Descending

Descending

Descending is the basic flight maneuver which results in a loss of altitude without gaining excessive airspeed and controlling the rate of descent with pitch attitude.

Objectives of the lesson

To learn how to descent:
  • At the recommended power-off descent speed.
  • At various power settings, airspeed, and flap/gear configuration to a selected altitude.
  • On a constant patch of descent towards a specific point of touch down.
  • At the published airspeeds required for an obstacle clearance approach.

Why ?

  • It’s essential to know how to descent properly in order to come back at the airport and landing.
  • Its part of every flight.
  • To avoid cloud layer.
  • etc.

Review questions: Climbing

  • How do you achieve a nose up attitude from straight and level flight
  • When forward pressure is to applied to the control column the nose….?
  • What is the effect of carb heat during a climb?
Before commencing descent:

Cockpit check (check list)

  • Mixture full Rich. If you descend from high altitudes to lower altitudes without enriching the mixture, the mixture will become leaner because the air is denser at low altitudes.
  • Carburetor heat as required (to avoid carburetor icing)
  • Look out ahead and below.

1:Power-off descent

Use:

  • During a forced approach
  • When we need to descend rapidly
Descent entry:
Power-attitude –trim (PAT)
  • Close the throttle smoothly but promptly.
  • Keep straight (the airplane will tend to yaw to the right due to reduced slipstream effect, apply left rudder)
  • Allow airspeed to decrease.
  • Pitch for the approximate attitude for best glide airspeed (cessna172(65 kts).
  • Trim
  • If necessary make any minor pitch adjustments to attain correct airspeed, and retrim.
  • Note the steady decrease in altitude on the altimeter and the rate of descent on the vertical speed indicator.

Don’t forget !!!

  • In a power-off descents, pitch control airspeed, raise the nose to decrease airspeed and lower the nose to increase airspeed.
  • Every 500 feets (or as req) add cruise power (2300 rpm) to avoid thermal shock in .

2:Power-on descent

Use:

  • When precise control of the rate of descent and distance attained is desired.
  • Most routine descent and approaches to landing are power assisted.
Descent entry:
Power-attitude –trim (PAT)
  • Reduce engine power to an rpm setting judged to give a desired airspeed and rate of descent.
  • Allow the airspeed to decrease to that desired airspeed
  • Lower the nose to an attitude that will give the desired rate of descent.
  • Trim to maintain this attitude

Rule of thumb :

  • 100 rpm = 5 kts = 100 ft per mn.
To decrase the rate of descent while in power-on descent:
  • Apply the amount of engine power that will give you the desired rate of descent and simultaneously
  • Adjust the attitude of the aircraft to maintain the best descent speed and retrim for new attitude.

3:En route descent

  • Any variation of airspeed and rate of descent maybe combined to obtain the effect desired.
  • An en route descent is usually a power reduction to provide a suitable rate of descent while still maintaining cruising airspeed.

Procedures to Level off

  • Power – advance throttle for cruise flight setting (2300 rpm cessna 172), check POH.
  • Carb heat off
  • Attitude – raise nose to the cruise attitude and let the aircraft accelerates to cruise speed.
  • Keep straight (the aircraft will tend to yaw to the left as the throttle is advanced due to raised slipstream- right rudder)
  • Trim   to relieve any pressure on the control column

Start leveling off at 10% of your rate of descent

  • If you are descending at 500fpm start your level off 50ft prior to your desired altitude

Descent: instruments

Factors Effecting Descent

  • Power: Attitude + Power = Performance (not simultaneous) 100RPM = 100FPM (as we saw).
  • Wind: The wind affect the maximum gliding distance greatly.
  • Flaps.
Flaps:
  • Flaps increase lift and decrease stall speed.
  • Flaps allow steep rate of descent for approaches without increasing airspeed
  • Better forward visibility

Obstacle clearance descents

  • Power-on descents requiring steep angle of descent, as in obstacle clearance approach, usually involve full extension of the flaps and lower than normal approach airspeeds.
  • Speeds for high angle of descent (steep angle) and low speed approaches for cessna 172 are around 40° of flaps and 55 kts (check POH).
Estimating Range
  • -Selected a spot on the ground as a fixed point on the windshield that you choose as a reference point.
  • -When stabilized in a constant power-on or power-off descent at a constant attitude and airspeed, visual observations of ground positions in relation to a fixed point on your windshield will provide information as follows:
    • Positions on the grounds that appear to move down from the fixed position on the windshield are ground positions that you can reach and fly over with height to spare.
    • The position on the ground that remains stationary is the position that your aircraft should reach.
    • The position on the ground that appears to move up are the position that your aircraft cannot reach.

Review questions

  • What does PAT stand for?
  • How do you estimate how far the airplane can glide?
  • What is the effect of flaps on our descent?
  • What does it mean if during descent a reference point is moving up on the windshield?
  • What is the best glide speed?