Page 273 - The Jet Engine

25: Overhaul

Contents Page

Introduction 263
Overhaul/Repair 265

Disassembly
Cleaning
Inspection
Repair
Balancing
Moment weighing of blades
Assembling
Testing
Preparing for storage/despatch

hours have been achieved, this concept is known as
time between overhauls (T.B.O.). Operators will often
remove engines in order to acquire 'fleet stagger'
thus preventing a situation when an unacceptable
number of engines require removal at the same
period of time.
INTRODUCTION
3. The length of time between overhauls varies con-
1. It is most important that the cost of maintaining
an engine in service is considered at the design siderably between different engine types, being
stage. All aspects of engine repairability are also established as a result of discussions between the
operator, the airworthiness authority and the manu-
considered, both to reduce the requirement for facturer, when such considerations as the total
overhaul or repair and to avoid, where possible, experience gained with the particular engine series,
designs which make repairs difficult to effect. Engine the type of operation, the utilization, and sometimes
construction must allow the operator to complete the climatic conditions, are taken into account. In
overhaul or repair work as quickly and cheaply as improving the overhaul period the airworthiness
possible. authority may take into consideration the background
of the operator, his servicing facilities and the
2. In service, the engine is inspected at routine experience of his maintenance personnel.
periods based on manufacturers' recommendations
and agreed between the operator and the relevant 4. When a new type of engine enters service,
airworthiness authority. The engine is removed from sampling (i.e. engine removal, dismantling and
the aircraft when it fails, during these inspections, to inspection) may be called for at a modest life. The
meet the specified standards. This concept is a form sampling will be continued until the life at which the
of 'on-condition' monitoring, reference para. 9, engine should be overhauled is indicated by the
however, regardless of condition, some engines are condition of the sample engines or by its reliability
removed when a stipulated number of engine flying record in service. In some instances, the ultimate life

263