The Nimrod Review - Official Documents

The Nimrod Review
Introduction
23.1
23.2
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‘Age’ in relation to aircraft is, understandably, an emotive subject. There is a perception that ‘old’ is bad and
‘new’ is good. The equation is, however, not as straightforward as that. Age is not ipso facto bad, so long as its
effects are understood, appreciated, and defied. Age may even bring some advantages in the form of reliability
and predictability. The average age of the US Air Force B52 Bomber fleet is 47 years, and I understand that it
is the current intention of the US Air Force to continue to operate this fleet until it is at least 75 years old, and
perhaps beyond.
Nonetheless, older aircraft generally need more care, not less. Ever greater rigour, resources, and vigilance are
required to ensure they remain airworthy. Given the right levels of care, repair, and maintenance, however,
there is no reason why many ‘legacy’ aircraft should not continue to fly safely for many years. It is a question of
resources, priorities, and unrelenting attention to detail.
‘Bathtub’ curve
23.3
(9) Determination of ‘baseline’ safety.
9. The lesson learned from the Nimrod XV230 case is that age matters. The following matters are all
the more important as an aircraft gets older:
(1) Good trend monitoring;
(2) Good husbandry;
(3) Good attention to spares;
(4) Good attention to skills;
(5) Good corporate knowledge; and
(6) Good forensic teardowns.
The ‘bathtub’ curve is a useful graphical representation to illustrate a common reliability trend during the life of
a product or piece of equipment (See Figure 23.1 below). The ‘bathtub’ curve comprises three periods: (a) the
‘infant mortality’ period with an initially high but (immediately decreasing) failure rate as ‘teething’ problems
are sorted out; (b) a ‘normal’ or ‘useful’ life period with a low, relatively constant failure rate (as the product or
equipment and its use settles down); and (c) a ‘wear-out’ period which exhibits an increasing failure rate as age
and reliability begins to take its toll.
Failure
Infant mortality
‘BATHTUB’ CURVE
Normal or ‘Useful’ life
(‘constant’ failure rate)
Figure 23.1 The ‘bathtub’ Curve
Wear-out
period
Time