Maule Fuel Starvation
Posted: Fri Apr 28, 2023 6:11 pm
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.
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.