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
- 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.
air cycle machine is the cooling section of the airconditioning
system and is comprised of a cooling turbine and heat
2 The Fokker F100 series aircraft are fitted with Rolls Royce Tay engines.
On 9 February 2005, the aircraft operator reported to the ATSB that it had already implemented, or intended implementing the following improvements to the maintenance of the airconditioning and bleed air systems:
- Reduced the mesh size of external filters on the flow control valve and reduce the replacement interval from 4,000 flight hours to 500 flight hours.
- Upgraded the turbine bypass valves to the latest standard.
- Introduced regular heat exchanger cleaning at 2,000 flight hour intervals.
- Cooling turbine overhaul period reduced to 9,000 flight hours.
- Cooling turbine oil level and contamination check introduced.
- Cooling turbine oil replacement interval reduced from 4,000 flight hours to 1,000 flight hours.
- Shut-Off and Pressure Regulating Valve (SOPRV) and Shut-Off and Temperature Modulating Valve (SOTMV)1 to be overhauled at every shop visit (previously completed on condition).
- Pressure regulating and temperature modulating valve filter replacement interval reduced from 4,000 flight hours to 500 flight hours.
- A six-monthly bleed air system verification check has been introduced to check for proper system operation.
- A 4,000 flight hour interval, complete system survey to check the entire bleed air and airconditioning system has been introduced.
On 24 February 2005, the aircraft manufacturer advised the ATSB that it had issued two optional maintenance tasks for cleaning the primary and secondary heat exchangers and additional maintenance for the air cycle machine. These tasks were sent to the operators by means of a Temporary Revision to the Maintenance and Planning Document (TR OPT-002, dated January 12, 2004) and were incorporated into the F70/100 Maintenance and Planning Document, issued by Fokker Services B.V. on July 1, 2004.
1 The SOPRV and SOTMV are components in the bleed air system used to control the system pressure and temperature.
|Date:||06 December 2004||Investigation status:||Completed|
|Time:||1355 hours EST|
|Location:||56 km SW Rockhampton, Aero.|
|Release date:||24 June 2005||Occurrence class:||Technical|
|Report status:||Final||Occurrence category:||Serious Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Fokker B.V.|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure point||Townsville, QLD|
|Departure time||1259 hours EST|