FPV and FPV Cage
IRS is currently summed and weighted with GPS & DME/DME (when available) inputs and corrections applied.
Instantaneously however the inertial accelerometers tell you where your HUD velocity vector is pointed in inertial space regardless of the fact that “where” you are, might not be exactly “where” you are.
Any error in any axis of the accelerometers would be miniscule instantaneously, or evident to the point that no baro refinement is going to instantaneously correct it and you would have fail annuciations. RLG’s are, however, highly reliable.
Thanks for the physics - the definitions may be helpful to someone. 
Can you see your error here? In the first sentence you (correctly) state that the IRS accelerometers measure acceleration. In the second sentence you say that this is velocity, and it isn't - it is acceleration (as you said).
If the accelerometers measure a change from level flight to a descent (a change in the direction of the aircraft = an acceleration), then a new velocity is calculated from that measured acceleration (and that calculated velocity can be shown on either HUD or PFD). If the accelerometers then measure another change in the velocity, and calculate from that measured acceleration that the aircraft has levelled out, that can also be shown on the velocity vector - however if there are any errors in any of the measured accelerations, there will be a consequent error in the displayed vector - the IRS has no way of measuring directly if the aircraft is actually flying level again without referring to barometric data. Hence the baro refinement in the vector, and the notes in both the Boeing and Airbus manuals that altimetry errors affect the FPV and VS.
Yes, all of the IRS platforms I have flown can be updated in flight - but it isn't usually (or automatically) done.
I never said this - the barometric refinement isn't to the IRS - it's to the system's calculation of the FPV.
Trust me.
However, at any given moment, INSTANTANEOUSLY if you will, the output of a three-axis accelerometer system can be fed to a Heads Up Display (HUD) computer. This INSTANTANEOUS output consists of speed and summed 3 axis direction (velocity, velocity vector) only, until the next speed and direction as a result of any ongoing acceleration is processed.
If the accelerometers measure a change from level flight to a descent (a change in the direction of the aircraft = an acceleration), then a new velocity is calculated from that measured acceleration (and that calculated velocity can be shown on either HUD or PFD). If the accelerometers then measure another change in the velocity, and calculate from that measured acceleration that the aircraft has levelled out, that can also be shown on the velocity vector - however if there are any errors in any of the measured accelerations, there will be a consequent error in the displayed vector - the IRS has no way of measuring directly if the aircraft is actually flying level again without referring to barometric data. Hence the baro refinement in the vector, and the notes in both the Boeing and Airbus manuals that altimetry errors affect the FPV and VS.
However, there ARE other systems whose IRS position and local level can be updated after the original IRS, INS, or IMS alignment while in-flight.
And, there is NO new data input, baro or otherwise, to inertial accelerometers, they are what they are. Mechanical platforms can be torqued in-flight in some systems and existing local level errors refined, however nothing is changed physically in the accelerometers, only their orientation. RLG's don't need this, no moving parts anyway.
Trust me.
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Checkboard
not all models have GPS available, unfortunately.
shortfuel
yes, We have that tool in the bus, too, so you can keep flying safe within the envelope, but I mean i would like to have some alternative means of knowing if I am flying at zero V/S so I can use the tables according to my GW and set the pitch (which thrust as needed to maintain it) and then find which is the mother****er erroneus instrument. (It would be useful to realise that altitude information is misleading, too). In pitot only scenarios, everything is simple once you come to the conclusion that speed is unreliable.
not all models have GPS available, unfortunately.
shortfuel
yes, We have that tool in the bus, too, so you can keep flying safe within the envelope, but I mean i would like to have some alternative means of knowing if I am flying at zero V/S so I can use the tables according to my GW and set the pitch (which thrust as needed to maintain it) and then find which is the mother****er erroneus instrument. (It would be useful to realise that altitude information is misleading, too). In pitot only scenarios, everything is simple once you come to the conclusion that speed is unreliable.
I said instantaneous inertial velocity is available from inertial accelerometers.
That calculated velocity gradually deviates from true velocity due to errors in measuring acceleration over time.
To obtain a useable, reasonably error free, vertical component of the velocity (for either the Vertical Speed or the FPV) the EFIS cannot use the raw IRS velocity calculation - I know this because both Boeing and Airbus say so (see the posts above).
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There's an easy way to prove that the FPV does or does not ADC data. Pull the CBs for the ADCs (well, at least this is possible on the Boeings which don't have ADIRUs).
Why would you need long term error correction on an instrument of this nature? You want to know what the aircraft is doing now. Of course, in the case of V/S, your IVSI may be damped so that that the needle isn't going crazy, but is this necessarily a requirement for the FPV? (and couldn't it be done electronically anyway, in the IRU?)
Perhaps the pilot should not use the FPV to maintain altitude simply because of HIS/HER accumulated errors. The pilot can't possibly keep the FPV exactly on the horizon every second of the flight, so the aircraft may go up/down over the long term. By the way, how does Boeing word its warnings?
Regards
NSEU
Why would you need long term error correction on an instrument of this nature? You want to know what the aircraft is doing now. Of course, in the case of V/S, your IVSI may be damped so that that the needle isn't going crazy, but is this necessarily a requirement for the FPV? (and couldn't it be done electronically anyway, in the IRU?)
Perhaps the pilot should not use the FPV to maintain altitude simply because of HIS/HER accumulated errors. The pilot can't possibly keep the FPV exactly on the horizon every second of the flight, so the aircraft may go up/down over the long term. By the way, how does Boeing word its warnings?
Regards
NSEU
Why would you need long term error correction on an instrument of this nature?
Of course, in the case of V/S, your IVSI may be damped so that that the needle isn't going crazy, but is this necessarily a requirement for the FPV?
(and couldn't it be done electronically anyway, in the IRU?)
Inertial = D/R
Hi OK465,
Despite Checkboard’s best efforts, you still seem to be missing a fundamental point in relation to the practicality of using raw IRS data to determine FPV many hours after IRS alignment. Inertial navigation is posh dead-reckoning: time erodes accuracy. It cannot be re-aligned on the hoof. (How did the Apollos ever get to the Moon?)
Since retirement, I’m not even allowed in an airline cockpit, but perhaps a current pilot will confirm that, by the end of a long flight, the GS readings on the captain’s and F/O’s displays typically differ by several knots. They are unlikely to read precisely zero when the aircraft stops. If the IRS MCDU is interrogated, comparable discrepancies will be found in the TRK readings. (Easier to compare them than establish the exact current TRK, even though TRK=HDG at his point.) So each IRU has developed an accumulated error of track and ground-speed, because of minute errors in each measured acceleration in azimuth.
Now: looking at the vertical axis, it is inevitable that the same problem will exist. Since one of the most common and important uses of FPA in airline ops is to fly a non-precision approach at a precise angle of around 3 degrees, an unknown error of even one degree would be unacceptable. (A similar error in TRK, however, might not even be noticed.)
When the A320 was launched, the FCOM (Tech) indicated that the FPA was purely inertial in vertical as well as azimuth (the azimuth being the TRK vector). I think the same had been true of the A310 and A300-600. Some years later, however, I lost an argument with a copilot, who showed me a current FCOM in which it was shown that barometric data was, by then, taken into account. Precisely how this is done is unclear to me, but the FPV vertical vector seems to be based on the same VS as that shown on the related VSI.
Chris
PS
Another aspect of FPV is that, as OK465 says, true altitude is not equal to pressure altitude. So the same applies to VS. If the FPA was purely inertial and precisely accurate, it might indicate a climb or descent when the aircraft is maintaining a steady flight-level.
On the other hand, to fly a precise instrument approach, a baro-based FPA would be inferior to an accurate inertial one.
Despite Checkboard’s best efforts, you still seem to be missing a fundamental point in relation to the practicality of using raw IRS data to determine FPV many hours after IRS alignment. Inertial navigation is posh dead-reckoning: time erodes accuracy. It cannot be re-aligned on the hoof. (How did the Apollos ever get to the Moon?)
Since retirement, I’m not even allowed in an airline cockpit, but perhaps a current pilot will confirm that, by the end of a long flight, the GS readings on the captain’s and F/O’s displays typically differ by several knots. They are unlikely to read precisely zero when the aircraft stops. If the IRS MCDU is interrogated, comparable discrepancies will be found in the TRK readings. (Easier to compare them than establish the exact current TRK, even though TRK=HDG at his point.) So each IRU has developed an accumulated error of track and ground-speed, because of minute errors in each measured acceleration in azimuth.
Now: looking at the vertical axis, it is inevitable that the same problem will exist. Since one of the most common and important uses of FPA in airline ops is to fly a non-precision approach at a precise angle of around 3 degrees, an unknown error of even one degree would be unacceptable. (A similar error in TRK, however, might not even be noticed.)
When the A320 was launched, the FCOM (Tech) indicated that the FPA was purely inertial in vertical as well as azimuth (the azimuth being the TRK vector). I think the same had been true of the A310 and A300-600. Some years later, however, I lost an argument with a copilot, who showed me a current FCOM in which it was shown that barometric data was, by then, taken into account. Precisely how this is done is unclear to me, but the FPV vertical vector seems to be based on the same VS as that shown on the related VSI.
Chris
PS
Another aspect of FPV is that, as OK465 says, true altitude is not equal to pressure altitude. So the same applies to VS. If the FPA was purely inertial and precisely accurate, it might indicate a climb or descent when the aircraft is maintaining a steady flight-level.
On the other hand, to fly a precise instrument approach, a baro-based FPA would be inferior to an accurate inertial one.
It cannot be re-aligned on the hoof. (How did the Apollos ever get to the Moon?)
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Aircraft fitted with IRS don't have IVSI's - because you don't need an Inertial Vertical Speed Indicator when you have an inertial platform (the IRS) already fitted.
Since retirement, I’m not even allowed in an airline cockpit, but perhaps a current pilot will confirm that, by the end of a long flight, the GS readings on the captain’s and F/O’s displays typically differ by several knots
On the 737 the FPV on airplanes without HUD is purely additional information, its use is not required by any kind of procedure or approach. NPAs are flown in approach mode following the exact same presentation as on an ILS or GLS approach.
By the way, how does Boeing word its warnings?
Additional Information
The flight path vector is based on inertial sources and may be used as a reference in maintaining proper path control.
The flight path vector is based on inertial sources and may be used as a reference in maintaining proper path control.
Afraid not, it will always read 0, nothing else.
Originally Posted by shlittlenellie (post #19, above)
The systems manuals for the 737 state that there's a barometric input to the FPV:
The Flight Path Vector (FPV) symbol represents airplane flight path angle vertically and drift angle laterally. The flight path vector is displayed on the PFD when the EFIS control panel FPV switch is selected on. The FPV shows the Flight Path Angle (FPA) above or below the horizon line and drift angle left or right of the pitch scale's center. The FPA uses inertial and barometric altitude inputs. The vertical FPA is unreliable with unreliable primary altitude displays. (C) Boeing Aircraft Systems Manual.
The Flight Path Vector (FPV) symbol represents airplane flight path angle vertically and drift angle laterally. The flight path vector is displayed on the PFD when the EFIS control panel FPV switch is selected on. The FPV shows the Flight Path Angle (FPA) above or below the horizon line and drift angle left or right of the pitch scale's center. The FPA uses inertial and barometric altitude inputs. The vertical FPA is unreliable with unreliable primary altitude displays. (C) Boeing Aircraft Systems Manual.
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Originally Posted by Checkboard
Aircraft fitted with IRS don't have IVSI's - because you don't need an Inertial Vertical Speed Indicator when you have an inertial platform (the IRS) already fitted.
On the 747-400 (at least) Baro from the ADCs is fed into the IRUs. The IRU's then mix baro and IRU info and send this as IVSI information to the PFD. This is why the IVSI information disappears when either the ADC or IRS CBs are pulled.
From my course notes.
"Vertical Speed
The IRU computes vertical speed from vertical acceleration. This is integrated with time to get inertial vertical speed. Inertial vertical speed is combined with pressure altitude (ALT) from the ADC after both have been filtered. This is the vertical speed output from the IRU.
Vertical acceleration provides fast reponse using the IRU accelerometers, and the long term correction for stability is provided by pressure altitude input from the ADC. The IRU must get valid pressure altitude from the ADC to compute vertical speed.
NOTE: The Altitude Rate [another output from the ADC] from the ADC is used to initialise the IRU vertical speed in ATT mode only. If the Altitude Rate is not available, the pressure altitude will be used for initialisation."
As I said, if you pull either the IRU CBs or the ADC CBs, this removes the IVSI.
If the FPV is removed if I pull the ADC CB's, I will be suitably humbled. I'll also check for the presence of the FPV during ATTitude mode.
Rgds
NSEU
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If the FPV is removed if I pull the ADC CB's, I will be suitably humbled.
As always, the guys who fix the aircraft are the last to know.
Cheers
NSEU
Quote from Denti:
On our 737 classics it sometimes had 1 or 2 kts difference after a 12 hour day, on our NGs not anymore, no matter how long the flight.
It was careless of me to assume that all pilot’s displays would use raw IRS data for GS, as does the A320 (unless things have changed recently). It may be that the B737NG is using GPS for the pilots’ displays of GS? Let’s stick with the readings quickly available on your IRS MCDU (or whatever it might be called on Boeings − the last Boeing I flew was the 707, with retrofitted dual-INS).
OK465,
Thanks for some more fascinating stuff about Alaska Airlines’s pioneering Cat 3A work, but you are still side-stepping the issue of the (non)-suitability of raw IRS data for FPA displays.
Whether this is displayed by HUD or EFIS (Air Inter A320s were fitted with HUD in 1988), is irrelevant to the inertial/baro argument. British Airways was one of the pioneers of continuous-descent non-precision approaches (SOP since the 1980s) and, when they obtained the A320 (courtesy of BCAL), the FPA made them that much easier and more reliable. The FPA FD function enabled us to fly them to MDA with AP, although many of us preferred to hand-fly using the raw-data bird. (Just for the record, we never level-off at the MDA.)
The fact that there is room for argument about how these relatively simple systems are organised on different aircraft types shows how poor is the documentation supplied by the manufacturers to pilots, and even engineers like NSEU.
Chris
On our 737 classics it sometimes had 1 or 2 kts difference after a 12 hour day, on our NGs not anymore, no matter how long the flight.
It was careless of me to assume that all pilot’s displays would use raw IRS data for GS, as does the A320 (unless things have changed recently). It may be that the B737NG is using GPS for the pilots’ displays of GS? Let’s stick with the readings quickly available on your IRS MCDU (or whatever it might be called on Boeings − the last Boeing I flew was the 707, with retrofitted dual-INS).
OK465,
Thanks for some more fascinating stuff about Alaska Airlines’s pioneering Cat 3A work, but you are still side-stepping the issue of the (non)-suitability of raw IRS data for FPA displays.
Whether this is displayed by HUD or EFIS (Air Inter A320s were fitted with HUD in 1988), is irrelevant to the inertial/baro argument. British Airways was one of the pioneers of continuous-descent non-precision approaches (SOP since the 1980s) and, when they obtained the A320 (courtesy of BCAL), the FPA made them that much easier and more reliable. The FPA FD function enabled us to fly them to MDA with AP, although many of us preferred to hand-fly using the raw-data bird. (Just for the record, we never level-off at the MDA.)
The fact that there is room for argument about how these relatively simple systems are organised on different aircraft types shows how poor is the documentation supplied by the manufacturers to pilots, and even engineers like NSEU.
Chris
I still don't understand it, but the FPVs did disappear
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Because the FPV (just like the VSI indication) cannot tell from IRS & accelerometer information alone when the aircraft has a zero rate of climb
As far as I know, the attitude of the aircraft is not corrected by baro (only by gravity over the longer term). From this, it seems I falsely deduced that the FPV circuits computed deviations from this frame of inertial reference.
Anyway, thanks for the insight (but I could do without the rolling eye icons, thanks)
Regards
NSEU
