Engine driven fuel pump failure...
 

Ok folks...I am trying to get some out of the box thinking here. And maybe a little words from people who have had experience with this first hand!

I personally have learned and the way that I understand aircraft systems to work is this...in the event of an engine driven fuel pump failure in a low wing aircraft FUEL INJECTED ENGINE, where the engine will quit and the emergency procedure to follow is the ABC's of that procedure. Now for the restart the electric boost pump is in every checklist that I have ever seen, however my REAL question is...Will it work? Does the engine roar back to life once you put that magical switch to the boost or on position? Are there any aircraft designed where the boost pump is not strong enough to restart the engine? Common sense would say no, but there might be some models out there that might have a subpar electric pump. Please any experiences would be appreciated!

Hey CFItillIDie,

How long have you been a CSIP and what airport do you fly out of. I left the airlines to be at home as a CSIP. Really enjoy flying the Cirrus and other high-performance GA birds. Cirrus says if you experience an engine driven pump failure the boost pump will not provide enough pressure to continue to run the engine.

Have you ever had a student shut the plane down and leave the boost pump on? I'm certain it would not create enough power for sustained flight, but I believe the engine would still be "operating" or turning over. The 4-6psi of fuel pressure just is not enough to create the power required to continue flight. That design is really for vapor suppression.

TYPICALLY for all piston engines (for specifics see your airplane's AFM)...

Carbs: The boost pump and/or gravity supplies fuel to the carb. All you're doing is filling the float bowl at near-ambient pressure.

On a low-wing you need the pump, on a high-wing gravity alone is usually enough. The boost pump on a high wing is a back up to gravity...ie not strictly required unless you fly with sustained low g's, have some kind of flow resistance in the lines/filter to the carb, or experience a total gravity failure.

On some high wings gravity might not provide enough flow through the filter for sustained high-power ops (ie takeoff)...in which case boost pump use is mandatory.



Fuel Injection:The boost pump and/or gravity delivers fuel from the tanks to the inlet of the main fuel pump which requires a slight positive pressure head. The main pump boosts that pressure much higher, probably to about 30-50 psi for the injectors.

A failed boost pump may not stop the engine, but a failed main pump will. The boost pumps or gravity cannot supply more than 2-3 psi, nowhere near the 30-ish required for fuel injectors.

Again, read for AFM.

Problem with FI engines and a restart is they can be hard to restart even when the edp is working proper . Especially with a bendix rsa system . The injector lines can vapor lock .Not a good place to be for sure !

I think if there were a total Gravity failure, a failed engine would be a mute point.;)

As others have noted, the purpose of boost pumps is several; boost pumps move fuel, they supply head pressure to reduce or eliminate vapor lock, they ensure a steady supply of fuel to higher-pressure pumps, they refill carburetor bowls, and in some cases (not all) they serve as backup supplies to the engine driven pump.

In many cases, boost pumps will not suffice to provide enough fuel flow to the engine at high power settings. It's for this reason that sometimes when a pump failure is suspected (or verified) one can operate at reduced power.

Lack of pressure to the engine driven pump can result in pump damage. It should be noted that where an engine driven pump fails, absent a shaft or drive failure, it may serve to damage or contaminate the accessory section of the engine. An engine shutdown may be necessary, or even eminent.

Boost pumps may have sufficient flow and pressure (two very different things) to supply the engine, or they may not. Auxiliary fuel pumps come in many flavors, and applications. In one fuel system, pumps may be used for nothing more than transferring fuel from one part of a tank to another, to ensure fuel flow or availability at various flight attitudes. Other pumps may exist to supply the enigne, or other tanks, and some pumps may be boost-override pumps that provide higher pressures than others, to ensure that the boost override pumps can supply fuel in the event other pumps are not available. Some pumps may be used for little more than to dump fuel (in advanced aircraft). In some aircraft, the engine driven pumps require fuel for cooling and lubrication, and need a boost supply to prevent cavitation or EDP damage.

In some cases, the EDP is necessary to provide the fuel flow necessary to activate certain engine controls, or the fuel control unit. Boost pumps in some cases do not provide adequate fuel pressure (or flow) to handle this, as well as to supply the engine.

Some aircraft use high and low boost positions, for various reasons. When you turn on your boost during an emergency procedure, you may not be doing so to supply the engine, but to restore flow to the engine driven pump or to other parts of the fuel system.

The Cessna 200 series of piston aircraft utilize a small header tank located at the base of the wing struts (or where the struts would terminate, in the case of the 210); these tanks receive fuel from the wings, and send fuel to the engine driven pump, fuel control, etc. They're important, because they're not just a point through which fuel passes when headed to the engine. They're a point where bypass fuel off the pump is returned, and that leads to a critical issue called "fuel flow fluctuation."

Cessna has a procedure for FFF, which involves applying boost pressure, adjusting mixture, and switching tanks. This procedure has been in the Cessna manuals for several decades, and it's dead wrong. In fact, it can assure that you don't get the engine restarted. A little systems knowledge will bear that out.

When the engine stops during fuel flow fluctuation, it's often because hot bypass fuel, returned to the header tank, is trying to climb back up the supply line from the wing tank. The result is stoppage of fuel flow, vapor lock in the feed line from the wing tank to the header tank, and an engine failure or intermittent power and fluctuation. Applying boost pressure cements the deal; it ensures that additional fuel is flowing to the pump, increasing the amount of bypass sent back to the header tank, making the problem worse, and often making a restart impossible.

The key to understanding what to do is knowing the system; the header tank feeds the fuel selector before going elsewhere. Rather than turning on the boost pump, reduce throttle and switch tanks. This situation doesn't occur often, but when I've experienced it, swapping tanks did the trick right away. Restoration of an undisturbed cool fuel supply fixed the problem. Then and only then could I concern myself with boost, adjusing the throttle and mixture, and referring to a checklist. Know your aircraft and what the various inputs do.

Ever applied high boost in flight and watched its effect on fuel flow, engine operation, CHT's, EGT's, etc? In some systems, it's possible to kill the engine with boost, or cause it to load up, and even lead up.

Even in the same type aircraft, significant variations in the fuel system (and other systems) can exist. I operated a certain type of piston twin in a hostile overseas location that had some very significant differences in the fuel system from civil-use counterparts. Those systems gave us up to a 12 hour loiter time at reduced power settings, but like many piston twins, also meant that under certain circumstances, not only would a portion of the fuel be unavailable, but the useable fuel might be bypassed and transferred to an unusable location. Not all pumps on board fed the engine, and not all pumps would run the engine, and when the engine was running, it might be crossfed from an unconventional tank, but would bypass fuel back to a conventional location...possibly making it unavailable later on.

Know your system, but know also that there are some aspects of operation that you may not learn until the actual failure of that engine driven pump, or other system failure.

What's the problem? Lift is now un-opposed and you're a balloon. Or at least a glider in a permanent thermal.

Some of the more complex piston turbocharged fuel systems have altitude limitations associated with partial fuel system failure. If one of the two (or three) pumps that supplies an engine fails, it may not have enough pressure to go higher than about 15,000 ft or so, effectively starving it for fuel above that. The engine will be fine below the altitude where the remaining fuel pressure is enough. Obviously it something to do with turbo boost, but I am not sure what exactly.

IO-520's in C310's, the boost pump on "high" will keep it running per the AFM, but notes that it might not provide enough pressure/flow to maintain full power.

This is recall, it's been over 5yrs since I've been in one, but I do remember this to be the case. There is a low setting for normal take-off/landing. High is also tapped when you use the primer for start

What you posted may be true for some aircraft but certainly not for all. Electric boost pumps come in all pressures and flow rates. On many aircraft the electric pump will run the engine just fine in the event the engine driven pump fails. In fact in the experimental world many are removing the engine driven pump and going with electric pumps only on injected systems. This approach has some real advantages with vapor lock reduction when using auto fuel.