Calculation of 3:1 Descent Ratio
As a backup to FMS generated TOD points, the 3:1 descent ratio can be used to plan and execute an idle descent. This descent ratio provides for a comfortable idle descent path by losing approximately 1,000 feet every 3 miles.
The basic equation is: (altitude to lose/1,000) x 3 = distance required
For example, if cruising at 35,000 feet and given a clearance to descend to 10,000 feet, the altitude lost equals 25,000 feet (35,000 - 10,000). In this example, the 3:1 calculation becomes 25,000 feet /1,000 times 3. Simply stated, 25 x 3 = 75. It will take approximately 75 miles to descend 25,000 feet using the 3:1 ratio. In order to arrive 30 miles from the field at 10,000 feet, add the 75 miles to the 30 miles resulting in a TOD point at 105 miles from the field. In order to properly use the 3:1 distance, a valid distance to the field or the crossing waypoint has to be available.
Continuously update the 3:1 calculation. For example, descending through 25,000 feet the aircraft is 60 miles from the field. 15,000 feet remains to be lost. 15 x 3 =45 miles. Adding 45 miles to the end of descent at 30 miles from the field yields 75 miles needed if on the 3:1 descent. Since the aircraft is only 60 miles out, the aircraft is high and the descent rate needs to be increased.
Prior to descent, modify the calculated distance for expected conditions. Subtract mileage for strong headwinds or add mileage if a tailwind exists. Add mileage if planning to use air-foil anti-ice or if a speed reduction is expected.
Use the Altitude Range Arc (green arc) to monitor the descent. The green arc displayed on the ND provides a visual indication of where the aircraft will intercept the altitude set in the MCP ALT window, based on the current rate of descent. If given a descent clearance to cross 30 miles from the field at 10,000 feet, create a corresponding fix on the LEGS page. Once the descent has started, monitor the green arc's position relative to the end of descent fix. The 3:1 descent ratio assumes a direct course to the field or waypoint. Any additional maneuvering adds distance to the descent allowing a shallower descent rate.
Early Descent from Cruise
When descending before the aircraft reaches the TOD, consideration should be given to choosing the best VNAV descent mode, either a Cruise Descent or a descent using DES NOW. The key factors to consider are distance from TOD, speed, and altitude to lose.
When changing altitude during the flight (ride or traffic) and the TOD is approximately 100 nm or greater ahead, perform a Cruise Descent. Set the lower cruise altitude in the MCP and CRZ page. Select VNAV and the aircraft will descend to the lower altitude at the speed that is active in the CRZ page. The autothrottles will retard the throttles and CLMP. The aircraft will begin a descent at approximately 1,500 fpm. The throttle may then be manually adjusted for the desired rate of descent. When ECON is the active mode, changing the cruise altitude will also cause the cruise speed to change; the lower the new altitude, the lower the new cruise speed.
Within approximately 100 nm of TOD the use of DES NOW is recommended. Set the lower cruise altitude in the MCP and select the DES NOW prompt on the Descent page. The autothrottles will retard the throttles and CLMP. The aircraft will begin a descent at approximately 1,500 fpm. Once the throttles are in CLMP increase power to maintain a shallow rate of descent of between 500 and 1,000fpm. This increases fuel efficiency by allowing the aircraft to intercept the idle path at a higher altitude. It will maintain this descent until intercepting the idle descent path, at which time the throttles will retard to idle and the aircraft will fly the calculated descent path. If
descending on the path the FMS will increase or decrease airspeed as necessary to stay on the vertical path. Path is primary, speed is secondary. ATC-assigned speeds may not be maintained. The FMS will correct any deviations from below the VNAV path through throttle position. For deviations above the path, an airspeed increase above the target of 15 knots when above 10,000 feet or 10 knots when below 10,000 feet will cause DRAG REQUIRED message to be annunciated on the MCDU scratchpad. If the deviation above the path is not corrected the aircraft will depart the path and may not make the target altitude constraint.
Ensure all variables involved such as wind, use of anti-icing, etc., are programmed so the VNAV function of the FMS can make its crossing restriction. Monitor PATH ERROR on the DES page.
Variables while on the idle descent path (ATC, traffic, winds) may cause the aircraft to deviate from assigned airspeeds or miss altitude constraints. The recommended technique for descending from cruise is to parallel or shadow the idle descent path.
• Reset MCP altitude window to a lower altitude.
• Select and execute the DES NOW prompt on the DES page at a point 5 to 7 miles prior to the top of descent point or 3 to 5 miles prior to the path if already in a shallow descent.
• When the throttles CLMP retard them to Idle.
The aircraft is now just short of the idle path and will maintain the speed displayed on the DES page. The 3 to 5 mile buffer/pad will help ensure the aircraft can make assigned altitude constraints.
VNAV Descent From Above Path
When a lower altitude is selected in the MCP and the aircraft is “above path”, e.g. when ATC has held the aircraft at cruise altitude beyond the planned TOD point, selecting VNAV causes the throttles to retard to idle and FMS IDLE to be displayed in the FMA speed window. The active DES mode command speed will be flown, but not exceeded, to regain the path. The pilot must use drag devices (and/or increase speed) to return to the path. Once within 150 feet, the path is regained and, if needed, airspeed will then be sacrificed in order to stay on the path. If the pilot chooses to accept an increase in airspeed to recapture the path by using FMS override, the pilot will not regain the path in the descent.
Other Modes of Descent
CAUTION: Do not actuate the pitch wheel when nearing altitude capture.
This will disable the ALT CAP mode and result in an altitude deviation. Confirm all MCP changes are annunciated on the FMA.
CAUTION: Changing the barometric altimeter setting while in the ALT CAP mode causes the autopilot to level the airplane above or below the selected altitude. However, resetting the barometric altimeter while in VNAV CAP will not interfere with level off at selected altitude.
When VNAV use is not desired, IAS/MACH mode is normally used for large changes in altitude (more than 2,000 feet). It will provide a rate of descent based upon thrust setting and will command a pitch attitude to acquire and maintain the selected IAS/MACH.
• Select a lower altitude in the MCP altitude window.
• Start the aircraft tracking down by actuating the pitch wheel. This will place the autopilot in the vertical speed mode.
• Select IAS/MACH on the MCP.
• Rotate the pitch wheel to the desired airspeed or mach.
• Retard the throttles manually to achieve the desired rate of descent.
When VNAV use is not desired, vertical speed (V/S) mode is normally used for small changes in altitude (approximately 2,000 feet or less) and for non-precision approaches. It will provide a constant rate of descent.
• Select a lower altitude in the MCP altitude window.
• Push the vertical speed button (this is only necessary if operating in the IAS/Mach mode).
• Rotate the pitch control wheel to the desired rate of descent.
• The autothrottles will retard only to the ATS low limit. Further throttle reduction must be accomplished manually.
Normally, descend with idle thrust and in clean configuration (no speedbrakes).
Maintain cruise altitude until 5 to 7 nm prior to the TOD and then descend 3 to 5 nm short of the descent path. Deviations from this schedule may result in arriving too high at destination and require circling to descend, or arriving too low and far out requiring extra time and fuel to reach destination.
The speedbrake may be used to correct the descent profile if arriving too high or too fast.
A good crosscheck is to be at 10,000 feet AFE, 30 miles from the airport, at 250 knots. Then use the 3:1 descent path based on flying distance to the runway to establish the descent for the approach.
Losing airspeed can be difficult and may require a level flight segment. For planning purposes, it requires approximately 60 seconds and 7 nm to decelerate from 320 to 250 knots in level flight without speedbrakes. It requires an additional 25 seconds and 2 nm to decelerate to flaps up maneuver speed at average gross weights. This equates to approximately 10 knots/nm at level flight. Using speedbrakes to aid in deceleration reduces these times and distances by approximately 50%.
Maintaining the desired descent profile and using the map mode to maintain awareness of position ensures a more efficient operation.
The PF should keep a hand on the speedbrake lever when the speedbrakes are used in flight. This helps prevent leaving the speedbrake extended when no longer required.
Use of speedbrakes does not appreciably affect aircraft roll response. While using the speedbrakes in descent, allow sufficient altitude and airspeed margin to level off smoothly. Retract the speedbrakes before adding thrust.
There is no maximum airspeed restriction for extension. Deployment of the speedbrakes will produce some buffeting and a pitch down tendency. Retracting the speedbrakes will result in pitch up. In either case, anticipation of the effect and proper use of the spoiler/speedbrake lever will result in a smooth transition between configurations.
Note: Speedbrakes must not be used with flaps extended in flight. Do not extend landing gear with speedbrakes deployed. Speedbrakes may be used with or without slats extended. Do not move spoiler/speedbrake lever to ground spoiler position in flight.
When descending with the autopilot engaged and the speedbrakes extended at speeds near VMO/MMO, the airspeed may momentarily increase to above VMO/MMO if the speedbrakes are retracted quickly. To avoid this condition, smoothly and slowly retract the speedbrakes to allow the autopilot sufficient time to adjust the pitch attitude to maintain the airspeed within limits.
When the speedbrakes are retracted during altitude capture near VMO/MMO, a momentary overspeed condition may occur. This is because the autopilot captures the selected altitude smoothly by maintaining a fixed path while the thrust is at or near idle. To avoid this condition, it may be necessary to reduce the selected speed and/or descent rate prior to altitude capture or reduce the selected speed and delay speedbrake retraction until thrust is increased to maintain level-off airspeed.
The LOW LIMIT setting on the autothrottles may not allow the aircraft to decelerate at adequate rates or maintain selected airspeed when at low altitudes and slow airspeeds. This occurs when a deceleration or descent is initiated with the speed select knob, VERT SPD, or while tracking the glideslope. In this instance, the throttles only retard to the LOW LIM, a higher power setting than idle, and the aircraft does not decelerate or the airspeed may increase during descent. Since the MD-80 engines exhibit a relatively high residual thrust while at LOW LIM, initiate terminal area descents with the pitch control wheel (vertical speed) for a smooth entry, then select IAS mode and retard the throttles to idle. Rate of descent can be adjusted with power and desired airspeed will be maintained. Approximately 500 feet before level off, increase the power setting to shallow out the descent (move the throttles forward until the red gear warning lights extinguish). This will provide for a smooth application of power by the autothrottles during level off and prevent a throttle burst. The use of VNAV precludes this issue.