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A Few FPV Questions

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1. I've noticed when using the FPV with the HUD that sometimes the smaller inner circle (don't know what that is called) which you are supposed to center inside the FPV symbol, does not appear. Why is that, is there something I need to do so the inner circle appears? Does it depend on a MCP mode?

 

2. The FCTM says:

"Note: When on final approach, the FPV does not indicate airplane glide path relative to the runway. ILS or GLS glide slope, VASI/PAPI or other means must be used for a proper glide path indication".

This surprised me because I had thought the FPV would serve as a type of Flight Director when using the HUD since there is no other FD type guidance on the HUD during an approach (or if there is I don't know how to activate it). So the FPV is not used during approaches by RW 737NG pilots?

 

3. How is the FPV typically used by RW pilots?

 

Thx,

Al

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Not a RW pilot;

 

But the FPV can also be used during raw data or NPA. Once you are established on the approach, just place the FPV 3° below the horizon to maintain a 3° FPA.

 

FLIGHT PATH VECTOR (FPV)

 

The FPV symbol represents airplane flight path angle vertically and drift angle laterally. The FPV 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 pitch scale’s center. The FPA uses inertial and barometric altitude inputs. The vertical FPA is unreliable with unreliable primary altitude displays.

 

The FPV symbol is displayed in two brightness levels. The FPV symbol is displayed dim when either the Flight Directors (FD) or a TCAS RA is displayed. The FPV symbol is displayed bright when FD’s are off and there is no TCAS RA.

 

The vertical axis shows FPA relative to the horizon. The lateral axis shows the drift angle relative to the center of the pitch scale on the ADI. The amount of drift cannot be determined from the display.

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I had thought the FPV would serve as a type of Flight Director when using the HUD

 

Too bad you didn't reference your quote... a page number would be helpful in understanding your question. Anyway..., Ch 10 Sec 42 beginning on pg 10.42.1 has every symbol used in the HGS described. The flight path symbol that you are referring to as a FPV is not analogous to the airplane marker or steering bars/single cue you see in a flight director.  It is analogous to the circle that appears on the PFD when you turn on the FPV.  Displays the actual flight path vector of the aircraft.  The small inner circle is called the flight director cue, it is in fact the flight director much like the single cue in the PFD.

 

The problem for beginning HGS users is to chase the cue with the flight path symbol.... you'll end up overshooting pitch and roll.  You have to look at the whole picture and include the actual aircraft pitch and roll in your focus.

 

Most of all, it takes both knowledge of what the symbols mean (use the FCOM) and practice.  Once you are used to it you end up missing the HGS in aircraft not equipped.

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To cover (3), in the RW the FPV is largely based on personal preference. Many have it on all the time - I generally have it turned off for takeoff, as the vertical climb path is largely inconsequential - in the takeoff we're flying an attitude to give us our desired performance for configuration and thrust, whether the aircraft is climbing on a 10 degree or a 12 degree path is not really important, and if we need to meet a tight altitude constraint, the VSD is the best tool to let you know how you're performing. I'll turn it on in the case of any engine problems - it's further confirmation one-inop that we're maintaining a positive climb.

 

Where the FPV is very useful is during raw data approaches, or visual maneuvers without FD guidance. Unlike light aircraft, in a jet it's very hard to accurately fly a profile based on the picture out the windshield. For instance, on a level downwind in the circuit/pattern, we might be at flap 5, 6 degrees nose up at 60% N1. In this case we use the FPV to assist us in flying level (by sitting it on the horizon). Once it's time to descend on a 3 degree path, regardless of attitude, thrust or configuration, sitting the FPV at -3 degrees will show us we're flying a 3 degree slope.

 

In short, it's confirming that the attitude and thrust you're using for the phase of flight is giving you the desired flight path angle.

 

Mike Archer

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Thanks for the inputs -- very helpful.

I still don't understand the following. On an ILS or RNAV LPV approach, what inputs are driving the HUD Guidance Cue (small circle) -- in other words, what flight profile will the a/c fly by closely following the Guidance Cue? By keeping the Guidance Cue centered inside the FPV symbol will the GS and LOC diamonds stay centered?

Thx,
Al

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Thanks for the inputs -- very helpful.

 

I still don't understand the following. On an ILS or RNAV LPV approach, what inputs are driving the HUD Guidance Cue (small circle) -- in other words,  what flight profile will the a/c fly by closely following the Guidance Cue (i.e., by keeping the Guidance Cue centered inside the FPV symbol).

 

Thx,

Al

The Guidance Cue is the equivalent of the Flight Director in the HGS. It will give lateral and vertical guidance based on your Pitch and Roll modes, the same as the FD on the PFD, with one key difference - on the HGS it's giving flight path vector information, whereas on the PFD it's giving attitude (pitch and roll) information.

 

To clarify using an example - if you're on the ILS and your modes are VOR LOC and G/S, the FD will be commanding, say, 1.5 degrees nose up to maintain the glideslope. The HGS guidance cue, however, will be commanding a flight path of -3 degrees, to achieve the exact same result. So think of them both as flight directors, driven off the same data, but representing their commands in different ways (attitude vs. flight path).

 

Hope that makes sense.

 

Mike Archer

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Mike,

 

Thanks -- your example clarifies things for me.  I wasn't sure if the HGS Guidance Cue would be getting localizer and glide slope inputs.

 

Al

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To clarify using an example - if you're on the ILS and your modes are VOR LOC and G/S, the FD will be commanding, say, 1.5 degrees nose up to maintain the glideslope. The HGS guidance cue, however, will be commanding a flight path of -3 degrees, to achieve the exact same result. So think of them both as flight directors, driven off the same data, but representing their commands in different ways (attitude vs. flight path).

 

 

And so if I understand correctly, another difference between a FD and the FPV/guidance cue in the case of an ILS approach is that when the FD is 'centered', it may just mean you have the correct attitude 'corrections' set to get back on the desired approach path, while if the FPV is centered around the guidance cue, you definitely are on  the correct path. In either case, I would keep a close eye on the localizer and glide slope indicators. 

 

Al

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And so if I understand correctly, another difference between a FD and the FPV/guidance cue in the case of an ILS approach is that when the FD is 'centered', it may just mean you have the correct attitude 'corrections' set to get back on the desired approach path, while if the FPV is centered around the guidance cue, you definitely are on  the correct path. In either case, I would keep a close eye on the localizer and glide slope indicators. 

 

Al

You are correct in your statement that if the FD is centered, this may be commanding a correction to get back on path - the guidance cue is no different, except it's shown as a flight path correction rather than an attitude correction.

 

Example: if you're one dot low on slope, the FD will command a nose up attitude change to regain the slope. Similarly, the guidance cue will command a shallower flight path (maybe 2 degrees instead of 3) to achieve the same. Once back on slope, the FD will command attitude to maintain on slope, and the guidance cue will do the same with flight path. You're also correct in that the LOC and GS deviation needs to be continuously monitored regardless.

 

In really simple terms, the FD tells you where the aircraft should point, and the guidance cue tells you where it should fly, and this includes corrections to path.

 

Mike Archer

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Mike,

My point was that if the guidance cue is within the FPV circle, it is not commanding a correction, put indicates you are flying where you should be. It is when the guidance cue is outside the FPV circle that it is commanding a correction. Correct?

Thx,

Al

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Mike,

My point was that if the guidance cue is within the FPV circle, it is not commanding a correction, put indicates you are flying where you should be. It is when the guidance cue is outside the FPV circle that it is commanding a correction. Correct?

Thx,

Al

If they're not aligned, yes, you need to correct your flight path so they are aligned. If they are aligned, yes, you're flying where you should be, but you're not necessarily on the approach path yet, if you've somehow left it.

 

The ONLY actual indication that you're on the localiser and glideslope are your LOC and GS deviation scales (or equivalent for non-ILS). Having the cue inside the FPV does not indicate that you're "on slope". If you keep them aligned though, you *will* end up on slope.

 

Another example: you've somehow ended up really low. You immediately correct attitude so the guidance cue is inside the FPV. You're still very low, but if you keep them aligned, the aircraft will return to the desired approach path, same as if you were following the FD bars.

 

Hope I'm not muddying things up for you.

 

Mike Archer

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It might be a good exercise for the student to fly the ILS approach using only GS and LOC deviation indications, no FD and no A/P.

 

Then add the FD and see exactly what it is helping you do.  I cannot think of any better way fully grasp the concept.

 

Once you are proficient with those two, then lower the HGS and give it a try.  You'll go ah-ha!

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The student has been an instrument rated RW pilot for more years then he would like to remember and is quite familiar with FDs in small GA a/c. It is understanding how using the FPV/guidance cue compares to using a FD that is the issue.

 

So what I understand now is that centering the FPV over the guidance cue is analogous to 'centering' a FD. In addition, the FPV 'points' to where the a/c is currently heading.

 

Thx all,

Al

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So what I understand now is that centering the FPV over the guidance cue is analogous to 'centering' a FD. In addition, the FPV 'points' to where the a/c is currently heading.

Thx all,

Al

Exactly!

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It is understanding how using the FPV/guidance cue compares to using a FD that is the issue.

 

Well...., not exactly.  Close to be sure but as pointed out the FD steering commands do not provide actual deviation (they do not replace the GS/LOC deviation pointers) but they guide pitch and bank to the pitch and bank that results in the desired flight path.

The flight path symbol is always presenting the actual path, not pitch and bank, and the FD cue guides the actual flight path to the desired path. The difference is subtle but important because neither path symbol or cue represents actual pitch and bank, which is why I first noted these as important parts of your focus when using the HGS.  A focus on trying to keep the cue centered in the flight path can lead to over controlling, and when that happens just remember pitch plus power equals performance (the old instrument training adage beat into me) and set pitch where you think it belongs and you'll overcome the over controlling.

 

Certainly didn't mean to imply you didn't have instrument qualifications, I could have worded that better.

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The key point to be understood here (which I believe now is) is that the guidance cue IS used in the exact same fashion as a conventional flight director (if you were responding to me saying "exactly"). You're very correct in saying that following the HGS cue is a different game to following the FD bars, but they're both command guidance to be followed to recapture or follow the desired flight path, which is where the confusion lay before.

 

From the HGS manual: "The guidance cue symbol is the "Flight Director". It functions in the same way as a single cue Flight Director, but is designed for control of flight path. For the pilot, the objective is to capture the guidance cue inside the Flight Path circle using pitch and roll control inputs".

 

Mike Archer

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For the pilot, the objective is to capture the guidance cue inside the Flight Path circle using pitch and roll control inputs".

 

Yup, agree. You have to manipulate pitch and roll to achieve a flight path that captures the cue ball. My emphasis is on understanding how one does not manipulate path directly.

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