Dual FMC failure Boeing 737 ng

[AVIATOR_JAGUAR]

what should be the course of action in case of Dual FMC failure in Boeing 737 ng ?

[BOAC]

Dust off the PLOG and QRP.

[iflytb20]

Use IRS for Lat-Long. Plot it on the Jepp say every 10 minutes or so. Since you would still have a Track Line available, you can use the ND to navigate using the following method:
*Set the Track Required in the MCP Course Window.
*Set the ND to CTR VOR mode.
*In the ND you will have the hollow Course needle and the white track line. The course needle is your Track Required[TR]. The Track line is your Track Made Good [TMG]. Use HDG SEL to turn the aircraft to keep the TMG line inside the TR line.
*This way you will be able to give wind corrections, make airway course changes etc with a bit of ease.

Hope this helps:)

[c100driver]

Or if you are within range of a VOR you could just use VORLOC and ALT HLD and resume conventional NAV

[Denti]

No FMC, no RNAV capability. Therefore not approved to use european airspace flying IFR except with special permission by every concerned ATC unit. So inform ATC and ask for vectors to the next airport.

Judging from our OFPs (LIDO) it isn't required anymore to have all waypoints or even all airways on it and therefore is not usable for navigation.

Actually had a case of FMCs being unable to "talk" to the rest of the airplane which resulted in autothrust always trying to achieve 104% N1 regardless of actual speed and MCP selected speed. If the same is true with completely failed FMCs you probably have basic autoflight modes, but no autothrust anymore which shouldn't be a problem though for any halfway competent pilot.

[exosphere]

All perfomance related calculations must also be conventionally done, like TOC (if relevant), TOD, actual weights and respective speeds.
Regarding this topic I recently had an FMS unable to give any performance outputs on a 737 classic: not a too big deal, but workload, especially on really short sectors, might slightly increase.

p.s. my first post (tech-log, I am pretty brave!) but a long-time reader. Hello everybody :O

[cosmo kramer]

what should be the course of action in case of Dual FMC failure in Boeing 737 ng ?

You do the "FMC Fail" non-normal checklist of course: :)

1 Resume conventional navigation. Without an operating FMC, LNAV and VNAV are not available.
2 When preparing for approach: Use the manual N1 set knobs to set the N1 bugs.
----

I had the pleasure of this a few weeks ago. With a "dual" FMC failure in a classic. Well... actually there is only one FMC in most classics, but the result is the same as a dual fail in an NG = "FMC FAIL" written on both CDUs and loss of map mode on both sides.

ATC was informed and we got vectors for the remaining 1 1/2 hour flight, since our route was mostly RNAV points. The checklist was read and approach speed and go-around N1 was looked up in the QRH. Flight progress was checked by looking in the Jeppesen charts and finding appropriate VORs where our flight plan points could be located with a radial and DME distance, so that we would have an idea when passing abeam them. During descend it started working again with the message "select active waypoint" which is normally assosiated with a power loss - probably condensation somewhere in the wiring I would guess - it was bloody cold that night.

We didn't have endless amounts of fuel so without the FMC prediction for fuel at destination, it was a bit uneasy feeling until we got a system going with VORs for cross checking our position. Otherwise it's no big deal.

[theficklefinger]

Yeah, don't bother with the peanut gyro, VOR, DME, and ADF. It's over.

Once you can't see your little airplane on the screen moving around, toast.

[STBYRUD]

Its no biggie other than that you are stuck with conventional navigation and manual performance calculations, as previously mentioned ;) Happened during my last sim ride (both boxes went blank, Map display blanked), interestingly enough without intent from the examiner - turned out that the sim became desynchronized, we continued flying nonetheless, became an interesting lesson after all...

[Checkboard]

Standard fit in an NG is the same as the -300: Two Control and Display Units (CDUs) feeding ONE Flight Management Computer (FMC), however dual FMCs are an option, as are Alternate Navigation Control and Display Units (ANCDUs)

ANCDUs are a CDU with the addition of a basic area navigation (i.e. LNAV, but no VNAV) computer installed. You will know if you have this option as you will have and Alternate Navigation Switch (ANS, with positions L and R) on the glareshield to direct the correct nav inputs to the autopilot.

If you don't have ANCDU backup:

Take-Off & Climb


Navigation is not normally permitted in B-RNAV (RNP 5) airspace, however is it normally OK to proceed to a repair airport with ATC informed. (Jeppesen Text, ATC pages 551-554)
.
MEL 2-34-27 is applied, some MNPS airspace routes must be avoided.
.
The Repetitive Flight Plan (RPL) must be cancelled, and a new plan filed with "S & R" in item 10 removed. "STS/NONRNAV" must be entered in item 18.
.
The phrase "Negative RNAV" is appended to each radio message on initial contact with each ATC frequency change.
.
Obtain the take-off N1 from the QRH "TAKEOFF N1%" page (it used to be PI 10.11), or the "ASSUMED TEMPERATURE REDUCED THRUST" PAGE (PI 10.12-13)
.
Set the calculated take-off N1 on the PF's N1 bug, using the manual bug procedure.
.
Calculate the MCT thrust (QRH PI 13.2) and set that on the PNF's N1 bug using the manual procedure (in case of engine failure)
.
MCP settings are normal (except no LNAV or VNAV), set the EHSI to VOR/ILS rose or expanded, as appropriate for navigation.
.
AUTOTHROTTLE is not available for take-off - set thrust manually prior to 80 knots.
.
Climb power may be set by selecting the AUTOTHROTTLE selector to ARM, and pressing the N1 button. (at thrust reduction altitude.)
.
Pitch modes available for flight are: TO/GA, LVL CHG and V/S (no VNAV)
.
You will need to navigate by HDG and/or VOR/LOC.
.
Once established on outbound track and above FL150 disengage the AUTOTHROTTLE (or deselct N1) and periodically set the N1 climb limit on both N1 guages as per QRH MAXIMUM CLIMB % N1 (PI 10.14) every 4-5000 feet.
.
(As the Power Management Computers (PMCs) are using inlet temperature and pressure data (P¹,T¹ sensed from the probe in front of the engine's fan) they have a very conservative schedule, so a significant performance degredation occurs as the aircraft climbs. There is no climb performance table information from the FMC, so the AUTOTHROTTLE will attempt to maintain the lower thrust if used in the climb)


Cruise


Approaching cruise level, extract the target N1 from (QRH PI 10.15 FLIGHT WITH UNRELIABLE AIRSPEED/TURBULENT AIR PENETRATION (Cruise), and set this number on the PNFs N1 guage.
.
Reaching cruise level, extract Limit N1 from the QRH MAXIMUM % CLIMB page, and set on the PF's N1 guage.
.
Autothrottle can be used to maintain cruise speed. Company rules vary, but generally without ECON cruise, LRC cruise is used (it could of course be a fixed MN cruise).
.
Navigate Raw data (or radar vectors if you have no shame about not being competent enough to fly an aircraft ;) ), and don't forget to inform ATC you are not RNP compliant.


Descent - Approach


Calculate your descent point from the chart M0.74/250 KTS DESCENT SPEED (ALL ENGINES) (PI 11.5)
.
Prior to commencing descent, bug the Vref speeds, using the VREF chart (QRH 10.3)
.
Once the ATIS is received, set the N1 guage cursors to cover a missed approach or Go-Around from the QRH GO-AROUND % N1 chart (page PI 10.14)
.
Descend at your company speeds - M0.74/280/250, for example.
.
Use Raw data below MSA, ensure the EHSI mode selector is at VOR/ILS
.
Auto-throttle may be used throughout the descent and approach.
.
In the event of a missed approach, pressing the TO/GA buttons will cause the AUTOTHROTTLE to disengage, and the thrust levers will have to be manually advanced to the previously buggeed N1 limit. The flight director will correctly display TO/GA commands.


N.B. : experience shows that FMC failures are usually of relatively short duration, if operating in ANCDU with IRS pages displayed on the CDU there will be no indication of FMC reset. Press any of the top two rows of keys on the CDU (INIT/REF, RTE, CLB etc) to test FMC status

[cosmo kramer]

Navigate Raw data (or radar vectors if you have no shame about not being competent enough to fly an aircraft ;) )
I do see the smiley, however maybe a bit too smart alec comment anyway.
Please do tell me how to fly direct to RNAV point "NOVOR" raw data? :=

Of course you can request direct to Im-da-man VOR, but if the airspace is complex enough, you find that ATC will prefer to keep you approximately on the filed RNAV route with vectors that you don't interfere with other airways. Have a look at your enroute charts for eastern-europe and you will find that most airways practically contains zero VORs or ADFs.

Calculate your descent point from the chart M0.74/250 KTS DESCENT SPEED (ALL ENGINES) (PI 11.5)
Or nm = height x 3, if you like to keep it simple :ok:

[Checkboard]

Please do tell me how to fly direct to RNAV point "NOVOR" raw data?:=

When you were flying little aeroplanes, how did you find a town, or turning point without a navaid at the point you are looking for?

It's called navigation - you have a working clock, a working heading reference, you can plot a fix to determine your current position ... ;) In your particular case - the dual IRS's in the 737 were still working, all you had to do was look up :rolleyes: for your actual track & ground speed, and get a chart out to see where the place was.

"Children of the Magenta line" indeed - that pilots today even have to ask how to do it, as though it is an impossibility without vectors, shows quite a bit. :uhoh:

http://i665.photobucket.com/albums/vv20/Checkboard/Picture1-1.png

In Australia, after the Omega navigation system was decommissioned, my airline had us flying with no area navigation system at all in a BAe 146 - just NDBs and VORs. On many of our routes we were completely outside navaid coverage (of any sort) for up to an hour. In one instance I was flying an aircraft with an unserviceable autopilot from Cairns to Darwin and had to divert north around a cyclone in the Gulf of Carpentaria.

http://www.aboutaustralia.com/a2it_package/images/travel/L46n.jpghttp://i665.photobucket.com/albums/vv20/Checkboard/cyclone.jpg

The solution (diverting left and right around thunder heads, outside navaid coverage of any sort, without autopilot) was simply to get a chart out, and run a series of air plots for each change of heading for an hour, then apply forecast wind vectors to ded-recon to a point back on the airway, and pick up the VOR radial.

[exosphere]

Or nm = height x 3, if you like to keep it simple

+1nm for each 10knots IAS deceleration, still to keep it simple :ok:

[cosmo kramer]

It's called navigation - you have a working clock, a working heading reference, you can plot a fix to determine your current position ... In your particular case - the dual IRS's in the 737 were still working, all you had to do was look up for your actual track & ground speed, and get a chart out to see where the place was.
I asked you how to proceed to an RNAV point raw data, not by proceeding VFR or Dead reckoning.

Neither hardly wise choices in congested RVSM/RNAV airspace. :ouch:

For data to be "raw", there needs to be data in the first place. That means that you have the ability to track some form of navaid which provide data.

Did you even consider the drift of the IRSs???
Such old ladies are not the most precise, and I may remind you that the updating from VOR/DME is applied to the FMC (which just failed). Hence you are left with a position that may have several hours of flight time and thereby drift - perhaps 10 miles or more. So what is the accuracy of your navigation when you include the accuracy of your DR?

Sometimes it's good to be cleaver, and sometimes it's better to leave out the completely irrelevant anecdotes (plus insults).

[Sciolistes]

Cosmo,
I asked you how to proceed to an RNAV point raw data, not by proceeding VFR or Dead reckoning.
Providing you have a VOR/DME in range, you can use the fix-to-fix (http://www.t37sim.com/fixtofix/fix_to_fix.htm) navigation technique. It takes a quite a bit of practice and mental gymnastics but for two crew should be doable. I'll conceed that it maybe a bit but unrealistic in modern airspace given that when vectors are unavailable, so are navaids!

As for IRS drift, the 737 Clasic is meant to be RNP 12 without updating. In my experience I wouldn't bank on it though!

And another exclamation mark for good measure!

[Checkboard]

I asked you how to proceed to an RNAV point raw data, not by proceeding VFR or Dead reckoning.

... and it still startles me that you think this is difficult !! I was pointing out that the IRS is available by switching the IRS panel to track mode, which will give you a very accurate track - i.e. no wind calculations!

So, to spell it out:

FMC fails.
Select HDG on the Mode Control Panel.
Complete QRH checklist(s).
Inform ATC NON-RNAV.
Get chart out of folder and determine current position, from last recorded fuel check on the PLOG initially then by local VOR.
Obtain airway track from chart and maintain that using HDG bug, aided by current track from IRS panel on the overhead (IRS position drift is irrelevant).
Approaching RNAV position, dial up suitable fix combination - VOR/VOR or VOR/DME.
As the aircraft approaches the fix, it will intercept one of the fix criteria before the other (assuming you are slightly off track) - ie one VOR radial before the other, or the DME arc before the radial. Slightly adjust the HDG bug to track the aircraft along the first intercept until you are at the second (fly the aircraft into the 'V' of the fix)*,
at the fix, turn HDG to next track - correct tracking using the TRACK display on the IRS panel overhead.
Write down time at fix. Note Groundspeed from overhead IRS panel. Calculate time to next fix (or just use the PLOG planned times - but personally I would work out my own).


The saying is that pilot's earn their larger-than-bus-driver wage not when things are going fine, but when things go wrong. Personally I would be professionally ashamed to ask for ATC help to navigate my aircraft, when no Navigation equipment on the aircraft had failed - just the box that did the navigation thinking.

*,(I was going to mention the fix-to-fix thing (although I never called it that) but thought it too complicated. Good link, though.) I would use that approaching the RNAV fix point.

[Jinkster]

The saying is that pilot's earn their larger-than-bus-driver wage not when things are going fine, but when things go wrong. Personally I would be professionally ashamed to ask for ATC help to navigate my aircraft, when no Navigation equipment on the aircraft had failed - just the box that did the navigation thinking.

And at the subsequent board of enquiry when the aircraft is laying in flames and the court says "Mr Pilot, why is the aircraft laying in flames?" and you can reply " I was ashamed to ask ATC for help when my navigational box failed, I know there is an X million pound radar system down there and I know I was wrong in my calculations but at least I tried"

Personally I'd get as much help as possible!! :ugh:

[Checkboard]

Perhaps given the standard of respondents here, and their level of confidence in their professional piloting abilities, failing to find and maintain a heading, given two IRS's to help, IS too dangerous for them.

To think that Area navigation has only been around in my lifetime ... Mick O'Leary is right - pilots today are only bus drivers, worth £20,000 a year. :(

[cosmo kramer]

You are not answering my question. You are just repeating yourself, with the same dead reckoning bs (and condescending tone). You are advocating to take an unprecise starting point, apply an even more unprecise methode, to find a point that ATC expects you to overfly with great accuracy. Great airmanship!
Personally I would be professionally ashamed to ask for ATC help to navigate my aircraft
Pride is a really nice quality to bring to an aircraft too.

It's not about being dangerous or difficult, it's about being inappropriate. I would happily fly DR if the FMC fails when no navaids or radar coverage is available - hardly the case over eastern europe.

Sciolistes, thanks for the link. Yes probably not very useful when you go 8nm pr. min, and again like DR too much of inaccuracy. And when starting to turn 30 deg off track to find an initial heading ATC it probably going to ask "confirm inbound XYZ".

[Checkboard]

You are advocating to take an unprecise starting point
In what way is a local VOR fix "unprecise"? :confused:
apply an even more unprecise methode
In what way is tracking information from a Dual IRS not precise? :confused:
to find a point that ATC expects you to overfly with great accuracy.
What part of "The phrase "Negative RNAV" is appended to each radio message on initial contact with each ATC frequency change." do you or ATC not understand? :confused:

[cosmo kramer]

A VOR fix jutted in a map on your lap, a line taken with a ruler to the next point. At track line obtained between said points...

So what is your accuracy in setting such a point? Are you sure you get radial 034 from the VOR or 037, the rose around the VOR on an enroute chart is pretty small. Are you going to actually draw on the map at all? Next from the inaccurate found point, perhaps just identified by holding a finger on the map, you are now going to draw(?) a line to the cleared point. How precise are you going to obtain that track in degrees (measured from the angle from the nearest median)?

(Don't say that you will obtain it from the flight plan, since of course you are aware that at most handoffs to a new sector you get a direct to the next sector boundary.)

So now you arrive near the next point, and of course one of the criteria are met first, because no way you will be bang on with the above approximation. When will you initiate the turn to the new track? How will you correct that track to get you back on precise route again.

Next you'll see that over the next points there will be an airway change. There is only 15nm between the next to points, unfortunately the controller doesn't provide you a direct too 200nm away. You are still to pround to ask ATC for radar vector and while you are looking the the map to find a new suitable VOR to tune, identify, jut a new unprecise point in your map, find a new unprecise track in the map (because you didn't turn over the required RNAV point you still can't use the flightplan).... all while moving at 8 nm pr. min.

While sitting heads down, ATC advice you to immediately turn right due to converging traffic in your 11 o'clock position. They then require to know why you strayed off your assigned route and inform you that they have to write a report.

Which part of unprecise don't you understand?

All may work well over Northern Australia, but over Europe? :hmm:

What part of "The phrase "Negative RNAV" is appended to each radio message on initial contact with each ATC frequency change." do you or ATC not understand?
Exactly! Only makes all your assumptions more hypothetical. When you say negative RNAV, ATC will send you off on a vector straight away - because they want to drink their coffee peacefully without "Omegaman" straying of his route and messing up their planned strategy.

Are you going to decline the vector and say.. negative RNAV but we are able direct RNAV point XYZ anyway?? :rolleyes:
Even more so when the vector given significantly shortens your route? ;)

[Checkboard]

The FMC is tracking the aircraft in NAV direct to your "direct to" tracking point. It fails, and you select HDG on the MCP, so you are still on track, tracking direct to the point. You call ATC and inform them you are no longer RNP capable due equipment failure.

If it was me:
ATC ask "Do you require vectors?", they get a cheery "No thanks."
ATC ask "Do you require routing via VORs?", they get a cheery "No thanks."

If they want to provide vectors to meet their operational requirements, due to traffic separation with a non-RNP aircraft in their sector, then of course that's fine.

Knowing I am on track, a quick VOR check just confirms the expected time over the next point ("confirms", as the PLOG is filled out, of course), and navigation continues as outlined above.

I would actually enjoy the diversion from the boring old normal, and have a pleasant time with a minor failure which can be easily dealt with entirely "in cockpit".

If it was you:
Apparently feeling unable to navigate your aircraft, you require Vectors - so a PAN call would be appropriate, to guarantee the extra handling and workload from ATC.

If it was Jinkster:
A MAYDAY call is appropriate, as he apparently feels that an FMC failure with him as pilot will result in "the aircraft laying in flames" if he doesn't get outside help. :rolleyes:

In terms of navigating with a map. Yes, I carry a Jeppesen plotter (specifically designed to be used in cockpit with Jeppesen charts) and a Jeppesen CR-2 circular slide rule. The method for using a ruler and pencil and whatnot to navigate about the place is beyond this discussion (i.e. - as a (presumably) licensed pilot, you shouldn't have to ask !)

[RAT 5]

treat it like a -200. What's the big deal? Has everyone forgotten how to fly and navigate? Not every a/c in the sky is an EFIS a/c. "don't panic, don't panic" comes to mind, as does KISS.

[moggiee]

treat it like a -200. What's the big deal? Has everyone forgotten how to fly and navigate? Not every a/c in the sky is an EFIS a/c. "don't panic, don't panic" comes to mind, as does KISS.

I'm glad I'm not the only person who was thinking that!

[de facto]

In terms of navigating with a map. Yes, I carry a Jeppesen plotter (specifically designed to be used in cockpit with Jeppesen charts) and a Jeppesen CR-2 circular slide rule. The method for using a ruler and pencil and whatnot to navigate about the place is beyond this discussion (i.e. - as a (presumably) licensed pilot, you shouldn't have to ask !)

Is that a CAA rule in the UK to carry such items?:E

If it was me:
ATC ask "Do you require vectors?", they get a cheery "No thanks."
ATC ask "Do you require routing via VORs?", they get a cheery "No thanks."

If it were me, i would say YES please and go on with my coffee:cool:

[Helen49]

I love this discussion between those who can navigate using first principles and those presumably brought up on the modern nav. systems! I remember the days when all pilots navigated in the way described by checkboard. It worked fine and many ATC units around the world had no radar and couldn't have helped even if asked.....apart from the odd QDM!

[Golden Rivet]

what should be the course of action in case of Dual FMC failure in Boeing 737 ng

No one has stated to blindingly soddin obvious....

Cycle the FMC CB's and keep your fingers crossed.

Dual FMC fail is a known issue. A Static Ram Fault in the failing FMC can flood the intersystem bus with garbled data. This causes the offside FMC to fall over when the data handler gets overloaded.

"All things being equal, the simplest solution tends to be the right one"

[greenslopes]

Dual FMC Failure......

Errrrrr.............. Handing over............

Not a biggy, used to happen not too infrequently on the classics. Re-plan via navaids advising ATC of amended route.

[Checkboard]

If it were me, i would say YES please and go on with my coffee:cool:

And what would you do over the Atlantic/Pacific/Africa/Australia/NE Asia? (i.e. No radar available - even if you need it.) :hmm:

[cosmo kramer]

First of all, the IDU shows True Track. You seem to have forgotten that, so that will send you straight off course with whatever magnetic variation in the area you are flying. Knowing your equipment may be a good starting point. :ok:

Assuming that you did read the instructions, let's be generous and say that because you manage to make some half precise fixes with your pencil along the way you only make a 3 degs error over e.g. a 300nm direct-to RNAV point. With basic MDR rules that would equate to 15nm. Considering the requirement for B-RNAV routes is 5 nm accuracy, I am sure ATC will be very happy about that. :=

You even mentioned yourself in your first post that the MEL says not to operate on BRNAV routes, but for some reason you decided to disregard it to go your own ways, because you know better and like to have fun!

Who said anything about panic or pan-pan, that would both be pretty ridiculous. It's hardly a state of urgency to request direct to a VOR (may I suggest brushing up on this subject as well). Like Greenslope this was exactly what I did and it was declined by ATC who instead provided a vector. Their choice not mine, their convenience, I don't care.
But I am not going to use my resources on ridiculous and unnecessary DR navigation and risking that my mistakes (of course some are infallible and never makes mistakes :hmm:) become someone else’s problem.

While you are sitting heads down drawing in your map, straying off your assigned route, I will calmly be drinking coffee like de facto ...or maybe even fly the aircraft or checking the odd volmet.

And what would you do over the Atlantic/Pacific/Africa/Australia/NE Asia? (i.e. No radar available - even if you need it.)

This was for my part covered before. I wrote that in the airspace above Europe DR navigation is not appropriate. With a failed RNAV system above the Atlantic it is on the other hand highly appropriate. Whatever resulting deviation must be made good when navigational aids are available again.

That you fail to see the difference shows lack of airmanship.

To Helen49 - SLF, I presume:
"The superior pilot uses his superior judgment to avoid having to use his superior skills". The macho "I'll show the world how good I am" pilot is luckily (almost) a thing of the past. :)

[theficklefinger]

Any stories of dark cockpits halfway to Hawaii?

[framer]

Interseting conversation.
My first four years airline flying were done with no area nav capability or autopilot and I feel pretty comfortable that I could get by ok going back to basics if I was without my dual gps, irs , fmc combo, and one of the first mental calculations i do upon reaching the cruise is my kg's per nm and my nm per minute for todays conditions, but.........my first priority is providing the safest travel to destination for my passengers. In this situation utilizing the capabilities of ATC and keeping the workload of myself and my copilot as close to normal as possible would achieve this. If for reasons of fun/ego etc I decided to do revert to old practices when it was not the only option, I would be
a) creating more workload than was required of the situation
b) increasing the chances of missing something normal (a checklist or wx change or atc clearance or transition altitude etc etc)
c)reducing my situational awareness by diverting my brain cells into calcs rather than leaving them free to observe how the flight was progressing.
d) creating a 'non-normal' situation and in non normal situations errors are much more likely to occur (lets face it, navigating like this is, now days, not a normal situation)
In addition to these things, my first officer may be more rusty than me or completely incompetant in this type of navigation (thats the world we operate in,I'm not saying it's right) and so I am effectively making it single pilot as well, taking him/her out of the loop, reducing the chances of effective cross checking.
I have no doubt that I could do it safely, but it wouldn't be the safest option, and unless there are significant costs to the safest option, I'm going to take it. If I want to practice that sort of navigation there is nothing stoping me from doing so when the fmc's are working fine, that way, one day when it is the only option available to me I will be proficient, but to do it when other options are available in order to have fun or stroke my ego is , in my mind, irresponsible.
Just my 2 cents.
Framer

[thocles]

hello,


I have fmc reboot after acars wind request, after that LNAV was lost and cannot re enter cost index, anybody had this problem?,

[MD11Man]

Old topic kicked up...

In case you have a double FMC failure when crossing the Atlantic, on a 737 NG with no CPDLC etc., and your flight plan data is something as:

5130N average true track 073
5220N average true track 081

And your FMC's fail at 25W. Which course do you fly?

Apparently, it's 083 true. I just don't know the correct procedure to get to that.