Air conditioning: How to improve cooling turbine reliability
Effectivity: Challenger 601/604/605/650 aircraft
ATA: 21
By: Costantino Raspa – CSE Bombardier strongly recommends cleaning the dual heat exchanger (DHX) P/N 191486-1/-2 and the pre-cooler exchanger (PCE) P/N 191482-2 every 4,800 flight hours or earlier, if operating in areas with high levels of pollution. This is especially true if there is a tendency to operate the air conditioning unit (ACU) for long periods during ground operations, as most air pollutants exist at ground level.
Although cleaning the DHX/PCE is not a scheduled maintenance task, it is still recommended to clean these units at the suggested interval or earlier as it will:
improve overall reliability of the cooling turbine P/N 586110-9
improve cooling and air flow performance
mitigate an overheat condition leading to a caution PACK TEMP HI message
eliminate odours in the cabin
The performance of the DHX and PCE is critical to the operation of the cooling turbine and overall air conditioning efficiency; therefore, its cleanliness is critical.
History has shown that the most common cause of the issues described above is often dirty, contaminated DHX/PCE and/or F.O.D., such as bird nests, beehives, dead animals and insects resting on top of the PCE or at the bottom plenum where the DHX is located.
These obstructions will block the passage of ram air (cooling medium) through the PCE and DHX, significantly affecting the ability to cool down the 10th stage bleed air before entering the cooling turbine.
The best prevention against F.O.D. is to install protective covers over each ACU EXHAUST PORT and the RAM AIR INLET when the aircraft is parked outside for an extended period.
For DHX/PCE cleaning, different options are available:
a) If the DHX/PCE units require cleaning and maintenance downtime permits, it is recommended that customers send the units to Honeywell, where both the ram air passages and the bleed air passages will be thoroughly cleaned. Furthermore, the units will be pressure tested for internal/external leakage. Honeywell can also make minor repairs if necessary.
The turn-around time is typically 10 to 15 days, but this can vary, so it is best to contact Honeywell for details:
Honeywell International Inc.
6930 N. Lakewood Ave, Tulsa, OK 74117-1807
Main business and AOG phone: 918-272-8000
b) If equipment and materials are available at the maintenance facility during maintenance downtime, procedures for cleaning the DHX/PCE can be found in the applicable Aircraft Maintenance Manual in TASK 21−51−06−160−802 (Cleaning of the Precooler) and TASK 21−51−08−160−801 (Cleaning of the Dual Heat Exchanger). These procedures do not include testing of the units as detailed in option (a).
c) If limited time is available, on-wing cleaning of the DHX/PCE is possible. On-wing cleaning procedures are described in RIL CL-0059 which can be accessed from the Bombardier Customer Portal. Go to Library, in Document Type select Engineering Documents, then select Reference Instruction Letters. Lastly, enter RIL CL-0059 in the search field.
It is important to note that the cleaning procedure described in RIL CL-0059 must not be used to clean the bleed air passages of the DHX/PCE, as there is potential risk of damaging the cooling turbine, valves and sensors with the cleaning solution. Furthermore, there is no practical way to ensure that all cleaning solution/rinsing water is removed prior to operating the air conditioning system. This procedure does not include testing of the units as detailed in option (a).
d) If maintenance downtime is limited, the alternative is to order serviceable DHX/PCE units from Bombardier Spares; however, proper planning is still required to ensure availability of the unit(s). Serviceable units are cleaned and tested by Honeywell.
Information on cleaning the DHX/PCE units is also found in the troubleshooting module of SMARTFIX PLUS under ATA 21-00 (Air Conditioning). Select “By Observed Faults” then select “215OBS001 – LOW FLOW IN CABIN OR FLIGHT DECK.”