Flight path vector

[JOBBA]

Hello! Can anyone please give me any info on how the flight path vector works (what it does & how you use it)? Its specifically the system on the 737ng although I would imagine the A/C type makes no difference! Many thanks in advance!

[FullWings]

As far as I understand it, the (Boeing) FPV is derived mostly from IRS output, thus allowing an instantaneous display of Flight Path Angle and drift information.

The FPV consists of a small aircraft symbol which moves over the 'attitude indicator' part of the PFD. If you were stationary on the ground, it would be on the horizon line and centered in the display. It shows the actual angle of climb/descent referenced to the Earth's surface.

If you took of in a 15Kt crosswind and rotated to 20degs pitch angle, you would see the 'Airplane Symbol' (pitch bar/wing bar) at the 20deg mark. Underneath it (normally!) and to one side would be the FPV, indicating maybe 13degs FPA and a bit of drift.

When you next levelled off, you would see the pitch bar at an attitude commensurate with your configuration (speed, weight, flap, etc.) but the FPV would be on the horizon line, indicating level flight.

When you descended on an ILS (assuming you followed the glidepath exactly), the pitch bar would move around quite a bit during flap/gear extension and speed changes but the FPV would stay at 3degs ND (or whatever).

That's the basics.

I find the FPV a very useful bit of kit and here's some reasons why:

It allows you, at a glance, to assess the performance of the aircraft. If the FPV is in the blue part of the AI, you are definitely going up. Vice-versa when you are 'in the brown'.

If you are unlucky enough to have a bad windshear encounter, the first instrument to warn you will be the FPV (assuming the PWS hasn't gone off) as it assumes an unusual position (drops away or rushes up). This is because your other cues (altitude, ROD, airspeed) have significant lag before they show the true picture. It also provides, as mentioned in the previous paragraph, an instant answer to "are we going up or down?"

It is a very good tool on non-precision approaches, especially once you have gone visual, or at night. Many airfields have no (visual) slope guidance and quick glances at the FPV will help stop deviations early.

You can set FPA on the MCP and get a sort of 'mini flight director' which brackets the place where you want the FPV to be to achieve the desired FPA. This is useful if you want a 3deg slope as the AI graduations are in 2.5deg increments...

Height keeping when flying S&L becomes simply a task of keeping the FPV on the horizon. You also become much more accurate as the other instruments (including the FD) only steer you back once you have made a deviation. The FPV registers the smallest trend immediately.

A 'fun' use is with crosswind landings. If you take a peek at the FPV every now-and-then (assuming use of the 'wing down' method), when you are fully aligned with the runway the FPV will be in the centre.

This is all from using the 777 FPV. I am told that the 737NG one is the same but I don't know if this is true...

Happy vectoring!

[JOBBA]

Very kind of you 'FULLWINGS'most comprehensive and helpfull.

[Port Strobe]

Is there usually some sort of a scale on the horizon in a/c fitted with a FPV to give an indication of drift? How accurate is it if (i.e every degree, two degrees etc) so and what value would it extend to?

Thank you.

[FullWings]

"Is there usually some sort of a scale on the horizon in a/c fitted with a FPV to give an indication of drift? How accurate is it if (i.e every degree, two degrees etc) so and what value would it extend to?"

On the 777, no - I don't know about other types.

I have never seen an azimuth scale on an AI, though I believe the drift representation is linear. I haven't found what 'full-scale deflection' is from the manuals but the FPV was starting to go off the edge of the display in a 200Kt crosswind the other day!

[Lemurian]

have never seen an azimuth scale on an AI, though I believe the drift representation is linear. I haven't found what 'full-scale deflection' is
On the '320 family,there is indeed an azimuth scale on the AI,matching the heading scale below.
The drift is directly read against that scale exactly like the flight path angle is read against the pitch scale on the ADI.
Therefore the drift max deflection is 25 degrees,reducing somewhat at pitch angles greater than 10 degrees up or down,due to the curve of the attitude indicator.

[FE Hoppy]

Those of you who will fly the EMB 170/190 in the near future will fall in love with the Flight Path Angle which is used in all phases of flight except Take Off. It is angle not vector so you still have to ref the HSI for drift but it is still very nice.

[earnest]

My experience of the Airbus FPV is that it is not as "instant" as Full Wings suggested, based on his B777 experience. (Perhaps the Boeing, being newer, has faster computers to figure it all out - the rest of his reply was pretty comprehensive). The Airbus' FPV have a bit of lag and wander - only a bit, mind - so it should be used as a performance instrument, ie set the pitch attitude with th AI, let it settle, then confirm that you are getting the flight path and track you require. It's a great tool but you can be suckered into "chasing" it and therefore not quite settling down on a desired flightpath. Any other 'Bus drivers agree or is it just my shaky hands?

[Port Strobe]

Thanks for the replies, after Lemurian's post I took a quick look on airliners.net and this is the best illustration I could find of what he said

http://www.airliners.net/open.file/437763/L/

It does show very clearly the scale on the AI, with the markings every 10 degrees of drift and how they line up with the HI below, with 25 degrees max deflection either side (time to go around maybe if the FPV wanders off the edge on approach??:) ). I couldn't find a close up with the TRK/FPA on, they're all "out the window" landing shots, but at least this shows the idea.

Thanks again for all the replies.