EB14-5 Factors Affecting Aircraft Performance — Comprehensive Notes

Lesson Objectives

  • By the end of this series, Ab Initio students should be able to express in their own words the factors affecting aircraft performance across the flight phases and related concepts:
    • Take-off
    • Climb
    • Cruise
    • Descent
    • Final approach and landing
    • Economic consequences of ATC changes on flight profiles
    • Performance restrictions due to ecological constraints

Take-off

  • The take-off distance is defined as the distance from the brake-release point to the point at which the aircraft reaches a defined height over the surface (35–50 ft).
  • For any take-off, the distance required under prevailing conditions must not exceed the take-off distance available at the aerodrome.
  • Major factors affecting take-off performance:
    • Aircraft take-off mass and balance
    • Temperature
    • Air density
    • Wind/windshear
    • Runway conditions (slope, surface)
    • Flap setting and airframe contamination
    • Bleed-air systems (air conditioning and anti-icing)
  • Take-off performance parameters:
    • A Performance Limited Take Off Mass or the Take Off Distances Required
    • A set of Take Off Speeds
    • A set of Thrust Limits

Air Density and Density Altitude

  • Density altitude is the ISA altitude at which the air density would equal the actual air density (corrected for non-standard temperature).
  • Effects of density altitude on take-off performance:
    • Increasing density altitude (i.e., decreasing air density) requires a higher take-off velocity (TAS).
    • Less thrust is available with higher density altitude.
    • When density decreases, take-off performance changes (the guidance notes a decrease in performance with lower density).

Aircraft Weight

  • Increasing gross weight has a three-fold effect on take-off:
    • Required take-off velocity increases
    • More mass for the same thrust → less acceleration
    • Increased rolling friction

Runway Conditions

  • Runway slope:
    • Upslope reduces the component of weight along the runway, reducing the net accelerating force and increasing take-off run; downhill slope has the opposite effect.
  • Runway length:
    • Longer runways allow greater time/distance to reach critical speeds and can offset some adverse effects of temperature and density on take-off performance.
  • Runway surface:
    • Soft or non-hard surfaces increase ground roll; tyres can sink; potholes/ruts degrade tyre movement; mud, snow, standing water reduce acceleration.

Wind

  • Taking off into a headwind allows reaching take-off velocity at a lower groundspeed, enabling shorter take-off runs.

Climb

  • Climb performance parameters:
    • Climb Gradient (or Climb Angle)
    • Rate of Climb
    • These are used to meet minimum regulatory requirements and/or ensure obstacle clearance.

Density

  • A decrease in density increases power required and decreases power available, so climb performance diminishes with decreasing density.

Altitude

  • Density decreases with increasing altitude, leading to diminished climb performance at higher altitudes.

Wind & Temperature

  • Stronger headwinds reduce groundspeed but can increase the climb angle (path angle).
  • Wind does not affect the Rate of Climb.
  • Higher temperatures reduce air density, which reduces climb performance due to lower thrust and higher drag.

Aircraft Weight

  • For a given weight, climb angle depends on thrust–drag difference.
  • Weight increases drag and power required, affecting both climb angle and rate of climb.
  • Higher weight reduces the maximum rate of climb; aircraft may need to fly at higher climb speed to achieve a given rate of climb (e.g., heavy aircraft may request high-speed climb > 250 kt).

Cruise

  • Cruise performance parameters:
    • Specific Range
    • Endurance
    • Optimum Speed and Altitude
    • Cost Index

Cruise Phase Definition

  • The cruise begins after level-off from the climb and ends when descent for landing is initiated by the crew.
  • During cruise, weight decreases as fuel is burned, affecting optimum speed and altitude:
    • Optimum airspeed and power settings may decrease over time, or the optimum altitude may increase as weight reduces.

Wind & Temperature

  • Headwinds reduce groundspeed and can increase fuel consumption, potentially prompting level changes.
  • Higher temperatures can increase moisture content in the air, reducing air density and thus climb/cruise performance; ice formation on wings can occur, increasing weight and drag; aircraft may request lower levels to reduce ice accretion.

Cabin Pressurisation

  • Not all aircraft are equipped with cabin pressurisation.
  • Unpressurised aircraft climb more slowly and may be limited to lower flight levels.
  • In cruise, cabin pressure remains constant except during level changes; no further ambient pressure change occurs.

Descent and Initial Approach

  • Descent performance parameters:
    • Descent Gradient (or Descent Angle)
    • Rate of Descent

Aircraft Weight

  • Weight reduces during descent as fuel is consumed; however, aircraft may be too heavy for landing and may require holding or extended routing to burn off fuel.

Speed & Rates of Descent

  • ATC may instruct speeds, influencing descent rates; higher speeds yield greater descent rates.

Aircraft Configuration

  • Descent is typically flown in a “clean configuration” for as long as possible within a continuous descent approach (minimum clean speed).
  • Lowering undercarriage and other configurations is delayed until appropriate speed bands near final approach.

Wind

  • Descending aircraft experience changing wind/air conditions that can cause turbulence and drift, affecting headings as instructed by ATC.

Cabin Pressurisation

  • Normal descents can be made if cabin pressurisation remains functional.
  • If cabin pressurisation system fails, the aircraft must descend quickly to equalise cabin pressure.

Final Approach and Landing

  • Landing performance parameters:
    • A Performance Limited Landing Mass or the Landing Distances Required
    • Landing Speed

Aircraft Configuration and Weight

  • Lift devices and other wing surfaces are deployed to maintain lift while creating drag to slow the aircraft, reducing the need for heavy braking and reverse thrust, especially on long runways.
  • Aircraft weight is a primary factor in determining landing distance; heavier weight requires higher landing speed to support the aircraft at the landing angle of attack and lift coefficient.

Wind

  • Strong winds increase the likelihood of wind-shear on final approach and associated turbulence.
  • Headwinds affect groundspeed and therefore landing run distance once on the ground.

Air Density

  • Higher density altitude increases landing speed (TAS), which increases lift and can reduce braking effectiveness.
  • The most critical landing conditions arise from combinations of high gross weight, high density altitude, and unfavourable wind.

Runway Conditions

  • Water on the runway reduces tyre–ground friction and braking effectiveness; hydroplaning can lead to a complete loss of braking and directional control.
  • Runway slope affects landing run: upslope reduces it; downslope increases it.

Minimum Dynamic Hydroplaning Speed

  • The minimum dynamic hydroplaning speed is given by:
    V{\min,\,dyn\,hydroplaning} = 9 \times P{\text{tire}} \quad (\text{knots})

    • Example: If tire pressure is $P_{\text{tire}} = 6\,\text{psi}$, then
      9 \times 6 = 54\ \text{knots}.

Interim Summary

  • Take-off
  • Climb
  • Cruise
  • Descent & Initial Approach
  • Final Approach & Landing

Economic Consequences of ATC Changes to Flight Profiles

  • Routing and re-routing can lead to longer distances flown, increased fuel burn, and higher operating costs.
  • Flight Levels:
    • Aircraft burn less fuel at higher altitudes
    • Airlines plan flights to operate at optimum speeds and levels to maximize range and minimize fuel burn
    • Lower fuel burn leads to reduced CO2 pollution

Ecological Factors Affecting Aircraft Performance

Fuel Jettisoning

  • Guidelines for jettisoning fuel:
    • Over the sea if possible, or above 10,000 ft
    • If operationally impracticable or safety concerns prevent dumping above those levels, it can be carried out above 7000 ft AGL in Winter and above 4000 ft AGL in Summer
    • Jettisoning below these levels should only occur if unavoidable

Noise Abatement Procedures

  • Procedures at most major and regional airports aim to minimize local disturbance and typically include:
    • Departing aircraft directed to fly a specific heading or maintain a track/altitude
    • Inbounds: use continuous descent approaches to minimize noise from airframes and engines
    • Specified minimum distances outside which to establish final approach for visual approaches
    • Encouraging operators to minimize noise impact on local residents

Environmental Protection

  • Aviation contributes to carbon dioxide (about 2–3% of world CO2 emissions), nitrous oxides, sulfur dioxide, and hydrocarbon pollution; hydrocarbons mainly affect passengers inside the aircraft.
  • Efforts to reduce environmental impact include both hard measures (technology improvements like more efficient engines) and soft measures (shorter taxi times, reduced holding, streamlined ground operations).
  • Example: A typical Boeing 747-400 in cruise emits about $6\times 10^2\text{ kg CO}2$ per minute in cruise but up to $9\times 10^2\text{ kg CO}2$/min when holding.

Water Pollution and De-icing

  • Fuel jettison and other discharges can contaminate water courses; de-icing fluids can also pollute watercourses and are subject to penalties.

Noise (continued)

  • Engine noise is categorized by engine Chapter standards (Chapter 4 are modern; Chapter 3 may be hush-kitted or restricted in certain areas).

Measuring Environmental Impact

  • Strategies include:
    • More continuous climb/descent operations to higher levels
    • More direct routes within airspace
    • Coordination with neighboring ANSPs and military users for more direct routes across the flight profile
    • Reducing airborne holding
    • Achieving or exceeding customers’ preferred cruise levels (FFL)

Military Flights

  • Military aircraft have different procedures and performance requirements but normally can comply with civil procedures.
  • Special handling procedures may be designed for military needs, including:
    • Reservation of a block of airspace (temporary segregated areas) for extensive vertical profiles, which may limit direct routings for civil aircraft
    • Special coordination with military controllers
    • In some cases, military jets may need to deploy a parachute on landing, which requires runway clearance
    • Special separation minimums for armed military aircraft or for formation flights depending on number of aircraft in formation

Calibration Flights

  • Calibration flights check the accuracy of ground-based navigational aids (e.g., VOR and ILS).
  • Three types of checks:
    • Installation
    • Routine
    • Post-accident
  • Routine check frequency depends on installation type; typical durations:
    • Several hours on small/medium airports
    • A few days on large airports (e.g., Paris Charles de Gaulle: about three days)

Calibration Flight Procedures

  • Before calibration, ATC briefing provides a schedule of procedures (types, altitudes, distances).
  • Calibration flights take priority; ATC should not interrupt measurements except in emergencies, which can cause inconvenience due to time requirements or special conditions.

Final Summary

  • Topics covered: Take-off, Climb, Cruise, Descent & Initial Approach, Final Approach & Landing, Economic Factors, Ecological Factors

Questions or Comments

  • Any questions or comments? (as per the slide)