Page 13 - April 2022

Come morning, however, we found fuel under the left engine. It was coming from the engine gang drain. This was not what I was hoping for, but I knew this aircraft had been sitting for some time. When we fired it up for the ground run and exercised the systems after months of dormancy, we probably exposed a dry seal somewhere.
We bagged every drain tube from the accessories that dump into the gang drain. This is routine fuel leak troubleshooting. Anything involving fuel, including the low-pressure fuel pump, the fuel control and the high- pressure fuel pump, got bagged. We didn’t have to wait long to see where the fuel was coming from. To my great dismay, the dripping drain tube came from the high-pressure (HP) fuel pump.
HP Fuel Pump and the FCU
The fuel control unit (FCU) is bolted on the end of the HP fuel pump. If that pump springs a leak,
it can spray fuel just about any- where. This poses a serious risk of fuel getting into the front bearing of the fuel control and washing out the lubrication. If that front bear- ing runs dry, the fuel control will fail, and the engine will run away. This is a “no-go” item in my book. Suddenly tomorrow’s acceptance flight was postponed as the shop scrambled to order an exchange HP fuel pump.
Blue Grease and Cotton Swabs
The front bearing of the FCU is packed with a special blue grease; it is quite viscous. If leaking fuel gets in there, the blue grease breaks down and is flushed downstream. A good test to see if the front bearing of the FCU was affected by leaking fuel is to disconnect the drain line from the fitting and poke a Q-tip® into the fitting. If there’s any blue grease in there, you know you have a problem with the FCU and the
HP fuel pump. This simple trick is a convenient way to prevent a runaway engine and subsequent shutdown.
In this case, I directed the shop to perform the Q-tip trick and the swab was clean. This meant the blue grease in the FCU front bearing was intact and the FCU didn’t need to come out – only the HP fuel pump had to be replaced. I think that pump started leaking right there on the ground run; the fuel went down the drain line, into the gang drain, and onto the floor. We didn’t run the airplane for very long, and we caught that fuel leak right away, so I say we dodged a bullet on that one.
Yellow Line, Blue Stain
Most drain lines going into the gang drain started out yellow and semi-transparent when new. They darken over time and can get spattered with oil to the point they appear almost black.
APRIL 2022
KING AIR MAGAZINE • 11