CLIMBS & DESCENTS

LESSON OBJECTIVES

  1. Describe the relationships and changes of the four forces of the flight in climbs/descends

  2. Understand the types of climbs/descents and the differences between them

  3. See the factors affecting the climb/descent performance

  4. Learn how to enter/maintain/finish a climb/descent

The most important thing in a climb is to maintain a constant airspeed and direction. As in S&L flight, the forces acting on the aircraft must be in equilibrium (Refer to Lesson Nº4 - Straight & Level)

The four forces when involved in a climb suffer some changes that affect the aircraft behaviour:

- Lift -> It is still perpendicular to the RAF (Relative Airflow), but in a climb, the relative airflow changes direction (remember that relative airflow depends on the flight path of the aircraft), so does the lift force that inclines backwards

- Weight -> Opposite to lift, the weight vector maintains its direction (pointing towards the Earth centre), but due to the aircraft nose-up position, now the weight is divided in two (a vertical component / a horizontal component acting in the direction of drag)

- Drag -> It is still parallel to the RAF, but acts in a different direction than in S&L, due to the change of direction of the relative airflow

- Thrust -> It is the most essential force in a climb, it has the job to counteract and overcome the drag and the rearward component of weight (RCW) making the climb possible. Also as the thrust inclines upward it starts counteracting part of the aircraft's weight reducing the amount of lift required

Looking at this diagram is straightforward to understand that the excess power is the variable that determines the climb performance:

- Excess Power = Thrust - (Drag + RCW)

- Higher Excess Power -> Higher Climb Performance

Climb Aerodynamics

Depending on the airspeed maintained, there are 3 types of climbs:

- Best angle of climb (Vx) -> At this airspeed, the aircraft gains height using less horizontal distance. For example, for the C172, this speed is 62 kts.

- Best rate of climb (Vy) -> At this airspeed, the aircraft gains height using less time. For example, for the C172, this speed is 75 kts

- Cruise climb -> A climb at a higher airspeed to improve visibility and engine cooling 

--- Vx < Vy < Cruise Climb ---

Best Rate vs Best Angle Climb

-Climb Aerodynamics-

As long as you remember the formula given to you in the last section, you will understand the factors that affect the climb performance:

- Excess Power = Thrust - (Drag + RCW)

- Higher Excess Power -> Higher Climb Performance

Every factor that affects thrust, drag or weight will affect the excess power and climb performance

- Altitude -> ↑ Altitude -> ↓ Air Density -> ↓ Thrust -> ↓ Excess Power -> ↓ Rate of Climb/Climb Angle

- Temperature -> ↑ Temperature -> ↓ Air Density -> ↓ Thrust -> ↓ Excess Power -> ↓ Rate of Climb/Climb Angle

- Humidity -> ↑ Humidity -> ↓ Air Density -> ↓ Thrust -> ↓ Excess Power -> ↓ Rate of Climb/Climb Angle

- Flap -> ↑ Drag -> ↓ Excess Power -> ↓ Rate of Climb/Climb Angle

- Weight -> ↑ Weight -> ↑ RCW-> ↓ Excess Power -> ↓ Rate of Climb/Climb Angle

- Wind -> It doesn't affect ROC but it affects climb angle. You will see it better with an example:

Wind Effect on Climbs

-- Aircraft 1 (No wind) -> Airspeed (75 kts), Groundspeed (75 kts -> 1.25 NM/min), ROC -> 500ft/min

-- Aircraft 2 (15 kts Headwind) -> Airspeed (75 kts), Groundspeed (60kts -> 1NM/min), ROC -> 500ft/min

-- Aircraft 3 (15 kts Tailwind) -> Airspeed (75 kts), Groundspeed (90kts -> 1.5 NM/min), ROC -> 500ft/min

-Climb Performance-

-Climb Procedure-

--- Entry in the climb---

-- Power -> Smoothly increase power to maximum                 

-- Attitude -> Apply back-pressure to the stick/yoke, select and hold the nose-up attitude required, maintain wings level with ailerons and coordination with rudder

-- Trim -> Trim for nose-up attitude to remove excessive loads

--- Maintain the Climb---

-- Lookout -> Keep scanning the sky to avoid other traffics that could be manoeuvring around you

-- Attitude -> As the power has been selected to full, the only control that is left to control the climb is the yoke/stick. So, in a climb, the elevator is the key to controlling the airspeed during the climb (For example, if our objective airspeed is 60 kts -> 65 kts (Nose-up to decrease airspeed) / 55 kts (Nose-down to increase airspeed)) 

                    -> Maintain wings level with ailerons and coordination with rudder

-- Instruments -> Check airspeed (airspeed indicator), attitude (attitude indicator) and coordination (turn coordinator)

--- Exit the climb and regain S&L flight---

-- Attitude -> Use the stick/yoke to lower the nose to regain a level attitude while maintaining full power, now the power that allowed us to climb is used to accelerate the aircraft. Due to inertia, the airspeed will not increase immediately but rather gradually. As the airspeed increases, the lift will increase (Lift formula -> 1/2dV² x CL x S), and the aircraft will pitch up, to counteract this effect more nose-down input is required to maintain the desired altitude

-- Power -> Once you reach the cruising airspeed desired, you must reduce power from full to cruise power setting (Remember to correct for pitch/yaw movements with power changes)

-- Trim -> Use nose-down trim to reduce control loads after you have the final attitude and power required for cruise

-Descends Aerodynamics-

As in climbs, to perform proper descends, the 4 forces of the flight must reach an equilibrium state:

- Lift -> It is still perpendicular to the RAF (Relative Airflow), but in a descend the relative airflow changes direction now coming from below the aircraft, so does the lift force

- Weight -> Its direction is the same (pointing towards the Earth centre), but due to the aircraft nose-down position, now it is divided into two (a vertical component / a horizontal component acting in the direction of thrust)

- Drag -> It is still parallel to the RAF, but in a descend the relative airflow changes direction, so does the drag force

- Thrust -> The amount of thrust used will determine the type of descent (Power-On/Power-Off). When thrust is used, it has the job to counteract the drag. In both cases, with or without thrust, there is a forward component of weight (FCW) that substitutes (without power) or helps (with power) the thrust force

 

Take a look at the next diagram to have a visual reference for the explanation:

Power-Off vs Power-ON Descents

Depending on the power setting used, there are 2 types of descends:

- Power-Off Descend -> A descend in which the power is at idle, also known as a glide

- Power-On Descend -> A descend in which the power is higher than idle

In the diagram, there is a comparison between power-on and power-off descent, in the last one, the aircraft has a steeper angle than the powered descend. Think about it, if the forces must be in equilibrium, then in both descends we need some type of force acting forwards to counteract the drag


- Power-on descend -> Thrust and Weight act together to overcome Drag
- Power-off descend -> There is no thrust so we need more weight acting forward to get the same result, so we must lower the nose descending with a steeper angle and at a higher rate of descent

-Descend Performance-

These factors are:

- Power -> ↑ Power -> ↓ FCW Required -> ↓ Rate of Descend (RoD) / ↓ Descent Angle / ↑ Range (Distance travelled over the ground in a descend)

- Lift / Drag Ratio -> A value to describe the wing efficiency -> ↑ L/D Ratio -> ↓ Drag -> ↓ FCW Required -> ↓ RoD / ↓ Descent Angle / ↑ Range 

- Flaps -> ↑ Drag -> ↑ FCW Required -> ↑ RoD / ↑ Descent Angle / ↓ Range 

- Wind -> It doesn't affect RoD but it affects descent angle. You will see it better with an example

Captura.JPG

---- Aircraft 1 (No wind) -> Airspeed (75 kts), Groundspeed (75 kts -> 1.25 NM/min), RoD -> 500ft/min

---- Aircraft 2 (15 kts Headwind) -> Airspeed (75 kts), Groundspeed (60kts -> 1NM/min), RoD -> 500ft/min

---- Aircraft 3 (15 kts Tailwind) -> Airspeed (75 kts), Groundspeed (90kts -> 1.5 NM/min), RoD -> 500ft/min

--- Entry in the descend---

Power -> Smoothly reduce power to the appropriate power setting depending on if you want to perform a power-off or power-on descent. (In this explanation, we will suppose a power-off descent)

Attitude -> With the elevator, hold the altitude by pitching up until you reach your desired descent airspeed, and once you reach that airspeed lower the nose to maintain it. For example, if you were cruising at 90 kts at 5000 ft and you want to perform a power-off descent at 60 kts. After selecting idle, pitch up to maintain 5000ft while the airspeed decreases, and once you reach 60 kts, lower the nose to maintain that speed. Maintain wings level with ailerons and coordination with rudder

Trim -> Use trim to remove excessive loads

--- Maintain the descent---

Lookout -> Scan the sky for traffic and as you are descending to a lower altitude, keep an eye for any obstacles that you could encounter

Attitude -> As we did in a climb, the pitch will control the airspeed, remember to lower the nose to increase the airspeed and raise the nose to decrease the airspeed. In case of a power-on descent, the pitch controls the airspeed and the power control the rate of descent

Instruments --> Scan the instruments to fly an accurate descent.

--- Exit the descent and regain S&L flight---

- Power -> Increase power

- Attitude -> Raise the nose to the required attitude to hold the altitude (4 fingers). As airspeed increases gradually, lift increases in consequence and higher forward pressure will be required to maintain altitude

Trim -> Trim to maintain the altitude selected

-Descend Procedures-