Reference · 63 terms

Glossary

Short, precise definitions of the words and symbols used in this module, each linked to the lesson where it is taught. Filter the list as you type, jump to a letter, or look a symbol up in Symbols at a glance.

Showing all 63 terms

A

Alternate airport: An airport the aircraft can divert to if it cannot land at its destination. The fuel rules require enough fuel to fly a missed approach at the destination, climb, cruise to the alternate and land there with a final reserve.; See: Lesson 6Related: Reserve fuel, Final reserve, Holding
Approach category: ICAO's grouping of aircraft by threshold speed (roughly \(V_{\text{REF}}\) at maximum landing weight): A below 91 kt, B 91 to 120, C 121 to 140, D 141 to 165, E 166 to 210 kt. A customer may require a category, which limits the approach speed and so the wing loading.; See: Lesson 4Related: Reference landing speed \(V_{\text{REF}}\), Wing loading \(\WS\)
Aspect ratio \(A\): Span squared over wing area, \(A = b^2/S\). A higher aspect ratio lowers induced drag; an airport span limit \(b_{\max}\) caps it at \(b_{\max}^2/S\).; See: Lesson 2Related: ICAO aerodrome code letter

B

Balanced field length: The runway length at the decision speed \(V_1\) for which the accelerate-stop distance and the accelerate-go distance (engine failure at \(V_1\), continue to 35 ft) are equal. Part 25's takeoff field length is the greater of it and 115% of the all-engines distance.; See: Lesson 4Related: Decision speed \(V_1\), Takeoff field length
Baseline: The one configuration, sized and analyzed, that comes out of conceptual design and goes on to preliminary design.; See: Lesson 1Related: Conceptual design, Preliminary design
Block time: The time from leaving the gate to arriving at the gate (“blocks off” to “blocks on”): taxi out, climb, cruise, descent and taxi in. Airlines schedule, and pay crews, by block time.; See: Lesson 5Related: Mission profile
Breguet endurance equation: The time a jet can stay aloft on a given fuel: \(E = (1/C)(L/D)\ln(W_{i-1}/W_i)\). Jets loiter at \(\LDmax\), where drag and fuel flow are lowest. It gives the loiter fraction \(\exp[-EC/(L/D)]\).; See: Lesson 6Related: Loiter, Holding, Breguet range equation
Breguet range equation: The cruise range of a jet at constant speed, \(L/D\) and \(C\): \(R = (V/C)(L/D)\ln(W_{i-1}/W_i)\) (Module 1). It builds the payload–range diagram and the cruise fraction.; See: Lesson 3Related: Payload–range diagram, Breguet endurance equation

C

Certification basis: The set of airworthiness standards an aircraft type is certified against: for example 14 CFR Part 25 or EASA CS-25 for transport aircraft, Part 23 or CS-23 for normal-category aeroplanes (up to 19 passengers and 8618 kg).; See: Lesson 4Related: Climb gradient \(G\), Landing field length
Climb gradient \(G\): Height gained per distance flown, \(G \approx (T - D)/W\). Part 25 sets minimum gradients with one engine inoperative: in the second segment 2.4%, 2.7% and 3.0% for two, three and four engines. The required thrust-to-weight ratio is \(\frac{N}{N-1}(1/(L/D) + G)\).; See: Lesson 4Related: Second segment, Certification basis
Combat radius: The one-way distance from base to a combat area that a military aircraft must reach, fight and return from. The aircraft flies at least twice the radius, plus combat and reserves.; See: Lesson 5Related: Mission profile, Ferry range
Committed cost: The share of an aircraft's eventual cost fixed by decisions already made, whether or not the money has been spent. It rises fastest in conceptual design, when design freedom is greatest and knowledge least.; See: Lesson 1Related: Conceptual design
Compliance matrix: A table of every requirement with the value the current design achieves, how it was shown (analysis, test, inspection) and the margin. Updated at each turn of the design wheel.; See: Lesson 7Related: Requirement, Trade study
Conceptual design: The first phase of design: what the aircraft will look like, weigh and cost, and whether it can meet the requirements. It ends with a sized baseline, and commits most of the eventual cost.; See: Lesson 1Related: Preliminary design, Detail design, Baseline
Contingency fuel: Fuel for the unexpected on the way: stronger headwinds, a longer route, a lower altitude. ICAO rules typically use 5% of the trip fuel; the US international rule uses 10% of the flight time at cruise fuel flow.; See: Lesson 6Related: Reserve fuel

D

Decision speed \(V_1\): The speed during the takeoff run below which the crew stops after an engine failure and above which it continues.; See: Lesson 4Related: Balanced field length
Derived requirement: A requirement on a part of the aircraft that follows from a top-level one by analysis: a takeoff field length becomes a maximum wing loading, a minimum \(\TW\) and a \(C_{L_{\max}}\) for the flaps.; See: Lesson 2Related: Requirement
Design payload: The payload of the design mission, usually a full cabin of passengers at a standard mass with baggage. Often less than the maximum payload, which the structure allows.; See: Lesson 3Related: Payload, Design range, Standard mass
Design range: The range the aircraft must fly with its design payload and reserves. Built from the great-circle distances of the target routes, with allowances for routing and winds.; See: Lesson 3, Lesson 6Related: Design payload, Equivalent still-air distance, Great circle
Detail design: The phase in which every part is designed for manufacture and the tooling is planned. It ends with the released design (a Critical Design Review), followed by building, flight test and certification.; See: Lesson 1Related: Preliminary design

E

Equivalent still-air distance (ESAD): The distance through the air that uses the same fuel as a route flown in a wind: \(d_{\text{air}} = d_{\text{ground}}V/(V - w)\), with \(w\) the headwind component. A headwind costs more than the same tailwind saves.; See: Lesson 6Related: Design range

F

Ferry range: The range with full tanks and no payload: point C of the payload–range diagram. The aircraft takes off below MTOW.; See: Lesson 3Related: Payload–range diagram
Final reserve: Fuel that should still be on board at landing after everything else: 30 minutes of holding at 1500 ft for turbine aeroplanes under ICAO and the US international rule, 45 minutes at cruise under US IFR rules.; See: Lesson 6Related: Reserve fuel, Holding
Fixed-engine sizing: Sizing an aircraft around an existing engine whose thrust is given; usually one requirement is then exceeded. Compare rubber-engine sizing.; See: Lesson 1Related: Rubber engine
Fuel capacity: The usable fuel the tanks hold (volume times density, about 0.8 kg/L for jet fuel). It limits the payload–range diagram beyond point B.; See: Lesson 3Related: Payload–range diagram, Maximum takeoff weight

G

Great circle: The shortest path between two points on a sphere. Distance \(d = R_E\theta\), with \(\cos\theta = \sin\phi_1\sin\phi_2 + \cos\phi_1\cos\phi_2\cos\Delta\lambda\) and \(R_E = 6371\ \text{km}\).; See: Lesson 2Related: Nautical mile, Design range
Growth factor: The increase in takeoff weight per kilogram added: \(1/(1 - \Wf/\Wo - \We/\Wo) = \Wo/(W_{\text{crew}} + W_{\text{payload}})\) with the fractions fixed. Typically 3 to 6 for transports.; See: Lesson 7Related: Range wall, Trade study

H

Holding: Flying a racetrack pattern while waiting to land. Holding fuel is estimated with the endurance equation; the final reserve is defined as holding time.; See: Lesson 6Related: Breguet endurance equation, Final reserve

I

ICAO aerodrome code letter: The class of aircraft an airport is built for, by wingspan: A under 15 m, B under 24, C under 36, D under 52, E under 65, F under 80 m. It limits span, and so aspect ratio.; See: Lesson 2Related: Aspect ratio \(A\)
Integrated product development (IPT): Organizing design work in teams that bring together design, analysis, manufacturing, maintenance and cost specialists for each part of the aircraft from the start, instead of passing the design from group to group.; See: Lesson 1Related: Conceptual design

L

Landing field length: The runway an airline needs: the demonstrated landing distance from 50 ft divided by 0.6, and 15% more on a wet runway.; See: Lesson 4Related: Reference landing speed \(V_{\text{REF}}\), Approach category
Launch customer: An airline whose order allows a manufacturer to commit to a new type, and which helps set its requirements.; See: Lesson 2Related: Request for proposal
Load factor (passenger): The fraction of seats filled, on average. Seats per aircraft \(= Q/(n\,LF)\) for \(Q\) passengers a day on \(n\) departures. (Not the structural load factor \(n = L/W\).); See: Lesson 2Related: Market study
Loiter: A mission segment flown for time rather than distance: holding, patrol, or a reserve. Its weight fraction comes from the endurance equation.; See: Lesson 5, Lesson 6Related: Breguet endurance equation, Mission profile

M

Market study: A forecast of traffic on city pairs and of the aircraft airlines will need, which sets the size and range of a new airliner, often as a family.; See: Lesson 2Related: Load factor (passenger), Design range
Maximum landing weight (MLW): The heaviest the aircraft may land, set by the landing gear and structure. The approach-speed limit on wing loading applies at this weight.; See: Lesson 3Related: Maximum takeoff weight
Maximum takeoff weight (MTOW): The heaviest the aircraft may take off: the design gross weight, \(\Wo\) for the design mission.; See: Lesson 3Related: Operating empty weight, Maximum zero-fuel weight, Payload–range diagram
Maximum zero-fuel weight (MZFW): The heaviest the aircraft may be without usable fuel. Fuel in the wing relieves bending, so the load without it is limited; MZFW − OEW is usually the maximum payload.; See: Lesson 3Related: Operating empty weight, Payload
Mission profile: The flight the aircraft is sized for, divided into numbered segments (takeoff, climb, cruise, loiter, combat, descent, landing), each with its distance or time, altitude and speed.; See: Lesson 5Related: Mission weight fraction \(W_x/W_0\), Segment weight fraction \(W_i/W_{i-1}\), Combat radius
Mission weight fraction \(W_x/W_0\): The product of the segment weight fractions, \(W_x/W_0 = \prod W_i/W_{i-1}\). The fuel fraction is \(\Wf/\Wo = 1.06(1 - W_x/W_0)\), with 6% for reserve and trapped fuel.; See: Lesson 5Related: Segment weight fraction \(W_i/W_{i-1}\), Mission profile
Morphological chart: A table listing each design choice (wing position, tail, engine location, number of engines) with its alternatives; concepts are generated by picking one option per row.; See: Lesson 7Related: Weighted decision matrix

N

Nautical mile (nmi): \(1\ \text{nmi} = 1.852\ \text{km}\), about one minute of arc of a great circle. A knot is one nautical mile per hour, \(0.5144\ \text{m/s}\).; See: Lesson 2Related: Great circle

O

Operating empty weight (OEW): Empty weight plus crew and the items needed to operate (catering, water, unusable fuel and oil): everything except payload and usable fuel. In sizing terms, about \(\We + W_{\text{crew}}\).; See: Lesson 3Related: Maximum takeoff weight, Maximum zero-fuel weight, Payload

P

Payload: What the aircraft is paid to carry: passengers, baggage and cargo, or weapons and sensors. Crew is not payload.; See: Lesson 3Related: Design payload, Standard mass
Payload–range diagram: Payload against range: flat at maximum payload to point A (MTOW), sloping to B (full tanks at MTOW), then more steeply to the ferry range C. Built with Breguet and a fixed reserve.; See: Lesson 3Related: Ferry range, Maximum takeoff weight, Fuel capacity
Preliminary design: The phase after conceptual design: the configuration is frozen in its main features and analyzed and tested in depth by specialists; the outer shape is lofted. Ends with a Preliminary Design Review.; See: Lesson 1Related: Conceptual design, Detail design
Pugh matrix: A simple concept comparison: each concept is marked better (+), the same (0) or worse (−) than a reference concept on each criterion.; See: Lesson 7Related: Weighted decision matrix

R

Range wall: The design range at which the fuel and empty-weight fractions leave nothing for payload: \(\Wo\) grows without limit as it is approached. Better technology pushes it out.; See: Lesson 7Related: Growth factor, Trade study
Reference landing speed \(V_{\text{REF}}\): The approach speed over the threshold, at least \(1.23\,V_{SR}\) under Part 25. An approach-speed limit is a wing-loading limit: \((W/S)_{\text{land}} \le \tfrac12\rho V_{SR}^2C_{L_{\max}}/g\).; See: Lesson 4Related: Approach category, Wing loading \(\WS\), Landing field length
Request for proposal (RFP): A customer's formal invitation to manufacturers to propose a design, with numbered requirements. Military programs usually begin with one.; See: Lesson 2Related: Requirement, Launch customer
Requirement: A statement of what the aircraft shall do, written to be specific, measurable, with its conditions, and verifiable by analysis, test or inspection.; See: Lesson 2Related: Threshold and objective, Derived requirement, Compliance matrix
Reserve fuel: Fuel carried beyond the trip for contingencies, a diversion to an alternate and holding. Set by operating rules; carried on every flight but not planned to be burned.; See: Lesson 6Related: Contingency fuel, Final reserve, Alternate airport
Rubber engine: An engine assumed to scale to exactly the thrust the aircraft needs, \(T = (\TW)\,\Wo g\), with its weight and fuel consumption scaled with it. Used early in sizing, before a real engine is chosen.; See: Lesson 1Related: Fixed-engine sizing

S

Second segment: The part of the takeoff climb after the gear is up, at \(V_2\) with takeoff flaps, usually the most demanding engine-out climb requirement: 2.4%, 2.7% or 3.0% for two, three or four engines.; See: Lesson 4Related: Climb gradient \(G\)
Segment weight fraction \(W_i/W_{i-1}\): The weight at the end of a mission segment divided by the weight at its start. Historical values: warmup and takeoff 0.970, climb 0.985, landing 0.995; cruise and loiter from Breguet.; See: Lesson 5Related: Mission weight fraction \(W_x/W_0\)
Service ceiling: The altitude where the best rate of climb has fallen to 100 ft/min (0.5 m/s). The cruise ceiling uses 300 ft/min (1.5 m/s).; See: Lesson 4Related: Certification basis
Specific excess power \(P_s\): \(P_s = V(T - D)/W\), the rate at which an aircraft can gain energy height; a typical military maneuver requirement.; See: Lesson 4Related: Service ceiling
Standard mass: An agreed mass per passenger (and per bag) used instead of weighing everyone. This module uses 100 kg per passenger with baggage for sizing; European rules use 84 kg for an adult with hand baggage, plus checked baggage.; See: Lesson 3Related: Design payload, Payload

T

Takeoff field length: The runway needed to take off under Part 25: the greater of the balanced field length (engine failure at \(V_1\)) and 115% of the all-engines distance to 35 ft.; See: Lesson 4Related: Balanced field length, Decision speed \(V_1\)
Threshold and objective: The two values of a military requirement: the threshold is the minimum acceptable, the objective what the customer would like if it is affordable.; See: Lesson 2Related: Requirement, Trade study
Trade study: An analysis of how the design, and its weight and cost, change as a requirement or design parameter changes. It puts a price on requirements and finds the best design.; See: Lesson 7Related: Growth factor, Range wall, Weighted decision matrix

W

Weighted decision matrix: A table that ranks concepts by a weighted score, \(S_j = \sum_i w_i s_{ij}\), with weights \(w_i\) adding to 1 and scores \(s_{ij}\) on each criterion. Its ranking should always be checked for sensitivity to the weights.; See: Lesson 7Related: Pugh matrix, Morphological chart
Wing loading \(\WS\): Weight per unit wing area (Module 1). Approach-speed, field-length and other requirements each limit it; Module 5 puts them on one chart.; See: Lesson 4Related: Reference landing speed \(V_{\text{REF}}\), Approach category

Symbols at a glance

Met a symbol in a lesson or in another textbook and not sure what it stands for? Find it here, then follow the link to its entry.

Symbols used in this module
Symbol	Meaning	Entry
\(\Wo,\ \We,\ \Wf\)	Takeoff gross, empty and fuel weight	Maximum takeoff weight
\(W_i/W_{i-1},\ W_x/W_0\)	Segment and mission weight fractions	Mission weight fraction \(W_x/W_0\)
OEW, ZFW, MZFW, MTOW, MLW	Operator weights	Operating empty weight
\(\phi,\ \lambda,\ \theta,\ R_E\)	Latitude, longitude, central angle, Earth radius	Great circle
\(Q,\ n,\ LF\)	Daily demand, departures, load factor	Load factor (passenger)
\(A,\ b,\ S\)	Aspect ratio, span, wing area	Aspect ratio \(A\)
\(V_1,\ V_2\)	Decision speed; takeoff safety speed	Decision speed \(V_1\)
\(V_{\text{REF}},\ V_{SR}\)	Reference landing speed; reference stall speed	Reference landing speed \(V_{\text{REF}}\)
\(G,\ N\)	Climb gradient; number of engines	Climb gradient \(G\)
\(\overline{RC},\ \gamma_d\)	Average rate of climb; descent angle	Mission profile
\(E,\ C\)	Endurance; specific fuel consumption	Breguet endurance equation
\(w\)	Headwind component	Equivalent still-air distance
\(w_i,\ s_{ij},\ S_j\)	Weights, scores and weighted score	Weighted decision matrix
\(P_s\)	Specific excess power	Specific excess power \(P_s\)

Historical segment fractions are from D. P. Raymer, Aircraft Design: A Conceptual Approach, Table 3.2. Regulations are summarized for conceptual design; the current regulations govern certification and operations.