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Virgin_A340

Descent rate question

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Hi,Just started flying the 737NG after a while on the 747. 737 is a really nice change from the heavy stuff. When going into Calgary earlier today I was at FL280 or something and had a crossing restriction of FL120 further along the approach and VNAV gave me a descent rate of something like 3000fpm which shallowed to 2700 after a bit. Is this normal in the 737? 747 seems to be MUCH easier to slow down and as a result has less dramatic descent rates.Also when I entered the FL120 and 250 knot restriction into the FMC during cruise a green decel and the waypoint in question flashed. I have not seen this happen before in the 744. Why did this happen?Thanks for any commentsAndy S

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Guest tmetzinger

>Hi,>Just started flying the 737NG after a while on the 747. 737 is>a really nice change from the heavy stuff. When going into>Calgary earlier today I was at FL280 or something and had a>crossing restriction of FL120 further along the approach and>VNAV gave me a descent rate of something like 3000fpm which>shallowed to 2700 after a bit. Is this normal in the 737? 747>seems to be MUCH easier to slow down and as a result has less>dramatic descent rates.>>Also when I entered the FL120 and 250 knot restriction into>the FMC during cruise a green decel and the waypoint in>question flashed. I have not seen this happen before in the>744. Why did this happen?>>Thanks for any comments>>Andy SFor the best results in a VNAV path descent, it helps to have input the winds aloft into the DES FORECAST page. That will allow the FMC to calculate the proper Top of Descent (TD) point. Usually a descent rate of around 2800 FPM keeps a clean NG within airspeed limits.When I fly using Southwest procedures (no VNAV), I usually dial in a VS between 2600 and 2800 at idle thrust, reducing to 2300-2500 or so as I pass through FL240, to keep the speed around 280 KIAS

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Higher descent rates occur at higher altitudes for a given indicated airspeed, due to thinner air. The thinner air decreases lift, drag, thrust and indicated airspeed and these effects are very apparent with greater altitude changes of 10,000 ft. or more.The "decel" point on the 737NG indicates when the FMC would start deacceleration from preset descent speed to the restriction speed. I have found that it is usually a little off but it provides a handy visual reference.An advantage to not using VNAV, as Tim pointed out, is you remain in the control loop instead of delegating a part of the flying to the computer. SWA has a lot of policies that are intended to keeping the pilot in the loop, such as (last I heard) no A/T during approach, which keeps the pilot-flying hands on the throttles.


Dan Downs KCRP

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