I know there has been a lot of discussion here on the pros and cons of Monarch caps. Today I was flying a 206 with Monarch Caps (which have been on this aeroplane for a long time and presumably with no issues) when my passengers asked why there was water coming out of the wing. I knew it wasn’t water and I thought the gauge on that tank (the same tank that was selected) was going down rather fast, but also know the old ordinal gauges are pretty in accurate. I thought for an instant I may have left the cap off after I dipped it. It was 3/4 full when dipped before flight. I landed to investigate and found the cap was on tight but the tank was now almost empty after 30 mins of flight, and there was fuel residual all over the wing. I was a bit horrified to say the least. What I am thinking (but dont know for sure) is that the cap itself does not do the sealing but the flapper below the cap is what does the sealing and stops any leakage. When I dipped that tank before flight I think the retaining chain for the cap got caught in the flapper and stopped it sealing. This is all presumption. I can’t think why else it would leak with the cap securely on when it has not done so before in the 50 or so hours I have done in it, and presumably not before that either. Can anyone tell me if my theory is accurate. ie That it is the flapper that does the sealing and not the cap?

I think both the cap and the flapper are supposed to seal. Could the chain have gotten twisted into a bunch and pushed the flap down. The only way I could get my chain down past the flap is to hold the flap open with my finger while feeding the chain in. Are you sure the cap was all the way down and the rubber seal nice and snug. Could it be the perimeter screws that hold the plate to the bladder have gotten loose?

Yeah, I'd guess the chain was caught in the flapper and perhaps the cap didn't seat all the way. The seal on the cap should seal it as well. As Marty noted, its a good idea to check the screws too. I've not seen those being loose cause the type of leaking you described, but just some streaking from screws. A quarter turn or so stopped it. So they can work loose. I'd fill it up, clean the area, make sure everything is where it's supposed to be, and then go do a short flight to see what happens. Good luck!

The flapper does not do any of the sealing... Its just the O ring on the cap. How new is the Monarch cap install? Several have complained about this.

I believe it can happen when the tab that the chain attaches to gets pulled out a bit and stops the O ring from sealing.

Agree, check the o-ring.

I’ve had Monarchs, excellent upgrade from those old caps that ride down in the water hole.

From the Monarch site:

“All three styles feature an anti-siphon flapper valve on the bottom of the inlet to prevent massive loss of fuel if the cap is left off or loose. (You can see this in the pictures below)”

So maybe not seal, but it’s supposed to lessen the flow.

Yes, it will keep a "massive" evacuation, but wont seal the tank in the slightest. In my experience, the little flap is best suited for capturing some kinds of fuel nozzles inside the tank.

The flapper didn't let a drop out when I flew around for a half hour without the cap. Probably started the flight 3/4's full. I'm a believer but you don't want to bugger it up either.

A spray pilot up here last summer has a 180 that has the Monarch fiberglass tanks and he said he got a fuel nozzle with a a catch ring deal on the end stuck in his one time. Think he finally got it out by using a knife to hold the flappe but maybe buggered it a little. Only time I've seen a nozzle like that was at a diesel pump.

I don't post often, but figured I'd chime in on account I have experienced this scenario with Monarch Caps. I have four of them on long range 185 tanks. The two outboard caps are at least 15 years old, and the inboard caps are 2-3 years old. Regardless, the design looks the same to me. I replace the o rings every couple years. Best place to pick those up is at the Alaska Airmen's Show. Anyway, on a few occasions after completely topping the tanks off I have experienced fuel jettisoning overboard from the left inboard tank filler cap. Note that I didn't actually open that cap to fill the tank, only the outboards. On one particular flight, once again after topping off, I noticed fuel jettisoning from the suspect cap on climb out. I landed to investigate (be cognizant of removing an inboard cap with full tanks, I suggest opening the respective outboard cap first). What I found was the keeper chain was stuck under the inboard cap flapper valve. Easy fix I thought. I made sure the chain did not impede the flapper valve, secured the cap, and off I went . . . Same thing on climb out. Landed again, opened the cap, and sure enough the chain was "sucked" under the flapper valve again. Go figure. This time I removed the chain before securing the cap. Proceeded to destination w/o a further recurrence. I only experience this event if I completely, and I mean completely top off the tanks to the flapper valve level on the outboard tank caps. The keeper chains are nice to have, especially on floats when its common to fuel from the inboard caps. I will say my customer service experience with Monarch has been excellent.

Leaky Monarch caps were discussed extensively here not long ago.
Consensus seemed to be that the eye thing that the cap-keeper chain hooks to was sometimes stamped out wrong from the factory, & the cap bottoms out on it instead of sealing.

Just think how much grief could have been saved if somebody didn't modify a great product with nothing punched in the side and a flapper that actually seals.

I believe this is the first airplane part that I've ever raged about, but I've got to, these things are downright dangerous to use.

Rubbish? Junk? Garbage? Crap? Rip Off? Idiotic?

So which of these best describes the Monarch Caps? Actually they all do. The fact that the company has the gall to still be selling these things is astonishing to me. The fact that the FAA approved the STC is well, the FAA isn't it.

This is my first experience with these wretched things and thank goodness will be my last as I can't get this junk off my 185 fast enough.

First off what a stupid design. How on earth did the FAA approve these trash can lids that sit on the top surface of the wing acting as spoilers disrupting the lift over a sizable width? And my 185 has four of these things. That is a lot of lift disruption.

And how about the fact that they don't fit tight to the wing, nor even. Allowing 165 mph wind under the lid, trying to rip them off the wing. Ever put your hand out a car window at 65 mph and tilt it up just slightly? How about trying that at 165 mph. Any wonder they leak fuel even with brand new seals?

And what kind of cheap ass rubber are they using for the "premium" o-rings? Does the rubber harden or crack with age? No, it turns to sticky gum which ends up down the throat of the filler neck and into the fuel where it will literally "gum up the works" in the fuel system with anything it comes in contact with. Sound safe?

Pictures below are from today.

First is the gumbo o-rings. I had to use carb cleaner to get the gum off the caps, the filler, and the screw in portion of the cap. I thought wow, that's why these things are leaking. But I was wrong.

Second is one of the caps with a new o-ring, chain inspected to ensure it's out of the way, and one can plainly see the cap doesn't sit flush but actually will allow air under which will lift it off the the Monarch filler plate, unsealing the caps allowing fuel to be siphoned out of the wing. All four caps resemble this one. (They looked like this with the old o-rings but the gumbo was sticky enough apparently to help keep them sealed...mostly). BTW, the photo's on Monarch's website show the same gap between the caps and filler plate. Did the FAA never see this?

Third is after today's two hour flight with clean contact surfaces and new "premium" o-rings. Be interesting to top off the tanks and see how much fuel I lost. You can bet I'll be requiring my money back for the purchase of these useless o-rings. And I can't wait to get the original Cessna fillers and caps back on. Can't happen fast enough.

Absolute rubbish these "Monarch Premium Cap"s.

I’ll do some testing tomorrow to try figure out what happened to cause our leak. It did freak me out a bit to loose that much fuel so quickly.

What I'm wondering is, how could the second mfg change the design of the originals. The second mfg doesn't have the machined aluminum casting that mine does and also the wire snap ring type chain holder. Instead they punched a chain holder loop. I didn't think you could change stuff. Notice the rubber flap seal is from 1990 and has even had fuel with you know what sloshing against it for 11 years.

All your guys that are having issues should be having you mechanics send in SDRs. It alu ss like this I definitely something that at the very least could use an AD...

So I went out and did some flying tonight to see if I could replicate the leaking I experienced the other day (after refilling the nearly empty tank). I had a hard job replicating it. The only way I could get it to leak at all was with the check chain jammed in the flapper AND the cap loose and not tightened down at all. Even then it wasn’t streaming over the back of the wing and visible from the cabin like it was the other day. I flew a few different scenarios.
With the check chain jammed in the flapper and the cap on TIGHT there was no leaking.
There was a slight dribble on the wing with the flapper properly sealed but the cap LOOSE. (The flapper will stop you losing lots of fuel if the cap is loose or off)
No leaking with both the flapper properly sealed and the cap TIGHT.
All O rings seem to be in good order and doing their job.
So I’m still at a bit of a loss to see how I lost 3/4 of a tank in 30 mins with the fuel visibly streaming over the back of the wing and the cap on. I can only think I had the check chain jammed in the flapper and the cap loose after I dipped it. Its hard to see how one would leave the cap loose as it always tightened down until the clicker clicks in. Hmmmm.

Interesting and thanks for the follow through report. In some ways, I'm not sure the chain is a great idea. A line boy broke one of mine and I never fixed it and the other cap is fine. I make sure the chain is straight and maybe reverse twist a little before screwing the cap on to make sure the chain isn't in a bind.

Simply have to get rid of the stupid chain and punch the chain retainer flat. The above pics show the cap sitting crooked - bet that side of the cap is bottoming out on the chain retainer tab.

I removed the chain and pushed in the tang on minbe... have had no issues since.

RE: Monarchs affecting the fly-a-bility of the wing and the dramatic drag penalty previously mentioned... No offense, but perhaps a bit dramatic.

Love these caps and have installed several with no issues. But thats just my .02

To be honest this thread has made me a bit nervous, i just bought a 182 with monarchs. I am happy to report no leaks.