Fuel pumps and fuel vaporization

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  • #83594

    I’m having a problem, when I get up to cruise and I shut the fuel pumps off the MP drops and the airplane yaws. My instructor said this is due to fuel vaporization and that I should wait a few mins to allow the fuel lines to cool off before shutting the pumps off. The problem I have is that sometimes when I wait a while the engine still stumbles when I shut the pump off. If I let the engine stumble it clears afteer a few seconds but this is very disconcerting for passengers…..

    Any thoughts? Thanks

    #95112
    rwelsh
    Participant

      Your instructor is sucking wind! These engines do not make vapor except maybe on the ground on a hot day after shutdown. You have other problems like maybe an air leak around an o’ring in the fuel control unit or your fuel pressure is too low. The reason they don’t make vapor is because over 20 percent of the fuel is returned to the main tank continually. You didn’t say what model plane you have but almost all twin Cessnas have TCM fuel injection other then the first three years of the 310.

      I never use the fuel boost pumps for anything except priming the engines for start and mine never stumble.

      Do you by chance still have the fuel pressure limiter installed? They were supposed to be removed over 20 years ago VIA a Service Letter. Also the fuel system should have been modified to eliminate the “auto” high boost pump when an engine driven fuel pump failed.

      Fill us in on what you have and what Service Letters/Bulletins have been accomplished on the fuel system.

      #95113

      Its a 1980 421C, and it has MEB 88-3 incorperated which has the three position swithch (high/off/low) without the auto high feature.

      I’ll have to see what other bulletins have been done (I don’t have the logs on me now).

      How can i tell if the fuel pressure is too low? There is adequare fuel flow during take off (45 gph on each engine) with the boost pumps on low or off.

      Thanks!

      #95114
      rwelsh
      Participant

        Tim, if you are getting 45 GPH, then your high pressure must be set right. The cruise fuel pressure is a mix of the low and the high setting and cruise seems to be where your problem is. The only way to tell is to perform the Service Letter on the fuel settings, I believe it is 96-3 but I am not sure. There aren’t many places the fuel system can get air and not show some external leak. There are 3 o’rings in the fuel control unit; two of which can get outside air. That is the extent of my guessing your problem.

        #95139

        My mech spoke with ram and they said that they have gotten a lot of reports of this from the field. They (ram) are now recommending leaving the pumps on low for all flight ops….

        My mech is going to run sb 97-3 on the left engine just to confirm everything is set correct, and they are going to do a leak test on the induction system to see if there are any air leaks….so we will see.

        #95140

        I agree that it’s not a vapor issue, but I don’t have the experience to know what it could be.

        A related question: On a MEB883 421C, what’s the thought on leaving the pumps on all the time versus turning them off in cruise?

        I’ve always just left them on, but don’t really know why!

        Robert

        #95141
        rwelsh
        Participant

          Robert, maybe you leave the pumps on all the time so you can wear out those $1900 pumps. If the fuel system is set up correctly, there is no need to leave the pumps on. The engine will still not run if the engine driven fuel pump fails. The aux pump needs to be on high to keep the engine running with an engine driven pump failure and even then the mixture control has to be reduced to keep from flooding the engine.

          #95447

          It is possible to get vapor lock in cruise flight with the electric pumps off. Fairly common on fast climbing singles and twins. Most likely scenario is taking off from a hot field where the plane and fuel is heat soaked. As you climb to altitude, the fuel slowly cools off, but the vapor pressure is a function of outside pressure and fuel temp. At the engine pump inlet, the pressure is even lower and this is the most likely point to get vapor lock.

          It is not the same as vapor lock on the ground, which is usually vapor in the fuel spider/lines. This is vapor lock at the engine pump inlet which could result in mild fluctuations in fuel flow or even a stumble. The reason the electric pump helps is it pressurizes the fuel to the pump to eliminate vapor lock. It is also more resistant to vapor lock itself since the electric pump is mounted lower than the engine pump.

          It doesn’t help on the Cessna twins that the fuel line passes close to the turbo wye.

          For the turbocharged cirrus fleet, those that like to turn off the pump (which is a small fraction) follow the instructions and leave the pump on low for 5-10 minutes after reaching cruise altitude. This let’s the fuel cool off to local temp. In my 421C, I turn the pumps on after start and off right before shutdown.

          The electric pumps are expensive, RAM lists ~ 1900 overhauled. Looks like RAPCO might have a FAA-PMA alternate,
          http://www.2aps.com/RAPCOFuel%20Pumps.htm for ~ $864. (2B7-29 for the 421C)
          The rapco cross reference list lists many Cessna twin part numbers.

          http://www.rapcoinc.com/rfs/pdf/fuelapp.pdf

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