Engine Instruments, II
The engine instruments give the pilot information about the health of the engine and how it is performing during the flight. This information is also required to keep the engine running within the specified operating limits so that it will remain reliable throughout its lifetime.
Engine data monitoring and logging tools create a history of data in which trends can be detected, this is helpful with preventive maintenance and keeping the engine running for years to come.
Some common instruments are: RPM, pressure and temperature gauges; fuel level and flow gauges are sometimes integrated in one unit. In multi-engined aircraft there is usually a set of gauges available per engine stacked next to each other.
After being watchful on RPM and temperatures the pilot must keep an eye out on fluid pressure and flow indicators, be that oil or fuel. As these two keep the engine smoothly running.
These are either made of a diaphragm or bourdon type sensors. This depends on the pressure they are working with, bourdon types are more suited to handle higher pressures. Reading is direct (with the pressure line running to the cockpit) or indirect with electrical wires and the sensor close to the engine, either on the engine or firewall.
The direct reading type is made of a bourdon tube and this is filled with a thin engine oil. The indirect type uses a resistance ratio meter and a firewall mounted sensor (sometimes on the engine, Rotax). This sensor is a diaphragm connected to a rheostat.
The ratio meter can be a millivolt meter or a electromagnet type with two coils which attract the pointer and the rheostat is connected with a wire in the middle of the coils so that the magnetic field is regulated with the varying resistance of the rheostat.
Manifold Pressure (MAP)
Engines with a controllable propeller of some sort need the MAP gauge to set the correct power setting with the selected propeller RPM. It reduces fuel consumption and by having a slight oversquare condition, for example 2200 RPM at 25" MAP, engine efficiency is also greatly improved.
Usually an aneroid capsule or diaphragm connected to a pointer with the scale calibrated in pressure, inches of mercury (inHG) or bar. The instrument can be made to show either only boost (pressure above atmospheric) or manifold pressure, from 10 to 30" (inHG) and for turbocharged engines the scale even goes higher, up 50" and more.
This instrument is similar to a remote oil pressure describer above. The sensor is normally a diaphragm type and connected to an indicator in the cockpit. Usually found on aircraft with fuel injected engines and low wing aircraft, although some high wing aircraft do have one too.
The sensor should be fitted in the pressurized fuel line after any fuel filter and pump to protect it from possible dirt in the fuel tanks.
Quantity & Flow
There are two types in use: the float and the capacitance quantity sensor. The float type measures fuel level by means of a float in each tank connected to a variable resistance. It is not very accurate due to turbulence, airplane attitude and such.
The capacitance type is much more accurate especially during flight in turbulent conditions. It consists of a probe made of two concentric metal rods isolated from each other. These rods have a certain capacitance and when the air between them is replaced by fuel that capacitance changes indicating fuel level on a calibrated instrument.
The capacitance can be measured electronically with high impedance sensitive circuits so that at no time a high voltage is present inside the fuel tank.
As with the float type the instrument must be calibrated as each fuel tank has a unique form causing the fuel level to drop non-linearly in the tank.
This is usually a vane type sensor located downstream behind the fuel filter and pump measuring the amount of fuel going to the engine. Fuel injected engines might have a fuel return to the tank in use so for a correct reading the fuel returning must also be measured with a flow sensor.
This instrument is even more important than fuel level as any change in fuel consumption is immediately visibile. It is usually manufactured in combination with a manifold air pressure (MAP) indication.