FPV and FPV Cage

[aviator521]

Hi,

Can anybody tell me the difference bbetween Flight path Vector and Flight Path Caged vector.

It will be helpful if there are some images or documents about it.

[Checkboard]

The Flight Path Vector (FPV) is a bit hard to say in the cockpit, so it is referred to as "The Bird", and looks thus:

http://www.airsimmer.com/snap/ags/PFD-FPV.jpg

When The HDG V/S – TRK FPA pushbutton on the Flight Control Unit (FCU - i.e. the autopilot control panel) is pushed, the flight director, normally either cross bars or V-bars (depending on customer option), is replaced by the Flight Path Director (FPD), a symbol which looks like a set of extended wings.

http://i665.photobucket.com/albums/vv20/Checkboard/Picture2-2.png

The pilot's task is then to fly the FPV onto the FPD bars, so that they look like one symbol:

http://vas.philippmuenzel.de/vasfmc_birdy.jpg

Calling the FPV "caged" isn't a term I have heard is this respect, but may be a colloquial term in some companies.

[BOAC]

Also known as 'the budgie' so perhaps the 'caged' is a light-hearted expression to do with birds? A bit like 'strangle the parrot' and 'shoot the rabbit'.

Perhaps when the FPD overlays the FPV?

[shortfuel]

Hello Checkboard,

Do you mind me asking where does your last screenshot come from?

That FMA looks completely awkward to me (i.e. the boxed HDG when FPV and D are ON and few other things...!)...

[Denti]

That description must be airbus ;)

On the 737 the FPV does not replace the FD and both can be used indepently of each other. And the FPV is of course switched on and off on the EFIS control panel. Anyway, no caged FPV on the PFD available, only on a HUD.

PFD Indications change a bit with activated FPV though, a track indication appears on the horizon line and selected course will be displayed as a small mark on the horizon line.

[Checkboard]

shortfuel - I just grabbed it from google images, I didn't even look at the site. Yes, it looks like a shopped image, the horizon should be at an angle, not the FPD etc etc.

Denti - yes, an airbus explanation. :)

[Checkboard]

The F18 (as in the twin tailed fighter) FPV can be caged by a pilot selection, in which case the drift element is removed, in order for the pilot to better determine angle of attack. In this case, when the drift exceeds a certain limit, a "ghost" FPV is also displayed.

[Denti]

OK465, very nice explanation. And yes, i thought about that case as well and honestly do not know how our FPVs react in such a situation. It was always an advise for us to use the FPV during unreliable airspeed/altitude flight as it is IRS generated and makes for a very easy way to see if you fly level or not, same for descending or climbing actually.

[Chris Scott]

aviator 521,

Checkboard's fine graphics show the FPV FD as provided on Airbus PFDs from the A320 (certificated 1988) and onwards.

The first EFIS PFD on Airbuses were on the A310 (certificated 1983) and A300-600. This includes a FPV symbol ("Bird") in the same form, but the FD is completely different from the A320 (and inferior). It consists of 4 dashes, arranged something like this:

|
_......._

|

(Sorry for the cheap graphics) :}

This was usually referred to as "the Cage", the idea being to fly "the Bird" into it. Perhaps this is what you are looking for. If you are unable to access an A310 Tech Manual, I might be able to dig-out mine. But OK465's idea may be the answer.

Chris

[fmgc]

Doesn't the A300-600 have the bird caged rather than a moustache?

[tubby linton]

The A300-600 does indeed have the cage .It looks like a hollow + sign.The FPA is set by the efis control panel and the course is set either by the selected VOR or ILS course.Unlike the FBW aircraft it cannot be directly controlled by the autopilot..

[Chris Scott]

Quote from fmgc:
Doesn't the A300-600 have the bird caged rather than a moustache?

Quite... :{ Like I said, delete the "moustache" ("........."). (The A310 and A300-600 are identical.)

EDIT
Have now used colour to camouflage the moustache and add realism.

tubby linton,
According to my 1984 A310 tech manual, the proportions are about right?
Regards,
Chris

[fmgc]

Ooops, sorry Chris!!

[Microburst2002]

Use of FPV is to be avoided if the reason for the unreliable speed are the static ports. Many simulators cannot simulate this failure, they only simulate pitot freezings and you may come to the conclusion that the FPV always gives a good indication.

Which makes me think of a question: in the ureliable speed procedure we have to level off and use the tables and then be able to troubleshoot. But how can we be sure that we are maintaining an altitude? If we have GPS available we can check the GPS page, but if not, how can we rule out a static problem?

[Denti]

Microburst, is that an airbus fcom advise? Just asking as we were told on the boeing that the FPV is purely IRS driven, no air data input at all and therefore a good tool (however pitch and power is primary) to be used in IAS disagree situations.

[shlittlenellie]

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.

This is contradicted by the QRH which states:
The flight path vector is based on inertial sources and may be used as a reference in maintaining proper path control. (C) Boeing.

[Checkboard]

Microburst, is that an airbus fcom advise?

Well, I'm not Microburst, but I may be able to help ;)

A320 series QRH: Page 2.16

UNRELIABLE SPEED INDICATION/ADR CHECK PROC

...


If remaining altitude indication is unreliable :
- Do not use FPV and/or V/S, which are affected.


Although the FCOM states that the FPV is an output of the IRS:

The system includes :

- three identical ADIRU's (Air Data and Inertial Reference Units).

Each ADIRU is divided in two parts, either of which can work separately in case of failure in the other :

• the ADR part (Air Data Reference) which supplies barometric altitude, airspeed, mach, angle of attack, temperature and overspeed warnings.


• the IR part (Inertial Reference) which supplies attitude, flight path vector, track, heading, accelerations, angular rates, ground speed and aircraft position.

It appears that the vertical element is barometrically aided in order to refine the path (i.e. the IRS vertical path is subject to drift, just as the horizontal navigation is, and so isn't accurate enough over time to drive a "pure" IRS-driven vector.)

[Microburst2002]

denti, yes it is specific airbus

thankyou checkboard

it seems however that in boeing you have to disregard it too, when static is erroneus.

It seems that they could easily develop a tool for unreliable speed situations based on inertial so that at least we can level off and maintain an altitude so we can troubleshoot the faulty indication/s.

[Checkboard]

The airbus unreliable speed QRH drill tells you to call up the GPS altitude on the MCDU - we all know where that little bit of information is?

[shortfuel]

they could easily develop a tool for unreliable speed situations based on inertial so that at least we can level off and maintain an altitude so we can troubleshoot the faulty indication/s.

That tool already exists: about 2.5° ANU and 77% N1 (Airbus twinjet FBW at CRZ lvl). Then you have all the time you need to try to understand what it is going on in the cockpit...
Disregarding/rejecting the erroneous indicated data are the most difficult and resource consuming parts here. And I am not talking about troubleshooting yet but just about the mental process involved and required by the pilot to decide he now needs to set the a/c in this safe configuration before doing anything else!

IMHO, procedure and training are specifically lacking here (i.e. immediate actions in case of unreliable data (speed and/or altitude) at CRZ level).

[Checkboard]

IRS is currently summed and weighted with GPS & DME/DME (when available) inputs and corrections applied.

I'm afraid not, OK465 - The FMS position is updated, by comparing the GPS position to the IRS position - but the IRS position remains as calculated from the last alignment.

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.
Also innacurate, I'm afraid - accelerometers measure acceleration, that is changes is velocity, and not velocity itself. Tiny errors in the calculated velocity build up to an incorrect value (known as drift), a change in acceleration can only adjust from that incorrect value.

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.
Errors are small, but they are cumulative. Errors from the alitimeter are not cumulative - so it appears that the system uses baro inputs to zero the vertical calculated flight path, and the IRS accelerometers then have a new, accurate, vertical velocity to work from.

[Checkboard]

Thanks for the physics - the definitions may be helpful to someone. ;)

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.

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.

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.Yes, all of the IRS platforms I have flown can be updated in flight - but it isn't usually (or automatically) done.

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.
I never said this - the barometric refinement isn't to the IRS - it's to the system's calculation of the FPV.

Trust me. ;)

[Microburst2002]

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.

[Checkboard]

I said instantaneous inertial velocity is available from inertial accelerometers.
Calculated velocity is available from the Inertial Reference System.

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).

[NSEU]

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

[Checkboard]

Why would you need long term error correction on an instrument of this nature?
.. because the long term which the aircraft has been flying before you want to use the FPV will have introduced an error in the presentation. :rolleyes:

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?
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.
(and couldn't it be done electronically anyway, in the IRU?) It is! This is what I am talking about - an electronic correction to the FPV and VSI presentation, using baro information.

[Chris Scott]

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.

[wiggy]

It cannot be re-aligned on the hoof. (How did the Apollos ever get to the Moon?)

Thread drift and a "AFAIK" but they used a combination of radio tracking, inertial nav ( especially for dynamic manoevres), and the platform(s) were realigned on the hoof, using old fashioned astro...(but the main navigator was above all Newton's laws).

[Denti]

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.

Dunno, the VSI in our 733s was called an IVSI, however the airplane was of course fitted with two IRS (strap down, not a platform of course).

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

Afraid not, it will always read 0, nothing else. 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.

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?

There is no warning in our checklists. In fact there is the following info in the IAS disagree checklist. Nothing at all in the ALT or AoA disagree checklists (AoA disagree leads to IAS and ALT disagree).

Additional Information
The flight path vector is based on inertial sources and may be used as a reference in maintaining proper path control.

[Checkboard]

Afraid not, it will always read 0, nothing else.
On the brand new A320 series I fly now, checking the difference (and there is one - a knot or two after a couple of hours) is part of the shut-down checklist to determine if it is necessary to re-align the IDIRS, or even to report them in the Tech-Log.

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.

[NSEU]

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.


I think our definitions of IVSI's differ. I call the vertical speed display on the PFD an "IVSI".

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

[NSEU]

If the FPV is removed if I pull the ADC CB's, I will be suitably humbled.

OK, I'm suitably impressed. I still don't understand it, but the FPVs did disappear when I pulled all the ADC CBs.

As always, the guys who fix the aircraft are the last to know.

Cheers
NSEU

[Chris Scott]

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. :ugh:
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

[Checkboard]

I still don't understand it, but the FPVs did disappear
Because the FPV (just like the VSI indication) cannot tell from IRS & accelerometer information alone when the aircraft has a zero rate of climb.

[NSEU]

Because the FPV (just like the VSI indication) cannot tell from IRS & accelerometer information alone when the aircraft has a zero rate of climb

I (originally) thought the beauty of the FPV was that it was independent of the baro system.

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

[Checkboard]

Did I put one in? I can't see it. :confused: