Issued: April 2017
ATA 28 – Pressure Build-Up in the Fuel Tanks
All
Flight crews learn very quickly in their flying careers not to open the fuel caps on the King Air when the fuel tanks are full; doing so risks you spending the rest of the day with fuel soaked clothes and the accompanying smell. Here are the reasons why the fuel rushes out of the fuel tanks and why the main fuel tank is pressurized under normal operations.
Fuel is generally stored in underground fuel tanks; this fuel tends to be cold. This fuel is then pumped into the airplane’s wing tanks and as the airplane sets out under the sun, the fuel will start to warm up causing thermal expansion. This fuel being trapped in the fuel tanks causes the internal pressure in the fuel tanks to rise. Opening the fuel caps under these conditions will relieve the pressure in the tanks an associated fuel stream comes with it. Even fuel stored above ground tanks can warm up while in the airplane’s fuel tanks causing the same effect. The only safe way to open the fuel caps is to do so when the main fuel tanks are less than full. This condition is more common on the main and auxiliary fuel tanks (for those models with aux tanks) and less common in the nacelle tanks (for those models with fuel caps on the nacelle tanks).
The condition described occurs on all King Air models; however, the King Air 300 and B300 are unique in that the pressure in the main fuel tanks can also increase any time the Transfer System is operating, which happens automatically anytime there is fuel in the aux tanks. The excess fuel being transferred to the nacelle tank from the aux tank is returned to the main fuel tank plumbing upstream from the flapper valve in the nacelle tank (see sketch). This excess fuel is what causes the pressure in the main and nacelle tanks to increase. This condition will exist until the fuel in the aux tank has been depleted and the transfer system is automatically shut off. Opening the fuel caps after a short flight (not long enough to deplete the fuel in the aux) will produce a stream of fuel and a sound of pressure being relief from the main tanks.
The fuel system is protected from over pressure by a pressure relief check valve located on top of the nacelle tank (see sketch on page 30). This Pressure Relief valve will open at approximately 1.66 psi. Essentially the fuel system can be pressurized up to 1.66 psi. Operators have blamed this pressure build-up to fuel leaks and spent a lot time finding the cause of the “over pressure that is causing the fuel leak.” Operators later find that the system is operating normally and that the leak they find is due to a leaky gasket, a loose clamp or faulty fuel bladder which cannot hold the normal pressure build-up.
NOTE: It is imperative that the orientation of the Pressure Relief Valve and the Suction Relief Valves are correct. A valve installed backwards can cause severe nacelle structural damage.
ATA 30 – Propeller De-Ice Wire Harness – Hartzell Service Bulletin
Serial Numbers LJ-1542 and after
Operators have reported that the propeller de-ice wire harness has been damaged during normal operation the airplanes listed above. Hartzell Propellers has issued Service Bulletin HC-SB-30-366 to address this issue. The Service Bulletin replaces the propeller de-ice wire harness and provides specific instructions on how to install and secure the wires to prevent the centrifugal forces of the propeller from damaging the wire harness. The Service Bulletin is available from Hartzell Propeller’s web site at http://hartzellprop.com/ or by contacting their Technical Support at techsupport@hartzellprop.com.
The above information is abbreviated for space purposes. For the entire communication, go to www.txtavsupport.com.
Leave a Reply