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The purpose of FLAPS-2-APPROACH is two-fold:  To document the construction of a Boeing 737 flight simulator, and to act as a platform to share aviation-related articles pertaining to the Boeing 737; thereby, providing a source of inspiration and reference to like-minded individuals.

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Saturday
Jul072012

Flight Path Vector (FPV) - Explanation and Use

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 are observant and note that an oddly shaped circle with lines has instantly appeared on the Primary Flight Display (PFD).

LEFT:  FPV button located on the Electronic Flight Instrument System (EFIS) unit on the Captain and First Officer side.  EFIS unit produced by CP Flight (Pro model).

What is the Flight Path Vector and What Does it Do - The Basics

The FPV is a small circular symbol which, when the FPV button on the EFIS is depressed, superimposes over the Attitude Indicator (AI) part of the Primary Flight Display (PFD). The circular symbol represents the aircraft's axis in relation to the vertical and lateral movement referenced to the Earth's surface.  If you were stationary on the ground, the circle would be on the horizon line and centered in the display.

The data received by the FPV is derived mostly from the Internal Reference System (IRS) of the aircraft; therefore, the Flight Path Vector provides an almost instantaneous display of flight path angle and drift information.

For example, if an aircraft took off in a 15 Knot crosswind the Flight Director (FD) bar would register the pitch of the aircraft while the circular FPV would be located above the horizon and to the right or left.  The lateral deviation of the FPV provides a visual indication of drift caused by the crosswind, while the vertical deviation shows the aircraft's attitude or pitch.

When the aircraft changes from climb phase to level flight, the FD bar is commensurate with the configuration of the aircraft (speed, weight, flap, etc.) and the FPV would be on the horizon line, indicating level flight.

Decending in approach phase on a 3 degree glidepath, the position of the FD and Horizon Heading Scale (aircraft symbol bar/pitch bar) is  dependent upon the speed, flap and gear extension, but the position of the FPV will stay at 3 degrees, unless the flight controls are used to alter the aircraft's pitch. 

The FPV will provide greater accuracy than the Horizon Heading Scale as it does not 'lag' behind real time as other instruments can do; therefore, it is sensible for flight crews to include this tool in their routine scan.

LEFT:  Flight Path Vector (FPV) in ProSim737 avionics suite.  The FPV symbol is in the blue part of the Altitude Indicator and reads roughly 2.5 - 3 degrees nose up.  The aircraft is in TO/GA command mode ascending at 1100 feet per minute to flaps up speed.  There is no crosswind so the symbol is does not show a deviation (drift) from center.

Boeing provides a caveat in their literature, stating that the FPV is not a primary flight instrument.  Therefore, information displayed by the FPV should be used to augment data from the primary instrumentation.

Flight Path Vector (FPV) Advantages

  • It allows you, at a glance, to assess the performance of the aircraft. If the FPV is in the blue part of the Primary Flight Display, you are definitely ascending. Vice-versa when you are 'in the brown'.
  • If you are unlucky enough to have a windshear encounter, the first instrument to warn you other than the  aural warning will be the FPV as it assumes an unusual position (drops away or rushes up). The other instruments (altitude, vertical speed and airspeed) have significant lag before they accurately show the true picture of what is occurring, but the FPV provides an almost immediate indication (live-time). 
  • It is an ideal tool to use during non-precision approaches as it provides the flight crew with additional situational awareness, especially during night operations.
  • The FPV is an ideal tool to gauge the accuracy with which the aircraft is flying a glideslope and can be used to cross check against other information.
  • The FPV is an ideal tool to monitor non-automation phases of the flight (manual flying) as the flight crew need only to keep the FPV on the horizon to maintain level flight.
  • The FPV registers the smallest trend almost immediately, while the flight director (FD) will only correct an issue after a deviation has occurred. 
  • The FPV can be used to provide additional information during crosswind landings. If you look at the FPV as part of your usual instrument scan, the FPV will provide visual display to whether you are correctly aligned with the centerline of the runway (the FPV will display the drift).

The last point requires expanding upon, as the FPV can be used to determine the correct rudder deviation to use when using the sideslip method for a crosswind approach and landing. A crosswind will push the FPV circle in the direction that the wind is blowing TO. Rudder inputs will cause the FPV symbol to move towards the the center of the Altitude Indicator.  Once the the FPV is centred in the Altitude Indicator, the aircraft is aligned correctly (no drift).

The Flight Path Vector is a small unobtrusive icon located on the PFD that pays large dividends when used correctly.  Not only can this device warn you of impeding problems but it can be used to facilitate greater flight accuracy in a number of conditions including 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.

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