Landing Performance Sequence and Approach Distance

Descent Phase

• The knowledge clip focuses on the first part of the landing performance sequence, including the conditions that influence approach distance and the method to calculate that distance.
• The approach part of the landing begins at the end of the descent flight phase.

Jet Plane Descent

• Jet plane descends from cruising altitude to initial approach altitude using gliding flight (no thrust).

• During descent, the sum of all forces in each axis is equal to zero (uniform descent).

• Lift is perpendicular to the mean aerodynamic chord, and weight points towards the center of the Earth.

• The lift is less than the weight (Lift = Weight * cos(gamma)).

• The drag balances with the sinus component of the weight in the longitudinal axis.

  • True airspeed ($Tas$) represents the direction of flight.
  • Vertical speed ($C$) represents the vertical speed of the aircraft.
  • Flight path angle ($\gamma$) is calculated as: $sin(\gamma) = \frac{C}{Tas}$.

Vertical Speed

• Vertical speed depends on the balance of forces in the X-axis:
  $Drag = Weight * sin(\gamma)$

• Rewriting for $sin(\gamma)$: $sin(\gamma) = \frac{Drag}{Weight}$

• Since $sin(\gamma) = \frac{C}{Tas}$, then $\frac{Drag}{Weight} = \frac{C}{Tas}$

• Rewriting for vertical speed ($C$): $C = \frac{Drag * Tas}{Weight}$

Approach Configuration

• Aircraft configuration changes during approach mainly to reduce speeds.
  • Slats and flaps are used.
  • Undercarriage (landing gear) is lowered.
  • This increases drag significantly, making it impossible to maintain the ILS glide path with idle thrust alone.

Takeoff vs. Landing Flap Settings

• Different configurations are needed during takeoff and landing.
  • During takeoff, the objective is to increase lift with minimal drag increase (slats extension and small flap setting) to avoid negatively impacting climb performance.
  • During landing, high drag is desired in addition to high lift (large flap settings) to slow down during the landing roll and approach, minimizing the total landing distance.

Flap Implications

• Flaps increase both the lift coefficient ($C<em>L$) and the drag coefficient ($C</em>D$).
• The minimum drag of the aircraft is determined by the maximum $C<em>L/C</em>D$ ratio.
• $C<em>L/C</em>D_{max}$ is found on the aircraft characteristic graph by drawing a tangent line from the origin.
  • A steeper tangent line indicates a greater $C<em>L/C</em>D$ ratio and smaller drag.
  • The slope of the tangent decreases with increased flap extension.
  • Large flap positions result in very high drag.

Fuel and Noise Considerations

• Flaps should only be used in appropriate situations (takeoff and landing).
• During Landing select the flaps as late as possible, retract flaps as soon as possible after takeoff.
• Delaying the flaps usage saves fuel and reduces noise pollution.

Drag Coefficient

• When the aircraft is in the landing configuration, its profile changes, and $CD_0$ (parasitic drag coefficient) increases significantly.
• Induced drag coefficient increases due to increasing $C_L$, while the Oswald factor decreases slightly due to wing shape changes (these changes are not significant)
• The total drag increases, and the minimum drag occurs at a lower speed.

Powered Descent Approach

• The landing configuration (full flaps, slats extended, gear down) makes an idle descent impossible during the ILS approach due to significant drag increase.
• Thrust must be used during the approach, which is known as the powered descent approach.
• Balance of force is similar to the descent performance, but with the addition of thrust.

Forces during powered descent

• Lift is perpendicular to the mean aerodynamic chord, and weight points towards the center of the Earth.
• The sum of all forces is zero in each axis (uniform descent).

Vertical Axis:

• Lift is countered by the cosine component of the weight.

Longitudinal Axis:

• Drag equals thrust plus the sinus component of the weight.
• $sin(\gamma) = \frac{C}{Tas}$, where C is the descent speed or vertical speed.

Equilibrium of Force Equations

• Equilibrium of force: $sin(\gamma) = \frac{Drag - Thrust}{Weight}$
• Since $sin(\gamma) = \frac{C}{Tas}$, then $\frac{Drag - Thrust}{Weight} = \frac{C}{Tas}$
• Rewriting for vertical speed ($C$): $C = \frac{(Drag * Tas) - (Thrust * Tas)}{Weight}$

Approach Distance Calculation

• The approach distance is calculated during a steady, uniform, powered approach.
• The aircraft starts at point A (end of descent without thrust) and begins the steady, uniform, powered approach until point B (end of approach, beginning of flare).
• Due to the steady uniform powered approach, the flight path is approximately a straight line.

Geometry of Flight Path

• Height at point A: $h_L$
• Height at point B: $h_F$
• The difference in height is: $h<em>L - h</em>F$
• $sin(\gamma) = \frac{Drag - Thrust}{Weight}$

Approach Distance Equation

• $tan(\gamma) = \frac{h<em>L - h</em>F}{S_A}$

• $S<em>A = \frac{h</em>L - h<em>F}{tan(\gamma)}$, where $S</em>A$ is the horizontal approach distance.