Maule Fuel Starvation

[PCH]

It's been about six weeks since I had a forced landing in a vineyard. I'm sharing this story in hopes that it may prevent someone else from my fate.

I was practicing short field touch and goes and had just taken off the airport and was in crosswind when the Franklin quit on my M-4. I was unable to restart the engine. Unfortunately, a very large vineyard abuts the airport and I was unable to return to the airport. I landed between the rows and only ended up with a minor scratch on my finger from flying plexiglass. Sadly, the Maule was totaled. Metal grape stakes are very unhealthy for airplanes.

My first thought was that I had run the tank dry even though I had dip-sticked it prior to flight and I have a instrumentation for fuel flow and usage.I was flying on the left tank and was surprised, when I dip-sticked after landing that tank contained 6 gallons. The Maule still had its original left/right/off fuel selector.

I have seen comments that there is a Service Bulletin or Service Letter that recommends replacing the early fuel selectors with the newer left/right/both/off selectors. I was unable to find either a Service Bulletin or Letter with that advice for a M-4 220c. There are however Service Letters and Bulletins for the fuel systems of the M-4 210c.

Searching the internet and this forum, I have found comments related to fuel starvation. There also seems to be a general belief that these fuel starvations were caused by a slip or a skid. Having been a glider pilot for 50 plus years now, I can guarantee my flight profile that day did not include a slip or skid. The viewing audience that Saturday morning concurred with that assessment.

The fuel selector was turned off on short "final". After returning the plane to its hanger, it was also found the gascolator had no fuel in it. So what caused the fuel starvation?

While studying the problem, I came across a placard on a Pacer: "Only straight and level flight when right tank is below 1/3 full". It turns out that the Pacers and Tri-Pacers had a problem with fuel starvation when drawing from their right tanks when the fuel level was below 1/3 full in that tank. Diving deeper, the problem arises because of the routing of the fuel lines from the right tank. It turns out that our Maules have essentially the same fuel line routing. Both Maules and Pacers have two inboard fuel inlets in the fuel tanks, one fore and one aft. These inlets feed separate fuel lines. The front line goes down the front door pillar and the back line goes down behind the door, them under the door till it T's together with the front line. From that junction, the fuel line from the right tank goes to the fuel selector. In the Maule, the left side fuel lines follow the same course around the pilot's door.

The Pacer, since it has no pilot's side door, has its left fuel tank fuel lines arranged without the loop necessary to get the rear fuel line around and under a door. Notice the Pacer has no flight restrictions when using the left tank.

As described in the Short Wing Piper forum, it's the "Hartford Loop" that fuel line takes under the door that can result in a stop in fuel flow from rear intake port when the front intake is unported. The result is the fuel line to the fuel selector and on to the engine is only filled with air. A mod for the Pacers is a fuel selector with a "both" position in addition to the right/left/off position.

At some point after the M-4, Maule supplied its planes with a fuel selector that also has a "both" position and from the routing of the fuel lines, it's clear the most prudent fuel selector position for critical operations would be the "both" position. I wish I would have known the design flaw, I would have happily upgraded my fuel selector.

And if anyone with a with an old style left/right/off selector is wondering if they should upgrade their fuel selector, when we took our fuel selector apart, we found the 50 year old nylon valve inside the selector to have cracks in it. It was only a short matter of time till it would have failed.

Fly safe.

[Mog]

Sorry to hear about your M4. I’m not sure there is an AD or what, but I replaced my fuel selector with the new Maule direct replacement part and it is equipped with a both position now. Same valve, it just has an additional detent. I have since then flown on both almost exclusively.

[Kirk]

Definitely no AD. Thought there was a Service Letter or Bulletin, but looked and didn’t find any for the selector valve. Installing the selector valve with the “both” position is a good idea though for this exact scenario.

For the M4-220, there is a Service Letter 23 for “low areas in fuel line crossover” that does seem to address the area you mentioned. A bit late now to check if it was complied with, but might satisfy some curiosity.

Sorry to hear about your loss but glad to know you escaped serious injury.

Kirk

[PCH]

Thanks guys for the kind words. I’ve seen Service Bulletin 23 before the accident. Even though my serial number was after the affected M-4s, I had checked that it had been complied with. It was.
To me, it looks like Maule realized they had fuel starvation problems but never they never fully understood what caused the problem.
Thankfully, the newer fuel selectors address the problem.

Anyone know of any M-4s that are sleeping in their hangers? I’m looking to replace mine.

[Curtis]

Sorry to hear about your loss. I have probably the nicest M4 220 in existence. Also one spare "0" SMOH Franklin 220 and the plane has a 60 hour SMOH Franklin 220 on it. I have been thinking about selling it since I am the only one usually flying in it and I have an RV7 that I am building and is almost ready to fly. Don't need to sell it but I may would. PM me.

[Kirk]

Been thinking about your mishap and occurred to me that a blocked or partially obstructed fuel vent could also be the cause. The point when the engine failed would be when the highest fuel flow demand was occurring. That, coupled with the forward fuel port in the tank being uncovered due to acceleration and nose up pitch. Wouldn’t be the first aircraft to have that occurrence.

Were you running the electric boost pump at the time? It’s not on the checklist for takeoff or landing but I always run mine for 3 reasons. 1: even though it’s a gravity feed system, I like having positive pressure to overcome any burps or slow feed. 2: It keeps me in the habit for the rare occasion I fly low wing aircraft. 3: Exercising the pump now and then is good for the pump.

Kirk

[1:1 Scale]

As described in the Short Wing Piper forum, it's the "Hartford Loop" that fuel line takes under the door that can result in a stop in fuel flow from rear intake port when the front intake is unported. The result is the fuel line to the fuel selector and on to the engine is only filled with air. A mod for the Pacers is a fuel selector with a "both" position in addition to the right/left/off position.

Sorry to hear about your mishap! I've been rebuilding my M4-220 since a mishap 5 years ago, and am in the final stages of getting everything sorted.

Can anyone explain the physics behind the "Hartford Loop"? I've had to put a couple of gallons in an empty left tank for ground tests, and both times didn't get any fuel flow until I opened the sump in the belly, then suddenly I got full flow to the gascolator and carb. This was with the fuel selector turned to the left tank, but I never tried the "both" setting. I've been trying to figure out the "why" behind this, as it seems that the weight of the fuel from the rear port should just push the air out of the system through the front port?

Once I've "burped" the belly sump, there's no more issue with fuel flow.

[Mog]

Didn’t occur to me until now, I had an incident a few years back on a stop and go in my M4. The engine hiccuped about at when the wheels left the ground and I aborted. Not sure what would have happened had I pushed through. Ultimately the engine cleared and passed a run up check just fine so I departed. Not sure what tank I was on, but maybe this was related to your incident. Not sure what value this info has, just has me wondering.

[PCH]

Kirk: I checked the forward facing fuel vents and they were clean and unobstructed. I've had a bug plug a pitot tube in flight before so it was one of the first things I checked.

I always check my electric boost pump on preflight but I don't use it for take off and landing. I might be incorrect, but I don't think having the aux pump on in my episode would have helped. The problem was that flow of fuel from the rear intake was interrupted by the air rushing into the forward unported fuel intake. Switching on the electric fuel pump and changing tanks did not restore power. I think I didn't have enough time for the fuel line and gascolator to fill with fuel before the forced landing was imminent.

[yanknbank]

This makes me want to replace the fuel selector on my m5-210c. I'd love to know more about this if anyone has anything to add. Do you think having a header tank would essentially preclude this issue from occurring? Surely the 1.5 gallon or so of fuel from the header tank would keep the engine running long enough for the air bubble to fix itself again. I'm wondering about low fuel high aoa climb outs too though.

[asa]

I don’t understand this whole Hartford loop and sucking air situation. Air should not enter the line simply because one port became unported. Why would it do that?

“Air is easier to suck than fuel” is what I’ve heard but I don’t understand how that applies.

I thought the pacers only had one port on the right tank due to it technically being an aux tank, which is why that placard exists - fuel can slosh to the end of the tank without one.

Which part am I wrong about?

Thanks
Asa

[yanknbank]

I'm having a hard time with it too. Fuel is not being sucked from these tanks anyway, unless perhaps at a sustained high level of consumption, like a 260hp at takeoff power full rich. The gravity feed is sufficient to back feed the aux or engine driven fuel pumps. All I can picture is an air bubble locking up a line, but I think you'd need bad line geometry for that to occur. Hoping someone will shed light on this.

[Kirk]

I do think replacing the fuel selector with the newer selector valve with BOTH position is a good safety enhancement. It might have helped in the OP’s situation.

I did some digging to refresh my memory on the PA 22/20 fuel system. The 2 systems are very different. In fact, there is an STC available to modify the PA22/20 to a system similar to the Maule: http://trimmeraviation.com/fuelsys.html

The “Hartford Loop” refers to the path from the right tank which has 2 outlets, fore and aft. From there, the line goes all the way down to below the floorboards where there is a gascolator and sump drain, it then travels up to the selector valve which is just below the pilot’s side window. I think “loop” is a bit of a misnomer there, but it does invite interruption thus the restriction against takeoff or landing on right main tank feed.
Some descriptions here:
https://www.shortwingpipers.org/forum/s ... uel-System
https://www.shortwingpipers.org/forum/s ... ne-diagram

This is unlike the Maule system that runs the right main fuel line along the upper part of the crossover structure that the seats attach to then forward to the selector valve. No low spot prior to the selector valve. That is why the selector valve is kind of an awkward reach down and forward. So, in my view, there is no “Hartford Loop” on Maule aircraft

I’d be glad to hear from a more experienced Piper person if I have any of that wrong.

Kirk

[PCH]

Hi Guys:
Interesting to see this tread revived just in time for the one year anniversary of my landout in the vineyard. I have spent the past year thinking and studying about what went wrong and I think I can answer some of the above questions.

To replace my ill fated 220C, I bought a nice M-4 210 that I am enjoying. Though I believe the 210's header tank will prevent the fuel starvation that occurred to me, the first thing I did was to replace the original fuel selector with a Left, Right and Both selector, On my 210, the fuel shut off is a separate panel mounted selector that shuts off the fuel after the header tank. I have done a cursory look at the NTSB accident reports for the 210s, and didn't find any reports that mimicked my event.

However, a review of the M-4 220C NTSB accident reports revealed at least 3 other accidents that appeared to be similar to my accident. It makes you wonder how many more unreported losses of power are out there.

The problem as I see it is this (and it affects both left and right fuel tanks): a climb out (say maximum performance take off) with 1/3 of a tank or less fuel unports the front fuel uptake. A climb (and perhaps a turn out) enhances the rear fuel line low point that occurs beneath the rear part of the door. On my 220C, fuel tank was pressurized by the forward facing air vent on top of the fuel cap. The pressurized air then rushes down the unported front fuel take-up blocking fuel from entering the line from the rear line. The air continues down the path of least resistance into the carburetor. The resulting engine stoppage is abrupt and sudden. Note that all this happened to me as I was on the left tank.

Safe flying to all!

Peter

[Kirk]

Peter,

Good to hear you are back in the air. The 210 HP I guess have some quirks, but have a dedicated following. HP to weight is about the best of all the engines they’ve tried. Maybe they will take the plunge on the Lycoming IO-390 some day. That would be a rocking combo.

Kirk