Aircraft Sensors Questions and Answers

Answer: C

Correct answer: The position of a movable core

An LVDT has a primary coil and two secondaries wound in opposition, with a ferromagnetic core free to slide within them. The core's position decides how much flux couples into each secondary.

At the centre the two outputs cancel and the net is zero; displacement to either side unbalances them, giving an amplitude proportional to travel and a phase indicating direction. Because nothing touches, there is no wear and the resolution is limited only by the electronics - the advantage over a potentiometer.

Answer: D

The sliding contact is a wearing part and dirt or wear produces intermittent readings. Its simplicity and low cost keep it in use where the travel is infrequent and the life requirement modest. Cost and simplicity keep it in service.

Answer: A

Correct answer: Inducing voltages in a stator

A synchro has a rotor winding excited with alternating current and three stator windings spaced at 120 degrees. The voltage induced in each stator depends on the angle between it and the rotor.

The set of three voltages therefore encodes the shaft angle uniquely, and connecting a matching receiver reproduces the angle at a distance without any digital conversion - which is why synchros were the standard means of transmitting position in earlier avionics. The output is absolute: the angle is known as soon as power is applied.

Answer: B

Correct answer: It reports change, not position

An incremental encoder emits a pulse for each increment of movement, and a counter accumulates them. The count gives position only relative to wherever counting began.

So at power up the position is unknown until a reference is found, which is why such systems run a homing sequence. An absolute encoder instead has a coded track giving a unique pattern for each position, and reports it immediately. Incremental encoders do sense direction, through two channels in quadrature, and either type may be optical or magnetic.

Answer: C

As a speed pickup it responds to passing gear teeth or magnets without any mechanical contact, and it works down to zero speed. A variable reluctance pickup by contrast produces no output at all when the shaft is stationary.

Answer: D

Fuel has a higher permittivity than air, so the probe capacitance rises as the tank fills. Adding a densitometer lets the system report mass, which is what actually determines the energy available. Volume alone would not.

Answer: A

Correct answer: The output depends on the direction

Hysteresis means the reading at a given input differs according to whether that input was approached from above or below - the output traces a loop rather than a single line.

The usual causes are mechanical friction, backlash and internal energy losses in the sensing element. It cannot be removed by calibrating at one point, since there is no single correct value to calibrate to; it is quoted as a percentage of span and either accepted or designed out. Drift, noise and saturation are three separate specifications.

Answer: B

Because it develops slowly, drift is not visible from a single reading and is found only by recalibration or by comparison with another sensor. It is the main reason calibration intervals exist at all. Nothing else reveals it.

Answer: C

Correct answer: Excitation and amplification

Most sensors are passive and produce a very small signal, so the conditioner must both supply the excitation the device needs and amplify what comes back - millivolts from a bridge or a thermocouple.

It usually does more besides: filtering, linearisation, cold junction compensation for thermocouples, and conversion to a standard output. Isolation is often provided but is not the whole of the function, and the conditioner does not power the indicator or support the sensor mechanically.

Answer: D

Correct answer: Avoid a common mode failure affecting both

Two identical sensors share every design fault, every manufacturing batch defect and every susceptibility - to icing, to a particular contaminant, to a software error in a shared conversion routine. A cause that defeats one may defeat both at once, and the pair would then agree while both were wrong.

Dissimilar sensors, working on different physical principles, are unlikely to fail in the same way for the same reason. That is dissimilar redundancy, and it protects against the failure mode simple duplication cannot.