Why does the engine often quit in my A23 (or A23A) when I advance the throttle? It seems like it will die if I turn the boost pump on and advance the throttle. One day the engine quit in the air, but we got it restarted. I can make it do the same thing on the ground. Is the Continental IO346 somehow unique, regarding boost pump operation?
Bob Steward, A&P-IA:
The IO-346A injection system is substantially unique among aircraft
injection systems. Why Beech/Continental didn’t go with the industry
standard Bendix RSA style injection can only be speculated as a cost issue.
Because of the differences in the injection from all other aircraft, you
have a really small number of Beech Musketeer injection experienced
mechanics to work with. And you may spend a lot of time (and money)
educating your mechanic of choice. (As Braden pointed out, it took them 8
months to find his problem.)
One common issue is the “boost pump”, which is NOT a boost pump on the
IO-346A injection, it is an emergency replacement for the engine driven pump.
Flipping on the “boost pump” will kill the engine in many situations. It
just floods it out. You use it to start the engine and prime the
cylinders, and in the event that the mechanical pump quits. Running both
will flood the engine.
So if your “problem” with advancing the throttle is that it dies, and you
had the “boost pump” on for safety because of low altitude or unusual
attitudes, then you are causing the problem by not following the factory
checklists and procedures.
Pilots not properly trained on the A23 or A23A will flip on the boost pump
like you do on a Cherokee — before T/O and on downwind, when changing
tanks, etc. Some of them are so lax, that they fly on for HOURS without
bothering to shut it down, because they don’t notice a difference (there is
none).
The IO-346A engine driven pump provides the required FLOW based on RPM with
about a 10% excess through a positive displacement pump, and the
throttle/mixture block leaks the excess back to the tank, based on the
control positions. Turning on the “boost pump” increases the flow
dramatically, and the “leak” in the throttle control can’t drain away
enough to run properly.
Going full throttle, as you suggest is the time it dies, floods out the
engine from the excess fuel.
Other injection systems are PRESSURE systems that depend on a steady fuel
pressure. The Continental IO-346A system varies the pressure as a function
of RPM, which changes the FLOW.
_____________________
Technical Editor’s note:
The RSA system used on Lycomings is pressure-based for both fuel and air, and varies fuel flow depending on the air flow that is entering the induction system through the air filter and Servo. It automatically meters fuel based on the amount of air that it ‘sees’ coming in, based on the air pressure generated in the ‘impact tubes’ that you can see in the Servo inlet. Raising the fuel pressure by turning on the boost pump will have relatively little effect on the fuel metering, if the air flow has not changed.
In the very unsophisticated Continental injection, fuel flow is based on throttle position and available fuel volume. The throttle position essentially determines the size of the opening that is feeding fuel into the engine. For any given opening, flow will vary if the available fuel feed volume and pressure varies. You actually set the full-throttle fuel flow by adjusting the fuel pump pressure regulator spring, or adjusting the fuel flow bypass valve. That is why adding boost pump flow adds fuel volume under pressure into the Servo, forcing more fuel through the opening. Since no extra air is entering the engine (still the same throttle position), the engine floods out. Along the same lines, if fuel flow increases more rapidly than intended as the throttle is opened, due to the boost pump being on, the engine will flood and die.
By the way, this is one reason why Bonanzas and Barons can be so difficult to start, and can tend to die out on roll-out. After priming, you have to find the ‘sweet spot’ of throttle position that matches the priming fuel; the Continental engine-driven gear pump can move very little fuel during cranking. And during roll-out, the throttle position says ‘idle’; but the forward speed is windmilling the prop above idle RPM. Since fuel flow is at ‘idle’ but the actual induction air flow is well above idle, the engine goes lean and dies. Continentals typically require an idle mixture setting that is on the rich side, to minimize this. That’s why they tend to have a ‘lope’ at idle, just like the big old radial engines. It is also why they can tend to foul spark plugs more rapidly, with carbon and lead.
_______________________
You can get the Continental Fuel Injection manual that covers the IO-346A
from www.TCMLink. com, the Continental web site.
Bob Steward, A&P IA
Birmingham, AL