Originally Posted by Tagron
As I recall a failed autoland or coupled ILS was one of the factors in the BOAC VC10 low fuel diversion to the military field at Bedford. Massachusetts, not Bedfordshire.
Do you have any more info on that incident? I haven't heard of it yet and am interested for my website.
From this same site, here's a bit from an ex-Avionics Engineer about the demise of the VC10 Autoland (his full story is here:
http://www.vc10.net/Memories/radio_development.html):
A Cat 3 autoland system was designed by BAC for the VC10. This was a duplicate fully monitored system of Byzantine complexity. Remember this was before the digital age, it was all magnetic amplifiers and analogue computing. Effectively this meant that each function was done four times, and all had to work for the system to meet Cat 3 standards which of course they rarely did. The Radio systems bore most of the blame at the monthly project meetings and as I was the sole wireless man at the meetings surrounded by twenty instrument and autopilot experts from BOAC and BAC I didn’t stand much of a chance. It was decided that the Radio Altimeters needed improving, not surprising really as they were made by STC to a design that must have been obsolete 20 years previously. The quote from STC for a modification package was horrendously expensive so I got lumbered to do it. I replaced all the coax cable with a special low loss version with a silver plated foil screen that was almost impossible to work with. We must have scrapped more cable than we used. We matched all the mixer diodes by individual selection and redesigned the monitor system completely. After about a year’s hard work the Radio Altimeters were working perfectly but the autoland system still did not perform. BAC then came up with a proposal for a package of modifications to all the other black boxes at a cost that made STC’s proposal seem small fry. By this time I felt a little more confident as I had proved the point with the Radio Altimeters and as I had some spare time I sat down at my desk and calculated the probability that all of the boxes would remain working for the four weeks required on average to certify an aircraft to Cat3 standard after a defect. As I recall this worked out that on average a maximum of one aircraft in the fleet would be certified for half the time. I could not believe this result, so I rechecked the maths, re-read the books but could not get a better result. We had already spent millions on the system and were planning on spending several more. I could not believe that my sums were correct, but I wrote it up anyway and went thus armed to the next project meeting where the big mod package was planned to be agreed.
I dropped the bombshell at the start of the meeting by asking the BAC experts what their planned system reliability was. I got a few blank stares, they seemed to have experts there on everything except reliability, but it was agreed that they would show my workings to their specialists at Weybridge and find the errors. Much to my surprise, the mod program decision was deferred. The next meeting they announced that my numbers were wrong, I had omitted to take into account the system wiring reliability so the situation was actually slightly worse than I predicted. VC10 Autoland was promptly scrapped and work started on another mod program to save weight by removing all the surplus equipment. I never got any thanks for saving the corporation all those millions, possibly because I asked the Project Manager who had signed the contract; which had no guaranty that it would work or compensation if it didn’t. 'Me' he replied and walked away.