Don’t use “Both” at altitude?

Discussion in 'Flight Following' started by Will Kumley, Apr 22, 2022.

  1. Will Kumley

    Will Kumley Line Up and Wait

    Joined:
    Aug 6, 2019
    Messages:
    545
    Location:
    Pacific Southwest

    Display name:
    Will
    I was looking at a Cessna 172 the other day and talking with the owner. He said he doesn’t use “both” on the fuel selector at altitude as it causes the engine to sputter and cough. I’ve never heard of this in my short time flying so it sounds odd to me but I’m curious if there is any validity to it? I’ve rented 172’s from the local flight school and always keep the selector on both unless I notice more fuel being pulled from one tank.
     
  2. Tantalum

    Tantalum Final Approach

    Joined:
    Feb 22, 2017
    Messages:
    8,555

    Display name:
    San_Diego_Pilot
    (A) - I've never heard of this
    (B) - I've never seen this in any Cessna 172 POH
    (C) - this is not part of any Cessna 172 checklist I've seen

    During fueling we'd throw it on left or right to make fueling easier, and during flight you sometimes get uneven burn causing a need to switch tanks

    BUT, if it's "sputtering and coughing" on both that would be a big cause for concern for me of some other issue in the fuel system
     
    Tarheelpilot likes this.
  3. WDD

    WDD En-Route

    Joined:
    Oct 16, 2019
    Messages:
    3,616
    Location:
    Atlanta / KRYY

    Display name:
    Vintage Snazzy (so my adult children say)
    Second what Tantalum says. If a 172 sputters on both tanks there is something wrong. Something to have your guy look at closely during the pre buy.
     
  4. dmspilot

    dmspilot Final Approach

    Joined:
    Oct 20, 2006
    Messages:
    5,058

    Display name:
    Display name:
    https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=36949

    Screen Shot 2022-04-22 at 1.54.05 PM.png
     
  5. mondtster

    mondtster En-Route

    Joined:
    Nov 25, 2011
    Messages:
    4,053

    Display name:
    mondtster
    Apparently you guys have never flown an older cessna with the selector valve placarded to fly on one tank or the other above 5,000 feet?

    It’s the subject of AD 72-07-02.
     
  6. geezer

    geezer Line Up and Wait

    Joined:
    Aug 8, 2019
    Messages:
    699

    Display name:
    Kruse'n
    I have flown 4 different 172's hundreds of hours each, on cross country flights. My preferred altitude range is 6,500 to 12,500 feet. I always select both, and have never had any issues in that operation. Ice has caused engine stumble many times, but not fuel selector position. I think that I recall doing the vent mod on 2 of the club planes, even though none of us had such an event while flying.

    The newest one was an M model, the oldest was simply a 172, no letter designator of model. It should be noted that Cessna tried under the most conducive conditions to replicate the alleged vapor problem, without success. Further, some of the owners reporting the problem had been on one tank, and cured the problem by changing tanks. Whether on one or both, the Cessna recommendation is simply to change tanks.

    The engine and carburetor have no way of being affected by the position of the fuel selector, so there is something serious wrong with that 172. It may be carburetor, mags, plug wiring, or something else Sooner or later, it will recur at an inconvenient time, and moving the fuel selector is not going to change anything, and you will need a place to land without power.

    That plane needs a skilled mechanic, unless the owner is simply reporting the Cessna Service Letter exists.
     
    Last edited: Apr 22, 2022
    Tantalum likes this.
  7. Tantalum

    Tantalum Final Approach

    Joined:
    Feb 22, 2017
    Messages:
    8,555

    Display name:
    San_Diego_Pilot
    Okay, so now I want to know the science behind this. Why should the engine care whether it's coming from one of two tanks or both at the same time? After the fuel selector is it not just one line? The mind boggles
     
  8. GeorgeC

    GeorgeC Administrator Management Council Member PoA Supporter

    Joined:
    Dec 5, 2010
    Messages:
    4,171

    Display name:
    GeorgeC
  9. geezer

    geezer Line Up and Wait

    Joined:
    Aug 8, 2019
    Messages:
    699

    Display name:
    Kruse'n
    That sounds like turbulence sloshing the fuel adds air to the fuel in the system. The side not in use would have time for the air to bubble up and out. Changing tanks would then transfer the bubble problem to the other side, and if trouble re occurs, another switch would be done.

    Fortunately, when I have flown in that much turbulence, I have landed with enough fuel in the tanks that the sloshing bubbles were not a problem. Another factor is that I have found that bad turbulence is rare at altitude, with VFR conditions.
     
  10. WDD

    WDD En-Route

    Joined:
    Oct 16, 2019
    Messages:
    3,616
    Location:
    Atlanta / KRYY

    Display name:
    Vintage Snazzy (so my adult children say)
    Wow - Never heard of this; love this place. Learn something new every day. How old is "Older" I wonder?
     
    Tantalum likes this.
  11. mondtster

    mondtster En-Route

    Joined:
    Nov 25, 2011
    Messages:
    4,053

    Display name:
    mondtster
    I'm guessing it has more to do with the tank venting system and how the fuel feeds from each tank under some environmental conditions. There have been several fuel system ADs over the years.
     
  12. Stan Cooper

    Stan Cooper Pattern Altitude PoA Supporter

    Joined:
    Aug 29, 2017
    Messages:
    1,903
    Location:
    Santa Rosa, CA

    Display name:
    Stan Cooper
    Cessna offered a service kit to modify the fuel system, but few took advantage of it. By the time I bought my 172D in 1999, the price of the service kit was in the stratosphere, so I just switched tanks every half hour when cruising above 5,000 feet.

    The problem was related to vapor lock and if you were on both tanks and had the problem, switching to single tank didn't help. OTOH, if you were on either left or right tank and had the problem, the fix was to simply switch to the other tank.

    Fuel Selector Placard.JPG

    Attached is the pdf for SK172-31B, "Improved fuel line vent-standard fuel tanks"

    Also attached is SE72-7 "Single Tank Operation" which describes the problem. The problem was limited to 172 models from 1956 to early 1970.
     

    Attached Files:

    Last edited: Apr 22, 2022
    FancyG and mondtster like this.
  13. Dan Thomas

    Dan Thomas Final Approach

    Joined:
    Jun 16, 2008
    Messages:
    9,022

    Display name:
    Dan Thomas
    Nope. Go to the link given in post #8 by GerorgeC. The venting issues are entirely separate, and different, from this issue. The uneven feeding/coughing was due to a design deficiency, with a phenomenon unforeseen by the designers. It took them awhile to figure it out, using clear fuel lines of the same ID as the aluminum lines, and with the interior out of the airplane, so they could observe the fuel in the lines to see what was going on. The kit Cessna offered was designed to bleed those bubbles back to the top of the tank at the vent connection before they got into the vertical section of the lines.

    There isn't anything "seriously wrong with the airplane." It's doing what it does, in some airplanes, and is the reason for the single-tank operation placard.

    It probably should have been an AD. The placard was a bandaid, and bandaids come off. I have done placard checks on airplanes and found multiple mandatory placards missing. Mandatory by the FAA's TCDS, not the manufacturer's "mandatory" SB or SL stuff.
     
    tonycondon likes this.
  14. Will Kumley

    Will Kumley Line Up and Wait

    Joined:
    Aug 6, 2019
    Messages:
    545
    Location:
    Pacific Southwest

    Display name:
    Will
    Wow, this is a very informative thread. Thanks. If I purchase this plane looks like I'll treat it like a Cherokee and switch tanks every 30 minutes once I'm above 5000 ft.
     
  15. Ghery

    Ghery Touchdown! Greaser!

    Joined:
    Feb 25, 2005
    Messages:
    10,805
    Location:
    Olympia, Washington

    Display name:
    Ghery Pettit
    Interesting. In all the 172s I've flown over the last 20+ years I've never seen this placard.
     
  16. Will Kumley

    Will Kumley Line Up and Wait

    Joined:
    Aug 6, 2019
    Messages:
    545
    Location:
    Pacific Southwest

    Display name:
    Will
    I haven't flown for 20 years, but I've rented more than a handful of 172's and I too have never seen this placard or had it told to me when doing a checkout flight with the rental/school that has the plane. But this explains the sellers comment about making sure he is on one tank or the other at altitude. Overall, I think the plane looks decent for its age. There is some bubbling paint that makes me want to ensure the pre-buy inspector takes a close look and the 30 year old paint looks pretty bad since its been sitting outside, but I can see its flying somewhat regularly and if all goes well on the prebuy we may own our first aircraft.
     
  17. Dan Thomas

    Dan Thomas Final Approach

    Joined:
    Jun 16, 2008
    Messages:
    9,022

    Display name:
    Dan Thomas
    Either the placards were missing, or you didn't fly the affected models, or the affected models had been upgraded with the Service Kit.

    upload_2022-4-22_19-6-15.png
     
  18. Ghery

    Ghery Touchdown! Greaser!

    Joined:
    Feb 25, 2005
    Messages:
    10,805
    Location:
    Olympia, Washington

    Display name:
    Ghery Pettit
    I would have to look to see, but my bet would be that the upgrade had been done. In any case, none of the planes exhibited a problem during the flights. If they had, there would be a note in my log.
     
  19. RickC

    RickC Pre-Flight

    Joined:
    Aug 1, 2016
    Messages:
    50

    Display name:
    RIckC
    I've had vapor lock occur a few times in my 1964 E model, but only on the ground. It happens pretty easily if you are running winter blend mogas on a hot spring or fall day with low atmospheric pressure. And especially after a long taxi or ground run gives the fuel plenty of time to heat up. Once they start dispensing summer blend gas the problem goes away. Because if this I always keep avgas in one wing during those times of year when susceptible to this set of conditions (like now). I always run on one tank above 5000 MSL as placarded so never had any problem up high with vapor lock.
     
  20. Salty

    Salty Touchdown! Greaser!

    Joined:
    Dec 21, 2016
    Messages:
    10,863
    Location:
    FL

    Display name:
    Salty
    What if you're taking off or landing above 5,000 feet?
     
  21. Dan Thomas

    Dan Thomas Final Approach

    Joined:
    Jun 16, 2008
    Messages:
    9,022

    Display name:
    Dan Thomas
    At full throttle the fuel flow should be high enough on (Both) to prevent the bubble-in-the-line problem. That problem is not vapor lock, which involves fuel boiling in the line and not only holding the fuel back but providing only vapor to the engine.
     
    Last edited: Apr 27, 2022
  22. nrpetersen

    nrpetersen Line Up and Wait

    Joined:
    Jan 4, 2013
    Messages:
    623
    Location:
    Minnetonka MN

    Display name:
    nrpetersen
    In 1977, two of us, both MechEngrs, very familiar with an O-300D powered 172H over the almost dark woods of N Wisconsin, about 2/3 full of pure AvGas, climbing for a several thousand feet on BOTH, in unusually smooth air. The engine stumbled briefly, and quit firing for at least 10 long seconds. Definitely a starve out.

    Both of us frantically "stirred the pot", but what seemed to get it back quite suddenly, was pulling the in-the-cabin fuel gascolator drain. It sure behaved like an under-the-floor over temperature and maybe related to heat and zero fuel flow in one of those rear door post lines.