On 6 December 2004, a Fokker Services B.V. F28 Mk0100 (Fokker
F100) aircraft, registered VH-FWI, was being prepared for a flight
from Townsville to Brisbane, Queensland, when the ground crew
noticed a rumbling noise coming from the left airconditioning pack
and notified the flight crew. No fault indications were present on
the flight deck, so the flight crew elected to depart for Brisbane
with both packs operational.
During cruise at Flight Level 350 (35,000 ft), the flight crew
noticed a burning smell and a loud noise coming from the
airconditioning system. Based on the earlier report from the ground
crew, the flight crew shut down the left airconditioning system.
The airconditioning/pressurisation system is designed so that the
left pack normally supplies the flight deck and the right pack
normally supplies the cabin area. In the event that one system
fails, or is shut down, the other system is capable of supplying
the required air to both the flight deck and the cabin. The noise
continued so the left pack was switched back on and the right pack
was shut down. The noise then stopped, confirming that the failed
system was on the right side.
Seven minutes later, the left pack produced similar symptoms and
was shut down by the crew. With both systems shut down, the
aircraft's pressurisation system was rendered inoperable and the
cabin altitude began to rise. The crew donned oxygen masks,
commenced an emergency decent to 10,000 ft and notified air traffic
control. The flight continued to Brisbane without further
incident.
Subsequently, both air cycle machines1 (ACMs) were removed from the
aircraft. A general inspection of the ACMs by maintenance engineers
found that the heat exchangers were in good condition so they were
returned to service. However, the cooling turbines were not
serviceable because the turbine shafts where difficult to rotate.
These cooling turbines were replaced with serviceable items.
A maintenance ground run was subsequently carried out to check
the aircraft bleed air system. The check found that both bleed air
temperature modulating valves and one of the pressure regulating
valves were malfunctioning. The malfunctioning valves were replaced
and the aircraft returned to service.
Since June 2004, the operator had sustained seven (including
these two) cooling turbine failures in its fleet of two Fokker F100
aircraft. The operator had previously noted the rate of these
failures and had investigated ways to improve the reliability of
the system.
The cooling turbines from this aircraft, along with four other
failed units, were sent to the ACM component manufacturer for
failure analysis. Those examinations found that all six units had
failed because they had been operated outside of the speed range
for which they had been designed.
A review of five of the six failed turbines found that at least
one of the aircraft's bleed air control valves (pressure,
temperature or flow rate) had also failed.
As a result of two overseas reports of the in-flight release of
engine fan case ice impact panels, the Australian Civil Aviation
Safety Authority (CASA) issued Airworthiness Directive (AD)
AD/F100/59 in January 2004. This Airworthiness Directive (AD)
included the following requirement:
Amend the Aeroplane Flight Manual, Section 5.05.01 to include
the following conditions on the use of engine and airframe
anti-icing systems by inserting the following:
Engine anti-icing must be switched ON during all ground or
flight operations when Total Air Temperature TAT is below +6
degrees C (+42 degrees F) down to and including -25 degrees C (-13
degrees F), irrespective of the presence of visible moisture.
The operator reported that this AD resulted in the use of the
anti-ice system increasing from approximately 20% of flights to
approximately 90% of all flights.
The cooling turbine manufacturer noted that the use of anti-ice
at altitudes above 30,000 ft can place the ACM outside the design
conditions, resulting in an overspeed. The design standard for the
aircraft (United States Federal Aviation Regulation Part 25)
defines the limiting icing envelope up to an altitude of 30,000 ft.
There was no requirement to design the system to operate in icing
conditions above this altitude.
Following the issue of engine Airworthiness Directive
AD/TAY/122 amendment
2, on 9 November 2004, CASA determined that the additional
operational requirements of AD/F100/59 were no longer required.
AD/TAY/12 amendment 2 required the following actions to be carried
out:
- Carry out an initial and repetitive examination of the bonding
of the low-pressure compressor ice impact panels in accordance with
Rolls Royce SB TAY-72-1638R2 or TAY-72-1639R2 as applicable.
- Repair or replace all low pressure compressor ice impact panels
if any visible movement, rocking motion or reappearing moisture on
the LP compressor case ice impact panel have been detected during
the examination.
- Replace all affected low-pressure compressor case ice impact
panels in accordance with Rolls Royce SB TAY-72-1638R2 or
TAY-72-1639R2 as applicable.
CASA stated in the AD that 'The actions specified by this
Airworthiness Directive are intended to make sure that the bonding
of these LP compressor ice impact panels complies with the design
intent'. CASA cancelled AD/F100/59 on 23 December 2004.
The aircraft operator has reported to the ATSB that there have
been no cooling turbine failures since AD/F100/59 was
cancelled.
