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Introduction. 1
Simplex. 1
Duplex. 2
Triplex. 4
Quadraplex. 7
The dual duplex. 8
Conclusion. 10
References. 11


Aircrafts are made up of complex subsystems that ensure that it is airworthy and also safe for transport. The subsystems are made up of anti-freeze equipment, carburetors, engines, propellers etc., all tailored to meet the specific needs of the different types of aircrafts. However, the most important systems in an aircraft are the following: the electrical and the power generation systems, the avionics which entail the systems that are used in the communication, navigation, radar systems, flight management systems and the airframe systems which are centered on the hydraulic (Moir & Seabridge, 2011).
However, it is important to discuss the underlying platforms through which the functioning of the aircraft is conducted. The architecture of the operational system represent the integration of sensor packages and computational performance that is able to compute aircraft data in a timely and efficient manner and therefore ensuring that it is airworthy (Vivekanandan, et al., n.d.). As such, there are various architectures that may be provided to the aircraft; simplex, duplex, triplex, quadruplex and dual duplex.


Flight control is a very important issue when speaking about navigation. The term simplex is used to refer to the architecture employed for the sensors and the control equipment of the aircraft. However, there is a built-in test is used to detect any failure in the architecture (McShea, 2010). The effectiveness of the built-in test is around 95% and the system is configured such that it may revert back to its original mode or to a safe value (Igloi & Karimi, 2005). One major application of this system is in the radar and communication systems of the aircraft.
The simplex architecture is composed of three components which are the safety controller, performance controller, and the decision logic. The three components work in sync with the performance controller offering control and if the safety is not adhered to, as determined by the decision logic, the safety controller takes over (Vivekanandan, et al., n.d.).
The main advantage of the simplex architecture is the simplicity and the ease of design. It is therefore useful for the simple functioning of the aircraft whereby failure does not result in humongous loss. However, the simplicity is also associated with its disadvantage whereby failure results in complete loss of operations
(Siurce: Vivekanandan, et al., n.d.)


The duplex configuration is used to describe an advanced architecture used in controlling the operations of the aircraft. Considering aircrafts that require higher safety levels, the duplex architecture may be employed to control the functioning of the various aircraft systems. In this, the sensor and controls are provided in pairs and therefore the failure of one does not hinder the movement and the control. It has to be noted that each set is identical to the other and this offers the aircraft about 100% proof to failure (Ying-qiu, 2002). There are two options offered by the duplex architecture in order to ensure and meet the safety requirements of the aircraft: high availability and high integrity.
The high availability duplex configuration may be the commonly applied and depends on the output of a single sensor and control (Garlan, et al., 2010). Therefore, the output depends on the lane that is operational and which presents the most viable option. On the other hand, the high integrity duplex configuration depends on the options presented by the two lanes. The architecture functions by determining the most appropriate course of action from the two lanes and control is only available if there is a mutual ground on which the two lanes meet (Sanchez-puebla & carretero, 2003).
One advantage of the duplex architecture is the continuity of operations when one system fails. However, the failure of a single lane reduces the reliability of the system because of the reduced margin of safety (Igloi & karimi, 2005). One major use of this system is in the interconnection between various aircraft systems such as the Air data inertial reference unit, the global positioning system and the radio NAVAIDS to the computers that are used in flight management (Rosero & Ortega, 2007)
(Source: Seabridge, et al., 2013)


The movement of an aircraft basically determines the safety levels accorded to the occupants. Therefore, it is important to determine the relative positioning as well as the location of the aircraft and this has been improved by using the triplex architecture. The triplex model is of the digital category and it utilizes the redundant hardware in order to ensure that the aircraft is reliable (Collinson, 2003). The software is used to determine the position and latitude and manage all the data related to the movement of the aircraft such as redundant air data and the inertia (dajani-Brown, et al., 2003).
The triple modular frequency is based on three systems which perform the required tasks. The three components however base the decision on the majority (2/3) which results to a single decision and output (Zhi-qiang, 2004). Another form of the triplex system is known as the monitored triplex and its configuration is that of three independent channels and each channel is continually monitored by the other channels. However, the failure is limited to only two channels and this limitation occurs when the system used for monitoring is very efficient and reliable. The major advantage of the system is mainly attributed to the decreased use of hardware. The general term used to define the triplex and the quadruple system is the redundancy. The failure of a single computer may result from various factors such as wrong input data, hardware failure, software failure etc.
The triplex architecture may be incorporated with a backup system (Kayton & Fried, 1997). The backup system is mainly introduced to reduce the chances of failure and as such, may put the triplex architecture in the same category as the quadruple architecture (Seabridge, et al., 2013). However, this backup is mainly mechanical and provides cushion as per the safety requirements, such as an emergency landing. The system comes to play when the primary systems to be used in this scenario fail.
The failures of the channels may be classified into the following: hard over, zero output, slow over, oscillatory, soft and intermittent failures (Spitzer & Spitzer, 2000). To begin with, the hard over failure refers to the complete breakdown of sensor output therefore demanding the other systems to resume full control. The zero output refers to a failure mode in which the output from the sensors is zero and as such there is no response and control. Thirdly, slow over refers to the increase in sensor output over time and may eventually lead to a hard over. Oscillatory failure is a failure mode that is dependent on the oscillation and the increase in amplitude and frequency are likely to lead to failure. The soft failure is whereby the sensor output is outside the range of the specifications. Finally, the intermittent failures occur in short briefs but the sensors are able to recover very fast. The application of the triplex architecture may be found in aircraft features such as the auto land of the autopilot version of steering. In this scenario, only the failure of the three lanes would result into a catastrophe.
Equal authority accorded to the three lanes is a major advantage in this architecture. The command is the average of the three lanes and as such, achieves high integrity. Furthermore, the decisions are based on the majority with a failure in one lane reverting the architecture to duplex (Moir & Seabridge, 2011). This ensures that the operations are not discontinued. On the other hand, the major disadvantage is mainly attributed to the reduced authority and control when two lanes fail.
(Source: Seabridge, et al., 2013)
(Source: Seabridge, et al., 2013)


 As with the triplex architecture, the fly by wire systems adopted must have a redundancy (bak, et al., 2009). The redundancy ensures that the system remains in operation when there are failures in some channels. The triplex and the quadraplex are epitomes of redundant systems whereby the configuration is in such a manner that failure in one channel does not limit the control of the aircraft. Redundant configurations require that the channels and the sensors are arranged in a parallel and independent manner in order to minimize the chances of failure. The quadraplex system is a form of extreme design whereby there are four channels that control the functioning of various systems of the aircraft. In this, a single system is replicated four times but there is a limitation to the number of channels that may fail prior to a safe landing. When the first channel fails, the system reverts to triplex and when the second channel fails, the system revers to simplex.
The architecture has been employed in unstable aircrafts whereby stability in the flight control systems is maintained by the four channels (McShea, 2010) .The failures can be in any form from electrical to hydraulic. This architecture has the advantage of providing backup options to failure mechanisms of the systems thereby ensuring that the safety is provided at a relatively higher level than the triplex and simplex. Furthermore, the architecture employs four lanes in its operations thereby ensuring effective control. On the other hand. The architecture is complex and therefore employed in aircrafts such as those in the military.
(Source: Seabridge, et al., 2013)

The dual duplex

The dual duplex architecture is more reliable than all the aforementioned architecture systems because each channel is accorded a monitor lane (Watkins & Walter, 2007). The two work in a synchronized manner with the command lane controlling the functioning while the monitor lane counterchecking the correctness of the functioning (Ananda, 2009). However, the implementation and design of the control and monitor system may be similar or dissimilar depending on the type of design used.
As with the functioning of the system, a command and monitor discrepancy may result into disconnection of the channel but the remaining command and monitor functioning will ensure that the aircraft continues its operations. However, this is subject to the cross monitor channel which is associated with individual channels. Therefore, the cross monitor channel determines the extent to which the lanes remain operational and any fault, such as a false warning, may result in the system failure.
The system is widely applied in the design of the major components of the aircraft such as the digital engine controls (Charana, et al., 2006). The application of this type of architecture in the critical organs of the aircraft is therefore based on the ability to easily detect and correct the errors that are easily found in control and monitor. However, two of the common failure modes are: failure of both the command and monitor lanes in opposite channels which results into loss of the system function, and the failure of one channel which result in no loss of performance (Schuster & verma, 2008). The second order failures are more critical and demand state of the art analysis because of the reliability (lopez, et al., 2007). The second order failure may be summarized as follows: a failure in the command and monitor lanes without the system detecting, and where the built in test fails to detect these errors. Furthermore, the cross monitor may fail to detect any discrepancy in the system (pignol, 2005).
The main advantages associated with this architecture is the ability to detect errors on a very high scale therefore ensuring the safety as well as the control of the aircraft. Furthermore, the integration of command and monitor functions ensure that it is very reliable.
(Source: Seabridge, et al., 2013)


Flight control and safety are some of the most fundamental operations of an aircraft but failure of these systems may result into catastrophic events. The failure may occur in the pilot section, autopilot section, landing, electrical, hydraulic, engine systems among others and therefore it is important to tailor the software that are used to control these functioning (mueller, et al., 2004).
There are basically five architectures that are employed to mitigate an aircraft from failure: the simplex, duplex, triplex, quadruples and the dual duplex architectures. The basic one is the simplex whereby failure in the channels may result in the failure of the aircraft. Higher order architectures are more reliable because the failure of one system does not necessarily mean failure of operations because there are back up options.
The higher order architectural systems which are the duplex, triplex, quadraplex and the dual duplex base their operations on redundancy. By definition, redundancy is the ability of the commands to be replicated and as such improve the reliability and effectiveness. However, the redundancy may be of two types: average output and command lanetypes.in the first instance, all the lanes are used in the operation with the command the average of the output. Furthermore, an erroneous lane is disconnected reverting the architecture to a lower form. On the other hand, the command lane type focuses on a single lane for commands and in case of any fault, an alternative lane is used.


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