Page 17 - Nov 2015 Volume 9, Number 11

With all of its complexity of floats and pumps, the Fuel Drain Collector system has proven to be surprisingly reliable and trouble-free. Malfunctions are rather rare, but when they occur they often take the form of a bad float or bad pump that prevents the tank from ever being emptied by being pumped into the nacelle. Usually it will take four shutdowns or more before the collector tank is so full that the overboard vent comes into play. Although we are now contributing to the ramp’s pollution just like in the early days, realize that no harm to the engine is taking place as the dumping fuel overflows onto the ramp. If you notice fuel draining at shutdown, you need to have maintenance find and fix the problem.
The other malfunction that can befall the Fuel Drain Collector system is more of a concern than the venting onto the ramp of a small amount of fuel at shutdown. Heaven forbid you receive the dreaded FBO call at midnight telling you that your airplane is spewing fuel onto their hangar floor and you need to get out there now to fix it!
The nacelle fuel tank holds about 60 gallons, most of which sits at a higher level than that 20-ounce collector tank. Remember I wrote that the line from the collector to the nacelle contained a check valve? Well, if that valve gets dirty, or hangs up, or won’t prevent backwards flow for some other reason...you’re trying to stuff the proverbial 100 pounds into a 10-pound sack. Sometimes you will be lucky enough to flush out the valve and have it reseal properly by making the collector pump do its thing. Battery on, fuel control heat switch on, if applicable, wait a couple of minutes and see if the overflow stops...as it surely will. Now turn the switches off and wait anxiously to see if the problem recurs. If it doesn’t, you were successful in cleaning out the check valve. If the flow starts again, better get an A&P on the way!
In 1980, beginning with LJ-901, LW-334, LA-58 and BB-666, Beech replaced the fuel drain collector system with a totally different and simpler design: the Bleed Air Purge system. A new, small line was added to the P3 tap-off from the engine and was routed, through a check valve, to a cylindrical accumulator tank mounted roughly where the collector tank had been installed – in the aft, lower area of the nacelle. A line from that accumulator proceeds through a second check valve to the fuel Flow Divider/Dump Valve assembly.
When the engine is operated at high power settings, high N1 speeds, P3 pressure can reach a level of about 100 psia, somewhat higher in the bigger PT6 models and somewhat less in the smaller ones. Since the pressure in the fuel manifold is usually much greater than this, fuel is always trying to back up into the accumulator tank but is prevented from doing so by the check valve. When fuel flow ceases at shutdown, the pressure in the manifold decreases rapidly such that soon the air in the accumulator can escape into the manifold. This shot of air pressure provides enough force to keep the fuel atomizing as it flows through the nozzles into the combustion chamber. Before all of the air shot is expended, all fuel has been purged into the combustion chamber with good atomization...hence, no worry about coking of the nozzles.
Unlike the collector system that the purge system replaced, now there are no moving parts and no electrical power required. In theory, this simpler system should be more reliable and trouble-free. In actual practice, that does not seem to be the case.
Here are two not-uncommon problems. First, if the check valve between the P3 source and the accumulator tank develops a leak, allowing air flow back into the engine from the tank, that trapped 100 psia of air pressure leaks down to near ambient pressure as the engine is operated at idle while taxiing in after landing. So now insufficient air pressure exists to provide the needed purge action and some or all of the residual manifold fuel does its old dribbling into the combustion chamber...with the consequent coking concern and white smoke. Second, if the other check valve leaks – the one between
NOVEMBER 2015
KING AIR MAGAZINE • 15