Descending lesson
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?
