Slowly airlines are planning a way forward in reactivating their fleets. (Re)starting operations with a reduced number of aircraft also triggers the question on which aircraft to choose for reactivation of the flight schedules. Has the time on ground been used to reduce deferred defect on aircraft? Do we want those aircraft with least amount of deferred defects open and how to handle those defects being deferred once a reduced flight schedule has recommenced? Even during normal operations, Deferred Defects on aircraft is something we would like to keep as low as possible. However, operational circumstances could cause engineers to make use of this approved method. In addition, are deferred defects a metric of organizational performance or aircraft reliability? How can you get proper insight in deferred defects on the fleet and what causes these defects to be deferred rather than rectified?

Any aircraft can fly with a set of predefined defects existing on that same aircraft without it affecting the airworthiness of the aircraft. This set of defects is laid down in the operator Minimum Equipment List (MEL) and is derived from the Manufactures Master Minimum Equipment List (MMEL). Within the MEL, defects are listed on aircraft systems which are allowed to exist, provided these defects are rectified on the aircraft within a predefined period of time. The time periods are classified as MEL categories and have 4 types:

Cat A:    Within a specific number of flying hours, cycles or calendar day

Cat B:    Within 3 calendar days

Cat C:    Within 10 Calendar days

Cat D:    Within 120 Calendar days

In addition, each listed defect might have additional operational restrictions resulting from it, or additional maintenance actions that need to be performed on the aircraft for the time the specific defects exist. Prior to each flight, the pilot needs to be aware of each defect existing on the aircraft and as such each deferred defect is recorded in the Aircraft Technical Logbook.

An increasing number of deferred defects on both individual aircraft as well the fleet as a whole, can result in additional planning challenges due to further operational restrictions on affected aircraft, and potential additional aircraft downtime in order to rectify the deferred defect within its given MEL category rectification time. Once the rectification time of a deferred defect has been exceeded and the defect has not been rectified the aircraft is considered non-airworthy (AOG, aircraft-on-ground) until that same defect is rectified.

Within Avilytics all information related to deferred defects is collated into the Deferred Defects workspace.

Avilytics Deferred Defects









The Deferred Defect workspace allows to identify for any particular trend around the creation as well as the closure of deferred defects on aircraft. Identification of trends on certain MEL reference, defects as well as deferral reasons provide intelligent insight into potential organizational or systems reliability actions that can be taken in order to reduce the deferred defects being raised and limit any operational impact these might have on overall aircraft & fleet availability

A quick metrics also provides the ability to spot seasonal trends, compare individual aircraft or even compare full years with one another.

How to track the reason for deferring a defect?

Knowing what the reason has been why specific defects get deferred rather than rectified will help to identify specific actions to take in order to reduce the rate of deferred defects. Reason for deferring defects by engineers can have various reasons, however the actions taken to reduce these deferred defect rate focus either on certain organizational measures or further technical investigations into the failing system as a potential systems reliability action.

Tip:        Adding short codes to the MEL deferral entry in the aircraft techlog will allow to spot trends on defect deferral reasons

Having a predefined set of short codes to be used by engineers whenever they defer a defect will help to identify any trends that corelate to deferred defects. It will assist in determining if organizational actions need to be taken or if further systems reliability investigations need to be launched. A typical list of short codes could include:

NPA:     No Parts Available

NMPR:  No manpower available

NGRT:   No groundtime available

TS:         Further troubleshooting required

NTLA:    No tooling available

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.