Owners of helicopters may have staff who utilise helicopter flight time which is not charged out. Not only is this practice dishonourable but is also dangerous as numbers of critical helicopter components are time expired and have a finite safe flight/use time.
Conventional engine hour clocks can be manipulated to obtain any desired reading. Battery operated flight time clocks can also be disconnected by for illicit use of the craft. To be absolutely sure or flight time and engine hours the monitoring device or clock must be totally tamper proof.
This design utilises the fact that helicopter blades and for that matter other propellors and engines make a large ammount of noise. It is feasible to have the engine hour/flight time monitor housed in a sealed metal box which is only open to the atmosphere via a water deflecting metal or other rigid material enclosure ( in a similar manner to a Dorado fresh air vent on a boat).
By allowing air pressure variations as noise to enter the enclosure ( which could only be opened by officials by breaking the seal) they act on a conventional cone type speaker of any conventional impendence (8-16ohm). Once the motor starts and the rotors start to generate lift the noise of both act to vibrate the speaker come in sympathy. The alternating current output of the speakers coil is passes through semiconductor rectifiers to generate DC which powers the electronics involved in the circuit. Once the noise starts and the blades generate thrust, the circuit can ' read' the frequencies involved and therefore make the electricity generation specific to the particular aircraft by active electronic filtering. The total 'on' time being stored in memory. When two or more aircraft are operated simultaneously in close proximity to each other they can be seperated by the simple means of placing the complete unit on another place on the helicopter( which changes the resonance and can be tuned and read by the electronic filter adjustment).
By this simple means owners and operators of fleet helicopters can ensure a better bottom line and the regulatory bodies can have total faith in flight/ engine time accuracy.