It is strange to read "when one part of a glideslope beam is switched off and the other is left transmitting" in Capt T A Davies's letter "No Mystery about ILS signals" (Flight International, 1-7 April). That means "interlocking" and means the very basics of electrical engineering are not taken care of in instrument landing system instrumentation.

Why is the system not designed with interlocking "either both or none"? This should work when either of the beams is under maintenance or unserviceable. It doesn't cost anything to do it. Another way could be to provide the system in aircraft that would identify the presence of both the beams and correct any deviation in the centralised glideslope to overcome such situations.

Raj Singh Rathee India

Source: Flight International