SUBJECT - BLEED AIR WARNING SYSTEM INSTALLATION IN RAYTHEON
(BEECH) TURBOPROP AIRCRAFT
SAFETY DEFICIENCY
Incorrect bleed air warning installation can lead the crew to
mis-identify a bleed air duct leak and isolate the wrong system
leading to pressurisation failure.
FACTUAL INFORMATION
Related Occurrences
199704165
During climb through FL180 the left bleed air warning light of the
Raytheon (Beech) B200 illuminated. The left bleed air system was
turned off when the aircraft was stabilised at cruise at FL220. The
cabin then slowly depressurised to 14,000 ft. A descent was
initiated and the aircraft returned to the departure
aerodrome.
A subsequent maintenance investigation found loose ducting and a
hole in the right bleed air system. In addition, it was found that
the left bleed air pressure transmitter was connected to the right
air bleed system and vice versa. The Australian Transport Safety
Bureau advised the Civil Aviation Safety Authority of these
findings when they became known.
The pilot in command subsequently stated that if the uncontained
bleed air leak had led to loss of the aircraft, the investigation
may have attributed this to his actions in not properly identifying
and dealing with a bleed air leak. This would have been the case
even though he had correctly identified and shut off the system
annunciating the fault. He reasoned that the cross-connected
warning system led him to shut off the serviceable bleed air
system. This then left the ruptured and leaking system still in
operation jeopardising the safety of the aircraft. If this had
resulted in an accident, the reason the pilot selected the wrong
system off may have been impossible to determine from examining the
wreckage and may have been incorrectly attributed to misidentifying
the failed system.
ANALYSIS
The B200 bleed air warning system is provided to give a visual
warning to the crew of a leak in the airframe bleed air ducting
behind the firewall of either engine. The bleed ducting is designed
to contain hot bleed air, which is under pressure. To protect
surrounding airframe structure from radiated heat the pipes are
wrapped in heat insulation lagging, which effectively prohibits
visual inspection of the piping for defects, such as pinholes, and
fatigue type cracks.
To monitor these pipes for leakage, a small diameter plastic
ethylene vinyl acetate (EVA) tube is routed in close proximity for
the entire length of each bleed ducting. This EVA tube is connected
to a normally open pressure switch located under the cabin floor. A
pressure of approximately 18 psi is supplied to this EVA tube
continuously from the de-ice manifold. When hot air from a ruptured
bleed air pipe contacts the EVA tube, the tubing melts. Depending
on which half of the system has failed, the released pressure
closes the pressure switch and illuminates the appropriate red left
bleed air fail (LH BL AIR FAIL) or right bleed air fail (RH BL AIR
FAIL) annunciator in the cockpit. The warning informs the crew that
there is a hot air leak somewhere in the left or right ducting,
which can then be isolated from the aircraft system by appropriate
checklist actions.
The EVA tubes, one for the left bleed air supply ducting and one
for the right, are routed forward to the pressure switches under
the cabin floor. The tubing from each side is identical. The
pressure switches are mounted side by side and are identical in
appearance and function.
It is possible that during the replacement of either the tubing or
the switches, the EVA tubing could be crossed and the indication
system would become cross-connected, displaying warnings for the
wrong half of the bleed air supply system plumbing. The crew can
then be misled into shutting off the remaining serviceable bleed
air source. This would result in a loss of cabin pressurisation and
the continued danger of leaking bleed air in a wing cavity. That
was the situation that presented itself in this occurrence.
In this situation with the bleed air system now completely
disabled, the pilot in command and passengers would be put at
additional risk of loss consciousness through the loss of
pressurisation after unknowingly turning off the remaining
serviceable system.
The pressure source for the EVA tubes, the de-ice manifold, is
pressurised by either engine operating and supplies both left and
right tubes. There is no provision for the pilot to test the
warning systems individually to ensure the warning generated is
correct.
An examination of the maintenance manual for the B200 series
aircraft revealed that the bleed air warning system does not have
any functional test procedure listed to verify the correct function
of the bleed air warning system after maintenance.
This type of bleed air warning system is used on, but not limited
to the B200 aircraft within the Raytheon product line, therefore
other models including the B300, B350 and B1900 could also be
affected.
