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 entails 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 operating system represents 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.


What is Simplex?

The term simplex is used to refer to the architecture employed for the sensors and the control equipment of the aircraft. The simplex architecture is used to control the systems in an aircraft that are generally simple and their loss may not be catastrophic to the operations 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)

How does Simplex work?

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.). They are therefore very useful in simpler aircrafts.
(Siurce: Vivekanandan, et al., n.d.)


One major application of this system is in the radar and communication systems of the aircraft.


  • 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 a humongous loss.


  • A major disadvantage of this system is that in the event of a failure, it results in the complete loss of operations.


What is Duplex?

The duplex configuration is a system that 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.

How does Duplex work?

In this system, 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 of failure (Ying-qiu, 2002).The failure reverts the architecture to simplex mode but this comes with the price of a reduced safety margin.


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 controls are only available if there is a mutual ground on which the two lanes meet (Sanchez-puebla & Carretero, 2003).


  • An advantage of the duplex architecture is the continuity of operations when one system fails.
  • The provision of an alternative ensures that the operations are not limited to one lane.
  • The precision and accuracy of the alternative lane are identical to the first ensuring reliability and sustainability.


  • The failure of a single lane reduces the reliability of the system because of the reduced margin of safety (Igloi & Karimi, 2005).

(Source: Seabridge, et al., 2013)


What is Triplex?

The triplex model is the digital category and it utilizes the redundant hardware in order to ensure that the aircraft is reliable (Collinson, 2003). 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.

How does Triplex work?

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 in a single decision and output (Zhi-qiang, 2004). 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.


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 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).
  • 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


  • A 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)


What is Quadruplex?

A quadruplex 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.

How does Quadruplex work?

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 quadruplex 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 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.


  • The architecture employs four lanes in its operations thereby ensuring effective control.
  • 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.


  • The architecture is complex and therefore employed in aircrafts such as those in the military.

(Source: Seabridge, et al., 2013)


What is 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.

How does Dual-Duplex work?

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 include failure of both the command and monitor lanes in opposite channels which result 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 (pignoli, 2005).


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 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.
  • Also, the integration of command and monitor functions ensure that it is very reliable.


  • The system is very expensive and is therefore required in aircrafts whereby the maneuverability and control are essential such as military planes.

(Source: Seabridge, et al., 2013)


In conclusion flight control and safety are some of the most fundamental operations of an aircraft but a failure of these systems may result in 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 is used to control this functioning (Mueller, et al., 2004).
There are basically five architectures that are employed to mitigate an aircraft from failure: the simplex, duplex, triplex, quadruplex 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, quadruplex 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 lane 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.


Ananda, C. m., 2009. General aviation and aircraft avionics; integration and systems test. Aerospace and electronic systems.
bak, S., CHivukula, D. K. & Odekunle, O., 2009. the system level simplex architecture for improved real-time embedded system safety.
Butland, J., 2012. Designing unmanned aircraft systems: A comprehensive approach. s.l.:s.n.
Charana, H., Scharbarg, J. L. & Ermont, J., 2006. Methods of bounding end-to-end delays of an AFDX network. Real-time systems.
dajani-Brown, S., Cofer, D., Hartman, g. & Pratt, s., 2003. Formal modeling and analysis of an avionics triple sensor voter. Lecture notes in computer science.
Garlan, D., Monroe, R. & Wile, D., 2010. Acme: an architecture description interchange language. s.l.:s.n.
Igloi, T. M. & Karimi, G., 2005. Aircraft avionics maintenance diagnostics data download transmission systems. s.l.:s.n.
Kayton, M. & Fried, W. R., 1997. Avionics Navigation System. Second ed. New York: John Wiley and sons inc.
lopez, j., Royo, p. & pastor, E., 2007. A middleware architecture for unmanned aircraft avionics. s.l.:s.n.
McShea, B., 2010. test and evaluation of aircraft avionics systems. s.l.:s.n.
Moir, i. & Seabridge, A., 2011. Aircraft systems: mechanical, electrical and avionics subsystems integration. s.l.:s.n.
mueller, G. E., Kohrs, D., Bailey, r. & Lai, G., 2004. Autonomous safety and reliability features of the k-1 avionics systems. s.l.:s.n.
Pignol, m., 2005. how to cope with SEU/SET at the system level. s.l.:s.n.
Rosero, j. A. & Ortega, j. A., 2007. Moving towards a more electric aircraft. s.l.:s.n.
Sanchez-Puebla, M. A. & Carretero, J., 2003. A new approach for distributed computing in avionics systems. s.l.:s.n.
Schuster, T. & Verma, D., 2008. Networking concepts comparison for avionics architecture. s.l.:s.n.
Spitzer, C. R. & Spitzer, C., 2000. Digital avionics handbook. s.l.:s.n.
Watkins, C. B. & Walter, r., 2007. Transitioning from federated avionics architecture to integrated modular avionics. s.l., s.n.
Ying-Qiu, C., 2002. An overview of the avionic integrated sensor (j). Telecommunications engineering.
Zhi-Qiang, H. E., 2004. Development and important supporting technology for integrated avionics systems. Telecommunication engineering.

Image References

I., Seabridge, A. & Jukes, M., 2013. Civil Avionics Systems. Second ed. s.l.: John Wiley and Sons.
Vivekanandan, P., Garcia, G., Yun, H. & Keshmiri, S., n.d. A simplex architecture for intelligent and safe unmanned aerial vehicles. Electrical engineering and computer science.