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Mission Statement 

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.

I am not a professional journalist.  Writing for a cross section of readers from differing cultures and languages with varying degrees of technical ability, can at times be challenging. I hope there are not too many spelling and grammatical mistakes.

 

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I use the words 'modules & panels' and 'CDU & FMC' interchangeably.  The definition of the acronym 'OEM' is Original Equipment Manufacturer (aka real aicraft part).

 

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Monday
Jul232012

Vertical Bearing Indicator (VBI) - How To Calculate A Controlled Idle Descent

Often you are requested by ATC to alter altitude, or must intercept a desired point in space at a certain altitude for operational reasons. There are several methods available to the pilot to initiate the change in altitude; outlined below are three methods.

A: Initiating Level Change or Vertical Speed on the MCP will activate an advancing and contracting green line arc (Altitude Prediction Line) on the CDU.  This green arc identifies the location that the aircraft will reach ,if the vertical speed is maintained, in relation to the active CDU waypoint.

B:  You can calculate the distance and vertical descent using mathematics, but this can be cumbersome and may illicit possible mistakes. 

C:  You can alter the LEGS page of the CDU keying in the new altitude constraints (this assumes you are using VNAV & LNAV.

The CDU Vertical Bearing Indicator (VBI) can help you.  The VBI is basically an angle calculator that provides "live" vertical speed information based upon a desired descent angle.  An example using the waypoint TESSI is provided.

  • Navigate to Descent page by pressing the DES key.
  • At lower right hand side of the DES page you will see the following: FPA, V/B, V/S.  This is the Vertical Bearing Indicator.
  • Key RSK3 (right line select 3) and enter the waypoint and altitude (TESSI/17000)

The VBI provides 3 fields: 

  • FPA (Flight Plan Angle) is the vertical path in degrees that the aircraft is currently flying.
  • V/B (Vertical Bearing) is the vertical path in degrees that the aircraft SHOULD be flying to reach the keyed waypoint (TESSI/17000).
  • V/S (Vertical Speed) is the vertical bearing (V/B) converted into vertical speed for easy input into the MCP.

Observe the V/B.  The idle descent in a B737 is roughly 3.0 degrees (PMDG use 2.7 degrees)

Wait until the V/B moves between 2.7 and 3.0 degrees (or whatever descent angle you require)

When the value is reached, dial in the required altitude and indicated Vertical Speed on the MCP

The Altitude Prediction Line will now intersect the selected waypoint (TESSI) and the aircraft should fly a perfect idle descent to TESSI.  Note that the original altitude selected for the pinpoint in the LEGS page does not reflect the new change.

Benefits

One of the advantages in using the Vertical Descent Indicator is that the pilot can instigate an accurate controlled idle descent, following a desired glide path to the desired waypoint.  This advantage can be used in a number of scenarios:

  1. Descent from cruise altitude.
  2. Approaching the runway from a straight-in approach course.
  3. Approach the runway from base or via an ARC approach.
  4. Approaching the runway for a downwind approach.

I often use the VBI from FL10 to FAF on approach, when other constraints are not required.

Video

I’ve made a short video showing the procedure. 

In the video, TESSI has been selected from the LEGS page and downloaded to the scratchpad.  Pressing DES opens the required page where the VBI resides.  In the scratchpad, the altitude constraint is entered for the waypoint – TESSI/17000 and uploaded to the WPT / ALT section of the Vertical Bearing Indicator (right line select 3). 

If you watch the indicator you will see the V/B and V/S changing as the aircraft approaches TESSI. 

Select the new altitude and vertical speed on the MCP (17000 & 780 - or nearest numeral) and you will note the FPA begins to change, indicating the new vertical path of the aircraft.  The Navigation Display (ND) will then show the Altitude Projection Line moving towards and stopping at TESSI.  The aircraft will now descend at the nominated angle of descent until reaching TESSI.  Note that the original altitude in the LEGS page does not reflect the new change.

To read a later post on the VBI, click here.

  • CDU is Flight Deck Solutions (FDS) and MCP is CP Flight (Pro model).  Double click video to view full screen.

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Reader Comments (2)

Ian,

Great tip and video! Will give this a try next time I'm flying. In the past I've done this manually, adjusting the V/S until the APL intersected the waypoint. This approach is far better.

Please keep up the work on these guides - very valuable.

FlyWJP

July 24, 2012 | Unregistered CommenterFlyWJP

Thanks for the note flyWJP.

Building has been slow due to some supply hassles with replacement parts (long story). Therefore, I decided to post some guides - it acts a good review for me too.

I was wondering what people thought, so your comment came in at the right time.

I have a selection of guides I've been working on. I'll use them as fillers between buidling posts.

Note, the the VBI is very accurate and gets you the angle of descent that you want (3 degrees or whatever). Using VS is good too - but you never know what the angle will be be until you do it!

I use VBI for approaches most of the time - it works well, especially for a long stright in approach to FAF. Take care, F2A

July 24, 2012 | Registered CommenterFLAPS 2 APPROACH
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