Marc Wolf, a Southern California long-time pilot, instructor, and King Air magazine reader, has requested that I write about the different King Air PT6 shutdown fuel purge systems that have been used through the years. I appreciate the suggestion of this interesting topic and will address it here.
First, realize that kerosene is actually more difficult to burn than most people realize. Did your high school chemistry teacher show you the trick of throwing a lighted match into a bucket of kerosene? It probably boggled your mind that the liquid extinguished the match harmlessly, and that no big “Kaboom!” resulted. Either the liquid must be very hot or atomized into a fine mist before combustion is easily achieved.
The fuel nozzles in the PT6 are the devices that atomize the jet fuel into a fine mist that is easily combusted. For this to occur, a tiny, properly-shaped orifice and very high fuel pressure upstream of the orifice are both necessary. The Minimum Pressurizing Valve in the Fuel Control Unit (FCU) won’t permit any fuel to leave the FCU and head for the nozzles without at least 80 psi, and in typical cruise operation we may see pressures near 400 psi.
But when the fuel flow is terminated for the purpose of engine shutdown, very quickly the pressure in the fuel manifold drops dramatically and we lose the shove that sent the fuel through the nozzles with enough force to achieve the desired atomization. Now the remaining fuel merely dribbles though the nozzles, entering the still-hot combustion chamber as liquid instead of atomized vapor. This causes more than one problem. First, the high combustion chamber temperature tends to boil off the lighter weight “hydro” part of this hydrocarbon fuel, leaving the heavier weight carbon behind. That carefully designed and meticulously manufactured nozzle orifice now can become partially clogged with leftover burned carbon residue…called “coke.” The overall result goes by the name of “coking” of the fuel nozzles and the result is a bad distribution of fuel, non-uniform temperature distribution in the combustion chamber, and eventual premature and/or expensive hot section repairs.
The second problem caused by having the last vestiges of fuel dribble, not spray, in the combustion chamber is the appearance of disconcerting white smoke coming out of the engine’s exhaust stacks following shutdown. Those dribbles of liquid jet fuel hit the hot combustion chamber liner surface and are evaporated or boiled into fuel vapor, or fuel “steam.” That is the white smoke we see – vaporized, unburned, jet fuel. The coking of the nozzles that likely preceded the appearance of the smoke is damaging. The smoke is harmless. But, it surely gives the passengers second thoughts about the safety of their transport since they’ve been taught that “Where there’s smoke, there’s fire.”
Recognizing the detrimental effects of allowing the fuel to dribble through the nozzles, turbine engine engineers have made provisions to eliminate this condition. In the Pratt & Whitney PT6, starting right from day one, the engines have incorporated a Dump Valve. This device – between the FCU and the manifold(s) feeding the nozzles, often right on the manifold – is held closed by positive fuel pressure, but opens due to spring force at shutdown when fuel pressure drops. It provides a path of lesser resistance allowing the last bit of fuel to dump harmlessly out of a vent tube onto the ramp. With that easier option available, no fuel dribbles through the nozzles since there is an easier way for it to escape.
The first 10 years of King Air production – like all other turbine engines of the time – had fuel venting onto the tarmac at every shutdown. How much? About a half-cup (four ounces) typically. The Environmental Protection Agency (EPA) was created in 1970 during the years of the Richard Nixon administration and one of its early directives was that all this dumping of raw jet fuel onto the airport ramps and then evaporating into the atmosphere or being washed into the storm sewers could not be a good thing for Mother Earth and her inhabitants. It had to be eliminated.
The King Air model 200 – the best seller of the entire series – was in its certification flight test program from October 1972 to November 1973. Facing the coming EPA mandate, Beech was working to incorporate a new shutdown fuel purge on this airplane. Nearly the entire Beech factory workforce was permitted to leave their normal posts and to move out by the runway at Beech field when the prototype, BB-1, made its maiden flight on October 27, 1972. We watched the white plane takeoff and were there for the landing about 45 minutes later. Ah, success, as the plane taxied up to the parking spot near where Mrs. Beech herself, Chairwoman of the Board, and Frank Hedrick, President, were waiting. As the engines spooled to a stop, it was a bit nerve-racking to see tons of white smoke pouring out of each set of exhaust stacks. Hmm, we all thought, are we going to watch BB-1 go up in smoke after just one flight?! The smoke eventually stopped and the engineers went back to their desks for a little more attention to this area of concern.
The system that was perfected and installed on all early 200s quickly made its way onto the other models of King Airs that were being built. Serial number LJ-672 in the C90-series, LW-124 in the E90-series, and B-208 of the A100-series were the first King Airs, other than the 200 model, to have a factory-installed “Fuel Drain Collector” system. Beech offered a kit to add this system to earlier models to bring them into compliance – known as the “EPA Kit” – and some operators got field-approvals for systems of their own design. This all happened in 1975.
The Beech system is comprised of the following elements: (1) a metal, rectangular, collector tank big enough to accept about 20 ounces of fuel, mounted on the lower portion of the aft cowling fire seal; (2) an Up-On/Down-Off float switch installed in that tank; (3) a line going from the tank to a pump; (4) a small electric fuel pump mounted in the cowling near the tank; (5) a line, containing a check valve, going from the pump back into the nacelle fuel tank; and (6) a vent line from the top of the collector tank going to a universal drain tube that vents (don’t tell the EPA!) onto the ramp.
The new fuel drain collector pump needed a power supply. It was found that the Fuel Control Heat circuit breaker/switches had enough unused capacity that the power for the collector pumps could be forthcoming through the respective side’s Fuel Control Heat switch. Since the switch is normally off prior to shutdown, the half-cup of fuel sits in the collector tank after shutdown. It does not get pumped into the nacelle tank until the Fuel Control Heat switches are turned on following the next start. This causes no problems whatsoever, so don’t buy into the misinformation that you should leave the switches on until after shutdown.
Beginning with serial numbers LJ-738 and LW-248 in 1977, Beech stopped wiring the collector pumps to the switches and began having dedicated left and right Fuel Drain Collector CBs installed on the cockpit’s right sidepanel. This is also the case with the F90, the LA-series that appeared in 1978.
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 the accumulator tank and the fuel manifold – then fuel can partially or totally fill the accumulator, displacing the necessary volume of air needed to provide the proper “shot,” providing the same result, smoking and coking.
There is a third problem that’s rare, but not unheard of. Namely, what if both check valves leak at the same time? Now the fuel that can migrate from the manifold into the accumulator tank can also find its way into the P3 air system. It can lead to a strong kerosene smell coming into the cabin from the environmental bleed air inflow, most noticeable when high power is added during takeoff.
The last few E90s (LW-334 through LW-347), as well as all of the F90s after LA-58 (but not the F90-1s) almost always exhibit smoking at shutdown even when everything is working properly. Why? Because these models have a unique FCU, unlike the FCU installed in other models. Instead of a single fuel line going from the FCU to the Flow Divider/Dump Valve where it then feeds the primary and secondary manifolds, the -28s on the E90 and the -135s on the F90 have two fuel lines exiting the FCU, going to a device called the Start Control, mounted on the aft accessory case. The Start Control does the sequencing of fuel to Primary and Secondary manifolds and also contains the shutoff valve operated by the Condition Lever. Two lines exit the Start Control to connect to the manifolds, and each of these lines is over two feet long. What this means is that there is a lot more fuel line volume and more residual fuel that must be purged at shutdown, yet the accumulator tank is exactly the same size as in all the other models. I am convinced there is an insufficient air charge to properly purge the last bit of fuel through the nozzles…such that smoking is almost inevitable.
There is a straightforward, but rather odd, procedure that should be used for the E90s and F90s with the bleed air purge system and this same procedure can be used on other models that exhibit smoking until check valve problems can be addressed and corrected. What’s the procedure? Merely have the ignition exciters sparking during shutdown by moving the Auto-Ignition switches to Arm before pulling the Condition Levers into Cut-off. Remember to turn Auto-Ignition off before leaving the cockpit. Having the ignition sources active at shutdown allows the dribbling fuel to be ignited and burned, solving the smoke issue.
All 300s and 350s were manufactured with the Bleed Air fuel purge system, none having a fuel drain collector system.
The P3 accumulator will not have enough pressure to provide adequate fuel manifold purging if N1 has never reached a high level. It is very common to see some smoke if the engine is started, reaches only idle speed, and is then shutdown.
Some of our readers have experience with the Honeywell (nee Garrett) TPE331 turboprop engine, a version of which is used on the King Air B100. All 331s also use a P3 fuel purge system, but theirs is a bit more complex with a normally-closed solenoid valve involved. When the shot of P3 air purges the fuel manifolds at shutdown, it causes so much fuel to be sent through the nozzles that engine speed actually increases noticeably before it starts to decrease. This phenomenon rarely if ever happens with the PT6. No N1 surge will be seen. What will be observed, however, is a momentary delay between pulling the Condition Lever and seeing N1 start to decrease…maybe just a half-second. It’s short but noticeable, and something that is not observed with the older collector tank system. If you do not experience the delay, you did not get a good purge and may expect to see some smoke.
I hope this discussion has increased your understanding of these systems and will help in troubleshooting any problems that may occur.
Correction: My article that appeared in the August 2015 issue of King Air discussed Pressurization System Abnormalities. One of the components I reviewed was the Preset Solenoid valve, and I stated that it was installed on C90 and later King Air models. No, it made its appearance with the introduction of the B90 in 1968; LJ-318 and after all have Preset Solenoids.
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at kblonigen@cox.net.
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