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LESSONS LEARNED
engine start detects a potentially dangerous failure condition,
control is passed to the backup channel. If it detects the same
fault, the EEC can shut the engine down to prevent further
damage. By design, the two engines' EECs operate independently.
The airplane's fault history database recorded a " VSV SYSTEM/
TRACK (L) " code corresponding to the initial " L ENGINE
MAINTENANCE " warning. This code indicates a discrepancy
between the commanded and observed positions of one or more
of the variable stator vanes (VSVs) in the engine's ten-stage highpressure
compressor. The four-stage VSV system is designed to
to compensate for operating conditions by optimizing airflow
through the compressor. All four vanes are controlled by a single
hydraulic actuator via jointed mechanical linkages. The actuator is
driven by fuel flow and relaxes to a default position if the engine is
shut down.
Though its siblings in the BR700 line triggered only a " DO NOT
DISPATCH " warning in response to a VSV mismatch, the original
BR725 software instead commanded an engine shutdown. A
similar episode in September 2018 prompted Rolls-Royce to revise
the BR725's operating logic to conform to the other members of
the BR700 family, but operators were given two years to make the
update. It had not yet been implemented on the incident airplane.
An on-aircraft torque check found that on both engines, the
maximum torque required to manually actuate the VSV system
exceeded the service limit of 50 Newton-meters (Nm). A
maximum of 76 Nm was measured on the right engine but was
brought within limits after lubrication. The left engine reached 129
Nm, which was not resolved by lubrication. A subsequent engine
strip found that while most of the first- and second-stage vanes
moved freely, about half the third-stage vanes and nearly all the
fourth-stage vanes were stiff; some of the fourth-stage vanes
could not be moved by hand. Some third- and most fourth-stage
vanes also lacked the designed clearance (or " float " ) between the
vane and the engine casing.
After the vanes were removed, heavy corrosion was found in the
third- and fourth-stage bores in the compressor casing, reducing
the diameter within the bushings close to that of the spindles
within them. Only light corrosion was observed in the first- and
second-stage bores. Crystalline deposits were found immediately
downstream of the fan at the entry to the engine core, and in the
compressor, particularly on the stator vanes between the thirdand
fourth-stage VSVs. Scanning electron microscope analysis
" determined that the deposits were largely composed of sodium
chloride, sodium sulphate, and sodium nitrate, all environmental
salts. " Like the airplane involved in the 2018 episode, this
Gulfstream was based in a coastal environment.
Rolls-Royce responded by expediting the software update over
the rest of the fleet; it was completed on all BR725 engines by
September 2019. The company also issued a service bulletin
mandating torque checks on all engines older than 24 months,
and added a scheduled maintenance task of re-lubrication every
1,200 flight hours or 48 months, whichever comes first.
Relevant EASA certification requirements include a maximum
risk per flight hour of " any individual component failure
developing into a hazardous engine effect " no greater than one in
100,000,000 and a maximum risk of one per 100,000 flight hours
of an in-flight engine shutdown. The 2018 episode demonstrated
that " an EEC-initiated shutdown was too severe a consequence
for a VSV track fault. " The generous initial schedule for correcting
the problem presumably reflected a belief that the occurrence
remained very unlikely-an assumption this second incident
served to correct.
-David Jack Kenny is a freelance aviation writer and
statistician.
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