Enroute Performance

Wing Commander Rajagopal

En-route Performance

En-route performance refers to the aircraft’s capability while flying between departure and destination. It includes maintaining the required speed, altitude, fuel efficiency during cruise. En-route performance is affected by aircraft weight, weather, altitude and engine efficiency.

Forces Acting on the Aircraft

  • Lift acts through the centre of pressure, while weight acts through the centre of gravity.
  • Lift and weight form a strong couple, producing a pitching moment.
  • Thrust and drag act along the aerodynamic axis of the aircraft.
  • Thrust and drag form a comparatively weaker couple.

Tail Load

  • The stronger lift-weight couple creates a natural nose-down pitching tendency.
  • The tailplane produces a downward force to create a balancing nose-up pitching moment.
  • This downward force is known as tail load or tailplane download.
  • Aircraft trimming generates the required tail load.
  • Tail load increases the effective weight and drag of the aircraft.
  • Both induced drag and parasite drag increase due to tail loading.

Fuel Consumption Technique

  • Tail load can be reduced by moving the centre of gravity (CG) rearwards.
  • This can be achieved through selective fuel consumption or fuel transfer.
  • Burning fuel from forward tanks moves the CG aft.
  • A rearward CG reduces the nose-down pitching moment.
  • Less tail load reduces aircraft weight and drag.
  • Reduced drag improves both range and endurance.

Maximum and Minimum Speeds

  • Constant speed is maintained when thrust equals drag.
  • Increasing thrust causes the aircraft to accelerate.
  • Acceleration continues until drag again equals thrust.
  • The aircraft then stabilises at a higher speed.
  • Maximum and minimum speeds depend on the balance between thrust and drag.
  • Maximum speed occurs when maximum available thrust equals drag.
  • Minimum steady speed occurs where minimum thrust balances minimum drag.

Back of the Drag Curve and High-Altitude Speeds

  • Very low speeds are referred to as the back of the drag curve.
  • At these speeds, drag increases rapidly while available thrust is limited.
  • Flying behind the drag curve should be avoided whenever possible.
  • At high altitudes, engine thrust decreases because of reduced air density.
  • Aircraft must therefore operate within a narrower speed range.

Factors Affecting Endurance or Loiter Time

Flight for Endurance

  • Endurance is the amount of airborne time obtained from a given quantity of fuel.
  • Maximum endurance is important during holding operations.
  • Greatest endurance is achieved when fuel consumption is minimum.
  • Endurance = Airborne Time (hours) ÷ Fuel Used (kg)
  • Specific endurance is the reciprocal of fuel flow.
  • Specific Endurance = 1 ÷ Fuel Flow

Flying for Endurance in Jet Aircraft

  • Fuel consumed per unit thrust is known as Specific Fuel Consumption (SFC).
  • Maximum endurance is achieved when both SFC and thrust are minimum.
  • SFC is lowest at high altitudes, low temperatures and appropriate engine RPM.
  • High altitude reduces fuel consumption per unit thrust.
  • Minimum thrust occurs at the minimum drag speed (Vmd).
  • Jet aircraft achieve maximum endurance by flying at high altitude near Vmd.

Flying for Endurance in Propeller Aircraft

  • Maximum endurance occurs at minimum power.
  • Lower power reduces fuel flow and Specific Fuel Consumption.
  • Piston-engine aircraft achieve minimum SFC at lower altitudes.
  • Turboprop aircraft achieve minimum SFC at medium altitudes.
  • Best endurance is achieved by flying at the minimum power speed (Vmp).

Effect of Weight and Speed on Endurance

  • Increasing weight shifts the total drag curve upward and to the right.
  • Drag and power required increase.
  • Fuel flow increases, reducing endurance.
  • Best endurance speeds increase with aircraft weight.
  • Jet aircraft achieve endurance at Vmd, while propeller aircraft achieve endurance at Vmp.
  • Higher weight also lowers the optimum operating altitude.
  • Lower cruising altitude further reduces endurance.

Effect of Configuration on Endurance

  • Flaps and landing gear increase parasite drag.
  • The drag curve shifts upward and to the left.
  • Fuel flow increases and endurance decreases.
  • Maximum endurance speeds decrease in dirty configuration.
  • Flaps should be extended only when operationally required.
  • Wind does not affect endurance because only airborne time is considered.

Effect of Altitude on Endurance

  • Jet aircraft achieve the best endurance at high altitudes.
  • Maximum endurance for jets is typically obtained above the tropopause.
  • Turboprop aircraft achieve maximum endurance at medium altitudes.
  • Piston-engine aircraft achieve maximum endurance at lower altitudes.

Factors Affecting Range or Distance Travelled

Flying for Range

  • Range is the distance travelled for a given quantity of fuel.
  • Maximum range is achieved by minimising fuel consumption over a given distance.
  • Range = Distance (NM) ÷ Fuel Used (kg)
  • Specific Air Range (SAR) = True Airspeed ÷ Fuel Flow
  • Maximum range requires high TAS together with low fuel flow.
  • Lower Specific Fuel Consumption increases Specific Air Range.

Range Flying in Jet Aircraft

  • Minimum drag occurs at Vmd.
  • Maximum Specific Air Range occurs at approximately 1.32 × Vmd.
  • Jet aircraft achieve best range by flying at high altitude near 1.32 Vmd.
  • Lower SFC increases Specific Air Range.

Range Flying in Propeller Aircraft

  • Minimum power occurs at Vmp.
  • A slight increase in speed produces only a small increase in power required.
  • Higher TAS improves Specific Air Range.
  • Maximum speed-to-drag ratio occurs at Vmd.
  • Turboprop aircraft normally achieve best range at medium altitudes.
  • Reduced SFC increases Specific Air Range.

Effect of Weight on Range

  • Increasing weight shifts the drag curve upward and to the right.
  • Total drag and power required increase.
  • Fuel flow increases, reducing range.
  • Best range speeds increase for both jet and propeller aircraft.
  • Higher weight results in lower cruising altitude.
  • Lower cruising altitude increases fuel consumption.
  • Aircraft selection should balance payload and fuel requirements.

Fuel and Payload Planning

  • Fuel and payload planning is commonly illustrated using a payload-range diagram.
  • From point A to B, payload is increased until the Zero Fuel Mass (ZFM) is reached.
  • At point B, range is zero because fuel has not yet been added.
  • From point B to C, fuel is added until Maximum Take-off Mass (MTOM) is reached.
  • Additional range beyond point C requires replacing payload with fuel.
  • At point D, fuel tanks are completely full.
  • Any further increase in range is possible only by reducing payload.

Effect of Configuration on Range

  • Flaps and landing gear increase drag and power required.
  • Fuel flow increases, reducing range.
  • The drag curve shifts upward and to the left.
  • Best range speed decreases in dirty configuration.
  • Best range speeds are approximately Vmd for propeller aircraft and 1.32 Vmd for jet aircraft.
  • Proper trimming and aircraft balance improve range.
  • Airframe icing increases weight and drag, reducing range.

Effect of Wind on Range

  • Cruising altitude selection should consider prevailing winds.
  • The benefit of higher TAS at altitude should be balanced against stronger winds.
  • Maximum range speed is approximately 1.32 Vmd.
  • Headwinds require a higher optimum range speed.
  • Tailwinds allow a lower optimum range speed.

Long Range Cruise Speed

  • Long Range Cruise (LRC) considers both fuel economy and operating costs.
  • A slightly higher cruise speed may reduce total journey time.
  • LRC speed is approximately 4% higher than maximum range speed.
  • Specific Air Range decreases by only about 1%.

Types of Cruise Techniques

Climb Cruise Technique

  • Maximum Specific Air Range is achieved by flying at the optimum altitude.
  • As fuel burns, aircraft weight decreases and optimum altitude increases.
  • An ideal cruise would involve a continuous climb.
  • Continuous climb is generally impractical in normal operations.

Step Cruise Technique

  • Step climb is the standard cruise technique for transport aircraft.
  • Aircraft climb approximately 2,000 ft whenever operationally appropriate.
  • The optimum altitude gradually catches up as fuel is consumed.
  • Further step climbs cease near the top of descent.
  • Cruising continuously at optimum altitude provides maximum Specific Air Range.
  • Step climb achieves approximately 99% of maximum Specific Air Range.
  • Maintaining one constant cruise altitude provides approximately 90% of maximum Specific Air Range.

Cruise Technique in Turboprop Aircraft

  • Turboprop aircraft cruise at lower altitudes than jet aircraft.
  • Increasing altitude increases TAS and reduces SFC.
  • However, higher altitude also increases power required.
  • Altitude selection is therefore largely influenced by wind conditions.

Cruise Technique in Piston Aircraft

  • Piston-engine aircraft have relatively constant SFC with altitude.
  • SFC increases with higher manifold pressure, lower RPM and incorrect mixture settings.
  • SFC has only a small influence on range.
  • Power required increases with altitude.
  • Higher TAS is largely offset by increased power required.
  • Full-throttle height is the altitude where full throttle is required to maintain a given speed.
  • Above this altitude, the aircraft cannot maintain the selected speed.
  • Maximum specific range is normally achieved just above the full-throttle height.

Summary of Cruise Speeds