Fuel smell in cockpit on takeoff

[Dnewt]

Hi all,

I’m looking for some advice for a UK based new Mooney owner.  The a/c is a 1987 M20J and a smell of fuel enters the cabin shortly after takeoff and then disappears after 3-5 minutes.

We’ve had two mechanics look over the engine and all seems fine, one replaced an induction gasket and the other found a minor seep in the cabin adjoining the pilot side fuel tank near the fuel lines which has been cleaned up and marked.  The fuel lines look fine and the staining is around 4 bolts.  I thought that this may be the cause but I now doubt it as the problem only occurs on take off after the clean up.

The a/c runs fine at all other times and there are no signs of leaks on the ground after thorough run ups.  Mags have been checked on the ground and in the air at cruise and fuel flow is as expected during all phases.

The only things I can narrow it down to are:

- something related to the landing gear being raised?

- occurring when the engine is at full power / rich.  However, I have done a T&G when I didn’t raise the gear and the problem didn’t occur which points to the first option.

Ive read a few posts on Mooney leaks but can’t find anything similar to this.  Any thoughts or areas worth investigating?

Thanks in advance,

David

[jaylw314]

Are you sure you're smelling fuel?  I was in a Cherokee that had a brake line leak from the parking brake.  I had never smelled hydraulic fluid before, and in the cockpit it was a weird "it sort of smells like fuel" smell that it really very strong.  If you've never smelled it before, it might be a good idea to check your master cylinder and take a CAREFUL whiff of it (it is really VERY strong)...

[N201MKTurbo]

It is a most likely a tank leak. Fuel may be pooling in the wing and flowing to the cabin when you rotate.

[carusoam]

Welcome aboard Dnewt.

Sounds like a fuel leak for sure...

Some are easier to find than others...

First...

• look around the outside of the plane at vent holes on the bottom of the wing...
• check under the carpets for blue stains...
• under the cowl, each fuel injector has a vent hole, often found to have blue stains running out of it...

Second start researching fuel leaks on MS...

Fill in your detail in your avatar. You won’t want to know about carbs when you have FI...

PP thoughts only.

Best regards,

-a-

 

[anthonydesmet]

Dnewt,

having gone through tank issues a few years back until my eventual reseal I agree with @carusoamand @201MKTurbo. You are getting a leak at rotation.  When your done flying do you top off before putting it in the hanger or are you a top off before next flight guy?  If you are then in addition to carusoam’s advice, top off and let it sit, then look for the stains under the carpet and under wings,etc.

[larrynimmo]

these tanks are built directly in the wing...no real separate tank, and they are sealed at the rivits by epoxy....overtime with stress the plastic cracks and weeps....

 

then you need to go through the costly expense of resealing the tanks...this is more or a winter issue...

I have one tank that weeps, and in the winter I keep the level down lower and that really helps.

[MARZ]

Pull your inspection panels close to the wing root - any seep will be apparent - to gauge the severity clean any stain and recheck after flying a few times.   

[Yetti]

There is also an inspection panel mid wing bottom leading edge.  Pull that one and look for fuel  staining.

[0TreeLemur]

I too agree that this sounds like a fuel leak near or into the cabin.  When we fly our Mooney's, the cabin is actually at a pressure slightly less than ambient.  After takeoff, the evaporation of fuel pooled near the wing root or thereabouts with vapors being pulled into the cabin is my guess.  The fuel is leaking from sender gasket, leaky tank sealant or both.  My aircraft just had its tanks resealed and fuel level sender gaskets replaced- and the avgas smell is gone.  Now it just smells like "Old Airplane".  Good luck.

[Shadrach]

Is there any sign of fuel staining on the underside of either wing? I would start with the lower under wing leading edge panels closest to the fuselage. Once removed fell around inside for fuel pooling.  You will notice small holes on the underside of the wing.  Those are there to act as drains when liquid pools inside the leading edge cavity.  If the plane is new to you, it might not be a bad thing to pull the lower interior panels and give everything a look.  Your plane is a late enough model that SB M20-208 does not apply, so it's possible that no one has seen the interior of the wing roots or the cage since 1987 (Good year for Mooneys and Napa Cabernets).

[Danb]

Somewhat related, I have long range tanks, recently when I fill my tanks the night before a long trip I have fuel smell in the hanger and in the cockpit....I haven’t found any fuel seepage  yet ?..

[mooniac15u]

When I first got my J the fuel caps did not seal properly.  If I took off with full fuel the combination of nose-up pitch and low pressure over the wing would cause fuel to come out past the fuel cap.  It caused a strong fuel smell in the cabin.  I suspect I could smell it in the cabin because the J has a fresh air intake near the tail.

[Zulee]

I had my tanks resealed and within a couple weeks I experienced the same thing...smell of fuel but once airborne the smell would dissipate as you've described.  As others have shared, you can look around your main gear well for signs of fuel but even if you do I would check the fuel tube that runs from the tank inside the cabin.  If you look where our bench seat meets the floor on each side...you can pull the floor carpet back from the side, or you may have to pull the side panel off to get a good look.

[bradp]

Put your nose down at the footwell during preflight. I bet you’ll catch a hint of fuel smell and it will blow into the cabin when the wing gets windy. 

The other place to check is the inboard fuel sender gaskets. 

Follow the smell to the leak... it will reveal itself. 

[blakealbers]

Fuel drain in cockpit could be weeping. I noticed slight stain years ago but didn’t seem to bother couldn’t find any actual leak. Finally last week I saw fuel welling up right next to the pull tab. Flew no further had it fixed at the shop where I landed. Reseal kit on gascolater, flew home today and realized my plane smelled weird, or rather that I have been smelling fumes a lot longer than I thought!


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[carusoam]

Keep in mind, fuel line leaks can be really bad... they show a little fuel on the outside leaking... what you don’t see is all the air the same leak is allowing into the fuel line...while running... air being less viscous than fuel, will leak a whole lot more than the few drips of fuel that are visible...

air bubbles caught in the FF sensor paddle wheel really cause funny readings... over estimating fuel used...

PP thoughts only...

Best regards,

-a-