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Topic Options
#19403 - 01/14/11 10:00 PM Re: Fuel balance [Re: wrecksum]
steve71e Offline
Second in Command

Registered: 02/22/10
Posts: 172
Loc: Atlanta, Ga
It ain't "Theory". It's fact ! It's right in system description and diagram. Cessna SB updates both caps to be vented. If you have an unvented cap on the right it is incorrect. If your parts book says a non vented cap goes on the right it has not been updated. New caps, new vent and innerconnect hoses, new transmitter gaskets (basically fix all your leaks) have fixed many unneven fuel burn squawks for me. If you have left and right vent inlets as in late 1970s and up 182s, the vent tubes behind the struts get adjusted to correct and even up fuel burn. Other wise, fly straight and level. My '79N burns evenly. Never more than a gallon difference. Put in in left or right position when parked on unlevel ground or gravity will level your fuel tanks for you.
_________________________

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#19407 - 01/14/11 10:21 PM Re: Fuel balance [Re: steve71e]
wrecksum Offline
SVP Public Relations
Platinum Pilot

Registered: 09/26/10
Posts: 2704
Loc: Dominican Republic
Originally Posted By: steve71e
It ain't "Theory". It's fact ! It's right in system description and diagram. Cessna SB updates both caps to be vented. If you have an unvented cap on the right it is incorrect. If your parts book says a non vented cap goes on the right it has not been updated. New caps, new vent and innerconnect hoses, new transmitter gaskets (basically fix all your leaks) have fixed many unneven fuel burn squawks for me. If you have left and right vent inlets as in late 1970s and up 182s, the vent tubes behind the struts get adjusted to correct and even up fuel burn. Other wise, fly straight and level. My '79N burns evenly. Never more than a gallon difference. Put in in left or right position when parked on unlevel ground or gravity will level your fuel tanks for you.


Hi Steve.
I've got two vented caps and no discernible leaks.
I don't have any smell of gas but I'll check the vent pipes anyway.Is it possible to gain access through the roof panel or does all the headlining have to come out?
_________________________
A person without a sense of humour is like a wagon without springs.It's jolted by every pebble in the road.

Cessna 172, C172, C-172, Cessna Skyhawk fan.
www.cessna172club.com

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#19415 - 01/14/11 11:19 PM Re: Fuel balance [Re: wrecksum]
RodneyHooverCFI Offline
The Cessna Sensei
Gold Pilot

Registered: 02/16/10
Posts: 1645
Loc: Pasadena, MD
Few posts back leaking fuel vent crossover line was mentioned. Flown a 172 that over several flights, would notice a hint of fuel smell when doing things like slow flight and such. Couldn't figure out where it was coming from as it was just a few minutes at a time that I could smell it, and never saw signs of a fuel leak. Few weeks back, flew the plane and during the initial climb, fuel was slowly dripping from the antenna cable that loops from the glideslope antenna, around a trim piece, then into the headliner. Drop of fuel hitting my hand while on the throttle got my attention. Reported it to mx, turned out the rubber connections on the vent had degraded enough to leak.

As for the cross-feeding while fueling, I've heard that a lot, followed the advice and even passed it on to students. Two days ago, doing preflight on 172, using a dipstick showed that left tank had about 2 gallons less than the right tank. Plane was parked in a level hangar since the previous day. Sure makes sense to me that the tanks would cross-feed and always equalize themselves, especially when the plane is parked for more than a few minutes. I don't see how the fuel tanks aren't more than large water levels, but doesn't seem to be the case. I've been around for countless preflight inspections, all of which involved using a dipstick to check fuel quantity when they weren't completely full. Over and over again, I've seen differences in left vs. right quantities of 0.5 to 2 gallons difference, all when the planes are on level ground. What I'm trying to get at here is has anyone ever actually noticed any cross-feeding from fueling while the fuel selector is in the "both" position?
_________________________
Flight Instructor- ASEL and Instrument Airplane
Commercial Pilot with Instrument Rating- ASEL & AMEL
Assistant Chief Instructor at W00, Freeway Aviation.
States I've flown to-

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#19417 - 01/14/11 11:29 PM Re: Fuel balance [Re: wrecksum]
EdW Offline
Club Sponsor
Gold Pilot

Registered: 11/21/10
Posts: 1222
Loc: Just east of Graceland
Steve, reread my post; 2 vented caps, sound vent system, servicable interconnect, no leaks...still uneven burn. External vent line checked for positioning per the manual. Still uneven burn. Everything checked, double checked, and cussed at. STILL UNEVEN BURN. So, your theory didn't cure the problem. And yes, I DO know how to read a maintenance manual, Service Bulletin and AD. I've been chasing this particular ghost for almost nineteen years.

The only thing I haven't yet done is going to the McFarlane STC'd modified vent line.

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#19424 - 01/15/11 01:02 AM Re: Fuel balance [Re: EdW]
EdW Offline
Club Sponsor
Gold Pilot

Registered: 11/21/10
Posts: 1222
Loc: Just east of Graceland
Cessna AD 79-10-14 requires only ONE vented cap,on the right wing. The text says to comply with Cesna SESL SE77-6, which says to install the cap on the right tank. This is applicable to models thru 1975.


Edited by EdW (01/15/11 01:03 AM)

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#20186 - 01/19/11 09:36 PM Re: Fuel balance [Re: EdW]
steve71e Offline
Second in Command

Registered: 02/22/10
Posts: 172
Loc: Atlanta, Ga
Cessna hasn't even made the unvented cap in years, Cessna SE77-6 was superseeded by SEB 92-27. Raised non vented caps C156004-0101 no longer available since 1976. Only raised vented caps p/n C156003-0101. Now this doesn't mean someone else may have them though. SEB 92-27 goes into great detail for installing raised and reduced diameter caps as well as SE77-6. These SBs did away with all large diameter fuel openings flush caps and all the old caps that looked like an automotive screw on caps. Ed, I just read your one post on water contamination in your fuel system and if a Skyhawk gets water in it's fuel system it is most likely coming in past the fuel caps. I don't know where else it would come from. Also, if your fuel system is letting in water, it's letting in air somewhere other than Cessna designed it too as well. There is a symptom of your uneven fuel burn issue. Think I'd definitely replace those caps. I replace mine every two years. When they get easy to turn, it's time to replace them. I never get water when doing my sumps and it lives outside.
_________________________

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#39936 - 06/21/11 12:24 AM Re: Fuel balance [Re: steve71e]
OldCrow Offline
Club Sponsor
Silver Pilot

Registered: 02/22/10
Posts: 890
Loc: STS, CA, USA
So with the discussion on flight above 5,000 and switching thanks---
Does anyone know what the Cessna Kit No. SK172-31B or SK172-32
contains or how it is installed??

I would like to be able to leave it on both.. As a newby I inadvertently turned it to OFF once (only once) and it got very quiet shocked shocked


RE: C-172 Power Loss at altitude
Airworthiness Directive
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 39
Amendment 39-1415; AD 72-07-02
Airworthiness Directives; CESSNA Models 172, 172A, 172B, 172C, 172D, 172E, 172F, 172G,
172H, 172I, 172K, Airplanes

To reduce the possibility of engine power interruption at altitudes above 5000 feet caused by vapor formation in the fuel lines, accomplish the following:
(A) Effective now, the airplane must be operated on a single fuel tank immediately upon reaching cruise altitudes above 5000 feet.
(B) On or before April 1, 1972, install at the fuel selector valve applicable Cessna placards P/N's 0509021-1, 0509021-2 or 0509021-3 as provided with Cessna Service Letter SE72-7, dated March 17, 1972, or any FAA-approved equivalent placard which reads as follows: SWITCH TO SINGLE TANK OPERATION IMMEDIATELY UPON REACHING CRUISE ALTITUDES ABOVE 5000 FEET.
c. Compliance with the provisions of Paragraphs A and B is no longer required when the fuel system has been modified by the installation of applicable Cessna Kit No. SK172-31B or SK172-32 referenced by Cessna Service Letter SE72-7, dated March 17, 1972, or by the accomplishment of any equivalent method approved by the Chief, Engineering and Manufacturing Branch, FAA, Central Region.
This amendment becomes effective March 25, 1972.
_________________________
Another day in paradise
1969 172k AirPlains 180hp

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#39979 - 06/21/11 12:40 PM Re: Fuel balance [Re: OldCrow]
joelaff Offline
Bronze Pilot

Registered: 04/08/11
Posts: 616
Loc: Missouri, USA
I always figured the uneven fuel burn between the tanks was due to flying solo... left side of the plane being tilted down slightly.

Anyone know why only the older 172s need to be switched to a single tank at 5000ft? This does not appear to apply to my 172M (1974), but wondering why...
_________________________


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#39998 - 06/21/11 07:16 PM Re: Fuel balance [Re: joelaff]
November X-ray Offline
Club Sponsor
Gold Pilot

Registered: 08/15/10
Posts: 1589
Loc: Brookshire, Texas, USA
This is due to a few items, all related to the fuel system. On the older model Cessna's the fuel system is externally vented on the left side only with the right tank vented via a crossover to the left tank. With the increase in altitude comes a change in both pressure and temperature thereby increasing the potential for a "vapor lock". In the older models, the reason for switching to a single tank is to increase the velocity of fuel flow thru the fuel line to reduce the possibility of vapor lock. Additionally the fuel line size was increased slightly on the newer models with vents on both tanks, either thru vented caps or external venting thereby reducing/eliminating this issue.
_________________________
N. X.




www.cessna172club.com

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#40035 - 06/21/11 11:36 PM Re: Fuel balance [Re: November X-ray]
Russ Farris Offline
Second in Command

Registered: 08/15/10
Posts: 148
Loc: Charlotte, NC
There was never a problem with the Continental powered 172s with vapor lock above 5,000 feet. The Cessna 170B has the same fuel system as the early 172s, and there is no single tank operation AD above 5K for them.

With the introduction of the O-320 in 1968, reports started to come in about power losses operating in the both tanks position. Cessna even modified a 172 with clear plastic fuel tubing in an effort to see what was going on - they never were able to find a problem. Operating on a single tank above 5,000 was deemed an acceptable way of dealing with the issue. The fuel line diameter was increased on subsequent models.

It was easier to have the AD include all early model 172s than to prove there was never a problem with the Continental powered airplanes.

The weight of the pilot is much too close to the centerline to have any effect on lateral balance.

Russ Farris


Edited by Russ Farris (06/21/11 11:42 PM)

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#40037 - 06/22/11 01:27 AM Re: Fuel balance [Re: Russ Farris]
OldCrow Offline
Club Sponsor
Silver Pilot

Registered: 02/22/10
Posts: 890
Loc: STS, CA, USA
suspicions confirmed -- but still I wonder what was the cessna fix that solved the problem??
bigger lines ?
another vent behind the right strut?
different Caps"
I cannot find any reference to the two part sets called out.
Must be a conspiracy ...
grin
_________________________
Another day in paradise
1969 172k AirPlains 180hp

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#41021 - 06/29/11 09:09 PM Re: Fuel balance [Re: OldCrow]
Bigdoggh Offline
Gold Pilot

Registered: 05/26/11
Posts: 1440
Loc: Georgia
Love this board!!! I just bought a 172L (1971) which consistently burns left tank more than right. I have been checking before and after each flight. This has been very informative. I think I have the unvented caps...they are relatively flush with the wing. Does this mean I have the wrong caps?
_________________________
George - 1972 C-172L


FTY ANB CTJ LGC GVL AHN 20GA PUJ ALX ACJ UOS DTS AVL I69 YUP DVK AIK DVT SAF AXX ROA DYL TTN LYH GSP DNN VPC PIM RYY JKA ASD MJD RHP MKL M05 HKA TYS CHA TYQ OSH

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#41032 - 06/29/11 11:33 PM Re: Fuel balance [Re: Bigdoggh]
OldCrow Offline
Club Sponsor
Silver Pilot

Registered: 02/22/10
Posts: 890
Loc: STS, CA, USA
If they look like these red ones that have a _|_ shape I believe you will find a flat rubber disk on the inside -- vented.
if they have a flat top and grips around the edge and no rubber flap in the middle of of the top inside they probably are original (from my limited experience)unvented.
cessna top view


Edited by OldCrow (06/29/11 11:33 PM)
Edit Reason: spelng
_________________________
Another day in paradise
1969 172k AirPlains 180hp

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#41047 - 06/30/11 08:02 AM Re: Fuel balance [Re: Russ Farris]
Propduster Offline
Gold Pilot

Registered: 08/30/10
Posts: 1072
Loc: Nova Scotia, Canada
Originally Posted By: Russ Farris
There was never a problem with the Continental powered 172s with vapor lock above 5,000 feet. The Cessna 170B has the same fuel system as the early 172s, and there is no single tank operation AD above 5K for them.

With the introduction of the O-320 in 1968, reports started to come in about power losses operating in the both tanks position. Cessna even modified a 172 with clear plastic fuel tubing in an effort to see what was going on - they never were able to find a problem. Operating on a single tank above 5,000 was deemed an acceptable way of dealing with the issue. The fuel line diameter was increased on subsequent models.

It was easier to have the AD include all early model 172s than to prove there was never a problem with the Continental powered airplanes.

The weight of the pilot is much too close to the centerline to have any effect on lateral balance.

Russ Farris


I was wondering about that - there's a sticker by the selector in my 172D, but I've never moved the thing from "Both" since the day I took possession. Flew at 5500 the whole way back from Winnipeg and never had any problem, nor did I notice any uneven fuel burn between sides beyon maybe a gallon either way.

Thanks, Russ!

Rick
_________________________
1963 Cessna 172D C-FPEC

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#41052 - 06/30/11 08:52 AM Re: Fuel balance [Re: Propduster]
Showboatsix Online   content
Gold Pilot

Registered: 12/04/10
Posts: 1035
Loc: KUAO, Aurora, Oregon
Originally Posted By: Propduster
Originally Posted By: Russ Farris
There was never a problem with the Continental powered 172s with vapor lock above 5,000 feet. The Cessna 170B has the same fuel system as the early 172s, and there is no single tank operation AD above 5K for them.

With the introduction of the O-320 in 1968, reports started to come in about power losses operating in the both tanks position. Cessna even modified a 172 with clear plastic fuel tubing in an effort to see what was going on - they never were able to find a problem. Operating on a single tank above 5,000 was deemed an acceptable way of dealing with the issue. The fuel line diameter was increased on subsequent models.

It was easier to have the AD include all early model 172s than to prove there was never a problem with the Continental powered airplanes.

The weight of the pilot is much too close to the centerline to have any effect on lateral balance.

Russ Farris


I was wondering about that - there's a sticker by the selector in my 172D, but I've never moved the thing from "Both" since the day I took possession. Flew at 5500 the whole way back from Winnipeg and never had any problem, nor did I notice any uneven fuel burn between sides beyon maybe a gallon either way.

Thanks, Russ!

Rick


I have a 56 Cessna 172, a July 56 airworthy date, I have forgotten to switch to single twice, once over Eugene Oregon, at 6500ft another time giving a first ride to Mt ST Helens, at 7500 ft, the engine does a couple of surges, and then rapidly dies down to about 1500rpm, switching to single tank and the roughness goes away in about 30 seconds. I have a 1/2" vent facing forward on the LH tank, and both caps are of the vented AD requirement. Don't know why, but it will do it on my plane.

Dean SB6

Quote:
So with the discussion on flight above 5,000 and switching thanks---
Does anyone know what the Cessna Kit No. SK172-31B or SK172-32
contains or how it is installed??


I know this about this kit, they were originally sold for $29.00, I thought to myself, I wonder if Cessna has any left, I did a Cessna parts search and YES!!!! they had several still in NOS inventory, so I asked how much to ship, I was quoted $900.00 and change for the same $29.00 kit.............. I told them that I was born at night...... but not LAST night! and they could keep their $29.00 kit right where they were..... on the shelf!


Edited by Showboatsix (06/30/11 08:59 AM)
_________________________

Note: "Speed and 172 are not used in the same sentence!"


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#41065 - 06/30/11 11:06 AM Re: Fuel balance [Re: Showboatsix]
Propduster Offline
Gold Pilot

Registered: 08/30/10
Posts: 1072
Loc: Nova Scotia, Canada
Originally Posted By: Showboatsix
[I know this about this kit, they were originally sold for $29.00, I thought to myself, I wonder if Cessna has any left, I did a Cessna parts search and YES!!!! they had several still in NOS inventory, so I asked how much to ship, I was quoted $900.00 and change for the same $29.00 kit.............. I told them that I was born at night...... but not LAST night! and they could keep their $29.00 kit right where they were..... on the shelf!


Sweet Whistling Jesus - now THAT's inflation! grin

I wonder if such a price hike might have something to do with liability rather than economics...AD seems to work, perhaps the "kit" was never tested to the level necessary to satisfy the leeches...er, lawyers...and that's a way to still offer it, so as not to raise any red flags, but also discourage its use.

As my old boss used to say, just because you're paranoid, it doesn't mean that they aren't out to get you! wink

Rick
_________________________
1963 Cessna 172D C-FPEC

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#41094 - 06/30/11 09:02 PM Re: Fuel balance [Re: Propduster]
agghopp Offline
Pilot in Command

Registered: 02/17/10
Posts: 348
Loc: Buckeye State
By the way, you can buy new seals for your fuel caps for a couple of bucks. Call your favorite parts shop like A/C Spruce or Lane Aviation. Take you <5 mins total to swap them out and get nice, firm seals again on your caps.

Secondly, I have a G model that says switch in cruise upon level 5000. Is there any reason I cannot switch tanks BELOW 5000?
_________________________
'66-G model, O-300 145 HP Conti, "Smooth Sixie"

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#41104 - 06/30/11 09:54 PM Re: Fuel balance [Re: Russ Farris]
DocOz Offline
Safety Pilot

Registered: 05/01/11
Posts: 70
Loc: Quincy, IL
I'm so glad you have made this comment. I'm a liitle on the nervous side of "switching tanks" over 5k since my plane and seals have a few years under her belt. I'm a both tanks kind of guy despite the AD. If the 170 doesn't have a problem then I don't have a problem.

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#41107 - 06/30/11 10:10 PM Re: Fuel balance [Re: DocOz]
California Flyer Offline
Platinum Pilot

Registered: 10/21/09
Posts: 4831
Loc: Groveland, California
Interesting discussion. In my new(er) 2000 SP model, I have noticed that when I start from full tanks, the left tank always draws first, for about the first several gallons, then the right tank kicks in. It's been that way every flight. I don't switch to just one tank or the other, I leave the selector at Both.
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http://www.cessna172club.com


Owner of N291ME, a 2000 Cessna 172SP

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#41109 - 06/30/11 10:59 PM Re: Fuel balance [Re: California Flyer]
flyer921 Offline
Bronze Pilot

Registered: 01/09/10
Posts: 556
Loc: Harrisonville, MO
Mine has the same issue sometimes. Will be interesting to see if my new McFarelane modified fuel vent line will change it. Wish mine had the dual fuel line at the tanks like the newer 172's have. Mine still has the single fuel line coming from each tank.

I have done fuel flow checks on mine while sitting on the ground and it flows even, only thing I noticed is there was air bubbles in the fuel flow, so I changed all the rubber hoses and still get bubbles, dosnt seem to hurt anything though.


Edited by flyer921 (06/30/11 10:59 PM)
_________________________
PPL, A&P/IA
Precision Aircraft Service LLC

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#41134 - 07/01/11 10:14 AM Re: Fuel balance [Re: flyer921]
wrecksum Offline
SVP Public Relations
Platinum Pilot

Registered: 09/26/10
Posts: 2704
Loc: Dominican Republic
Ours always uses the left tank first and on long flights we switch to right to equalise.
A couple of times in cruise we noticed a brief whiff of Avgas in the cockpit. All the pipes are perfect as we checked them when we had the interior out so I can't figure where it comes from.
Any body noticed this before? Any ideas?
_________________________
A person without a sense of humour is like a wagon without springs.It's jolted by every pebble in the road.

Cessna 172, C172, C-172, Cessna Skyhawk fan.
www.cessna172club.com

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#41147 - 07/01/11 12:39 PM Re: Fuel balance [Re: wrecksum]
bushpilot Offline
Second in Command

Registered: 01/30/11
Posts: 135
Loc: north eastern Ontario, Canada
This left tank feeding first or faster on the 172s seems to have always been there. My 172 G always went down a 1/4 of a tank ahead of the right tank and was never a problem. A few years ago the vent tube somehow during maintenance got moved down about quarter of and inch and then the left tank would show almost empty before the right tank started to come down on the gauge. This was handy for my home made fuel transfer system that I made to extend the range on long trips. When the left tank gets down to about 1/4 we pump fuel from an outboard motor fuel tank up into the left tank. I had the maintenance engineer adjutst the vent tube back to where it used to be (you could see the paint line) and now the left tank is back like it used to be, only a quarter of a tank or so ahead of the right. I have the vented cap on the right tank as per the AD. We were told that a large insect like a bee could hit and lodge into the vent tube and lead to fuel starvation and this was the reason for the vented RH cap, nothing to do with uneven fuel feed. Just what I know, suspect, believe. bp

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#41174 - 07/01/11 02:13 PM Re: Fuel balance [Re: bushpilot]
Bigdoggh Offline
Gold Pilot

Registered: 05/26/11
Posts: 1440
Loc: Georgia
I have been documenting my fuel levels before and after each flight and will have more data later, but had to at least share this...

Topped off in Anniston (KANB) Alabama the other day and flew home to KFTY Fulton County in Georgia. After the flight, I had:

17 gallons in right tank
13 gallons in left tank

When I did my pre flight this morning, I had the following (AND DID NOT FLY THE PLANE IN BETWEEN!!!):

14 Gallons in right tank
16 gallons in left

So, I burned 6 gallons in left tank and 2 in right on my way home from Anniston, and somehow, with the plane on the ramp, the fuel ended up with more in the left tank. That's a bit bizzarre to me, but then I'm a rookie.

I have a lot more data in my log and will post again soon.
_________________________
George - 1972 C-172L


FTY ANB CTJ LGC GVL AHN 20GA PUJ ALX ACJ UOS DTS AVL I69 YUP DVK AIK DVT SAF AXX ROA DYL TTN LYH GSP DNN VPC PIM RYY JKA ASD MJD RHP MKL M05 HKA TYS CHA TYQ OSH

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#41185 - 07/01/11 03:28 PM Re: Fuel balance [Re: Bigdoggh]
bushpilot Offline
Second in Command

Registered: 01/30/11
Posts: 135
Loc: north eastern Ontario, Canada
The plane may appear to be sitting level but with even a very slight tilt to one side the fuel will cross feed through the fuel valve until the fuel is level from one side to the other. If you put the valve to Off after shutting down, the fuel will not cross feed and you will have the same amount in each tank tomorrow as you had when you shut down today. bp

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#41221 - 07/01/11 09:00 PM Re: Fuel balance [Re: wrecksum]
flyer921 Offline
Bronze Pilot

Registered: 01/09/10
Posts: 556
Loc: Harrisonville, MO
Originally Posted By: wrecksum
Ours always uses the left tank first and on long flights we switch to right to equalise.
A couple of times in cruise we noticed a brief whiff of Avgas in the cockpit. All the pipes are perfect as we checked them when we had the interior out so I can't figure where it comes from.
Any body noticed this before? Any ideas?



I had the same issue when my tanks were full. I had fuel quantity sending units leaking fuel, so I replaced the gaskets and still had the same issue. I finally noticed that there was fuel leaking around the wire terminal on the sending unit itself, so I called up McFarelane and ordered 2 new sending units and problem fixed.
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PPL, A&P/IA
Precision Aircraft Service LLC

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