An AOG event, or Aircraft On Ground, is one of the most disrupting events to an airline operation. In addition such AOG events pose potential costs to the airline in order to get passengers where they need to be, denied boarding compensation, additional maintenance costs as well as network effects on subsequent flights. AOG’s are caused by technical defects on the aircraft which are required to be fixed prior to the aircraft taking off. Being able to monitor the risk of such technical failures occurring on an aircraft can help significantly to mitigate the impact of an AOG event or even prevent the event from happening in the first time. In EXSYN we call this AOG risk monitoring and when used correctly can be a great asset to airline Maintenance Operations Centres

An Aircraft On Ground  event is one of the most costly occasions within any airline operations. An unexpected technical defect on an aircraft can result in the subsequent flight from being delayed or even worse, cancelled. The result being potential denied boarding compensation for passengers, costs to arrange accommodation for stranded passengers, booking replacement flights and/or potential network effects of subsequent flights being delayed. Different industry bodies have developed metrics on how much a minute of delay can cost an airline and its save to say that, regardless of their different numbers, they are all significant.

So being able to prevent such AOG situations is critical to keeping costs in check. But how can we prepare for unexpected technical defects? Some aircraft have been equipped with Aircraft Health Monitoring systems (AHM) being able to process data from sensors on the aircraft and detect if certain components are showing errors or non-critical malfunctions which over time could grow into a technical defect which needs to be corrected before the aircraft can fly again.

However, monitoring this aircraft health is just one piece of the puzzle. Afterall, not every technical malfunction directly results in the aircraft being grounded until the defect is fixed. In addition, not every component on the aircraft that can pose a risk to the aircraft being grounded is tracked in an Aircraft Health Monitoring system.

In order for airlines to completely visualize the risk of each aircraft in the fleet encountering a technical defect resulting in an AOG event, additional data of the aircraft is required. Think of the Minimum Equipment List in order to determine the impact of a technical failure on the airworthiness of the aircraft. Or the component life value data of parts installed in order to be able to determine condition degradation curves.

Once airline have the AOG risk per aircraft visible in its turn it also does not mean that every component that poses a risk needs to be replaced. The goal of AOG risk monitoring is to provide Maintenance Operations Centres a decision-making tool in the day of operation. Using it to way the costs and benefits of replacing components to prevent unscheduled replacements due to failures or accepting the risks accepts and catering for this within the operational process.

Within Avilytics, the AOG risk of each aircraft in the fleet is displayed based on each component installed on the aircraft (all ATA chapters), by algorithm calculated component condition degradation and the impact of a potential technical failure on the airworthiness of the aircraft based on its Minimum Equipment List. Together with OEM AHM systems this gives a full picture to MOC controllers on AOG risks existing on every aircraft in the fleet.

AOG risk dashbaord

Figure 1: AOG Risk Fleet Overview

AOG Risk component reliability

Figure 2: Detailed Aircraft Risk Board

AOG Risk component condition

Figure 3: Component Condition Deegradation Modelling

How to determine the impact of a potential technical failure on the AOG risk of an aircraft?

Not every technical failure directly poses a threat to the airworthiness of the aircraft, ad subsequent AOG events to fix the failure. Leading in this is the Operator Minimum Equipment List. This document states which components and systems need to be operative before flight and what the consequence is of a component or system does not function properly. Each upcoming technical failure detected, either by AHM or by component condition monitoring always needs to be seen in context of the Minimum Equipment List and any potential already existing deferred defects that might have a further deteriorating effect on the AOG risk of the aircraft itself

Tip:        Existing deferred defects have a large effect on rapid raise of AOG risk on an aircraft

Once a defect has been deferred on an aircraft, a subsequent related system or component failing can directly mean that the aircraft is AOG. Hence embedded your existing deferred defects in any AOG risk monitoring function is paramount to being able to visualize the actual full AOG risk. As an example, an aircraft can have one VHF system unserviceable, however as soon as the other VHF system fails it means that the aircraft is AOG. Additionally, operational consequences of deferred defect can have a further effect. Having an unserviceable APU might not necessarily impact the airworthiness of the aircraft, however as soon as the aircraft needs to operate on an ETOPS flight, it means its AOG until the APU is operable again.

Do you have any questions?

Feel free to contact us or give us a call on +31 20 8200 7600. We would be happy to help you further. 


AVILYTICS is a fully out-of-the-box aircraft reliability management solution that focuses on providing insights in technical reliability, upcoming potential technical failures as well as organizational efficiency analytics. It combines the traditional scope of aircraft and fleet reliability management with advanced techniques from predictive analytics to also build AOG risk profiles of aircraft, identify aircraft based reoccurring defects and measure organization performance. A full holistic approach to using data in order to increase aircraft availability and fleet performance.