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Acceleration and Stop Distance (ASD)
The distance which, for a given weight, ensures that an aircraft can come to a stand safely on the runway if the take-off is aborted at the most critical point. Includes runway length and stopway.

Adaptive Aircraft Assignment
Sometimes referred to as A3. The practice of dynamically optimising aircraft capacities to a schedule in order to reflect changes in demand. (See Dynamic Capacity Management and Demand Driven Dispatch.)

Adjusted Present Value (APV)
A discounted cash flow methodology that recognises that cash flows carry different risks and should therefore be assessed using different discount rates.

Aeronautical Information Publication (AIP)
A publication issued by or with the authority of a state containing aeronautical information of a lasting character essential to air navigation. It contains details of regulations and procedures pertinent to flying aircraft in the country to which it relates.

Aircraft Classification Number (ACN)
A number that expresses the relative structural effect of an aircraft on a pavement of a specified standard sub-grade in terms of a standard single wheel load. Calculated by the aircraft manufacturer. (See Pavement Classification Number.)

Aircraft kilometres
Distance flown. The total amount of flying is equivalent to the number of flights performed on each stage multiplied by the stage distance.

Aircraft life cycle
The span of time embracing all phases of an aircraft life, including research, development, testing and operation. Often associated with Life-cycle costing.

Aircraft utilisation
The average number of block hours flown by each aircraft in operation. It is usually expressed as either a daily, monthly or annual value.

Airport Planning Document
A document provided by Original Equipment Manufacturers to provide guidance for airport operators and airlines concerning the handling of the basic version of a particular aircraft.

Allowances
An amount of fuel included in the reserves to take account of factors that were unforeseen in the planning stage of a mission. These factors include: deviation of an individual aircraft from the expected performance level; en-route winds which may have been higher than expected; and deviations from planned routeings or flight levels. The allowance is often based upon a percentage of trip fuel for the mission, or mission flight time. Also known as contingency fuel.

Ambient conditions
Common, prevailing and uncontrolled atmospheric weather conditions.

Assumptions
A set of operating conditions and configurations provided by an airline to a manufacturer under which aircraft performance should be measured. Also known as ground rules.

Audio Video On Demand (AVOD)
An in-flight entertainment system that allows users to select, then watch and listen to video or audio content on demand.

Auxiliary Power Unit (APU)
A device located in the tail of an aircraft for the purpose of generating electrical energy for functions other than propulsion.

Available Seat-Kilometre (ASK)
Equivalent to one available seat being flown one kilometre. An expression of passenger capacity on offer.

Available Tonne-Kilometre (ATK)
Equivalent to one tonne of capacity being flown one kilometre. An expression of total capacity on offer. A useful way of measuring all productive capacity, including passenger and cargo operations. Requires an estimation of the average weight of passengers and their baggage.

Block time
The time taken between 'blocks off' and 'blocks on'. Engine start-up, ground manæuvres, or taxi times are included.

Brake energy speed
The maximum speed from which a take-off can be aborted.

Brand
A name, term, symbol, design, or combination of these elements that identifies goods or services and differentiates them from competitors.

Break-even load factor (BELF)
The percentage of capacity filled so that total costs incurred exactly equal the revenue generated.

Buyer Furnished Equipment (BFE)
Equipment that must be provided by an aircraft buyer for installation in the aircraft. Typically includes communications and navigation equipment, galleys and seats.

Cabin Configuration Guide
A document describing an uncustomised aircraft to aid the customisation process.

Cash operating costs
Direct Operating Costs but with cost of aircraft ownership removed, leaving purely cash-incurred costs. Useful to measure the real economic performance of an aircraft. (See Direct Operating Costs.)

Clearway
An additional space beyond the end of an airport-controlled runway, with no obstacles, that can be overflown before the take-off safety speed is attained.

Cocoon seats
Premium class airline seats that recline within a fixed shell, with no interference to the space afforded the occupant of the seat directly behind.

Cross Crew Qualification
An Airbus designation for the process of qualifying flight crew for a new aircraft type of another family by focusing on the differences between the new and currently-qualified aircraft, rather than undergoing a full type-rating for the new aircraft.

Discount rate
The rate at which future money will be discounted in a discounted cash flow calculation. The calculation is basically the reciprocal of compounding. As the discount rate rises, future values diminish in value.

Dynamic Capacity Management
An ORTEC designation for the process of dynamically optimising aircraft capacity to match late changes in demand. Also known as Demand Driven Dispatch, and Adaptive Aircraft Assignment.

Equivalent-cost modelling
The practice of normalising the seat-count of different aircraft types with different configurations of seats to a common parameter (such as an equivalent Business Class seat) in order to remove bias in economic comparisons.

Equivalent Still-Air Distance (ESAD)
Distance flown after modification of ground distance to take account of the effect of either a headwind or tailwind. ESAD equals ground distance only in zero wind conditions.

European Aviation Safety Agency (EASA)
An agency of the European Union which has been given specific regulatory and executive tasks in the field of aviation safety.

Extended Twin Operations (ETOPS)
A set of regulations that govern the design and operation of twin-engine aircraft to enable them to fly further than one hour from an airfield at their one-engine inoperative speeds. (See Long Range Operations.)

Fan diagrams
A visual representation of comparative aircraft operating economics where the variables are trip cost and unit operating cost.

Federal Aviation Administration
An agency of the United States Department of Transportation with authority to regulate and oversee all aspects of civil aviation in the United States.

Fleet planning
The process by which an airline acquires and manages appropriate aircraft capacity in order to serve anticipated markets over a variety of defined periods of time with a view to maximising corporate wealth. In the case of a leasing company, the acquisition of a fleet of aircraft is a means to provide a portfolio of opportunity to profitably rent to organisations requiring flexible solutions to their aircraft capacity needs.

Flight Crew Operating Manual (FCOM)
A document issued for each aircraft and engine model. It does not require the approval of the certificating authority and includes the certified performance found in the Flight Manual, plus non-certified performance, such as the en-route performance. (See Flight Manual.)

Flight Information Regions (FIR)
Describes airspace of specific dimensions in which a Flight Information Service and an alerting service are provided. FIRs may include the territorial airspace of several countries.

Flight Manual
A document issued for each aircraft and includes certified performance and operating procedures. It is the official level of performance recognised by certificating authorities and the only one that can be used in real operating calculations.

Flight profile
A procedure that describes the operating envelope of an aircraft mission in terms of phases of flight (such as aircraft speeds in climb, cruise and descent), definitions of terms, altitudes, fuel load, and reserve policies.

Flight time
The time for a mission from start of take-off run to the landing. Flight time is mostly used as a basis for the calculation of maintenance costs.

Fly-by-wire
A flight control philosophy that relies upon the electrical signalling of both primary and secondary flight control surfaces, rather than mechanical controls.

Geometric altitude
The geometric height above sea level.

Great circle distance
Also known as the orthodromic distance, it is the shortest distance between any two points on the surface of a sphere. Colloquially known as, 'as the crow flies'.

Ground Support Equipment (GSE)
Equipment designed to facilitate the servicing of aircraft at airports. GSE is categorised into equipment for ground power operations, aircraft mobility, and the loading operations for passengers and cargo.

Hub-and-spoke systems
The design of a route network so that traffic moves from feeder airports along 'spokes' of the wheel towards the 'hub' of the wheel, which is an airport providing connections to other hub airports.

In-flight entertainment and connectivity (IFEC)
Any form of entertainment made available to aircraft passengers. Includes video, audio, gaming and communications technologies. Regarded as as important product differentiator.

Incremental cash flow modelling
An analytical process that simplifies the evaluation of competing products. Instead of considering the performance of an entire business, the analysis is restricted to those elements where competing products directly influence the outcome. Factors common to all competing products are ignored.

Indirect Maintenance Cost (IMC)
Sometimes referred to as 'burden', includes overheads, administration, tooling, testing equipment and facilities, record keeping, supervision and quality control. IMCs are usually fused with direct maintenance costs to form a part of overall Direct Operating Costs. (See Direct Operating Costs.)

Indirect Operating Costs (IOC)
Operating costs not directly influenced by the type of aircraft being operated. Typically includes handling, passenger services, catering, ticketing, sales and promotion, general and administration costs.

'Intelligent misuse' of aircraft
An expression thought to originate from a former chairman of Cathay Pacific called Duncan Black. A principle whereby aircraft are deliberately deployed on non-optimum routes in order to achieve a more than compensating benefit by reducing overall system costs due to benefits that accrue through a more streamlined fleet.

Internal Rate of Return (IRR)
An investment appraisal methodology that portrays the return on capital based upon the time value of money. The IRR is an expression of the discount rate where the Net Present Value of all future cash flows would be equal to zero. (See Net Present Value, discount rate.)

Isochrones
A line connecting places of equal journey time to the same location. In ETOPS, used to draw 'time circles' around diversion airfields to represent the potential endurance of an aircraft.

Layout of Passenger Accommodation (LOPA)
A cabin layout drawing showing the arrangement and dimensions of seats, galleys, toilets, and other cabin equipment.

Life limited parts
Any part for which a mandatory replacement time limit is specified in the type design, instructions for continued airworthiness or, in some cases, the maintenance manual.

Logit models
A popular probabilistic model representing the discrete band of choice behaviour of individuals. Used in flight scheduling and revenue management systems.

Long Range Operations (LROPS)
A European initiative to develop a regulatory framework to embrace all aircraft involved in long-range operations, irrespective of the number of engines on the aircraft. Involves enhanced systems redundancy to further reduce the number of en-route diversions. LROPS is based on the idea that a flight crew should be able to choose the safest alternate airfield to land and not the closest. (See Extended Twin Operations.)

Maintenance Planning Document (MPD)
Contains a manufacturer's maintenance programme for its products, with tasks identified for an aircraft utilisation that is not exceptionally high or low. The MPD forms the basis for an individual airline to compile its own maintenance programme in accordance with national requirements.

Manufacturer's Weight Empty (MWE)
The weight of the aircraft as built by the Original Equipment Manufacturer, without incorporating elements necessary for revenue earning payload to be carried. Typically, seats, galleys, pallets and containers are not included. Similarly, fuel and other items necessary for the operation of the aircraft are also excluded.

Master Minimum Equipment List (MMEL)
A document describing items that are additional to the aircraft standard specification, and approved by the relevant airworthiness authority. (See Minimum Equipment List.)

Maximum Design Take-Off Weight (MDTOW)
The maximum certified weight at which the aircraft can take off, as measured at the brake release point on the runway.

Maximum Zero Fuel Weight (MZFW)
The weight of the aircraft with payload added, but no fuel. The MZFW minus the Operating Weight Empty give the 'structural payload'.

Minimum Equipment List (MEL)
A document applicable to an operator and based upon the Master Minimum Equipment List, which is approved by the relevant airworthiness authority.

Mixed Fleet Flying (MFF)
An Airbus designation that describes the operational practice where one pool of pilots is permitted to fly different types of aircraft concurrently.

Net Present Value (NPV)
The summation of all future cash flows in a study period, minus the initial investment.

Operating Weight Empty (OWE)
The weight of an aircraft prepared for service, composed of the Maximum Zero Fuel Weight plus the operator's items. (See Operator's items.)

Operator's items
Elements necessary for an aircraft to be operated in service. Includes items such as unusuable fuel, oil for engines, water for galleys and toilets, aircraft documents, passenger seats, galleys and fixed equipment, passenger service items, emergency equipment, and crew and their baggage.

Pavement Classification Number (PCN)
The number that expresses the relative load carrying capacity of a pavement in terms of a standard single wheel load. Calculated by the airport owner. (See Aircraft Classification Number.)

Pressure altitude
The indicated altitude when an altimeter is set to a pre-determined pressure setting of 1013.25 millibars (or 29.92 inches of mercury). This setting is equivalent to air pressure at mean sea level in the International Standard Atmosphere.

Profit profiles
A modelling tool, originally developed by Boeing, that measures route profitability by taking into account operating costs, revenues and demand spill. It enables economic comparison of different aircraft types, with differing sizes, against a common measure of original demand.

Quality of Service Index (QSI)
A market share indicator originally developed by the former Civil Aeronautics Board of the United States. QSI enables passenger behaviour to be modelling in terms of preference for elements such as travel time, frequency of service, number of stops and airline image.

Reclearance
A flight planning technique designed to decrease the amount of reserve fuel loaded for a mission.

Reduced Vertical Separation Minima (RVSM)
An operating procedure whereby the vertical separation required between aircraft flying between FL290 (29000 feet) and FL410 (41000 feet) is reduced from 2000 feet to 1000 feet.

Request for Change (RFC)
A process in aircraft definition where a customer communicates to the manufacturer changes to the standard specification.

Request for Information (RFI)
An informal document submitted to a manufacturer by a potential buyer comprising general questions concerning an aircraft type, in terms of its generic characteristics and product development.

Request for Proposal (RFP)
A document intended to formally outline an airline's expectation of the content of the manufacturer's offer.

Reserve policies
Formally agreed policies that ensure that sufficient fuel is on board an aircraft to safely reach an airfield other than the originally intended destination.

Residual value
An expression of the expected future value of an aircraft or other asset according to either an appraiser's or market opinion.

Return on Investment (ROI)
An investment appraisal technique comparing average annual profit generated as a percentage of the original investment.

Revenue management
The integrated control of capacity and price with the objective of maximising overall financial benefit.

Revenue Passenger-Kilometre (RPK)
Equivalent to one occupied seat being flown one kilometre. RPKs are commonly used as an expression of passenger traffic.

Revenue per Available Seat-Kilometre (RASK)
Revenue achieved for each available seat being flown one kilometre. A particularly useful measure of economic performance as it integrates the concepts of both yield and load factor.

Revenue Tonne-Kilometre (RTK)
Equivalent to one tonne of revenue payload being flown one kilometre. An expression of overall economic performance, integrating passenger and cargo operations.

S-curves
A graphical representation of the relationship between market share and frequency of service, either on a route or on a network, often characterised by a curve shaped liked the letter 'S'.

Same Type Rating (STR)
An aircraft type rating where aircraft share the same Type Certificate. Award of an STR depends upon the degree of functional equivalence and handling characteristics.

Seat pitch
The distance between any point on an aircraft seat and the identical point on either the seat in front or behind. Usually expressed in inches.

Seller Furnished Equipment (SFE)
Certificated equipment on an aircraft and installed by the manufacturer. Includes, for example, air conditioning, autoflight, fuel, electrical and hydraulic systems, plus the brakes, auxiliary power unit, engines, and cabin items such as toilets, overhead stowage, public address and smoke detection systems.

Specification Change Notice (SCN)
A wording alteration to the standard specification produced by the aircraft manufacturer in response to a Request For Change. Typically includes changes to cabin furnishings and materials, and external livery, for example. Once agreed between the parties, the SCN becomes a contractual document.

Spill
An expression of average demand which, for a particular time period, exceeds the capacity offered.

Stopway
An additional levelled area beyond the end of an airport-controlled runway that can bear the weight of a decelerating aircraft in the event of an abandoned take-off, but is not to be taken into account for the take-off calculation.

System Configuration Guide
A document produced by the aircraft manufacturer that provides information about optional aircraft systems, their function and alternative equipment vendors.

Take-Off Distance (TOD)
Runway length plus clearway.

Take-Off Run (TOR)
Runway length without clearway or stopway.

Tankering
An operational procedure in which an aircraft is loaded with sufficient fuel to perform the following stage, with reserves considered. Usually applied where no fuel supplies exist at the destination, or for economic reasons where the cost of carrying additional fuel is less than the cost of purchasing fuel at the destination.

Weighted Average Cost of Capital (WACC)
A method for selecting the discount rate for a discounted cash flow study that blends the company's borrowing rate with the shareholders' expected return on investment.