Single point failure discussion ?

[Larry DeCamp]

We debate about ignition frequently. Why is there is no discussion about fuel ?
We have wing root filters, electric pumps and mechanical pumps in series, high quality filters, but it all goes through a float/ main jet or a distribution spider or fuel rail. The fuel rail may be the answer where the failure rate is divided by 4 or six injectors unless the ECU uses a common source of fuel.
It may be valuable to add your insight to this topic

 

[TS Flightlines]

Larry, that is interesting. Even with a servo, you could possibly have a single point failure, I guess. Don and Kyle would be much better to answer that that me. Remember back in the day when we build Chevy small blocks with 4 Weber Carbs, basically 1 carb per 2 cylinders? I doubt we would want 2 or 4 servos, or throttle bodies, so each cylinder is basically an independent fuel system. Sure would look cool though!

The delivery system and plumbing would be a nightmare, but it would be redundant at the engine. Tanks, selector valve (s) (or not), pumps, etc, boy it would be fun to plumb this, but lets face it, not practical for us.

Tom

 

[N804RV]

You wanna know the biggest single point failure in our little airplanes? Crankshaft! Don't overthink, just enjoy the flight!

 

[David Z]

Single pilot aircraft. There's a single point of failure in the one person operating the aircraft.

As we move up the size and performance capabilities of aircraft, redundancy requirements increase. This is to the point where airliners have 2, 3 or 4 of everything.

There are tons of single points of failure in our planes. One tire per gear leg, one pilot, one crankshaft, etc. I don't see this as an excuse to reduce the single points of failure where possible. We can't (easily) have a 2 engined RV, so we accept that. We can have two fuel pumps, so we do. Some of us have taught a spouse how to land providing some redundancy for the pilot. Single points of failure should be minimized where reasonable and efforts taken to reduce probability or consequence of failure where there is a single point of failure.

This thread started out comparing ignition to fuel. There are two ignitions because magnetos are unreliable. Fuel servos and carburetors are really quite reliable. I have had enough magneto failures to loose count. I had one fuel servo failure, and even with that, the failure mode was to run excessively rich. The engine still ran, choking on the fuel, but made more than adequate power to fly.

Do we need two electronic ignitions? I don't have data on EI failure rates to have that answer. It's easy to install, so it's done and provides redundancy. It's very difficult to install two fuel servos, or two carburetors, so we don't.

 

[Walt]

I have had enough magneto failures to loose count. I had one fuel servo failure, and even with that, the failure mode was to run excessively rich. The engine still ran, choking on the fuel, but made more than adequate power to fly.

How many of those mag failures were on properly maintained units?
From what I hear most mag failures are due to just running them till they fail, can't blame the mag for that!

 

[David Z]

I flew for a Navajo charter outfit for a while, and before that a C185 in floats, also commercial charters. The point is they followed an approved maintenance schedule. Usually these fall prety close to the manufacturer's recommended maintenance intervals. Considering much of it is on a piston twin, that's over 3000hrs of "engine time", and I was running 4 magnetos at once.

 

[Larry DeCamp]

Appreciate the comments guys

In hind sight, my real question was to explore any debate/insight about servos and distribution for classic FI. It appears that fuel rail and individual injectors significantly reduce the possibility of a dead stick event, assuming electrical design is appropriate. However
FWIW, I opted for AFP 150 in lieu of SDS injection because I didn't want to spend all my time fiddling with parameters instead of flying. In hind sight, the rail solves the distributor concerns but not the servo. Is filtration the only defense against servo caused fuel starvation ?

 

[z987k]

I flew for a Navajo charter outfit for a while, and before that a C185 in floats, also commercial charters. The point is they followed an approved maintenance schedule. Usually these fall prety close to the manufacturer's recommended maintenance intervals. Considering much of it is on a piston twin, that's over 3000hrs of "engine time", and I was running 4 magnetos at once.

Roughly same experience, and in navajos to. The only failures I've ever had on piston engines are magnetos that were professionally maintained through an approved maintenance schedule.
I've also had 1 mag failure in my own airplane.

 

[rv8ch]

... In hind sight, the rail solves the distributor concerns but not the servo. Is filtration the only defense against servo caused fuel starvation ?

I think it would be helpful to know the various failures on fuel servos and fuel systems that have been seen over the years. I think clogging is one of the main failures, and the other is parts coming loose. Both failure modes can be reduced but almost impossible to eliminate - you can't test every fuel station you visit for contamination, for example. Parts can vibrate loose even if correctly installed.

 

[lr172]

I think it would be helpful to know the various failures on fuel servos and fuel systems that have been seen over the years. I think clogging is one of the main failures, and the other is parts coming loose. Both failure modes can be reduced but almost impossible to eliminate - you can't test every fuel station you visit for contamination, for example. Parts can vibrate loose even if correctly installed.

I had a run of problems with fuel contamination resulting in a dead stick landing and a few other rough engine landings. Contamination was coming from the inside of fuel pump and finally over ran the finger strainer and clogged injectors. No way to address with PM unless you are pulling the finger strainer every month. I do it every year, but that did not address my issue that seemed to come out of nowhere. Main filter looked brand new with no debris, so it didn't come from the fuel source.

My pump was TOTALLY COATED with white growth inside of it and also found small black chunks, presumably from rubber. My running theory is I got a load or two of fuel that contained chemicals not normally found in 100LL and it attacked the Alum and ruibber, causing the growth of whatever that white stuff was. Unfortunately it is still a mystery. There was a thread not too long ago, discussing possible 100LL formulation changes.

 

[Larry DeCamp]

Why crickets on water in fuel line low spot ?

Obviously dynamic flight pressure and fuel pump suction have overcome fuel levels and even condensate historically. But water in a low spot at 33F is ice at 31F . The question was why Vans vent design, or the Rocket coils that can have water in the low spot, when down pointing drain line only suffers from drainage from a full hot tank.

This discussion went silent . What am I missing.

 

[PilotjohnS]

Water

Water in vent line I think is very rare. And enough water in vent lone to cause blockage when frozen is probably even rarer.

In the vent line, I suspect there will be fuel vapor since the vapor pressure of fuel always creates some vapor on a continuous basis. So at all times, there is an evaporation of the fuel forcing vapors out of vent line. The vent line is small, so this would take years to empty tank.

In my mind, the only way for water to enter is through the nightly temperature changes. As the fuel tank changes in temperature, some normal, outside air can be drawn into the vent line and cause condensation at the low spot. But this will be surrounded by fuel vapor so would probably not be sufficient to block the line.
JMHO

 

[Larry DeCamp]

Thanks John S for commenting

I was thinking the 1/4" tubing in the tank plus vans vertical line in the fuse (total about 5 feet) could condense enough to cover the low spot in some climates like Alaska and Florida. Apparently not a problem !

 

[Ralph Inkster]

Condensation is not only an in tank problem. Moisture can end up in the lines, filters, gascolators, fuel pumps, & SERVO too.

I had a Rocket in the shop one time, he lost power on take off & had to circle around & land, engine would idle at that point but would die out on runup. Checking the obvious potential causes, we eventually found white chalky deposits in his gascolator (one of the main arguments of equipping a plane with a gascolator, Canada eh?). Further investigation found a LOT of white goop in the servo inlet filter & alarmingly in behind & thru out the whole servo. This demanded us checking out all the other components in the whole fuel circuit.
White deposits thru out the mechanical fuel pump, same inside the bores of the Andair fuel valve, we didn't even bother to check the aux fuel pump, kust replaced it. Following the fuel lines ahead of the fuel valve, we found another Andair gascolator buried under the pilot seat, totally unaccesable, & so corroded that I needed a big vice grip to break it open, so bad that inside annodized surfaces were stripped & lots of pitting in the aluminum bowl. In the end we replaced the whole fuel system, plumbing & all from the tank right up to the cylinders.
So, condensation corrosion can definitely collect & happen in those small fuel line areas too.. This plane was parked in a hangar in Tennessee for about 3 years before coming up to Canada.

& the vent lines had water deposits in them too.

 

[moosepileit]

How many of those mag failures were on properly maintained units?
!

I learned on my first plane to avoid maintenance on dual redundancy items on the same schedule by the same person-

Bendix mags on my first plane- the fastener holding down breaker cam let go on each an hour apart.

Too young n dumb to pull the second when finding the first.

Technically, my first failures as an owner were the very yoing magnetos. At least the Pitts S1S was a good glider if needed...