CHAPTER 3 BASIC CONCEPTIONS & RELEVANT DEFINITIONS
3.1 D EFINITION OF R ELEVANT C ONCEPTIONS
Some terminologies that referred in this thesis are defined in details as below:
Reliability
It can be simply defined as the probability that a system or product would perform in a satisfactory manner for a given period of time when used under specified operating condition. Reliability can be measured in term of mean time between failure (MTBF), mean time to failure (MTTF), or mean time between maintenance (MTBM). Thus, the aspect of time is the critical in reliability measurement (Smith, 2001).
Dispatch Reliability
Dispatch reliability is normally defined as the probability that an aircraft can departure in a satisfactory manner during a given period of time at the specified airport or area. Satisfactory departure means a successful takeoff without failure.
Failure
It is the termination of the ability of an item to perform its specified function. Or, Non-conformance to some defined performance criteria (Smith, 2001).
Failure Rate
A value expressing the frequency of failure occurrence over any specified time interval or cycles of operation.
Failure Mode
It means the various manner or ways in which failures occur and the resulting operating condition of item at the time of failure.
Departure failure
It can be defined as an aircraft fails to take off successfully or departure delay more than 15 minutes comparing to scheduled departure time due to any accidents and incidents happen in departure period, but except the unexpected factors and scheduled human actions, such as strikes and wars. Departure failure can be expressed as a distribution function [F (t)], which is related to various risk factors.
For the civil aviation, the departure failure includes: flights cancellation, flight divert and flight departure delay.
Pushback
The point in time when an aircraft is pushed away from the departure gates so that it may commence taxi-out. This is also known as the gate departure time.
R (t) – It is reliability function, over the time.
F (t) – It is failure distribution function, or the unreliability function. If the random variable t has a density function of f(t), then
∫
∞ considered, the mean life θ for the exponential function is equivalent to mean time between failures (MTBF). t is the time period of interest, e is the natural logarithm base (2.7183). The reliability at time t ist
It refers to the time of pushback, especially when the parking brake is released and may commence to taxi-out. It is also known as the gate departure time.
Off Time
It refers to the takeoff time at which the aircraft weight is no longer borne on the landing gear.
On time
It is associated with the touchdown time, it is the time that aircraft’s weight is borne on the landing gear again.
In time
It is related to the moment the parking brake is applied at the gate. These times are recorded and reported by the respective airlines.
Departure Queue
It refers to the line consisting of aircraft waiting for their turn to take off.
Demand for service
It is the time when an aircraft is ready to be granted access to the runway. This does not imply that the runway is available for this aircraft to use. If other aircraft are already waiting for the service, then the occurrence of a demand for service means that an aircraft has entered the end of the departure queue to wait its turn for take off.
Roll-out
It is the time interval between pushback and the start of the aircraft take-off. This time includes taxi-out time and time spent waiting in the queue.
Taxi-out
It is the time interval between pushback and demand for service.
Taxi-in
It is the time interval between touchdown and the aircraft parking brake is applied at the gate.
Down time
Also called outage, it is the period during which equipment is in the failed state.
Availability (A)
It is the probability that an item, when used under stated conditions in an ideal support environment (i.e., ideal spare parts, personnel, diagnosis equipment, procedures, etc.), will be operational at a given time (Modarres, 1993). or degree to which an equipment will be ready to start a mission when needed. Availability is divided into up-time availability, steady state availability, and instant availability.
When u is uptime during total time T, and d is downtime during total time T, the availability A can be expressed by:
d u
u TotalTime
UpTime
A= = + (3-5)
Maintainability
A characteristic of design and installation which is expressed as the probability that a failed item will be restored to operational effectiveness within a given period of time when the repair action is performed in accordance with prescribed procedures and resources. This, in turn, can be paraphrased as “the probability of repair in a given time”.
Dependability
It is the probability or degree to which an equipment will continue to work until a mission is completed.
Mean time between failures (MTBF)
For a stated period in the life of an item the mean value of the length of time between consecutive failures, computer as the ratio of the total cumulative observed time to the total number of failures.
Mean time to failure (MTTF)
For a stated period in the life of an item, it is the ratio of cumulative time to the total number of failures.
Mean life
It is the mean of the times to failure where each item is allowed to failure.
Fault tree
It is a graphical method of describing the combinations of events leading to a defined system failure. In fault tree terminology the system failure mode is known as the top event. The fault tree involves essentially three logical possibilities and hence two main symbols. The three types are: The OR gate whereby any input causes the output to occur; The AND gate whereby all inputs need to occur for the output to occur; The Voted gate, similar to the AND gate, whereby two or more inputs are needed for the output to occur. Two symbols are for the AND and OR gates.
RAMS
It is the abbreviation of Reliability, Availability, Maintainability and Safety-integrity.
Type I delay
Also named original delay, it can be defined as the flight delay occur due to the factors of a specific flight itself – not related to any previous flight delay. The delay factors can be mechanism, weather, airport operation, passenger, crew human factor, maintenance.
Type II delay
Also named propagated delay, it can be defined as the flight delay occurs due to the delay of its previous flight. the delay propagated through airlines schedule, since the operation resources, such as aircraft, crew operation schedule, and passengers or luggage are limited. In the other word, this sort of delay happens due to delay
multiplication.
Runway capacity
It can be defined as the number of ‘slots’ available at an airport in a given period (often one hour). A slot is the right to schedule either a landing or take-off within a particular period, and has specific time dimensions which define where occurs within a day, within a week, and with in a year.
Turnaround time
For a short-haul flight, it is defined as the time for an aircraft to complete full off-loading, loading and where required, catering and cabin cleaning procedures; For long-haul flights, the time including comprehensive technical and cabin services should be considered instead (international Air Transport Association, 1997). The scheduled ground time of a turnaround aircraft is defined as consisting of two portions, namely the standard ground service time and schedule buffer time (if applicable)