Listen for the WHOOSH

[pelmet]

I had read about this sort of thing and it actually did happen to me once but after a longish flight. A high wing Cessna on its first flight after a fairly substantial maintenance inspection. We landed and when I opened a fuel cap to refuel, I head the sound which made me wonder about whether there was a problem. I went inside to call the place we had picked the aircraft up from and when I came back out, the other guy said that he had taken a piece of wire and put it up the vent and found a small piece of masking tape in a ball(although for some strange reason he threw it away and I never saw it). The maintenance company had done a lot of work to get the fuel guages accurate.

I suspect it was only a partial blockage as we flew over two hours. I believe I got new fuel caps or had them subsequently checked.

Anyways.....it can be a difficult one to catch on a walkaround but if you hear a Whoosh when you open the fuel cap......be suspicious.

C-GDON, an Insignia College Cessna 172, was conducting flight training circuit exercises at
Vancouver, Boundary Bay airport (CZBB), BC with a student and instructor on board. The aircraft
departed Runway 13 after the student performed a touch and go landing. At approximately 400
feet AGL the engine surged and then lost power (AVCO Lycoming O-320-E2D). The instructor took
control of the aircraft, declared an emergency, and attempted to return to land on Runway 25. The
aircraft did not reach the runway and landed near the airport perimeter fence in a farmers field. The
aircraft sustained no damage. There were no injuries. The ELT did not activate.
Subsequent maintenance actions included adjusting the LH fuel tank vent tube in accordance with
the maintenance manual and replacing the LH fuel cap with a vented filler cap. The aircraft has
been returned to service.

[photofly]

Vented fuel caps are a good idea. (Aren't they all vented by now?)

[pelmet]

Vented fuel caps are a good idea. (Aren't they all vented by now?)

I have taken a look at vented caps on a Cessna before and it was difficult to see any venting. I wonder if they can become blocked.

[photofly]

Mine are like this:

06-00985b.jpg (37.07 KiB) Viewed 2384 times

Peel the red rubber bit back, and there's a hole that goes through to the top. The idea is that air can get in from outside, but positive pressure in the tank pushes the rubber against the cap and can't escape.

You actually don't want to equalize the pressure to the top of the wing in flight - that's where the lift is, so the pressure there is lower than the static pressure.

A whooshing noise can be fuel vapour escaping when you release the lid in hot weather - that's ok. The regular underwing vent has a flap valve and the fuel system seals, so you can certainly build up some positive pressure in the tank when things warm up.

You would have to work pretty hard to block both the underwing vent and the fuel cap to get negative pressure in the tank while you fly.

[photofly]

Here's what's under the rubber:

Screen Shot 2021-07-19 at 11.16.13 AM.png (107.5 KiB) Viewed 2374 times

[pelmet]

A whooshing noise can be fuel vapour escaping when you release the lid in hot weather - that's ok.

Interesting. I wasn’t aware of that. Has anybody else heard a normal ‘whoosh’ in hot weather. Is it certain individual types or most light aircraft.

I do remember that little engine rubber flap thing now.

[photofly]

I looked under the rubber flap. Looks like there are four little holes, spaced around the centre.

If you had suction in the tank it would be hard to get the cap off. If there’s a bit of excess pressure then the cap will pop off without effort. It should be very straightforward to tell the difference.

[pelmet]

I looked under the rubber flap. Looks like there are four little holes, spaced around the centre.

If you had suction in the tank it would be hard to get the cap off. If there’s a bit of excess pressure then the cap will pop off without effort. It should be very straightforward to tell the difference.

Thanks,

Don't remember any particular difficulty in removing the cap.Do remember a bit of a whoosh and then the piece of tape was found.

Still curious if anyone else has heard a 'whoosh' from vapour in hot weather. Went Cessna flying today on a just returned aircraft at 30 degrees and fueled up. Nothing heard this time.

[Dry Guy]

Yes, it's quite common to hear vapor escaping when you open the cap in the summer. As others have said it makes it easier to lift up the cap. What you don't want to feel is vacuum holding the cap in position or hear air rushing into the tank.

[pelmet]

Yes, it's quite common to hear vapor escaping when you open the cap in the summer. As others have said it makes it easier to lift up the cap. What you don't want to feel is vacuum holding the cap in position or hear air rushing into the tank.

Thanks,

I appreciate the info.

However, what I don’t understand is why it would be normal to have positive pressure in the tank from vapour. The main vent should allow any excess pressure to escape.

[photofly]

The Cessna vent line has a flap valve on the end in the tank: it lets air in but not fuel (or vapour) out, as long as it’s working properly.

1579C660-4D9D-48DD-A836-402A41F4F3E2.jpeg (143.14 KiB) Viewed 1956 times

Item 11 in the diagram.

I read that (according to Macfarlane) that valve should have a tiny pressure relief hole; maybe mine is blocked. Something to look into, anyway. Here’s a whole article on the subject:
http://acversailles.free.fr/documentati ... enting.pdf

[PilotDAR]

There should not be a pressure differential in the tanks of GA airplanes. If there is, it should be fixed before further flight. Though aluminum or wet wing tanks don't collapse, bladder tanks will. For rigid tanks, failure to vent could cause a fuel flow problem.

Valve 11 above should have a little hole in it to allow the pressure to equalize. That's why most unmodified high wing Cessnas will dribble fuel from the left wing vent when the tank is full, it's flowing through the little hole.

The vented fuel cap is required by AD. Two of them are better.

[pelmet]

Yes, it's quite common to hear vapor escaping when you open the cap in the summer. As others have said it makes it easier to lift up the cap. What you don't want to feel is vacuum holding the cap in position or hear air rushing into the tank.

The Cessna vent line has a flap valve on the end in the tank: it lets air in but not fuel (or vapour) out, as long as it’s working properly.

I read that (according to Macfarlane) that valve should have a tiny pressure relief hole; maybe mine is blocked. Something to look into, anyway. Here’s a whole article on the subject:
http://acversailles.free.fr/documentati ... enting.pdf

It just doesn't make sense that the flap valve is somehow able to differentiate between air and vapour and then let one through while stopping the other. And liquid would get through if air can. I remember back in the day, the Cessna's at my flight school would be put in the warm hangar overnight during the winter after being fully fueled for flight the next morning. Each one would have a bucket placed on the hangar floor, under the fuel vent so venting fuel would not go onto the hangar floor. It was amazing how much fuel was in each bucket in the morning.

The second full paragraph of page 11 in the article you published mentions the orifice that allows air, vapour, or liquid to pass through(outward from the tank) in order to equalize fuel tank pressure. This seems to makes sense, otherwise, expanding liquid and gasses will put significant pressure on the tanks. I would suggest that a whoosh when you open the fuel cap is not normal and should be written up whether due to positive pressure or negative pressure.

Listen for the Whoosh.

[pelmet]

Found an old but interesting thread with a Whoosh example……..

viewtopic.php?p=832017#p832017

[ohoh]

The vents on most GA light aircraft are simple. They allow slight and equalized positive pressure(good) and prevent negative pressure (bad).
A bit of positive pressure helps ensure fuel is being delivered to the engine, with or without a fuel pump. Many light aircraft don't use a fuel pump, especially high wing. The following is applicable to both bladders and metal or integral tanks. The vent on the bottom of the wing for Cessnas with struts for example, have very specific instructions for position behind the strut. The article linked above shows this. (Many on aged aircraft are NOT in the correct position). The vent on 180 -185's for example should peek out from below the strut by about a third of the hole so it's getting some impact or ram air but not full ( not the entire end of the pipe exposed below the strut). The manual has a good illustration and measurement for anyone that knows this is an important adjustment. You'd be amazed and disappointed to count the number of maintenance engineers NOT aware of this simple five minute adjustment. If this vent inlet becomes blocked the vented cap is a backup, only allowing negative pressure to be released by that little rubber flapper on the cap being sucked down and drawing air INTO the tank through the cap. That flapper is there to contain or stop positive pressure from being able to be pushed out. When the aircraft is sitting stationary on the ground, positive pressure should bleed off through the lower vent. If the vent system is operating properly you should NOT have a whoosh from air entering or escaping.
Without slight positive pressure in the tank, in flight, you're now relying on gravity alone. If the lower vent is plugged, or not positioned correctly this will eliminate the primary source of good equalizing positive pressure. The safety net and backup is the vented caps allowing air into the tank, but not allowing fuel vapors out. If the vented caps are old, flapper stuck to the cap or hard as a rock then the lack of air coming into the tank to replace fuel being used by the engine will cause a vacuum in the tank, starving the engine of fuel. Chances are good that if the flapper is in bad shape, so is the main seal for the cap. This will allow air to be sucked OUT of the tank past the seal or o-ring on the cap, due to top of wing = low pressure. At this point you'd have multiple failures, lower vent plugged, backup vents (caps) plugged and vacuum created by poor gas cap seal. Unlikely but possible if the fuel system vents and caps are ignored. Result - your looking for a safe place to land, unscheduled.