Tools To Assist in Approach: Using the B737-800 Vertical Bearing Indicator, Altitude Range Arc and Vertical Deviation Scale
On 12 February 2012, the flight crew of a Boeing 737 aircraft, registered VH-TJS and operated by Qantas Airways Limited, was conducting a scheduled passenger service from Sydney, New South Wales to Canberra, Australian Capital Territory. Due to scheduled maintenance the instrument landing system at Canberra was not available and the crew prepared for an alternate instrument approach that provided for lateral but not vertical flight path information. The flight was at night with rain showers and scattered cloud in the Canberra area.
Shortly after becoming established on the final approach course with the aircraft’s automatic flight system engaged, the flight crew descended below the minimum safe altitude for that stage of the approach. The crew identified the deviation and levelled the aircraft until the correct descent profile was intercepted, then continued the approach and landed. No enhanced ground proximity warning system alerts were generated, as the alerting thresholds were not exceeded.
During those phases of flight when terrain clearance is unavoidably reduced, such as during departure and approach, situation awareness is particularly crucial. Any loss of vertical situation awareness increases the risk of controlled flight into terrain. This occurrence highlights the importance of crews effectively monitoring their aircraft’s flight profile to ensure that descent is not continued through an intermediate step-down altitude when conducting a non-precision approach (Australian Transport safety Bureau, 2013).
Determining the correct rate of descent (RoD) or vertical speed (V/S) is a critical attribute if an aircraft is to arrive at the correct altitude and avoid excessive descent rates. Control of the vertical path uses two different methods: the step-down method and the constant descent method. Both methods assume that the aircraft is being flown in landing configuration at the final approach speed (VaPP) from the final approach fix (FAF) to the landing initiation of the missed approach.
Non Precision Approaches (NPA)
Historically non precision approaches reference ground navigation aids that exhibit a degree of inaccuracy, which is often enhanced by the poorly defined vertical path published on an approach chart; NPA charts typically provide only an assigned altitude at the FAF and the distance from the FAF to the MAP. Thus, flight crew awareness of the aircraft’s vertical position versus the intended vertical path of the final approach can be quite low when executing traditional style step-down approaches.
To determine the best vertical speed to use during a non precision approach, flight crews use a number of 'back of the envelope' calculations.
Rate of Descent & Glideslope Calculations
There are several calculations that can be used determine rate of descent – some more accurate than others. Search ‘determine descent rate’ in Google. Some of the more commonly used rules of thumb are:
- Divide your ground speed by 2, then add a zero (120 kias / 2 = 60, add 0 = 600 fpm).
- Rate of descent (RoD) in ft/min should be equal to 5 times the ground speed in knots (same as above but different calculation).
- To maintain a stabilized approach, add a zero to your indicated air speed and divide by two (150 kias + 0 = 1500 / 2 = 750 fpm).
- To determine distance from threshold to start a 3 degree glideslope, take the height above ground level and divide by three hundred (600 ft AGL / 300 = 2 nm).
- To maintain a 3 degree glideslope (ILS), multiply your ground speed by 5. The resulting number is the rate of descent to fly (110 kias x 5 + 550 fpm on 3 degree glideslope).
- If the glideslope is not operational on an ILS approach with DME, multiply the distance ‘to go’ by 300. This will provide the height in feet above the threshold of the runway (4 nm to the threshold; multiply x 300 = 1200 ft).
Flight crews today, especially those flying in and out of busy intercity hubs, rarely execute step down approaches as computer and GPS-orientated systems have replaced traditional methods of navigation. However, as the flight into Canberra revealed, the best system may at times be inoperative or fail and it is good airmanship to understand and be able to remember one or more of the above equations.
Today's systems provide a high level of redundancy and the Boeing 737-800 NG incorporates a number of integrated aids to assist a flight crew during descent and approach. In this post some of less commonly understood aids will be discussed.
A method often overlooked is to use the Vertical Bearing Indicator (VBI) which is functionality available in the CDU. The VBI can calculate an accurate rate of descent to a particular spatial point. It is basically an angle calculator that provides ‘live’ vertical speed information based upon a desired descent angle, the current speed of the aircraft and a end location.
LEFT: CDU showing DES Page, waypoint/altitude and VBI interface (Key RSK3 & RSK4).
A flight crew enters into the VBI the final altitude that the aircraft should be at (for example, the Final Approach Fix or runway threshold). This figure is determined by consulting the appropriate approach chart for the airport. The CDU will then calculate the descent rate based on flight variables. As the aircraft descends, the VBI readout will continually update the descent rate based upon aircraft speed and rate of descent.
The flight crew can either manually fly the descent rate or use part or full automation to maintain the rate of descent. A common method is to use the Vertical Speed (V/S) function on the MCP.
It is important to underatnd that the VBI has nothing to do with VNAV. The VBI takes the raw distance between the aircraft and a selected altitude point and calculates a vertical bearing to that point. If that point is part of a route in the CDU, then the next altitude constraint will be displayed, unless the user changes this.
Accessing the VBI
Navigate to Descent page on the CDU 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) allows manual entry of a waypoint and altitude or altitude restriction. Type the waypoint and altitude separated by a / slash symbol into the scratchpad of the CDU and upload to the correct line. (for example, MHBWM/200).
The VBI provides three fields:
- FPA (Flight Path Angle) is the vertical path in degrees (angle of descent) that the aircraft is currently flying.
- V/B (Vertical Bearing) is the computed vertical path in degrees that the aircraft SHOULD be flying to reach the CDU waypoint or altitude restriction.
- V/S (Vertical Speed) is the vertical bearing (V/B) converted into a vertical speed (RoD) for easy input into the MCP. The V/S is the vertical speed (RoD in feet per minute) required to achieve the displayed vertical bearing.
Observe the V/B. The idle descent in a B737 is roughly 3.0 degrees. Wait until the V/B moves between 2.7 and 3.0 degrees (or whatever descent angle you require based upon your approach constraints) and note the descent rate (V/S). At its simplest level, the V/S can be entered directly into the MCP and is the rate of descent required to achieve the computed vertical path..
The VBI can be used for any waypoint, fix and altitude and acts in conjunction with the AFDS.
Automation will attempt to follow the vertical bearing indicated by the CDU; for example, if a VNAV descent is activated before the Top of descent (ToD) is reached, the Flight Management System (FMS) commands a 1250 fpm descent rate until the displayed V/B is captured while maintaining VNAV connection.
The vertical bearing when the aircraft is on final approach calculates data from the Final Approach Fix (FAF) to the runway threshold.
To read an earlier post concerning the Vertical Bearing Indicator.
Other Approach Aids
A handy feature often overlooked is the Altitude Range Arc (ARA). The ARA is a green coloured half semicircle which can be viewed on the Navigation Display (ND). The ARA indicates the approximate map position where the altitude, as set on the mode control panel is expected to be reached. Once the aircraft is well established on the vertical bearing (V/B) calculated by the CDU, the ARA semicircle should come to rest on the targeted waypoint.
LEFT: Altitude Range Arc and Vertical Deviation Scale and Pointer B737-800NG
Vertical Deviation Scale and Pointer (VDS)
The Vertical Deviation Scale is another feature often misunderstood. The scale can be found on the lower right hand side of the Navigation Display (ND).
The VDI will be displayed when a descent and approach profile is activated in the CDU (such as when using VNAV). However, the tool can be used to aid in correct glideslope for any type of approach (RNAV, VNAV, VOR, etc). To display the VDI, an appropriate approach be selected in the CDU; however, the flight crew fly a different type of approach without VNAV engaged).
The Vertical Deviation Scale presents the aircraft’s vertical deviation from the flight management computer’s determined descent path (vertical bearing) within +- 400 feet. It operates in a similar way to the Glideslope Deviation Scale on the Instrument Landing System (ILS).
The VDS is a solid white-coloured vertical line with three smaller horizontal lines at the upper, lower and middle section, on which a travelling magenta-coloured diamond is superimposed. The middle horizontal line represents the aircraft’s position and the travelling diamond represents the vertical bearing (V/B).
When the aircraft is within +- 400 feet of the vertical bearing the diamond will begin to move, indicating whether you are above, below or on the V/B target. When the aircraft is on target (middle horizontal line) with the indicated vertical bearing, the FMA will annunciate IDLE thrust mode followed by THR HLD as the aircraft pitches downwards to maintain the V/B.
In some literature this tool is referred to as the Vertical Track Indicator (VTI).
Vertical Development (VERT DEV)
The Vertical Development (VERT DEV) is the numerical equivalent of the vertical deviation scale and is found on the Descent Page of the CDU. The VERT DEV allows a flight crew to cross check against the VBI in addition to obtaining an accurate measurement in feet above or below the targeted vertical bearing. The VERT DEV will display HI or LO prefixed by a number which is the feet the aircraft is above or below the desired glideslope.
The Vertical Deviation Scale and pointer (VDS) will remain visible on the Navigation Display (ND) throughout the approach, and in association with the Vertical Development display on the CDU are important aids to use for Non Precision Approaches (NPA).
The traditional method of a step down approach, which was the mainstay used in the 1970s has evolved with the use of computer systems and GPS. In the 1980s RNAV (area navigation) approaches with point to point trajectories began to be used, and in the 1990s these approach procedures were further enhanced with the use of Required Navigational Performance (RNP) in which an aircraft is able to fly the RNAV approach trajectory and meet specified Actual Navigation Performance (ANP) and RNP criteria. From the 1990s onward with the advent of GPS, the method that non precision approaches are flown has allowed full implementation of the RNP concept with a high degree of accuracy.
Although the nature of non precision approaches has evolved to that of a 'precision-like' approach with a constant descent angle, their are operators that widely use these techniques, despite their flaws, weaknesses and drawbacks. Even if modern navigational concepts are used in conjunction with traditional methods, aids such as the VBI, ASR and VDI should not be overlooked. Appropriate cross checking of the data supplied by these aids provides an added safety envelope and avoids having to remember, calculate and rely on ‘back of the envelope’ calculations.
The flight crew landing in Canberra, Australia did not use all the available aids at their disposal. If they had, the loss of vertical situational awareness may not have occurred.
ANP - Actual Navigation Performance
ARA - Altitude Range Arc
CDU – Control Display Unit (used by the flight crew to interface the with the FMC)
FAF - Final Approach Fix
FMS – Flight Management System
FMA – Flight Mode Annunciation
FMC – Flight Management Computer (connects to two CDU units)
ILS – Instrument Landing System
KIAS - Knots Indicated Air Speed
MAP - Missed Approach Point
MCP – Mode Control Panel
ND – Navigation Display
NPA – Non Precision Approach
RoD – Rate of Descent
RNP - Required Navigation Performance
RNAV - Area Navigation
ToD – Top of Descent
V/B – Vertical Bearing
VBI – Vertical Bearing Indicator
V/S – Vertical Speed
VDS – Vertical Deviation Scale and pointer (also called Vertical Track Indicator)
VERT DEV – Vertical Development