with HDG HOLD, TRK SEL, TRK HOLD
4) HDG REF switch was 'Normal'
5) LNAV engagement was perfect
6) this could have happened earlier in flight but was experimented near 80W

I am assuming this is to do something with magnetic unreliability.

Q1) Is this A/P behaviour related to AMU?
Q2) Can anyone post a link to this where this degradation in A/P behavior (specific for b777) can be better understood.

Apologies if I have overlooked something too obvious
Regards

[wiggy]

Late evening so not going to get into details but any chance at some point it was anything to do with the last two lines of this (from our FCOM):

. Heading Reference (HDG REF) Switch
Pushing alternately selects the heading reference for the PFDs, NDs, AFDS, and FMCs.
NORM –
"normally references magnetic north"
( then a reference to tail numbers)
"automatically references true north when north of 82°N latitude (or north of 70°N between 80°W and 130°W) or south of 82°S latitude (or south of 60°S between 120°E and 160°E) for PFDs, NDs, and FMCs."

(Another reference to tail numbers)
"provides no reference for AFDS roll modes other than LNAV when north of 82°N latitude (or north of 70°N between 80°W and 130°W) or south of 82°S latitude (or south of 60°S between 120°E and 160°E)."

FWIW under our SOPS we would normally have the HDG REF in TRUE after TUFLY, so can't say I've ever seen what you describe.

[Ex Douglas Driver]

777 FCOM1 34. Navigation Systems
FMC POLAR OPERATIONS
When entering the polar region, automatic switching to a true north reference is annunciated by a white box around the word TRU on the ND. A TRUE heading reference can be selected with the heading reference switch inside or outside the polar region. The ND displays a green box around the word MAG to annunciate the change back to magnetic reference when leaving the polar region. If the heading reference is TRU in the descent phase, the ND displays a flashing amber box around the word TRU.
The current GRID heading displays near the top of both NDs when the aircraft is north of 70°N or South of 70°S.The GRID heading is not used by any aircraft system.

Note:
When operating the autopilot in the polar region in other than LNAV, the TRUE position on the heading reference switch must be selected.

What you saw is normal for selecting HDG SEL in the polar areas without manually selecting the heading reference switch to TRUE.

[wiggy]

Thanks for tracking that down, just had another look and with exception of the comments about GRID I found the exact same wording in our FCOM, albeit under 11.31 FMC Operations.

[JammedStab]

Concerning IFSD......For our older 777's, the FCOM says "Heading information is not available in polar regions". For our newer models, they display true heading.

But my FCOM shows the polar region in the vicinity of 80W(when approaching from the west) as starting at 73N, not 72N as you experienced.

[ACMS]

Agg Karan:- I guess you should have been trained by your company on what to expect in Polar regions.
Might be useful information should you need to divert and land at a Polar airfield. So I'd be prepared if I was you, 02:00 ain't a good time to workout how to land in Greenland whilst on fire....

Have fun.

[agg_karan]

# Wiggy - thanx for the info, interestingly I don't recollect seeing this in my FCOM but will check again-

(Another reference to tail numbers)
"provides no reference for AFDS roll modes other than LNAV when north of 82°N latitude (or north of 70°N between 80°W and 130°W) or south of 82°S latitude (or south of 60°S between 120°E and 160°E)."

# Douglas driver THANKYOU for last info

# jammed stab thanx didn't know IFSD's can show true heading, our text says it simply blanks but thanx for replying

# ACMS - need Info not advise :) Your answer is really no answer at all :)

THANKYOU all for sharing :)
Grateful and regards

[JammedStab]

# jammed stab thanx didn't know IFSD's can show true heading, our text says it simply blanks but thanx for replying

More correctly....SOME ISFD's will show true while others will blank.

[agg_karan]

Understood Thank you

[RVF750]

ACMS possibly wasn't being rude...., Polar ops should have been trained. None of us should be expected to be familiar with stuff unless we've been properly trained. If there's a failing in your company's system, then perhaps a quiet word somewhere might help them.. If you are in a part of the world where this isn't socially advisable, then just take it that ACMS is from a part of the world where it is.

No insult or failing on your part is implied....

[agg_karan]

# RVF : the short coming is on my part for not having read the book in wholesome :) was intending to have info here regardless of the company training, background etc etc. ACMS is one of the most knowledgeable persons here :) no offense taken.