Flight Management Navigation -
FMS Vertical Navigation (VNAV)

Vertical Navigation (VNAV)

VNAV provides vertical profile guidance through the climb, cruise, and descent phases of flight.

Speed/Altitude Constraints

VNAV controls the path and speed to comply with waypoint crossing constraints. Waypoint crossing constraints are entered on the LEGS page waypoint line by pushing the applicable key on the right side of the CDU. Barometric altitude constraints must be below the cruise altitude to be valid. Values entered as part of a procedure and manually entered constraints are shown in large font. FMC predicted values do not act as constraints, and are displayed in small font.

Waypoints can have altitude or airspeed/altitude constraints. Speed constraint entries require an altitude constraint at the same waypoint. All speed constraints are considered by the FMC as at or below constraints.

At or above altitude constraints are entered with a suffix letter A (example: 220A). At or below altitude constraints are entered with a suffix letter B (example: 240B). Mandatory altitude constraints are entered without any suffix letter (example: 270).

Altitude constraints with two altitudes may be entered in either order. The lower altitude constraint, followed by a suffix letter A, and the upper altitude constraint, followed by a suffix letter B (example: 220A240B or 240B220A).

Takeoff and Climb

1

Takeoff
The takeoff (TO) pitch mode is used for the takeoff and initial climb. VNAV is normally engaged at 1000 feet and pitch guidance continues to maintain the target airspeed.

During takeoff the target airspeed is between V2 + 15 and V2 + 25 knots. When VNAV is engaged the FMC airspeed is set to the target airspeed.

Climb thrust must be selected before VNAV can be engaged.

2

Acceleration Height
At the acceleration height, flap retraction, or AFDS altitude capture before acceleration height, VNAV commands an airspeed increase to 250 knots or the speed transition associated with the origin airport, limited by configuration.

3

VNAV Climb
The VNAV climb profile uses VNAV SPD or VNAV PTH at the default climb speed or pilot selected climb speed to remain within all airspeed and altitude constraints that are part of the SID entered into the active route. Autothrottle uses selected climb thrust limit.

4

Climb Constraints
VNAV enters the VNAV PTH mode to remain within departure or waypoint constraints. Speed maintained during this time can be:

  • Procedure based speed restriction
  • Waypoint speed restriction
  • Default VNAV climb speed
  • Manually entered climb speed

If the FMC predicts the airplane will not reach an altitude constraint, the message UNABLE NEXT ALT is displayed on the CDU. Speed intervention can be used by pushing the IAS/MACH selector and manually setting a lower airspeed to provide a steeper climb or climb derates can be deleted.

5

Top Of Climb (T/C)
The point where the climb phase meets the cruise altitude is called the top of climb. Approaching this point, the FMC changes from the climb phase to the cruise phase. The T/C is displayed any time the FMC calculates a change from a climb phase to a cruise phase, such as a step climb.

The T/C point is displayed on the map as a green open circle with the label T/C.

1

Takeoff
The Takeoff (TO) pitch mode is used for the takeoff and initial climb. VNAV is normally engaged at 1000 ft and pitch guidance continues to maintain the target airspeed.

During takeoff the target airspeed is between V2 + 15 and V2 + 25 knots. When VNAV is engaged the FMC airspeed is set to the target airspeed.

  • As installed - (G-FCLC) When VNAV is engaged (above 400 feet) the thrust reference changes to climb.
  • As installed - (G-FCLE) Climb thrust must be selected before VNAV can be engaged.
2

VNAV Climb
The VNAV climb profile uses VNAV SPD or VNAV PTH at the default climb speed or pilot selected climb speed to remain within all airspeed and altitude constraints that are part of the SID entered into the active route. Autothrottle uses selected climb thrust limit.

3

Climb Constraints
VNAV enters the VNAV PTH mode to remain within departure or waypoint constraints. Speed maintained during this time can be:

  • Procedure based speed restriction.
  • Waypoint speed restriction.
  • Default VNAV climb speed.
  • Manually entered climb speed.

If the FMC predicts the airplane will not reach an altitude constraint, the message UNABLE NEXT ALT is displayed on the CDU. Speed intervention can be used by pushing the IAS/MACH selector and manually setting a lower airspeed to provide a steeper climb or climb derates can be deleted.

4

Top Of Climb (T/C)
The point where the climb phase meets the cruise altitude is called the top of climb. Approaching this point, the FMC changes from the climb phase to the cruise phase. The T/C is displayed any time the FMC calculates a change from a climb phase to a cruise phase, such as a step climb.

The T/C point is displayed on the map as a green open circle with the label T/C.

Cruise

At cruise altitude, the FMC commands economy cruise speed or the pilot entered speed until reaching the top-of-descent (T/D) point. Alternate cruise speed options are:

If the cost index is set to zero the FMC commands maximum range cruise speed. Cost index modifications are allowed until within ten miles of the top of descent.

Cruise Climb

When VNAV is engaged, resetting the MCP to an altitude higher than the current cruise altitude causes the new altitude to be displayed in the scratchpad of the CDU. The altitude can then be entered on the CRZ ALT line on the CRZ page. When the modification is executed the airplane will climb to the new cruise altitude. The CRZ page displays ACT ECON CRZ CLB.

Step Climb - PEGASUS FMC

Fuel and ETA predictions assume the airplane climbs at each predicted step climb point as airplane weight decreases. FMC predicted step climb increments are based on the step size entered on the CRZ page. Entering a step size of zero causes the FMC to assume a constant altitude cruise. Flight crew entry of a step altitude on the CRZ page overrides the FMC step climb predictions.

Entry of a planned step point and altitude on the RTE LEGS page overrides a "Step To" entry made on the CRZ page.

Predicted or planned step altitudes are displayed on the RTE LEGS page. The distance and ETA to the next step point is displayed on the CRZ and PROGRESS pages. Step points are displayed on the HSI map with a green circle and S/C label.

Step Climb - Non-PEGASUS FMC

Fuel and ETA predictions assume the airplane climbs at each predicted step climb point as airplane weight decreases. FMC predicted step climbs are based on ICAO cruising altitudes or the crew entered STEP TO altitude on the CRZ page. Flight crew entry of a step altitude on the CRZ page overrides the FMC step climb predictions.

Predicted step altitudes are displayed on the RTE LEGS page. The distance and ETA to the next step point are displayed on the CRZ and PROGRESS pages.

Cruise Descent

Resetting the MCP to an altitude below the current cruise altitude causes the new altitude to be copied to the scratchpad if the altitude change is 4000 feet or less. The new cruise altitude can be entered on the cruise page. When the modification is executed the CRZ page displays ACT ECON CRZ DES. If the altitude set in the altitude window is below the speed transition (SPD TRANS) or restriction (SPD RESTR) altitude displayed on the DES page, those altitudes and speeds are deleted and the airplane will maintain cruise speed during the descent.

Note: A cruise descent will not provide speed adjustments to comply with speed restrictions or transition altitude speeds. Transition or speed restrictions must be maintained by flight crew action.

Mode Control Panel Speed Intervention

With VNAV engaged, pushing the IAS MACH selector enables speed intervention. Speed intervention allows the flight crew to change airplane speed with the IAS/MACH selector.

The above illustration shows VNAV mode for each phase of flight during speed intervention.

Note: The FMC does not use the speed set on the MCP for fuel or ETA predictions so FMC predictions are not accurate if speed intervention is used for an extended period.

In VNAV PTH mode, thrust controls speed; in VNAV SPD mode, pitch controls speed.

During a VNAV path descent the FMC will maintain the descent path even if speed intervention is selected. Therefore speed intervention has limited affect when the airplane is on the VNAV descent path.

In a path descent, VNAV PTH changes to VNAV SPD during speed intervention. In all other phases, the pitch mode remains the same. In VNAV PTH mode, thrust controls speed; in VNAV SPD mode, pitch controls speed.

In approach phase during speed intervention, pitch mode is in VNAV PTH after speed intervention and the vertical path is kept regardless of IAS MACH selector changes.

VNAV changes to approach phase when:

If a "Direct To" waypoint on the approach is executed, VNAV transitions to approach phase when the airplane passes the "Direct To" waypoint. If a new waypoint is added to the approach and executed, VNAV may not transition to approach phase.

Descent

The FMC calculates a descent path based on airspeed and altitude constraints and the end of descent (E/D) point. An E/D is created when an altitude constraint is added to a waypoint in the descent phase. The altitude constraint can be entered manually or may be part of a selected VFR or instrument approach procedure. The E/D can be created at one of these positions:

Entering an arrival procedure provides an E/D point.

The FMC calculates the top of descent (T/D) point after the E/D is entered. T/D is the point where the cruise phase changes to the descent phase. The T/D is displayed on the HSI as a green circle with the label T/D. The descent path starts at the T/D and includes waypoint altitude constraints. The path to the first constraint is based on:

With the MCP altitude set below the current airplane altitude and at the T/D point, the FMC commands idle thrust for the level deceleration segment and pitch to track the descent path. Usually, the descent speed is economy above 10,000 feet and 240 knots below 10,000 feet. Final deceleration is commanded to arrive at the final approach fix or the outer marker at 170 knots.

If the airplane passes the T/D and the altitude window has not been set lower or if the airplane reaches the MCP altitude in descent, the pitch mode changes to altitude hold. The MCP must be reset and VNAV re-engaged to continue the descent.

Target speeds are changed by entries on the LEGS or DESCENT pages. Forecast descent winds and TAI/ON ALT for approach idle thrust may be entered on the DESCENT FORECAST page.

If an unexpected (not entered on the DESCENT FORECAST page) head wind results in a significant reduction in airspeed to maintain path, thrust increases. The CDU message THRUST REQUIRED displays if the A/T is disconnected and thrust is required to maintain path. The CDU message DRAG REQUIRED displays if an unexpected tail wind results in a significant increase in airspeed. If the airspeed deviation to maintain path is excessive and the limit speed may be exceeded, the FMC directs the airplane to depart the path.

Early Descent

An early descent is initiated by forcing the FMC into the descent phase before reaching the top of descent point. VNAV commands a descent at a reduced descent rate until the idle descent path is intercepted.

1

DES NOW
Use the DES NOW prompt on the DESCENT page. VNAV starts an early descent and captures the idle descent path.

Note: When more than 50 NM from the top of descent point, perform a cruise descent rather than a descend now for descent to intermediate altitudes. During cruise descent the FMC computes a new top of descent for the new cruise altitude and accurate destination fuel predictions. Using DES NOW more than 50 NM from the top of descent point can cause a fuel computation error.

2

Within 50 NM of Top of Descent Point
Use the MCP altitude selector to start an early descent. Within 50 NM of the top of descent point, VNAV starts an early descent and captures the idle descent path.

3

More than 50 NM from Top of Descent Point
Use the MCP altitude selector to start a cruise descent. If the distance from the top of descent is more than 50 NM, VNAV begins a cruise descent to the new cruise altitude and the new T/D is displayed. Monitor the descent profile to ensure the new cruise altitude is reached before T/D.

Approach

For VFR and non-precision approaches displaying a runway waypoint on the RTE LEGS page, the FMC calculated path is to a point 50 feet above the runway threshold.

For a non-precision approach without a runway waypoint on the RTE LEGS page, the VNAV path is calculated to the MDA or a calculated altitude at the missed approach point. The calculated altitude may be below the MDA to ensure flight path angle and normal threshold crossing height.

Note: It is the flight crew's responsibility not to descend below the MDA until adequate visual contact is achieved.

Pushing a GA switch activates go-around. The Autothrottle increases thrust and the autopilot pitch maintains airspeed.

Cruise and Descent Profile (Non-Precision Approach)

Cruise and Descent Profile (Non-Precision Approach)
1

Cruise
Before the top of descent, FMC is in cruise mode and uses VNAV PTH and ECON cruise speed.

2

Level Descent Phase
After top of descent, FMC is in descent mode, VNAV decreases airspeed to ECON descent speed, maintains altitude in VNAV PTH.

3

Descent
Upon reaching descent speed, VNAV descends in VNAV PTH at ECON descent speed.

4

Speed Limit Protection
If a tailwind which was not entered on the descent forecast page causes the airplane to accelerate the DRAG REQUIRED scratchpad message will be displayed. If the speedbrakes are not deployed, the pitch mode will change to VNAV SPD and depart the path before the speed reaches the limit.

5

Speed Restriction Deceleration
Before the speed restriction altitude, VNAV decelerates to commanded speed using VNAV PTH.

6

Descent and Approach
When at restricted speed, VNAV descends and starts approach in VNAV PTH at commanded speed.

7

Minimum Descent Altitude
When the MDA is reached with VNAV engaged the airplane will maintain the MDA altitude in VNAV PTH.

If the missed approach point is crossed without selecting GA, VNAV will start a VNAV SPD climb to the missed approach altitude.

If the missed approach point is crossed without selecting GA, VNAV will maintain the missed approach point altitude until GA is selected.

8

Go-Around (GA)
The missed approach go-around is commenced by pushing a Go-Around switch. Pushing a Go-Around switch

  • Starts a missed approach.
  • Sets go-around thrust.
  • Deletes altitude constraints between the airplane and the missed approach waypoint.
9

Missed Approach Level Off
If VNAV is selected during missed approach, VNAV engages in VNAV SPD. At the missed approach altitude the pitch mode changes to VNAV PTH.

VNAV Engine Out Operation

The FMC provides single engine performance guidance which is accessed with the ENG OUT prompt on the CLB or CRZ page. After the engine out page is selected the execute key must be pushed to activate the single engine guidance.

The Autothrottle system does not have a single engine capability and the Autothrottle must be disconnected after an engine failure. VNAV thrust settings and thrust reference modes must be manually set when operating single engine.

Climb (Engine Out Above Engine Out Max Alt)

When the airplane is above the engine out maximum altitude, selection of the ENG OUT> prompt on the VNAV CLB page creates a modification and displays the applicable engine out drift down (D/D) performance data to enable the airplane to descend to the engine out maximum altitude. Execution of the modification activates the engine out drift down function.

Cruise (Engine Out Above Engine Out Max Alt)

Selection of ENG OUT> may also be selected on the CRZ page. If the current altitude is above the engine out maximum altitude, the FMC will command a cruise drift down.

Selection of ENG OUT> may also be accomplished on the XXXX ALTN page in conjunction with a diversion modification.

VNAV EO
VNAV EO
1

Engine Out Modification
Select the ENG OUT> prompt on the VNAV CRZ page. Disconnect the Autothrottle and set maximum continuous thrust on the operating engine.

Result: The FMC creates a modification and displays the applicable engine out driftdown (D/D) performance data to enable the airplane to descend to the engine out maximum altitude.

2

Drift Down Execution
Set the MCP altitude at or below engine out maximum altitude and execute the FMC modification. This assumes clearance is approved to descend slowly to a non-standard altitude; for example, FL233.

Result: VNAV commands a drift down, and the engine out MAX altitude becomes the cruise altitude at 1L. The descent rate is controlled to maintain at least 300 feet per minute (fpm). Time and distance for the D/D to engine out MAX altitude are displayed at 2R.

The initial drift down speed defaults to EO (minimum drag) speed. Prompts for LRC (long range cruise) and CO SPD (company speed) are displayed or a manual speed entry may be made.

The FMC sets the drift down speed to the EO (minimum drag) speed. Speed adjustment cannot be made on the FMC.

3

Engine Out Cruise
When VNAV captures the engine out maximum altitude, the page changes to the engine out cruise page and the pitch annunciation is VNAV PTH. Predictions for engine out step climb are displayed at 2R.

The VNAV single engine speed can be adjusted to LRC speed, company speed, or a speed entered by the crew. Any change in the single engine speed will change the maximum altitude.

The VNAV single engine speed is set to long range cruise speed and can not be changed.

4

Subsequent Cruise Descent
With the FMC in engine out mode more than 50 nm from T/D, set a lower MCP altitude, select the entered altitude from the CDU scratchpad to the CRZ ALT line on the CRZ page and execute.

Result: VNAV cruise descent at approximately 1,250 fpm at the current speed. When the engine out cruise descent intersects the planned descent profile, descent mode becomes active.

FMS Data Entry