Descent Rates
Descent Rates tables provide typical rates of descent below 20,000 feet with idle thrust and speedbrakes extended or retracted.
Rate of Descent (Typical)
Target Speed Clean With Speedbrake
757-200 757-300 757-200 757-300
M 0.78 / 290 knots 1800 fpm 2000 fpm 2700 fpm 3200 fpm
250 knots 1500 fpm 1500 fpm 2000 fpm 2400 fpm
VREF 30 + 80 1200 fpm 1300 fpm 1600 fpm 2000 fpm
Normally, descend with idle thrust and in clean configuration (no speedbrakes). Maintain cruise altitude until the proper distance or time out for the planned descent and then hold the selected airspeed schedule during descent. 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. The Approach checklist is normally initiated during descent and should be completed passing 10,000 feet MSL or transition level, whichever is lower.
Plan the descent to arrive at traffic pattern altitude at flaps up maneuvering speed approximately 12 miles from the runway when proceeding straight-in or about 8 miles out when making an abeam approach. A good crosscheck is to be at 10,000 feet AGL, 40 miles from the airport, at 250 knots.
Losing airspeed can be difficult and may require a level flight segment. For planning purposes, it requires approximately 45 seconds and 3 NM to decelerate from 290 to 250 knots in level flight without speedbrakes. It requires an additional 45 seconds and 3 NM to decelerate to flaps up maneuvering speed at average gross weights. Using speedbrakes to aid in deceleration reduces these times and distances by approximately 30%.
Maintaining the desired descent profile and using the map mode to maintain awareness of position ensures a more efficient operation. Maintain awareness of the destination weather and traffic conditions, and consider the requirements of a potential diversion. Review the airport approach charts and discuss the plan for the approach, landing, and taxi routing to parking. Complete the approach briefing as soon as practical, preferably before arriving at top of descent. This allows full attention to be given to airplane control.
Speedbrakes
The pilot flying should keep a hand on the speedbrake lever when they are used in-flight. This precludes leaving the speedbrakes extended.
Use of speedbrakes does not appreciably affect airplane roll response. While using the speedbrakes in descent, allow sufficient altitude and airspeed margin to level off smoothly. Lower the speedbrakes before adding thrust.
To avoid buffeting, use of speedbrakes with flaps greater than 5 should be avoided. If circumstances dictate the use of speedbrakes with flaps extended, high sink rates during the approach should be avoided. Speedbrakes should be retracted before reaching 1,000 feet AGL.
757-300
Note: With speedbrakes fully extended, flaps up maneuver margin to stick shaker
is reduced. Additional airspeed (up to 5 knots) may be added to flaps up
maneuvering speed to ensure full maneuver margin (40° bank capability).
The flaps are normally not used for increasing the descent rate. Normal descents are made in the clean configuration to pattern or instrument approach altitude.
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 selected speed and delay speedbrake retraction until after level off is complete.
Flaps and Landing Gear
Normal descents are made in the clean configuration to pattern or instrument approach altitude. If greater descent rates are desired, extend the speedbrakes. When thrust requirements for anti-icing result in less than normal descent rates with speedbrakes extended, or if higher than normal descent rates are required by ATC clearance, the landing gear can be lowered to increase the rate of descent.
Extend the flaps when in the terminal area and conditions require a reduction in airspeed below flaps up maneuvering speed. Normally select flaps 5 prior to the approach fix going outbound, or just before entering downwind on a visual approach.
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本文链接地址：757 Flight Crew Training Manual 机组训练手册(42)
