Saturday, June 11, 2005

Fuelish Wisdom

Talked to Bill at Ichneumon. He knew who I was, acknowledged that he was supposed to schedule me into a course, but seemed in no hurry to do so. I talked to a couple of Ichneumon pilots and they indicated that courses get scheduled at the last moment. So my next few days or weeks will consist of studying the airplane and bugging Bill.

And you get to study along with me.

Today's topic is the glorious fuel system of the airplane I hope to be flying soon. (No prizes, Canadian aviators, for guessing which company Ichneumon is, so please don't name it in the comments. You can just smile smugly at how obvious it is.) The engines can burn pretty much any kind of turbine fuel, it can even burn avgas formulated for piston engines, althought the lead isn't good for the turbine blades, so it's limited to 150 hours per engine overhaul cycle. You could probably run it on barbeque lighter fluid in a pinch but I wouldn't want to be the one to have to open all those little bottles to pour the contents into the tanks. And explaining it to maintenance afterwards might be tricky, too.

The manufacturer even designed it so I wouldn't have to climb up on the wing to pour the lighter fluid in, as the airplane has its two fuel tanks in the belly, and the filler necks along the left side. The forward tank feeds the right engine and has a capacity of 151 imperial gallons, while the 164 gallon aft tank feeds the left engine. Fuel cannot be pumped from one tank to the other, but one can choose to crossfeed, supplying both engines from one tank.

To prevent all the fuel from sloshing to the front or back of the airplane, each fuel tank is divided into four interconnected cells, numbered one through eight from front to back. Each cell contains a fuel capacity probe and has two vents, but cells #4 and #5 are the collector cells. and also contain electric boost pumps to send fuel up to the engine. The way they collect fuel from the other cells is quite clever. The boost pumps can deliver 450 pounds of fuel per hour at 22 psi, more than the engine can possibly burn. The excess flow is pumped around a loop going back to the collector cell again, but on the way it goes through an ejector, sort of a one-way T-joint connected to the manifold between the four fuel cells. The force of the fuel being pumped past actually draws fuel out of the other three cells, so that if everything is working properly, the collector cell should remain full until the other three cells are empty. A level control valve in the collector tank inlet standpipe prevents overfilling of the collector tank.

If the ejector becomes blocked, the boost pump (and its back up) fail, or the level control valve jams shut, the fuel will not transfer and the fuel level in the four cells will become equal. With an inoperative ejector system, extreme nose up or down attitudes produce greater changes in centre of gravity and risk uncovering the boost pumps, but normal aircraft operations can continue. The FUEL LOW LEVEL light will illuminate early, but that's part of the fuel indicating system, and a topic for another blog entry.


SimonG said...

If there are four cells, how are they numbered one to eight?

Aviatrix said...

The four cells in the forward tank are numbered one to four, then the cells in the aft tank are numbered five through eight.