WARRANTY PERIOD AND INSPECTION POLICY
IN AIRCRAFT SERVICE

Nicholas A. Nechvala, Konstantin N. Nechvalb, Edgars K. Vasermanisa, Kristine Rozitea

a) Department of Mathematical Statistics
University of Latvia
Raina Blvd 19, LV-1050, Riga, Latvia,
e-mail: nechval@junik.lv

b) Department of Computer Science
Transport and Telecommunication Institute
Lomonosov Street 1, LV-1019, Riga, Latvia
e-mail: konstan@tsi.lv

KEYWORDS
Aircraft Structure Components; Warranty Period; Fatigue Cracks;
Inspection Policy.

ABSTRACT
Fatigue is one of the most important problems of aircraft arising
from their nature as multiple-component structures, subjected to
random dynamic loads. Fatigue damage is considered to initiate in a
structural component when cracks develop, whether or not they are
detected. In this application warranty period of the aircraft structure
component is defined as the operational life to initiation of a fatigue
crack of detectable size. The fatigue process then continues by crack
propagation, resulting in strength degradation. Periodic inspections of
aircraft are common practice in order to maintain their reliability above
a desired minimum level. This paper proposes a methodology to
determine warranty period and (if, after this period, a crack in aircraft
structure components is not detected) appropriate non-periodic
(decreasing) inspection intervals for fatigue-sensitive aircraft structures,
so that their reliability remains above a prespecified minimum level
throughout their service life. The methodology, based on assumption
that the time to crack initiation (measured in number of flight-hours) is
a random variable with density function following the Weibull
distribution, is applicable for situations where it is difficult to quantify
the costs associated with inspections and undetected cracks. This
methodology allows one to find the inspection policy for detection of
initial cracks in critical structural components of aircraft when only the
functional form of the distribution of the time to crack initiation is
specified but some or all of its parameters are unspecified; this
constraint is often met in practice. The numerical examples are given.