View Full Version : Flight Path Vector - behavior

19th Sep 2012, 16:45

Could anyone confirm this. When initiating a turn, should the flight path vector be going in the opposite direction of the turn. Same things when pitching down abruptly, should the FPV be going up a bit?



Gonzo point
19th Sep 2012, 16:51
I?m not sure about what exactly you mean...could you be more precise.

Lord Spandex Masher
19th Sep 2012, 18:52
No. Twice.

20th Sep 2012, 22:58
Could you be more precise as where the FPV should point. From what I recall, when pushing on the stick the FPV moved above the command bar and then, as the descent was stabilized, it would come down.

Mercenary Pilot
21st Sep 2012, 00:43
In basic terms, the FPV should point in the direction the aircraft is travelling through the air (relative to the pitch and horizon indications on the Primary Flight Display).

So if you push forwards on the yoke, the aircraft will pitch down but initially the aircraft will still be travelling in the same direction due to its inertia, once the aircraft starts to descend in this example, the FPV indicator will start to move towards the 'Airplane symbol' on the PFD.

21st Sep 2012, 04:03
I still don't get the point, are you talking about the HUD or the PFD??
be more precise in your question helping other people been able to give you the correct reply .;)

21st Sep 2012, 10:57
"the FPV indicator will start to move towards the 'Airplane symbol' on the PFD."

I don't think this is possible. The FPV displays in pitch the angle of the Flight Path relative to the horizon. The a/c symbol displays a/c attitude. The FPV will be displaced sideways from the centre position on due to drift and this reflects the Track Line. We all know that B737 flies level between 6 degrees & 2 degrees nose up depending on speed. The FPV will be at ZERO degree on the PFD. The a/c will climb at low/medium levels about 7-10 degrees and the FPV will be significantly below that. It'll descend from high level 290kts with about 0 degrees; the FPV will be below that. On approach the attitude is 1-2.5 degrees and FPV is -3 degrees. In my experience it is impossible to stabilise the FPV on top of the a/c symbol.
I once saw a PPL'er in a fun sim session. He'd done the apptitude tests where you move a small square to follow a moving dot on a screen using a joy-stick. He tried to do the same with the FPV and a/c symbol similar to a Flight Director. It is impossible and he was very confused, plus we ended up in some very interesting positions.

21st Sep 2012, 11:24

Yes, all true, but what MP was saying is that, when you initiate a descent, the attitude leads the flight path initially, so the Attitude will indicate nose down BEFORE the FPV indicates a downward flight path. As the vertical speed inceases, the FPV will indicate a descent, thus moving "Towards the AC symbol".

It may or may not GET TOO the aircraft symbol (in most aircraft attitude is seldom the same as FPA, so it probably won't) but it will move in that direction.

21st Sep 2012, 12:31
Wiz, in level flight, the aircraft bars ride above the horizon, while the FPV sits on the horizon.

As you pitch down, the FPV drops below the horizon as the pitch bars approach it (and continue below it, if necessary).

So the pitch bars approach the FPV, then the FPV reacts and moves away from them (in a pitch down). Vise Versa in a pitch up.

Mercenary Pilot
21st Sep 2012, 12:56
WizofOz got what I was trying to say but in hindsight, using a descent was a bad example from me, as on a normal descent on a Boeing type would of course be made at around 0 degree pitch hence the FPV would actually move AWAY from the airplane symbol.

However Acheo mentioned an abrupt pitch down which put in my mind a steeper than normal descent i.e from level flight, push forward to pitch 5-10 degrees nose down, what happens to the FPV? It would move toward the airplane symbol.

I've over complicated this now haven't I? :p

21st Sep 2012, 14:33

No, no necessarily.

The FPVs show where the aircraft is going, the PFD where it is pointing.

Flight Path doesn't instantly change with attitude as it has to accelerate to whatever VS the new Attitude/Power combo will command steady state.

With sufficient rate you can have the nose below the horizon before there is any appreciable descent rate and thus descent path shown by the FPV.

It's an extreme example but think of something like Puchakovs Cobra- the attitude all over the shop while the Flight Path is basically stable.

No you CAN'T do that in a 737, but you can have a transitory state in which attitude is temporarily lower than the flight path.

It's all actually a little academic, as the attitude on the PFD is set to an arbitrary datum in any case.

21st Sep 2012, 16:39
You can buy a Garmin G-1000 trainer for $35 and play with the path vector to your heart's content.

Lord Spandex Masher
21st Sep 2012, 19:13
Guys read his questions again. Does the FPV move/point/go in the opposite direction? No it does not.

21st Sep 2012, 20:18

The FLP can be located on both the HUD or the PFD. Mine is located on the PFD.

To all,

Here's the issue I have. I was looking at the PFD in a coordinated stabilized turn while the FPV on my PFD was pointing a way inside the turn. It tought it was wrong since the flight path in a turn can be, at best, 90 degrees of the turn radius. With strong winds, maybe slightly inside the turn.

Now I'm testing my PFD in a SIL and I'm using various simulators which could send wrong inputs. I mentionned the "descent" scenario and most answers confirm was I already knew. My real problem is the "steady level turn" scenario.

I hope it clarifies my initial question.


21st Sep 2012, 22:23
The dictionary I'm looking at defines "Vector" as "a line representing a physical quantity that has magnitude, and direction in space as velocity and acceleration.
The aircraft attitude is in relation to the horizon. It could be indicating one degree nose up attitude but descending at a three degree attitude towards the earth. What you would see then is a slight nose up (one degree) attitude on the flight director and at the same time a three degree indication on the FPV. If you could attach a length of string to the nose of the 'plane and extend it to the point of impact on the earth, that would be the plight path vector. Forget about the shape of the 'plane. Think of it as a round ball. So the vector is the path of the ball through the air. It has nothing to do with the attitude of the 'plane.
If you look at the FPV as you fly with a cross wind in flight, you will notice the FPV off to one side, representing the vector of the cross wind. Nothing to do with the direction (heading) of the 'plane.

airline man
22nd Sep 2012, 08:52
acheo (http://www.pprune.org/members/53557-acheo) your observations are correct but use of the words "opposite direction" to describe the initial movement if the FPV is wrong. As someone said earlier, when u initiate a change in pitch or go off wings level into a roll, the aircraft takes a fraction of a second to actually change direction.

So take the "steep turn" exercise in the simulator. upon initiating a turn to the left, the fpv continues going straight momentarily with inertia but will start changing heading very soon. this delay is visible on the pfd by a slight increase in the gap between the aircraft symbol and the fpv. this isn't because the fpv is going away from you, but you are going away from the fpv.

during the turn, the fpv doesn't come to the centre of the pfd because there is a slight lag which is caused by the minor but constant difference in the direction of the aircraft longitudinal axis and the direction that the aircraft is actually flying.

however, when you stop banking and return to wings level, you will find that the fpv returns to the centre of the pfd. Or as per your observation, goes towards the direction of the turn. (no crosswind conditions of course).

So to sum up, you are going away from the fpv and it is trying to follow when you initiate any manouver.