Engine Instrumentation — Page 540, Lesson 639

All right, let's pick up with engine instrumentation. We've already covered how we measure the quantity of fuel in the tanks. Now I want to show you how we measure the rate at which that fuel is being used, and then we'll look at how we transmit control positions around the aircraft. So, beyond just how much fuel is in the tank, we can show the rate of fuel consumed and the instantaneous rate of fuel flow. That's the job of the Fuel Flowmeter. Now, a key point here: this instrument can display either volume flow or mass flow. Volume flow is how much physical space the fuel takes up per hour, while mass flow is how much actual fuel weight passes per hour. These are different, and we'll come back to why that distinction matters. Here's the fundamental physics behind it: flow is proportional to the square root of pressure drop across an orifice. Let me unpack that. An orifice is simply a precisely-sized hole or restriction in the fuel line. As fuel passes through it, there's a pressure drop—the pressure on the upstream side is higher than on the downstream side. The greater the flow, the greater that pressure drop, but it's not a straight-line relationship. If you double the flow, the pressure drop doesn't double—it quadruples. So to get flow, you take the square root of that pressure drop. Now, a very simple flowmeter is just an adaptation of a pressure gauge that uses this principle. That's used on many light piston engine injection systems. But most modern engines don't use that. They use an electrical sensor that utilizes the change in torque or speed of a turbine, which is also called an impeller. Let me walk you through the construction of this modern sensor, because it's quite elegant. The typical construction consists of a light alloy casting with guide vanes and an electrical 'pick-off' coil. Inside that casting, there's a helical vane impeller—imagine a propeller twisted like a screw thread—and embedded in that impeller is a magnet. Here's how it works. The fuel flows through the guide vanes, which straighten the flow, and that flow strikes the helical vanes, causing the impeller to rotate. As the impeller spins, the embedded magnet rotates past the pick-off coil. That rotating magnetic field induces a sinusoidal signal in the coil. The frequency of that signal is proportional to the speed of the rotor, and the speed of the rotor is proportional to the rate of volume flow. So, by measuring the frequency of that AC signal, we know the volume flow rate. Now, remember I said we can display mass flow as well? To get mass flow, the signal is corrected for temperature. Why temperature? Because fuel density changes with temperature—hot fuel expands and becomes less dense, so a given volume weighs less. By measuring temperature and correcting the volume flow signal, we get an accurate mass flow reading. There's a figure here that shows this whole setup—the casting, the impeller, the pick-off coil—it's , Figure 38.23, a typical fuel flowmeter remote signal transmission system. Now, total consumption. The total consumption is obtained by integrating the rate of fuel consumption over time, and that time is one hour. So we're summing up the flow rate continuously to get a total. The units we use depend on whether we're measuring volume or mass. For volume flow, the units are gallons/hour or litres/hour. For mass flow, the units are pounds/hour or kilograms/hour. A flowmeter that displays fuel consumed as well as fuel flow is broadly defined as an Integrated flowmeter. So it's not just showing you the instantaneous rate; it's also totalling it up for you. One more critical point about placement: the flowmeter is located in the high pressure fuel line to the fuel spray nozzles, which are the burners. It's not in the low-pressure side; it's right up near the engine where the fuel is being injected. Now, let's shift gears to a different topic: remote signal transmission systems. To control an aircraft system, you often need to move a valve, a flap, or a lever on the engine. And the pilot needs to know the position of that control. Early systems had mechanical feedback to a position indicator in the flight deck—literally a physical linkage that moved a needle. But most aircraft flying today employ remote indicating systems that can be either DC or AC operated. Whichever system is used, each data transmission system employs two key components: a Transmitter, located at the source to be measured, and a Receiver, which acts on the information received. So the transmitter senses the position of the valve or lever, sends a signal—either DC or AC—and the receiver interprets that signal and displays it to the pilot. Finally, let's touch on the Flight Hour Meter. Some aircraft have a time monitor to record the usage of the engines and systems in flight. This can be automatically switched via the "weight-on-wheels" switch—which tells you if the aircraft is on the ground or airborne—or, more commonly, by an airspeed switch. So when the aircraft reaches a certain airspeed, the meter starts recording engine usage time. So to recap what we've covered: the fuel flowmeter measures rate of flow using the square-root relationship with pressure drop, modern ones use a turbine impeller with a magnet and pick-off coil to generate a frequency proportional to flow, mass flow is corrected for temperature, total consumption is integrated over an hour, and we transmit control positions using a transmitter-receiver pair that can be DC or AC operated.