The Nimrod Review - Official Documents

The Nimrod Review
76
5.17
determined age. Following the impetus given by the BOI, research was undertaken on the proclivity of seals to
leak. The results have suggested, however, that whilst there are a number of factors influencing leak rates, ‘age
and usage’ may not be the primary factors.
Components which are not lifed are maintained ‘on condition’ on the basis that: (a) the safety and operational
consequences of failures are ‘acceptable’, i.e. not catastrophic; and (b) it is reasonable to assume that the nonlifed
components are fit for purpose for the assumed life of the aircraft. It follows, therefore, that even where
there are no apparent assessed safety or operational reasons to life seals, it is essential to re-evaluate previous
lifing assumptions when a fleet is extended in service. The numerous delays to the Nimrod MR2 replacement
programme are a matter of public record (see Chapter 14). It is clear that the year-on-year delays to that
programme have extended the life of the Nimrod MR2 aircraft by many years. In these circumstances, therefore,
it is particularly disappointing that the MOD failed at any stage to observe the trend of an increase in the fuel
leak rate and failed to properly consider whether the previous lifing assumptions and maintenance policy for
fuel seals remained valid. An appreciation of the increasing trend of fuel leaks may have given pause for thought
by those responsible for compilation of the Nimrod Safety Case (NSC) when they came to consider the risk
posed by the Cross-Feed/SCP duct (see Chapter 10).
Categorisation of Leaks
5.18
Leaks are not simply noted and ignored. Leaks from wing tanks are regarded as far less of a problem than leaks
within the fuselage or in the vicinity of the aircraft’s engines. This is because leaks from wing tanks are not near
a potential heat source and will disperse into the airflow. While there is a zero-tolerance policy for fuel leaks
from pipes and couplings, which are always rectified as soon as they are discovered, the rectification of leaks
from integral fuel tanks may be deferred, based upon a categorisation system.
19
5.19 Leaks in integral wing tanks are categorised in increasing order of seriousness:
5.20
In summary:
“STAIN. Where fuel wets an area around the leak source not over 50mm (2 inches) in diameter in 2
hours”.
“SEEP. Where fuel wets an area around the leak source not over 160mm (6 inches) in diameter in 2
hours”.
“HEAVY SEEP. Where fuel appears to spread very slowly to cover an area larger than 160mm (6 inches)
in diameter. However, it does not flow or drip”.
“RUN. Where fuel is running and dripping at a rate less than 10 drops per minute. Alternatively, an area
where fuel appears immediately to flow or run, following the contour of skin where the area is wiped
dry.”
“UNACCEPTABLE LEAK. Where fuel is running or dripping at a rate greater than 10 drops per minute.
Repair or exceptional limitation action must be undertaken.”
“Stains” and “Seeps” in wing tanks are termed “minor leaks” and require recording and documenting
“during pre and post scheduled maintenance fuel leak mapping”, and will not necessarily require
rectification at the next tank opening or scheduled maintenance unless there is a “large concentration”
of them.
“Heavy Seeps” and “Runs” in wing tanks require rectification at the next tank opening or scheduled
maintenance and must be fully documented in the aircraft’s F700, F704 Acceptable Deferred Fault
Register.
Immediate repair of “Unacceptable leaks” may only be deferred “in exceptional circumstances”.
19 See AP101B-0500-2(R)1 Part 1, Leaflet 013, Annex 1, page 6-10 (BOI Report, Exhibit 50).