Fokker 70 and Fokker 100 product development program for a APU bleed air filter.
Introduction:
Service experience of the Fokker 100 and Fokker 70 has shown that soot deposits contaminate key-components of the bleed air system and air-conditioning systems. This contamination reduces system performance.
Analysis of the deposits has indicated that a large fraction consists of carbon dust. Air pollution at airports (diesel engine exhaust of ground service vehicles) and, to a lesser extent, carbon brake dust thrown up by jet blast are considered to be the primary sources of this contamination. The contaminated air is sucked in by the APU air intake. Subsequently, polluted air is distributed into the bleed air system and air-conditioning systems during ground operation.
Restoration and/or cleaning of components is difficult and time consuming. However, up to now, it is the only way to keep the performance of the air-conditioning system and bleed air system to an acceptable standard.
In 1991, an APU bleed air filter was tested on a Fokker 100, but the filter characteristics and contamination required a replacement interval of the filter of 70 FH. This interval was judged to be too low for day-to-day airline operation and did not provide an acceptable return of investment.
With the use of the latest filtration techniques, a new and larger filter has been developed, taking into account the experience gained during the 1991 test. The aim was to demonstrate feasibility of a filter replacement interval of at least 350 FH.
A preliminary cost-benefit analysis indicates that the pay-back period for this modification is less than 2 years under the assumption that a conservative improvement of 20 % MTBUR (Mean time between unscheduled replacements) on components is obtained. In addition, the filter will have a positive effect on passenger comfort because degradation of the Environmental Control System (ECS) is reduced.
Installation:
The APU filter will be installed in the rear fuselage of the Fokker 100 and Fokker 70 behind the rear pressure bulkhead. The conical shaped APU filter housing and two small ducts will replace a section of the existing APU bleed air ducting. No electrical or additional structural interfaces are required. Therefore, installation can be accomplished during an overnight check.
Filter replacement on aircraft can be accomplished in approximately 15 min.
Availability and program status:
The trial is in progress and has demonstrated that the filter allows a replacement intervals of 350 - 500 FH without noticeable degradation of the bleed air supply. 
Some product improvements were considered necessary to improve the filter resistance to the high temperatures. Two improved versions of the filter have been developed and tested successfully in the laboratory.
The improved filters are to be installed in June/July 2002. After installation, about 350 - 450 FH are needed for evaluation (which equals approx. 2 months). Assuming that the trail results confirm the laboratory tests, the APU filter modification will be available as a modification for all Fo100/70 operators in the third quarter of 2002.
The modification will be introduced per Fokker Service Bulletin SBF100-36-028.
Please contact your Fokker Services Account Manager for further information.