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Jet Descending and Ascending procedure using Autopilot

Asked by: 1727 views Aircraft Systems, Commercial Pilot

Hi

 

***Q : Is there a special procedure for Jet Airliners for  descending and ascending from level flight?

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I'm now studying to be a pilot. I now working on for pilot licenses. 

Being while, I play MS flight simulator x for fun and experiences, even though I'm a novice pilot and never flied Jets before. (I flied GA-aircraft with propellers for several hundreds hours)

 

I flied 737 or A320 with Autopilot in MS flight simulator x.

 

My question is

 

***Q : Is there a special procedure for Jet Airliners for  descending and ascending from level flight?

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I mean that If I fly C172, I usually do like this.

  • From level to Climb : Full power - Pitch up - Set for Vy or Vx or Desired VS
  • From level to Descend : Power to decrease rpm - Wait until Airspeed decreases - When reached at my airspeed - Start Descending - Descend  at specific speed or VS

 

I want to know the specific procedure for B737 or A320 especially using Autopilots.

For example, it was difficult for me to reduce airspeed fast with spoilers when needed and How to use it correctly, such as, when to disarm spoilers and when to arm it and what are cautions, etc. Another one is that Auto-throttle was unable to make Airspeed stable, I mean it was higher than set speed for about 10~20kts when descending.

 

Thank you.

 

2 Answers



  1. KDS on Nov 03, 2020

    I don’t know if this will answer your question, but there is very little hand flying in commercial airliners. It is pretty much done through the Flight Management System (FMS) and autopilot.

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  2. awair on Nov 04, 2020

    Autopilot or not, you would still be using the Flight Director, so the technique is the same.

    Descent is the easy half to answer: the FMC prompts for an idle descent from Cruise Altitude, including deceleration to 250 KIAS below 10,000′.

    Descent speed will depend on your ‘Cost Index’, set by the airline, to minimise fuel usage vs airframe/engine time. You might be descending from FL410 at anywhere between 250 to 320 knots, but somewhere between 280-310 would be more typical.

    The two modes used would be VNAV or FL change, VNAV should be idle thrust, unless the predicted winds are in error (drag required or thrust required), FL change calls for thrust to idle, and your descent path may then go above or below the calculated optimum.

    Rule of thumb, start descent 3nm per 1000′ altitude prior to your target, and allow 10-15nm for deceleration. A good ‘gate’ is 250 knots, 10,000′, 40nm from touchdown.

    Climb speed is also a function of cost index, but does not vary as much. Expect to climb at 310-320 knots above 10,000′ with full thrust. V/S is sometimes used to reduce roc before levelling to avoid TCAS alerts.

    For smaller step climbs (ATC restricted climb due traffic), V/S would be acceptable.

    This applies to the 737 & is mostly similar to the 777. However, the 777 moderates the Idle/Full power of FL change, to try to achieve small altitude changes in 2 minutes. Not sure if the NG AFDS was updated to be similar?

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