The passive or non-emissive Airband Receiver is basically an amplified "crystal radio" designed to receive nearby AM aircraft transmissions (121 - 133 MHz).
The "passive" design uses no oscillators or other RF circuitry capable of interfering with aircraft communications; so it should receive on board VHF "comms" from within the cabin of the aircraft.
Useful for "listening-in" to the pilot if you are on a commercial flight.
Because of low sensitivity, it is a short-range receiver only.
You cannot 'pick-up' two-way aircraft transmissions with this receiver!
The input tuned cct 'L' is a 2 turn loop, 30mm dia (measured at 0.15uH on my L-C Meter ) which intercepts RF directly as opposed to an LC cct fed with external aerial
Tuning capacitor is a 30pF Philips Beehive trimmer, with a short length of plastic tube glued - as a tuning shaft.
Capacitance (meas' on LC meter) runs from 28 to 7pF; which by formula gives a freq' range of 77 ~ 155MHz
Detector uses a biased 1N5711 (or similar) schottky diode (lowest forward-biassed voltage drop). The two 10M resistors bias the detector diode and the op-amp input near mid-rail for better detector efficiency
The LM358 dual op-amp draws less than 1 ma so the battery drain is minimal.
Insertion of earphones plug completes supply circuit and therby acts as an on/off switch.
The componentry (inc battery) fits neatly inside a 30mm PVC tube 130mm long

I acknowledge inspiration from the following webpages:
Passive Aircraft Receiver
Non-Emissive Airband Receiver
Non-Emissive Air-Band Receiver (Implementation & Use)