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Output Number
R20000043
Approval Date
Published Date Time
Recommendation type
Recommendation
Mode
Aviation
Date released
Background Text

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.

The Australian Transport Safety Bureau recommends, that Raytheon
Aircraft ensure some positive means of preventing the possibility
of system cross-connection of the bleed air failure pressure
transmitters.



As a result of the investigation of this safety deficiency, the
Bureau simultaneously issues the following recommendations:



R20000042



The Australian Transport Safety Bureau recommends that Raytheon
Aircraft review the Engine Bleed-Air Warning System post
maintenance procedures to include a functional test of the system
to verify that the failure warning correctly annunciates the failed
system.





R20000044



The Australian Transport Safety Bureau recommends that the United
States Federal Aviation Administration monitor the aircraft
manufacturer to ensure that this safety deficiency is
addressed.



R20000045



The Australian Transport Safety Bureau recommends that the
Australian Civil Aviation Safety Authority liaise with the United
States Federal Aviation Administration to ensure that this safety
deficiency is addressed by the manufacturer.



R20000284



The Australian Transport Safety Bureau recommends that the
Australian Civil Aviation Safety Authority alert all owners and
operators of Raytheon (Beech) turboprop aircraft fitted with the
EVA tube type bleed air warning system, to adopt post maintenance
procedures which include a functional test of the system to verify
that the failure warning correctly annunciates the failed
system.

Organisation Response
Date Received
Organisation
Raytheon Aircraft
Response Text

From: Associate Manager, Airframe and Servicesw

Wichita aircraft Certification Office, ACE-118W



This is our response to Safety Recommendations 01.041 and 01.042.
which pertain to failures of the bleed air warning system on a
Raytheon Model B-200 King Air.



Raytheon Aircraft Company (RAC) was requested to review the bleed
air warning system on the Model B200 and consider corrective
measures as proposed in the above safety recommendations. We have
reviewed RAC's proposed plan for corrective action and concur with
their plan. Below is a summary of the proposed corrective action to
address SR 01.041 and a rationale for not implementing corrective
action for SR 01.042.



SR 01.041 - "The Australian Transport Safety Bureau recommends that
Raytheon Aircraft review the Engine Bleed-Air Warning System post
maintenance procedures to include a functional test of the system
to verify that the failure warning correctly annunciates the failed
system."



Proposed Corrective Action: A review of the maintenance manuals for
the King Air Series airplanes revealed that the 300 and 1900 series
manuals instruct the maintenance personnel to label or tag wires,
cables, hydraulic lines, and tubing for proper reconnection. Each
of these manuals also provides instructions for functional testing
to ensure proper hookup within the Bleed Air Warning System
maintenance procedures. These items are not currently in the 90 and
200 series maintenance manuals, which include the Model B200. The
next revision of the Model 90 and Model 200 maintenance manuals
will include the same instructions that are presently included in
the 300 series and 1900 series manuals. The next revision to the
Model 90 maintenance manual is scheduled to be released at the end
of December 2001, and the next revision of the Model 200
maintenance manual is scheduled to be released.



SR 01.042 - "The Australian Transport Safety Bureau recommends that
Raytheon Aircraft ensure some positive means of preventing the
possibility of system cross-connection of the bleed air failure
pressure transmitters."



Rationale for Not Implementing Corrective Action: The request for a
design change is not considered necessary in view of changes to the
maintenance manual described above, Which are considered adequate
to address the safety concern. Additionally, if this type of design
change was deemed necessary for the bleed air warning system, then
other systems on the airplane would require evaluation and
corrective action as well in a similar manner. While some systems
(e.g., flight controls) have specific regulations that address
design requirements for minimization of the possibility of
incorrect assembly,there is no such regulation that applies
specifically to other systems, such as the bleed air warning
system.



Therefore, we recommend that the subject safety recommendations be
closed out as discussed herein.