Mooney Flapper Valves

M

meritflyer

Well-Known Member
In the M20K there are 'flapper valves' that are installed inside the gas tanks. To further describe their presence, if you pull off the gas caps, the valves act as an intermediate stop between the gas cap and tank.

Does anyone know their purpose?

(Hopefully that is clear)
 
I've never seen them, but based on your description could they be just a small piece of rubber meant as a back up in case a dingle berry pilot didn't put the caps back on?
 
The BeechJet has the same thing. Basically just a way to keep junk from the surface of the wing from falling into the fuel tank when you pop open the cap. Also a nice "barrier" between you and the fuel if you happen to have a volume "full" tank and you open the cap after the fuel warms up a bit.

I did a long flight to FL and a sharp fuel noticed that there was some milky white liquid sitting in that little bit of space and called our attention to it. It was nothing more that JetA that had started to gel since we were high enough for long enough that the fuel in the tanks was getting close to -40.
 
meritflyer said:
Does anyone know their purpose?
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It seems these are typically referred to as "antisiphon valves" or "antisiphon flapper valves." For instance, here's an AC describing some maintenance alerts:
Beech; Model B300C; King Air; Fuel Cell Antisiphon Valve Damage; ATA 2810
Be aware that damage to the fuel cell antisiphon valve can occur during refueling operations.
​
When the fuel nozzle is inserted into the fuel filler port, the nozzle opens the antisiphon, valve-flapper door which rests against the upper flange (pan) inside the tank. This area of the antisiphon valve may be damaged (bent) by pressure from the fuel nozzle and/or rough handling of the nozzle. The flapper door cannot seal properly if the pan is bent, and the valve will be less effective. It is recommended fueling personnel exercise extreme care while handling the fuel nozzle.
​
Part total time not applicable.

​
And another:

Beech; Model 200; King Air; Fuel System Defect; ATA 2810
This aircraft fuel system tanks are designed with an "antisiphon" valve located at the fuel cap. In the process of replacing an "antisiphon" valve, the technician discovered the new part was defective.

The technician performed a receiving inspection on the new "antisiphon" valve (P/N 101-920061-37) which he had received from Raytheon. During the inspection, he discovered the "flapper" assembly gasket and backup plate were incorrectly trimmed, and the hinge was exposed. This condition left a gap, which would allow fuel siphoning during flight, if the fuel cap came loose.
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mikecweb said:
Taking a stab but if the cap came off in flight fuel wouldn't get "sucked" out by the airflow over the wing.
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Trust me even if a cap isn't on all the way it will suck about half the tank out before it stops(about 18 gallons). I'll go look in my POH or my M20K service manual see if I can find what they are officially for, but I believe its to prevent debris while refueling. Or being a PITA when trying to get the last few gallons in.
 
meritflyer said:
What would the purpose of an anti-siphon valve be?
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The maintenance alert above tells you:
condition left a gap, which would allow fuel siphoning during flight, if the fuel cap came loose.

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I find it hard to believe that the flap is there to prevent debris falling into the tanks; the flap must open to allow the fuel nozzle to be inserted or at least let the fuel flow into the tank. Any debris collected would be dumped in along with the fuel. Filtering out debris requires, well, a filter.
 
tgrayson said:
The maintenance alert above tells you:
condition left a gap, which would allow fuel siphoning during flight, if the fuel cap came loose.

​
I find it hard to believe that the flap is there to prevent debris falling into the tanks; the flap must open to allow the fuel nozzle to be inserted or at least let the fuel flow into the tank. Any debris collected would be dumped in along with the fuel. Filtering out debris requires, well, a filter.
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Yes, because removing the debris before putting the fuel nozzle in, is outside the realm of complete possibilities. :whatever:
 
400A said:
Yes, because removing the debris before putting the fuel nozzle in, is outside the realm of complete possibilities.
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Probably wouldn't happen very often. This would be a poor engineering solution to something that really isn't a problem anyway.

I've already established that at least some Beech aircraft have these flappers for their antisiphoning effect. Without evidence to the contrary, it seems a reasonable inference they serve the same purpose on the Beechjet and the Mooney.
 
WalterSobchak said:
...I believe its to prevent debris while refueling. Or being a PITA when trying to get the last few gallons in.
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400A said:
Yes, because removing the debris before putting the fuel nozzle in, is outside the realm of complete possibilities. :whatever:
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Guys, you're overlooking the obvious.....they are anti siphon valves. Where is your "debris" going to come from? The fuel cap keeps debris out. The only way to get debris in the tank is to leave the caps off. The flapper valves are not pressure seals....hence the "flapper" name. They will "slow" the siphoning in flight, not eliminate it.

Mechanics will leave more debris in a tank than will ever get under the fuel cap.
 
NJA_Capt said:
Guys, you're overlooking the obvious.....they are anti siphon valves. Where is your "debris" going to come from? The fuel cap keeps debris out. The only way to get debris in the tank is to leave the caps off. The flapper valves are not pressure seals....hence the "flapper" name. They will "slow" the siphoning in flight, not eliminate it.

Mechanics will leave more debris in a tank than will ever get under the fuel cap.
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I agree that the flappers aren't there for depris, but debris can and will get in your tanks. That's partly the reason why you have an unusable amount of fuel in your tanks is to allow the sediment to rest at the bottom of the unused fuel. When you get fuel cells replaced there is an ungodly amount of crap at the bottom of the tank and gets worse with age.
 
ackeight said:
When you get fuel cells replaced there is an ungodly amount of crap at the bottom of the tank and gets worse with age.
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The same is true for tank type and wet bays as well, regardless of whether they have flappers or not. The tanks aren't usually washed out. Imagine if you kept using a drinking glass for a month without rinsing it out. Residue dries to the sides, gets recoated and dries again.
 
ackeight said:
debris can and will get in your tanks.
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But nothing a flapper valve would prevent. You'd need a pretty fine filter at the point where fuel in introduced into the system. It would greatly slow the rate of refueling and would have to be cleaned regularly. Probably a lot of the contaminants could pass through the fuel system without any problem, as long as they were widely dispersed in the fuel.
 
tgrayson said:
But nothing a flapper valve would prevent. You'd need a pretty fine filter at the point where fuel in introduced into the system. It would greatly slow the rate of refueling and would have to be cleaned regularly. Probably a lot of the contaminants could pass through the fuel system without any problem, as long as they were widely dispersed in the fuel.
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Not directed at you tgray, just this discussion in general.

Pointless anyway IMO. The fuel is filtered various times before ever leaving the nozzle. The only dirt likely to be introduced, pending proper filtration is followed, is that on the nozzle or near the tank opening on a dirty plane.

Sure some fuel gets through the filters with minor dirt, but here: http://amd.nbc.gov/library/dm/fuel_hb.pdf Chapter 4.
 
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