Joined: 11 Dec 2005
|Posted: Thu Feb 07, 2008 5:54 am Post subject: Maintainng The Descent Profile of 737-300
Proper decent planning is necessary to arrive at the desired altitude at the proper speed and configuration. The distance required for the descent is approximately 3nm per 1000 feet altitude loss for no wind condition using ECON speed. Rate of descent is dependent upon thrust, drag, airspeed schedule and gross weight.
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 30 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 25 seconds and 2 NM to decelerate from 280 to 250 knots in level flight without speedbrakes. It requires an additional 35 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 50%.
Use of speedbrakes between the down detent and the flight detent can result in rapid roll rates and normally should be avoided. While using the speedbrakes in descent, allow sufficient altitude and airspeed margin to level off smoothly. Lower the speedbrakes before adding thrust. Do not extend the speedbrake beyond the flight detent in flight.
Use of speedbrakes with flaps extended should be avoided, if possible. With flaps 15 or greater, the speedbrakes should be retracted. 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 1000 feet AGL. 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.
Flaps and Landing Gear:
Normal descents are made in clean configuration. If greater descent rates are desired, extend the speedbrakes. If higher than normal descent rates are required then the landing gear can be lowered to increase the rate of descent.
Extend the flaps when in the terminal area and conditions requires 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. Avoid using landing gear for increased drag. This minimizes passenger discomfort and increases gear door life.
At high gross weights, minimum maneuvering speed may exceed certain speed restrictions. Consider extending the flaps to attain a lower maneuvering speed or obtain ATC clearance for maintaining the higher speed. Pilots complying with speed adjustments are expected to maintain the speed within plus or minus 10 knots.