Mooney 252 max fuel uplift

[turbotrk]

Hi, I have spent one day trying to find out if my fuel flow readings and fuel indication on my 252 were right or wrong.

After refill and several hours flying I took all the fuel off the tanks and found out that there was much less then expected.

I started to refill gradually and got overreading informations on both televels, then I adjusted the fuel indicators to match the quantity exactly.

Finally I refilled completely the tanks and it took a lot of patience as once it starts to get full you must preceed slowly for a long time.

I managed to put in just 71.85 US Gallons of usable fuel , no way to get to 75.6 and it looks the same on both tanks.

how is it possible to have such a big difference from what's reported on the AFM?

[carusoam]

A typical Mooney tank challenge would be caused by the air that is at the top of the tank and how it is vented.

some Mooney tanks have two fill ports on each side with a procedure on how to fill them.

some Mooney tanks have different hardware at the top of the tank where the fuel nozzle enters the tank.

the 252 has a real special piece of hardware that includes a spring loaded flap to keep gas from leaving the plane when the cap is left off.

i helped somebody remove a a fuel nozzle from a 252 tank the other day.  The nozzle had a funny ring on it and got stuck at the bottom of the fuel neck and locked into the spring loaded flap...  It took two people with long fingers to hold the flap open to remove the nozzle.

if you look at this device.  You will find that it is different than other Mooneys.  

Are you only filling to the bottom of the flap?

can you vent all the air out from above the flap?

make sure both fuel vents under the wing are clear from any potential blockage...

i would expect that if you only fill to the bottom of the flap, you will be missing about five gallons per side for ten gallons total.

this is experience I have with the fuel neck differences between my M20C, R and my flight instructor's M20S.  Each have different fuel neck pieces.  The 252 has the most complex one to vent properly with the flap.

keep in mind, I am only a PP, not a mechanic.

best regards,

-a-

Edited September 20, 2015 by carusoam

[turbotrk]

I filled up to the top of the flap, I could not get any more than that. I tought that there might be air trapped inside but it is not practical to fill above the flap. Now what I have done is just changing the max value from 75.6 to 72gal and I will plan my flights according to that.

[Nemesis]

The 231 has the same anti-siphon valve in the fuel neck that your 252 has. When I want a full load of fuel, I commonly fill to about 1 inch above the bottom of the anti-siphon valve. Yes, it is slow going but I have done it many times and would guess it is reasonable that 1.5 gallons per side can be added after the bottom of the anti-siphon valve. The biggest problem is when filling above the bottom of the neck, the air has to be ejected and much of that air comes out around the bottom of the open valve and makes a bit of a splash.

Dave

[kortopates]

Full tanks are actually to the anti-siphon flap, nor above it. It requires a lot of patience to overfill the tanks above that because the tanks were not designed to be filled above that anti-siphon flap and now you are trying to force the trapped air out. That's not to say you can't add more fuel above the certified full level, but I think the only reliable way to know how your are doing is watch the number of gallons dispensed above the anti-siphon flap precisely.

As Anthony said, some of the  newer longbody's allowed for modifying the filler neck to increase capacity by  Mooney drawing, but there is no such approved modification for K's (it didn't even apply to all the longbody's) except for the Monroy extended tanks which of course is entirely different.

Anyway, you are to be commended for making the effort to go through and actually measure your tank capacity. And unfortunately you are not the first to report you couldn't get near certified capacity. Its not such an easy task without actually emptying the tank till totally dry by removing the sump drain and then making sure its really dry inside (its suppose to the lowest point on the wing, but even that depends on other factors) and then verify its dry before adding measured fuel.

To adjust your K factor, it really takes a lot more than just a partial tank. I suggest running more like a 100 gallons through it, without  re-zeroing the used amount (or carefully tracking it elsewhere) then run the  numbers and adjust the K factor and run another 100 gallons. One is too likely to jump around from error of not refilling to the exact same point every time based on just running a much more modest amount of fuel through. But multiple refills will help to minimize out that error.

Remember, when you start out, the only  real trustworthy data you have to start with is how much fuel you have added. Everything else is an unknown or estimate that you are trying to validate. It gets awfully hard to try to solve all of them at the same time. 

 

 

[Nemesis]

I once saw either Stacey Ellis or Bill Wheat write that the full fuel state in any of our machines was the top of the tanks - as much fuel as you can put in and still put the cap on. I remember that after reading the question, even they did not hot have the answer. Then, a couple of days later they wrote the answer. I will look on the email lists and see if I can find the response.

In the newer machines, there is even a hole that is drilled into the top outside portion of the anti-siphon valve so that air can pass without making all of the bubbling and splashing.

Dave

[kortopates]

 

I once saw either Stacey Ellis or Bill Wheat write that the full fuel state in any of our machines was the top of the tanks - as much fuel as you can put in and still put the cap on. I remember that after reading the question, even they did not hot have the answer. Then, a couple of days later they wrote the answer. I will look on the email lists and see if I can find the response.

In the newer machines, there is even a hole that is drilled into the top outside portion of the anti-siphon valve so that air can pass without making all of the bubbling and splashing.

Dave

Nope, talk to Stacey again if you like (I have directly). For the entire fleet its the bottom of the filler neck. If it was to the top as you suggest, then we have a major contradiction shown in the mod you referenced for the ventilation hole near the top of the filler neck to increase capacity in the longbody's - how could you increase beyond the top of the tank?. Better yet look at the diagram in the Mooney POH supplement for this modification that shows original capacity (89 gal) being full at the bottom of the filler neck and the new whole adding another 11 gals to capacity (100). That's the best illustration you'll find of where Full is. To make it easy, I'll just attach the one for the R and TN (there is another one for the Bravo that gave 51 gals a side)).

50gal_useable_fuel-rtn.pdf

[Nemesis]

I can't find the reference that I remember, so I will have to go with your insights Paul.

I will however keep filling beyond the bottom of the anti-siphon valve to get closer to my 72 usable.

--- Or maybe I had better keep my mouth shut or else the next time I am with Paul for flight instruction he will give those really rough unusual attitudes. ---

Dave

[kortopates]

I can't find the reference that I remember, so I will have to go with your insights Paul.

I will however keep filling beyond the bottom of the anti-siphon valve to get closer to my 72 usable.

--- Or maybe I had better keep my mouth shut or else the next time I am with Paul for flight instruction he will give those really rough unusual attitudes. ---

Dave

LOL's Dave, At least you are one of the few that has measured your tank capacity. Most have not. I wonder if somebody like Paul B that seals the tanks can vouch for a freshly sealed tank actually holding the TCDS specified capacity.  The only thing I can imagine is lots of patches, or even worse perhaps, lots of sloshing sealant added to tanks over the years to fix leaks robbing from the capacity.  I know my prior 231 had a gallon of sloshing sealant added to fix a leak once. But some of the reports just seem to far off.

[turbotrk]

Well, in the end it is not a big deal, as long as I am able to know how much I have inside and how much I burn it is fine.

Now my full tanks value will be set at 72gal istead of 75.6 remembering that this requires getting it filled up to the top of the flap and a few minutes of patience. I think that a/c manufactures should recommend such pratcices and the results should be put into the AOM as it is done with aircraft weighting procedudures. 

I am glad I did not rely on televels and added extra fuel always until performing this tasks, now knowing the exact amount of fuel inside the a/c I will keep the total count and after a couple of refills I will dry the tanks again checking the difference between the remaining fuel and shadin FF indication. The difference between the two values will be used to adjust the K-factor proportionally. Should be easy maths ...

Televels already adjusted, the were overreading , low fuel lights were ok, but less fuel in the tanks than expected could have leaded to nasty results.

[kortopates]

 I think that a/c manufactures should recommend such pratcices and the results should be put into the AOM as it is done with aircraft weighting procedudures. 

Be careful, our Mooney tanks should never be filled entirely because the regulations governing the fuel design in part required Mooney to allow for expansion space:

14 CFR Part 23.969 Fuel Tank Expansion: "Each fuel tank must have expansion space of not less than two percent of the tank capacity, unless the tank vent discharges clear of the airplane (in which case no expansion space is required). It must be impossible to fill the expansion space inadvertently with the airplane in normal ground attitude"

[turbotrk]

Yes, I agree with you filling completely is not recommended, what I wantet to say is that they should recommend that periodically the real total capacity should be assessed and reported on the AFM by getting the tanks empty and then refilling to the top, maybe once every two or tree years. On some aircrafts that have bladders inside like beechcraft it happens with time that capacity is reduced due to rubber deformation. Not the case on the mooney of corse but even among the same models total capacity differs and it needs to be assessed.

[Deb]

I have read on Beechtalk that the APS course suggests running your tanks dry while flying (each tank separately on different flights.) That way, when you fill your tank it will reflect what can be used. I am not recommending this, merely reporting what I have read.