Ww2 Lancaster Aircraft Instrument Standard Beam Approach Indicator

This is a WWII and Cold War-era Beam Approach Visual Indicator, Ref 10Q/4, Mod 27 was used in the Avro Lancaster

Per the Rochester Avionics Archives:

The Standard Beam Approach System is similar to the ILS system used in Modern aircraft. Beam Approach Beacon System – BABS – was a homing device used to help pilots land in poor visibility; it was fitted to the Lancaster, Halifax, and Whitley. Radio beacons on the airfield transmitted signals, which were picked up by aircraft on their approach to land.

The Main Beacon at the end of the runway transmitted a signal in the form of a narrow beam. When the pilot was lined up correctly on this beam the signal received was steady and he knew he was heading straight towards the runway. If he was too far to the left or right, the signal received was a stream of dots or dashes and he corrected his course until the signal changed to a steady pulse. So, azimuth and glide slope are both checked.

When the aircraft crossed the Outer and Inner Marker Beacons the pilot also detected their distinctive signals, which told him how far he was from the point of touch-down. He used this information to check his rate of descent until the ground came into view and he could make a safe visual landing. The system had some usage but the poor accuracy of the aircraft Altimeters made the system unreliable. A description of the system appears in Flight International in January 20th 1938.

This indicator is in exceptional condition given its age. The glass and all markings are intact. The two neon bulb holders are in place, but without the bulbs. Measures 3.25 inches diameter and ~4 inches deep.