Engine Instrumentation — Page 540, Lesson 639

I want to walk you through engine instrumentation now, and we're going to start with the fuel flowmeter. We've already talked about measuring how much fuel is in the tanks, but that's only part of the picture. As well as the quantity of fuel measured, we can also show the rate of fuel consumed and the instantaneous rate of fuel flow. So the fuel flowmeter can display either volume flow or mass flow — two different ways of expressing the same thing, and I'll come back to that distinction in a moment. The key physical principle here is that flow is proportional to the square root of the pressure drop across an orifice. Let me unpack that. An orifice is simply a restriction, a small opening in the fuel line. When fuel passes through it, there's a pressure drop — the pressure on one side is higher than on the other. And the rate of flow through that restriction is proportional to the square root of that pressure difference. That's the fundamental relationship that makes flow measurement possible. Now, a simple flowmeter can be an adaptation of a pressure gauge. This is used on many light piston engine injection systems. So on those smaller engines, you're essentially reading a pressure difference and converting it into a flow reading. But most modern engines use an electrical sensor, and this is where it gets interesting. This sensor utilizes the change in torque or speed of a turbine — and the turbine here is also called an impeller. Let me describe the typical construction, because it's a neat piece of engineering. You have a light alloy casting with guide vanes and an electrical 'pick-off' coil. Inside that casting sits a helical vane impeller — so the impeller has vanes arranged in a helix, like a screw thread — and embedded in that impeller is a magnet. When fuel flows through, it pushes against the vanes and the impeller rotates. As it rotates, the embedded magnet sweeps past the pick-off coil, and that induces a sinusoidal signal in the coil. The frequency of that signal is proportional to the speed of the rotor, and the rotor speed is proportional to the rate of volume flow. So the faster the fuel flows, the faster the impeller spins, and the higher the frequency of that induced signal. That's how we get a volume flow reading. Now, to measure mass flow, the signal is corrected for temperature. That's the key difference. Volume flow tells you how much fuel by volume is passing, but fuel density changes with temperature. So to get mass flow — how much fuel by weight — you take that volume-based signal and correct it for temperature. That correction gives you the mass flow. That figure shows you a typical fuel flowmeter remote signal transmission system — you can see the impeller and the pick-off arrangement I just described. Now, total consumption is obtained by integrating the rate of fuel consumption over time, and that time is one hour. So you're taking the flow rate and summing it up over an hour to get total fuel used. The units matter here. For volume flow, we use gallons per hour or litres per hour. For mass flow, we use pounds per hour or kilograms per 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 accumulating and showing you the total consumed. One more important detail: the flowmeter is located in the high pressure fuel line to the fuel spray nozzles, which are also called burners. So it sits right before the fuel goes into the combustion area, measuring what's actually being delivered to the engine. Now let's shift to a related topic — remote signal transmission systems. To control an aircraft system, you may need to move a valve, a flap, or a lever on the engine. And the pilot may need to know the position of that control. Early systems had mechanical feedback to a position indicator in the flight deck. So there was a physical, mechanical link from the engine control back to a gauge in the cockpit. But most of the aircraft flying today employ remote indicating systems, and these can be either DC or AC operated. Whichever system is used, each data transmission system employs a transmitter located at the source to be measured, and a receiver, which acts on the information received. So the transmitter is at the engine, sensing the position or the movement, and the receiver is in the flight deck, acting on that information to show the pilot what's happening. Finally, there's the flight hour meter. Some aircraft have a time monitor to record usage of the engines and systems in flight. This can be automatically switched via the 'weight-on-wheels' switch — that's a switch that senses whether the aircraft is on the ground or airborne — or, more commonly, by an airspeed switch. So the meter only runs when the aircraft is actually in flight, triggered by one of those two switches. That's the core of engine instrumentation for this section — the fuel flowmeter with its volume and mass flow measurement, the remote transmission system, and the flight hour meter.