Lecture 6: CRUISE PERFORMANCE AIRCRAFT WEIGHT & PERFORMANCE

Introduction To Cruise The cruise phase of flight starts after aircraft has leveled off from the climb and it ends when the descent for landing is initiated by the pilot. In short word, cruise is the phase of flight that falls between climb and descent. Climb Cruise / En-route Descend Approach & Landing Take-off Cruise is the level portion of aircraft travel where flight is most fuel efficient. It occurs between ascent and descent phases and is usually the majority of a journey. Technically, cruising consists of heading (direction of flight) changes only at a constant airspeed and altitude. It ends as the aircraft approaches the destination where the descent phase of flight commences in preparation for landing. For most commercial passenger aircraft, the cruise phase of flight consumes the majority of fuel. As this lightens the aircraft considerably, higher altitudes are more efficient for additional fuel economy. However, for operational and air traffic control reasons it is necessary to stay at the cleared flight level. On long haul flights, the pilot may climb from one flight level to a higher one as clearance is requested and given from air traffic control. This maneuver is called a step climb. Commercial or passenger aircraft are usually designed for optimum performance at their cruise speed or VC. There is also an optimum cruise altitude for a particular aircraft type and conditions including payload weight, center of gravity, air temperature, humidity, and speed. This altitude is usually where the higher ground speeds, the increase in drag power, and the decrease in engine power and efficiency at higher altitudes are balanced.

Cruise The cruise phase involves level flight most of the time and very few level changes. The largest percentages of trip time and trip fuel are consumed typically in cruise phase. The factors that affect the total time and fuel burn are: Speed selection and Altitude selection.

Cruise There are three options for cruise which are: Best range cruise Best speed cruise Best endurance cruise

Range Range refer to distance aircraft can fly on a given amount of fuel. The maximum range means to fly the greatest possible distance with available fuel. In other word, how far can the plane travel on a given amount of fuel.

Specific range Specific Range (SR) express the range performance of the aircraft at a moment in time.

Endurance Endurance is the time that aircraft can remain airborne with the fuel available. In other words, how long an aircraft is able to remain airborne on a given amount of fuel. It will be greatest when the fuel is used at the lowest possible rate, that is, the fuel flow is minimum.

Specific range Specific Endurance will have units of time/units of fuel.

Short Range Jetliner Characteristics Boeing 737-300 Airbus 320 Passengers (pax) 149 150 Maximum Fuel 20,000L 22000L Maximum Range 4,180 km 4,815 to 5,555 km Cruise Speed 910km/hr 845 km/hr Service Ceiling 40,000ft 39,000 ft

Medium Range Jetliner Characteristics Boeing 767-300 Airbus 330 Passengers (pax) 218 in three classes 253 in three classes Maximum Fuel 90,770 L 139,090 L Maximum Range 11,320 km 11,850 km Cruise Speed 850 km/h 880 km/h Service Ceiling 39,000 ft 39,370 ft

Long-Range Jetliner Characteristics Boeing 747-400 Airbus 380 Passengers (pax) 416/524 525 (max:800) Maximum Fuel 216 800 L 310 000L Maximum Range 13 450km 15 200km Cruise Speed at FL35 910km/hr 900km/hr Service Ceiling 41,000 ft 42,980 ft Ability to fly longer distances non stop. A380 has range about 15’000 km (it can fly non-stop from New York to Hong Kong).

Factors Affecting Range & Endurance There are several factors will have effect on the maximum possible range which are aircraft’s mass and speed, density altitude and wind.

Aircraft mass Increased aircraft mass, increases the drag. This requires greater thrust to balance the drag, which increases the fuel flow and reduces the specific range. Approximately 10% increase in mass will require 10% increase in thrust and fuel flow and 5% decrease in range. Since fuel flow increases, endurance also decreases.

Air density (altitude) Range increasing with altitude Increasing altitude, decreasing air density As the air became less dense, so drag is less. Less drag means that aircraft can fly faster. And engine also burn less fuel to create thrust Thus fuel flow decrease and range increase.

If drag keeps decreasing as density decreases and the aircraft can fly faster and faster, then why don't planes fly at even higher and higher altitudes until the density, and the drag, become zero?

Maximum Altitude (Service Ceiling) Engine performance that limits the maximum altitude that an aircraft can reach. Engine process becomes more difficult as the air density decreases . Compressor less efficient and the burning of fuel and air is also impossible. When this occurs, the engine will "flame out" and the plane falls into a dive until density increases enough for the engine to be restarted.

Altitude The optimum altitude for best range increases as weight decreases. The procedure to give maximum range would therefore be to allow the aircraft to climb as the weight decreases during the flight. This is achieved with few step climbs during the cruise phase of the flight. For a given weight there is an optimum altitude that gives the best speed. This altitude is different from the optimum altitude for the best range;

Speed Selection High-speed Cruise. Best range speed. Low-Speed cruise.

Speed High-speed Cruise Means Cruise at the maximum indicated airspeed (IAS). Provides the quickest en-route time, but sacrifices fuel efficiency. Because, when flying at higher speeds, drag increases, thus the fuel flow also increases. Not normally used by airlines. .

Speed Best range speed is the most efficient speed in terms of distance and fuel usage. Also known Long-range Cruise speed. It is the speed that will provide the, Furthest distance traveled for a given amount of fuel burned. Minimum fuel burned for a given cruise distance. Typical cruising speed for long-distance commercial passenger flights is 878-926 km/h The speed which gives the maximum range for a given aircraft weight and altitude is called best range speed. Cruise Speed= The most efficient speed in terms of distance, speed and fuel usage (optimum cruise speed). Most jet airliners fly at speeds of around 500 mph, depending on a lot of things. Airliners do travel in a straight line, when it's practical. But often the route isn't exactly straight because of traffic issues, weather (thunderstorms must be avoided), or prevailing winds (you don't want to fly against a wind blowing the opposite direction, so you might prefer to fly around it). The routes over oceans are often straighter than those over land, but they still vary in order to take wind and weather into account.

Speed Low-Speed cruise. This speed reduce the drag and the fuel flow, but they also reduce the distance traveled per time, and therefore reduce the range

Performance Speed TAS (True Airspeed) – aircraft speed in relation to the air mass in which it is flying. IAS (Indicated Airspeed) – aircraft speed as indicated on the ASI (Airspeed Indicator). CAS (Calibrated Airspeed) – IAS corrected for installation and instrument errors

Wind The best range will be reduced in a headwind condition. The opposite occurs in a tailwind condition, the best range will be increased.