Few things are more annoying than finding a puddle of fuel on the hangar floor the morning after a flight. A B200 I was consulting on recently manifested just that. Fuel leaks that crop up suddenly can result from recent maintenance where a seal wasn’t seated properly or where a fitting was left loose. Another cause is running an aircraft after it has been sitting a long time. Both circumstances could apply to this particular B200.
Of course, time was of the essence. A buyer was waiting in the wings, eager to take possession after a prolonged wait. The shop, which had done a great deal of maintenance over many months, was equally eager to send this King Air off to its new home. I was there on the buyer’s behalf to ensure the aircraft was running properly and that nothing had been missed.
When I first arrived on the scene, I found several squawks. Most of them were typical. As soon as those issues were addressed, it was time to pull it out of the hangar and complete a ground run. Not surprisingly, a couple of other squawks cropped up, so we pushed it back in the hangar to tackle those last few issues. The acceptance flight was scheduled for the next day.
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 anywhere. This poses a serious risk of fuel getting into the front bearing of the fuel control and washing out the lubrication. If that front bearing 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.
The drain line in question runs between the front of the fuel control and the back of the HP fuel pump; it connects to a drain fitting on the pump (refer to photo on page 10). If blue stains are noted in this drain line, you know the front bearing in your FCU has been compromised. The migration of the blue grease is a dead giveaway.
There is the remote possibility that the FCU bearing was washed out by a fuel leak, yet the Q-tip in the drain fitting came out clean, and no blue stains are visible because the drain line has darkened. In this case, I would poke a clean swab into the drain line and scrape around.
That blue grease is thick stuff and will adhere to the inside of the line. The chances of all the blue grease being dissolved and washed away without a trace are slim. It would take a leaky HP fuel pump spewing fuel onto the FCU for some time.
This drain line is easily seen in the King Air model 200s, 300s and 350s. Open the right-side cowl door and look in at the HP fuel pump filter housing; then look aft, and you’ll see a drain line. That’s it. On a model 90, it’s harder to see because the cowl doesn’t open low enough to get a straight shot at it.
The bottom line is this: If you have fuel leaking from the gang drain, you must immediately investigate where it is coming from. Pay attention to the HP fuel pump. A mere Q-tip could save you from a runaway engine and subsequent shutdown.