Climb Restrictions
VNAV enters the VNAV PTH mode to remain within departure or waypoint restrictions. Speed maintained during this time can be:
.
procedure based speed restriction
.
waypoint speed restriction
.
default VNAV climb speed
.
manually entered climb speed.
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 shown 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 shown on the map as a green open circle with the label T/C.
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Flight Management, Navigation -Flight Management System Operation 737 Flight Crew Operations Manual
MCP Altitude Intervention YD651 - YD655
The altitude intervention switch may be used to resume climb after a temporary level off.
TEMPORARY
Whenever the airplane levels off at an MCP altitude that is not in the FMC, VNAV ALT engages. In the illustration above, FMC cruise altitude is FL250 and the clearance altitude, FL190, is set in the MCP. Pitch maintains altitude and thrust maintains FMC target speed. In the illustration above, the speed after the temporary level off would be ECON CLB SPEED.
To resume the climb, put the clearance altitude into the MCP altitude window and push the altitude intervention switch. VNAV SPD engages. Pitch maintains FMC speed and thrust increases to the climb limit. In the example, the airplane climbs to FMC CRZ ALT and then levels off in cruise.
The altitude intervention switch may be used during climb or descent to delete altitude restrictions between the current altitude and the MCP altitude. When level at a restriction altitude, and cleared to a higher altitude prior to crossing the restriction waypoint, reset the MCP altitude to the new clearance altitude and push the altitude intervention switch.
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Operation
737 Flight Crew Operations Manual
In the illustration above, the current altitude restriction is deleted and the airplane continues VNAV climb to the cruise altitude. T/C moves to match the new climb profile.
The altitude intervention switch may be used to increase cruise altitude. When level at a cruise altitude, and then cleared to a higher cruise altitude, reset the MCP altitude to the new cruise altitude and push the altitude intervention switch.
T/C
In the illustration above, the cruise altitude is increased and the airplane enters a VNAV cruise climb at the economy cruise speed.
Altitude intervention cannot be used to decrease cruise altitude. Setting a lower altitude then pushing the altitude intervention switch causes the FMC to enter an early descent in the selected descent mode.
Cruise
At cruise altitude, the FMC sets cruise speed at the default or pilot entered speed until reaching the top–of–descent (T/D) point. Alternate cruise speed options are:
. long range (LRC)
. flight crew entered speed. Cruise thrust is set as required to maintain level flight at the target speed, with the autothrottle engaged. The FMC uses maximum range cruise speed if cost index is set to zero.
Fuel and ETA predictions are based on a constant altitude cruise unless a step climb altitude is entered.
Step Climb
If a step climb altitude is entered in the CRZ page STEP altitude, the FMC
calculates the point where the step climb should begin.
The distance and ETA to the next step point are shown on the CRZ and
PROGRESS pages. The next step point is shown on the map as a green open circle
with the label S/C.
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Flight Management, Navigation -Flight Management System Operation 737 Flight Crew Operations Manual
MCP Speed Intervention YD651 - YD655
YD651
YD652 - YD655
The above illustration shows VNAV mode for each phase of flight during speed intervention.
With VNAV engaged, pushing the speed intervention switch enables speed intervention. Speed intervention allows the flight crew to change airplane speed with the IAS/MACH selector.
In a path descent using speed intervention, VNAV PTH changes to VNAV SPD when the active descent segment is an idle thrust segment.
YD652 - YD655 Note: Aircraft equipped with geometric descent path will transition to VNAV PTH after the first altitude restriction.
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