I often get asked what the FPV button does on the EFIS unit. Pressing the button doesn’t do anything grand or remotely obvious, unless you’re observant and note that an oddly shaped circle with lines has instantly appeared on the Pilots Flight Display (PFD) above the attitude bar.
So what does the Flight Path Vector (FPV) do and how can it improve the accuracy of your flying.
LEFT: FPV button located on the Electronic Flight Instrument System (EFIS) unit. The FPV button is duplicated on the First Officer EFIS unit for redundancy. This is the EFIS produced by CP Flight (Pro model).
My understanding is that the data received by the FPV is derived mostly from the Internal Reference System (IRS) of the aircraft, thus allowing an instantaneous display of Flight Path Angle and drift information.
The FPV consists of a small circular 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 off in a 15Kt crosswind and rotated to 20 degrees pitch angle, you would see the pitch bar/wing bar at the 20deg mark. Underneath the bar and to one side would be the FPV, indicating maybe 13degs attitude and a bit of drift caused by the crosswind.
When you leveled off, you would see the pitch bar at an attitude commensurate with your configuration (speed, weight, flap, etc.) and the FPV would be on the horizon line, indicating level flight.
When you descended on an ILS (assuming you followed the glide path exactly), the pitch bar would move around quite a bit during flap/gear extension and speed changes but the FPV would stay at 3 degrees ND (or whatever).
The last comment is particularly important when flying an approach, as the FPV will provide greater accuracy in determining angle of attack than the pitch bar.
That's the basics.
I find the FPV a very useful tool 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, other than the PWS audio warning, will be the FPV as it assumes an unusual position (drops away or rushes up). The other visual cues used (altitude, ROD, airspeed) have significant lag before they show the true picture, but the FPV should be more or less immediate. 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 the angle needed in the FPA by using the Main Control Panel (MCP) and get a sort of 'mini flight director' This is useful if you want a 3deg slope.
- Height keeping when manually flying straight and level is simply a task of keeping the FPV on the horizon. The FPV registers the smallest trend immediately, where the flight director (FD) will only correct an issue after a deviation has occurred.
- Another use of the FPV is during crosswind landings. If you look at the FPV as part of your usual instrument scan and you are approaching the runway using the wind down method, the FPV will provide visual information to whether you are correctly aligned with the centerline of the runway.
The Flight Path Vector is a small unobtrusive icon that pays large dividends when used correctly. Not only can this device warn you of impeding problems but it can be used to facilitate accurate flight in a number of condition which include approach, straight and level flight and crosswind landings.
BELOW: Schematic of the Flight Path Vector showing how it relates to aircraft axis, angle and drift.