Question for 767 drivers
Guest
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Question for 767 drivers
Just a quick question on the 767 FMGS.
Whilst studying for the dreaded JAR exams, I've come across the following:
"Both FMC's normally use the left Air Data Computer (ADC). If the left ADC fails, both FMC's switch to the right ADC."
Is this correct, and if so why don't the left and right ADC's feed their respective FMC's?
Futhermore it states "Both FMC's normally use the left IRU for altitude, vertical celocity, acceleration, and heading data processing."
Again, why do they both use the left IRU for this, and what altitude output is obtained from the IRU.
Whilst this is of little relevance to the forthcoming exams, it has caused a bit of lively discussion, so a definitive answer would be good! Having studied an Op's manual we still can't clarify the position so any input would be appreciated.
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CW
Whilst studying for the dreaded JAR exams, I've come across the following:
"Both FMC's normally use the left Air Data Computer (ADC). If the left ADC fails, both FMC's switch to the right ADC."
Is this correct, and if so why don't the left and right ADC's feed their respective FMC's?
Futhermore it states "Both FMC's normally use the left IRU for altitude, vertical celocity, acceleration, and heading data processing."
Again, why do they both use the left IRU for this, and what altitude output is obtained from the IRU.
Whilst this is of little relevance to the forthcoming exams, it has caused a bit of lively discussion, so a definitive answer would be good! Having studied an Op's manual we still can't clarify the position so any input would be appreciated.
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CW
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"why don't the left and right ADC's feed their respective FMC's?"
You're not the only one wondering this, CW !
From my engineering training notes, I see that both L and R ADC's feed each FMC...and that the ADC that is used is dependent on AP/FD selection (as well as failures).
In your airline, when the Captain is the pilot flying, does he/she select the L A/P or F/D.. and when the F/O is flying, does he select the R A/P or R F/D? If so, the pilot flying would get to select the baro setting which goes to the active FMC....rather than, say, the FMC using an average of the two baro settings(not good if one pilot is incapacitated)
"Again, why do they both use the left IRU for this, and what altitude output is obtained from the IRU."
The altitude data (actually vertical velocity integrated) is the primary altitude source for the FMC (I suppose the ADCs provide long term correction). For the parameters you mention, the FMC's use the IRS corresponding to the autopilot in command. Is there a clue in there, somewhere? 8-)
Hopefully the info I have stated here is applicable to your aircraft.
Rgds.
Q.
You're not the only one wondering this, CW !
From my engineering training notes, I see that both L and R ADC's feed each FMC...and that the ADC that is used is dependent on AP/FD selection (as well as failures).
In your airline, when the Captain is the pilot flying, does he/she select the L A/P or F/D.. and when the F/O is flying, does he select the R A/P or R F/D? If so, the pilot flying would get to select the baro setting which goes to the active FMC....rather than, say, the FMC using an average of the two baro settings(not good if one pilot is incapacitated)
"Again, why do they both use the left IRU for this, and what altitude output is obtained from the IRU."
The altitude data (actually vertical velocity integrated) is the primary altitude source for the FMC (I suppose the ADCs provide long term correction). For the parameters you mention, the FMC's use the IRS corresponding to the autopilot in command. Is there a clue in there, somewhere? 8-)
Hopefully the info I have stated here is applicable to your aircraft.
Rgds.
Q.
Guest
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I'd have to agree with QAVION on this one. My notes state that FMC's can receive CAS,TAS,BARO ALT and PRESS ALT from both ADC's. The FMC will normally select the first valid ADC on line.
With reference to baro select, correct me if I'm wrong but all baro settings are referenced to captains altimeter unless selcted to F'O's via L/R baro selct switch on P55 panel.
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Fools rush in where Angels fear to tread
With reference to baro select, correct me if I'm wrong but all baro settings are referenced to captains altimeter unless selcted to F'O's via L/R baro selct switch on P55 panel.
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Fools rush in where Angels fear to tread
Guest
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Bit of a wobbly question from the JAR people, I think not entirely correct ( having just asked our tech people ). Both FMC's cross-ref both ADC's, but take primary info from their respective ADC. Normally, the LEFT FMC is the "master" and the right is the "slave" for inputting data, etc. This is the other way round when the rt autopilot is in command or during manual flt with left FD off and RT FD on.
So, during ops with left or centre AP in command, both FMC's are primarily using LEFT ADC info. This was the best we could figure they were asking about.....
Clear as mud ,eh?
So, during ops with left or centre AP in command, both FMC's are primarily using LEFT ADC info. This was the best we could figure they were asking about.....
Clear as mud ,eh?
Guest
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In our airline, (Talking about the 757 here but I think it is the same), Captain selects Left Instrument source. The F/O has Right AInstrument Source. Consequently we use the centre autopilot all the time. This still leaves the Captain's altimeter as the controlling baro. It also means that both flight directors are in view. On the 757 when the left or right autopilot is selected, the corresponding flight director disappears.
The IRUs provide ATTITUDE, ACCELERATION, GROUND SPEED, TRACK, TRUE AND MAGNETIC HEADING, LAT AND LONG AND WINDSPEED AND DIRECTION.
Eff Oh.
The IRUs provide ATTITUDE, ACCELERATION, GROUND SPEED, TRACK, TRUE AND MAGNETIC HEADING, LAT AND LONG AND WINDSPEED AND DIRECTION.
Eff Oh.
Guest
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"With reference to baro select, correct me if I'm wrong but all baro settings are referenced to captains altimeter unless selcted to F'O's via L/R baro selct switch on P55 panel."
You have a baro select switch on your 767 Glareshield panel? Is this a Boeing option... or was I asleep in class when they talked about this? :-)
BTW, which series 767 are you operating?
Rgds.
Q.
You have a baro select switch on your 767 Glareshield panel? Is this a Boeing option... or was I asleep in class when they talked about this? :-)
BTW, which series 767 are you operating?
Rgds.
Q.
Guest
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I was on the 757 for three years and don't remember needing to understand this or why pilots need a very in-depth grasp of such system wiring, unless they decide not to look up a system fault in the correct (COM) cockpit operating manual etc. Normal procedures had the FO using the right, maybe the center autopilots. Knowing why not to change a normal procedure can be important, no doubt.
Do you have certain abnormal procedures done only from memory?
In later airplane systems, to reset or turn something off should only be per checklist, whether normal or abnormal, i.e never switch off the left recirc fan inflight, since the ovbd exhaust valve will be locked open and the cabin will climb over 1,000 feet per minute, with no way to close it! A captain tried this when the very hot cabin could not be cooled off, even in manual. Luckily, we were over land not far from the destination, considering that we descended immediately to "bring the cabin down" and the fuel burn increased a good bit.
Do you have certain abnormal procedures done only from memory?
In later airplane systems, to reset or turn something off should only be per checklist, whether normal or abnormal, i.e never switch off the left recirc fan inflight, since the ovbd exhaust valve will be locked open and the cabin will climb over 1,000 feet per minute, with no way to close it! A captain tried this when the very hot cabin could not be cooled off, even in manual. Luckily, we were over land not far from the destination, considering that we descended immediately to "bring the cabin down" and the fuel burn increased a good bit.
Guest
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Had an interesting situation a few months back regarding dispatch with both FMC's inoperative (767-300ER). To my knowledge, our MEL is a virtual copy of the Boeing master MEL, but the procedures outlined to cover this situation did not work.
Our paperwork indicated dispatch (short domestic flight) with both FMC's inoperative. On arrival at aircraft, we had two placarded inop (Pegasus) FMC's, but both appeared to operate normally (maintenance procedures do not call for disabling FMC's in this situation. Write up for the problem was 'keyboard problems" with both FMC's. Crew must have been new to the Pegasus and not familiar with the slow(er) keypad entry to the acratchpad on the Pegasus units. This causes some crews to double enter keystrokes.
Maintenance took the easy way out and deferred the FMC's for the short domestic leg. MEL procedures (don't have the book in front of me)call for programing present position in IRS overhead panel and switching to CDU L/CDU R to activate STANDBY NAV. Followed procedures and - no standby (IRS only) nav. After discsussion, we elected to operate the flight using /A procedures with the operating (but deferred) FMC's programmed for the flight.
Further study of the manuals gave no clue as to why we could not get into Standby Nav. The only thing I could think of was that, while deferred as inop, the FMC's did not know they were inop (no fail light). To test this theory, after block in we pulled the FMC breakers and - WALLA - we could access the Stanby Nav function. ONE CANNOT ACCESS THE STANDBY NAV FUNCTION UNLESS BOTH FMC'S KNOW THEY ARE INOPERATIVE (FAIL LIGHT ON).
I wrote this up to our MEL folks (no response yet) and conferred with a couple of our overwater check airman, who were also unaware of this scenario. My concern would be that if FMC's are degraded or Inop on an overwater flight, and do not realize it (no fail lights), access to STANDBY NAV is not possible (without pulling the FMC circuit breakers, not an inflight procedure I am aware of).
Any comments out there?
Our paperwork indicated dispatch (short domestic flight) with both FMC's inoperative. On arrival at aircraft, we had two placarded inop (Pegasus) FMC's, but both appeared to operate normally (maintenance procedures do not call for disabling FMC's in this situation. Write up for the problem was 'keyboard problems" with both FMC's. Crew must have been new to the Pegasus and not familiar with the slow(er) keypad entry to the acratchpad on the Pegasus units. This causes some crews to double enter keystrokes.
Maintenance took the easy way out and deferred the FMC's for the short domestic leg. MEL procedures (don't have the book in front of me)call for programing present position in IRS overhead panel and switching to CDU L/CDU R to activate STANDBY NAV. Followed procedures and - no standby (IRS only) nav. After discsussion, we elected to operate the flight using /A procedures with the operating (but deferred) FMC's programmed for the flight.
Further study of the manuals gave no clue as to why we could not get into Standby Nav. The only thing I could think of was that, while deferred as inop, the FMC's did not know they were inop (no fail light). To test this theory, after block in we pulled the FMC breakers and - WALLA - we could access the Stanby Nav function. ONE CANNOT ACCESS THE STANDBY NAV FUNCTION UNLESS BOTH FMC'S KNOW THEY ARE INOPERATIVE (FAIL LIGHT ON).
I wrote this up to our MEL folks (no response yet) and conferred with a couple of our overwater check airman, who were also unaware of this scenario. My concern would be that if FMC's are degraded or Inop on an overwater flight, and do not realize it (no fail lights), access to STANDBY NAV is not possible (without pulling the FMC circuit breakers, not an inflight procedure I am aware of).
Any comments out there?
Guest
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Airplanes with HMCDU/MCDU:
a. LEFT FMC INOPERATIVE - When operating in flight director mode with both F/D switches ON, LNAV and VNAV will engage if the Captain’s NAV source switch is selected to R FMC or L CDU position. Otherwise, LNAV and VNAV will engage if the right autopilot is engaged or only the right F/D is ON. Turning off the left F/D switch would allow use of LNAV and VNAV, but only the right flight director would operate.
Procedure: After liftoff (with F/D in TO mode), use HDG SEL and FL CH modes as desired until Captain’s NAV selector is moved to an alternate source, or right autopilot is engaged. Then use LNAV and VNAV as desired.
b. RIGHT FMC INOPERATIVE - When operating in flight director mode with both F/D switches ON, LNAV and VNAV will engage. If the First Officer’s NAV source switch is selected to the L FMC or R CDU position, LNAV and VNAV will engage with any autopilot engaged. Otherwise, LNAV and VNAV will engage if the left or center is engaged or the right F/D is ON.
Procedure: After liftoff (with F/D in TO mode), use HDG SEL and FL CH modes as desired until First Officer’s NAV selector is moved to an alternate source, or left or center autopilot is engaged. Then use LNAV and VNAV as desired.
In addition to above, prior to takeoff accomplish the following as appropriate:
1) For an inoperative left FMC:
a. Select the FMC-R position with the Captain's FMC NAV Source Select Switch to load data from the right FMC into the left CDU.
NOTE: The FMC switch should remain in this position for at least one minute after flight
planning is completed or after any flight plan modifications prior to takeoff.
b. After a minimum of one minute, select the CDU-L position with the Captain's FMC NAV Source Select switch.
c. Observe that Captain’s HSI (-200/-300) or ND (-400ER) Map display changes to a simplified route display without normal navigation database symbology. “CDU-L” should be displayed on the HSI/ND. This confirms availability of the Alternate
Navigation system.
d. Prior to takeoff, select the FMC-R position with the Captain’s FMC NAV Source SelectSwitch.
For an inoperative right FMC:
a. Select the FMC-L position with the First Officer's FMC NAV Source Select Switch to load data from the left FMC into the right CDU.
NOTE: The FMC switch should remain in this position for at least one minute after flight
planning is completed or after any flight plan modifications prior to takeoff.
b. After a minimum of one minute, select the CDU-R position with the First Officer's FMC
NAV Source Select switch.
c. Observe that First Officer’s HSI (-200/-300) or ND (-400ER) Map display changes to a
simplified route display without normal navigation database symbology. “CDU-R”
should be displayed on the HSI/ND. This confirms availability of the Alternate
Navigation system.
d. Prior to takeoff, select the FMC-L position with the First Officer’s FMC NAV Source Select Switch.
The ability to confirm availabilty of Standby Nav System should stop crews from operating with "Failed" FMC's and SNS not operating.
This should be carried out before flight and if not avail you should be staying put !
Hope this helps
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Fools rush in where Angels fear to tread
[This message has been edited by Golden Rivet (edited 11 March 2001).]
[This message has been edited by Golden Rivet (edited 11 March 2001).]
a. LEFT FMC INOPERATIVE - When operating in flight director mode with both F/D switches ON, LNAV and VNAV will engage if the Captain’s NAV source switch is selected to R FMC or L CDU position. Otherwise, LNAV and VNAV will engage if the right autopilot is engaged or only the right F/D is ON. Turning off the left F/D switch would allow use of LNAV and VNAV, but only the right flight director would operate.
Procedure: After liftoff (with F/D in TO mode), use HDG SEL and FL CH modes as desired until Captain’s NAV selector is moved to an alternate source, or right autopilot is engaged. Then use LNAV and VNAV as desired.
b. RIGHT FMC INOPERATIVE - When operating in flight director mode with both F/D switches ON, LNAV and VNAV will engage. If the First Officer’s NAV source switch is selected to the L FMC or R CDU position, LNAV and VNAV will engage with any autopilot engaged. Otherwise, LNAV and VNAV will engage if the left or center is engaged or the right F/D is ON.
Procedure: After liftoff (with F/D in TO mode), use HDG SEL and FL CH modes as desired until First Officer’s NAV selector is moved to an alternate source, or left or center autopilot is engaged. Then use LNAV and VNAV as desired.
In addition to above, prior to takeoff accomplish the following as appropriate:
1) For an inoperative left FMC:
a. Select the FMC-R position with the Captain's FMC NAV Source Select Switch to load data from the right FMC into the left CDU.
NOTE: The FMC switch should remain in this position for at least one minute after flight
planning is completed or after any flight plan modifications prior to takeoff.
b. After a minimum of one minute, select the CDU-L position with the Captain's FMC NAV Source Select switch.
c. Observe that Captain’s HSI (-200/-300) or ND (-400ER) Map display changes to a simplified route display without normal navigation database symbology. “CDU-L” should be displayed on the HSI/ND. This confirms availability of the Alternate
Navigation system.
d. Prior to takeoff, select the FMC-R position with the Captain’s FMC NAV Source SelectSwitch.
For an inoperative right FMC:
a. Select the FMC-L position with the First Officer's FMC NAV Source Select Switch to load data from the left FMC into the right CDU.
NOTE: The FMC switch should remain in this position for at least one minute after flight
planning is completed or after any flight plan modifications prior to takeoff.
b. After a minimum of one minute, select the CDU-R position with the First Officer's FMC
NAV Source Select switch.
c. Observe that First Officer’s HSI (-200/-300) or ND (-400ER) Map display changes to a
simplified route display without normal navigation database symbology. “CDU-R”
should be displayed on the HSI/ND. This confirms availability of the Alternate
Navigation system.
d. Prior to takeoff, select the FMC-L position with the First Officer’s FMC NAV Source Select Switch.
The ability to confirm availabilty of Standby Nav System should stop crews from operating with "Failed" FMC's and SNS not operating.
This should be carried out before flight and if not avail you should be staying put !
Hope this helps
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Fools rush in where Angels fear to tread
[This message has been edited by Golden Rivet (edited 11 March 2001).]
[This message has been edited by Golden Rivet (edited 11 March 2001).]
Guest
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Golden Rivet,
Thank you for the excellent response. Is this from an MEL or from a troubleshooting/operations guide of some sort? Don't have the MEL with me (ships library), but recall similar procedures for dealing with one FMC inop. Also, could you post the guidelines for our situation, both FMC's (placarded) inop.
My main point is that to accomplish the appropriate procedures for FMC failure, the FMC must recognize that it has, in fact, failed (Fail light on). Normally equipement of this type is either working or not, but if it was operating in some degraded fashion (eg. lock up), I think it would be difficult to follow some of the procedures for FMC/FMC's inop while the FMC is still powered and believes it is functioning properly (no fail light).
Thank you for the excellent response. Is this from an MEL or from a troubleshooting/operations guide of some sort? Don't have the MEL with me (ships library), but recall similar procedures for dealing with one FMC inop. Also, could you post the guidelines for our situation, both FMC's (placarded) inop.
My main point is that to accomplish the appropriate procedures for FMC failure, the FMC must recognize that it has, in fact, failed (Fail light on). Normally equipement of this type is either working or not, but if it was operating in some degraded fashion (eg. lock up), I think it would be difficult to follow some of the procedures for FMC/FMC's inop while the FMC is still powered and believes it is functioning properly (no fail light).
