MAINTENANCE MINUTE WHEN THE HEAT IS NOT ON
By Scott Skola
Here’s a tip to help troubleshoot a customer’s bleed air system. Different manufacturers have their own nomenclatures, but this is the system that takes hot engine bleed air and routes it for cabin heating, cooling turbines, particle separators, and the like.
On occasion, the bleed air problem manifests itself only when the aircraft is running. One way to check this without running an engine is to apply shop air to the aircraft bleed air system. The picture below is a work-aid version used for Bell 212/412, MBB Bo105, Bell 222/230, or any aircraft with a 209-type bleed air “deck” valve. It consists of an AN919 fitting, an AN894 fitting, and standard compressed air fittings assembled with a pressure- rated rubber hose. But you can use any type of fitting or hose combination to connect a specific aircraft or bleed air system.
Connect the work aid to a bleed air system hose or tube at the engine supply side. Plug the work aid into an adjustable air pressure regulator, then plug the regulator into an air compressor. Adjust the regulator to the required bleed air system pressure and troubleshoot away.
Some common problems associated with bleed air systems: bleed air supply valves sticking under pressure, corroded electrical connections, excessive system leakage, and faulty temperature sensors.
On valves that stick, one of the primary causes is engine compressor wash soap that remains in the bleed air valve after the wash. When possible or convenient, disconnect the bleed air
system line upstream of the valve prior to engine wash. If unable, use a second rinse bottle per engine to flush out as much soap as possible. Be sure to turn on the bleed air system during the engine wash drying run for a full two minutes or as recommended by the manufacturer.
Another culprit on sticky valves is water quality used for the engine rinse/wash. Deposits build up on valve internal parts causing intermittent operation. Most engine maintenance manuals define water quality requirements, but for a quick check fill a glass jar with a small amount of water. Leave it in the sun to evaporate and see what’s left.
Corroded electrical connections can be prevented with Dow Corning 4 or Stabilant 22 applied to the contact pins and a light coat of an LPS product, or equivalent, to the exterior of the connector.
Short of a loose connection, excessive bleed air system leaks are normally a remove/replace option. Verify system integrity with a commercial leak-check product or a DIY dish soap-and-water blend that will indicate bubbles at suspected air leaks. Be sure to check the entire system plumbing and not just the connections.
If the bleed air system checks out good with the work aid and shop air, look to the system temperature sensors. If accessible, you can use a heat gun and thermometer on the temperature sensors to see if they operate at the required setting.
EDITOR’S NOTE :
On occasion we will bring back timeless popular maintenance tips previously published in past issues of Rotor Pro Magazine. This tip is from May 2018.
20 Nov/Dec 2022
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