The PT6A engines bleed valve lives a tough life! Its job is to get rid of excess air pressure generated by the engine compressor’s first three stages at lower RPMs or power settings. If it doesn’t do its job, the compressor can stall, emitting loud bangs when the pilot tries to accelerate the engine or reduce power, perhaps on approach! If it fails completely, large amounts of power can be lost, even enough to result in off airport landings!
The valve uses compressor discharge air pressure (P3), ported to one side of a piston, to close the valve. On the other side of the piston is the pressure that exists about two-thirds of the way through the compressor (P 2.5). The piston is sealed with a fabric impregnated high-temperature rubber diaphragm on engines up to the -60 series and with a carbon ring supported by a steel expander ring on the 65 and 67 PT6s. P3 pressure is higher than P 2.5 and the valve uses a rather fancy orifice to regulate P3 pressure for closing the valve! The orifice is called a “Convergent – Divergent” orifice! Basically, it’s a miniature, stainless steel venturi tube! This orifice is screwed into the top cover of the bleed valve and vents P3 pressure. As the engine accelerates, both P3 and P2.5 pressures increase. As the P3 pressure passes through the Convergent – Divergent orifice, the air accelerates. The orifice is sized to cause the accelerating air to reach supersonic speed at about 86% to 91% gas generator speed! The supersonic air causes a shock wave in the orifice, which in turn, blocks the orifice and causes the valve to close! I think someone stayed up late designing this orifice!
Whatever air the engine is inhaling is being vented by and used by the bleed valve! If your compressor gets dirty, your bleed valve is getting dirty too! Compressor washes can be fairly effective in cleaning up a compressor but isn’t effective, at all, in cleaning and maintaining the bleed valve! The maintenance manual doesn’t give much information on maintaining the bleed valve although it is quite a star in the performance troubleshooting charts! We find it to be good maintenance practice on PT6A engines to remove the Bleed Valve at the annual (100 hour) inspection and bench check the valve. It’s checked for its cleanness, freedom of operation, piston-sealing surface fretting (a good indication of engine vibration levels!), if the valve is closing at the right pressure, and Airworthiness Directive status.
The AD Note focuses on a loose guide pin normally tight-fit to the cover. If the pin is loose, its
safetying cotter pin can fracture and the pieces interfere with the valves operation! The AD Note, # 97-
04-12, makes the statement: “To prevent engine power reduction due to malfunction of the compressor
BOV, which could result in a forced landing and loss of the aircraft, accomplish the following: (then follows the technical stuff)” Bleed valves which are affected by the AD have a manufacturer’s code of: 8070 on the cover. Placing “RE71” on the cover following the part number identifies the bleed valve as having had the inspection complied with.
The compressor bleed-off-valve is “Out of Sight, Out of Mind”, but it sure deserves your mechanic’s attention come maintenance time!
Y’all Fly Safe!