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.
- 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 is the vertical path in degrees that the aircraft is currently flying.
- V/B is the vertical path in degrees that the aircraft SHOULD be flying to reach the keyed waypoint (TESSI/17000).
- V/S 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.
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:
- Descent from cruise altitude.
- Approaching the runway from a straight-in approach course.
- Approach the runway from base or via an ARC approach.
- Approaching the runway for a downwind approach.
I often use the VBI from FL10 to FAF on approach, when other constraints are not required.
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.
- CDU is Flight Deck Solutions (FDS) and MCP is CP Flight (Pro model). Double click video to view full screen.