Originally Posted by IO540
OK, I see it now. That wouldn't help. As a designer of electronic gear for some 30 years, I can say the system could fail with a half-dot indication almost as easily as with a perfectly zeroed indication. Especially if the half-dot offset was designed-in as the default indication when captured.
There is no way to solve this problem comprehensively, other than some kind of digital verification (with appropriate security) transmitted by a separate monitoring unit.
Do airline autopilots fly with the indicator (the HSI, or the flight director) perfectly centred? GA autopilots certainly don't; well not unless it is dead calm.
Thanks for the reply. Apologies for not getting back sooner... been busy.
I think what you are saying is that if the lobe transmitter was re-calibrated as I suggested then a failure would still show half a dot fly down. Actually that's not what I meant.
I merely suggest that if you intercept the G/S as normal, but delay the descent until you get a 1/2 dot fly down indication then this would provide a failure indication of the G/S. The reason being that if the lobe transmitter had failed, you would always get a centered needle on the G/S indication even after you had flown right through the glideslope.
My suggestion to re-calibrate the display is simply to do with the aircraft. If the display were recalibrated so that a centred indication was given when the aircraft was, in fact, flying 0.17 degrees above the glideslope, then pilots (and autopilots) would not have to modify their normal procedures. In this case if the lobe transmitter failed, then the pilot would see a permanent 1/2 dot FLY UP indication.
Either way, no descent could occur because the aircraft would continue to fly straight and level trying to intercept, until someone twigged the DME was indicating they had flown through the glideslope. Much safer, I think, than starting a descent before the DME starts screaming at you.
In terms of failure indication, you are quite right that separate monitoring is the only sure way to know. But in the case of this incident, the ILS was notamed as "unmonitored" - meaning that the normal fail-safes were out of the loop. Since the pilot has no failure indication other than the flags, this particular mode of failure was impossible to identify in the cockpit.
My suggestion to fly the procedure with a 1/2 dot fly down indication would provide an additional means of identifying such a failure.
You're also right of course that it would take a very skillful pilot to fly a perfect profile and perhaps the "perfection" of a "perfectly centred" G/S should be treated with suspicion... However this particular crew were coupled to autopilot and would be used to flying a very accurate profile, which in itself did not indicate anything unusual.
Regards