At a guess either the autoland was inop or one of the crew wasn't currently trained for autoland. Having just typed this, a quick look in the anorak log has 15 Tridents diverted in total so maybe there was another reason. I don't know the landing capabilities of the autoland equipped Tridents but I doubt it was better than the current ILS Cat 3A limit of 200m RVR touchdown, so perhaps the LHR vis was below that.

A number of ducks have to be in a row for autoland, or Cat IIIA/B as we now call it. Both crew qualified and current, aircraft systems on line, all required ground equipment and lighting serviceable. Take any one duck out and it cannot happen. Not sure of the state of play of Heathrow's kit back in 1975.

Incidentally, Luton was the first airport where the technical services were not operated by NATS to go CAT III in 1989 or 90. We sweated blood.

Edit: cj beat me to it. Wiki suggests the Trident fleet went from Cat IIIA to IIIB in 1975 so we are talking of 75m RVR touchdown.

For the Trident I think all three flight crew would have to be Cat III qualified.

Were all the Trident variants autoland capable or just the later types?

Since all the work was done on Trident 1 aeroplanes and it was certificated before the other variants entered service it is fair to say that all Trident marks could have been fitted with autoland. Not all examples were, of course, and I'd say it was not Cat IIIC on all those that had autoland.
Here's the famous 1968 film with Jimmy Phillips talking to the camera while the aeroplane gets on with a landing:
https://www.youtube.com/watch?v=C6ilhtHLqOI

OY-APU leased to Monarch from Maersk as G-BBZG from 02-02-74 to 12-12-75. The aircraft then returned to Maersk. That would tally with the December 75 date for the photo.
Temps.

Looks like a SABENA 737-200 pointed out also.

Here is another photo taken on the same day. Noted is another DC8 performing short haul duties but this one seems shorter than the KML example on the last page. The small size might be an optical illusion due to what seems like large well spaced windows so there is less of them compared to the 737 behind it. I always assumed the DC8 was built for long haul.

http://i1320.photobucket.com/albums/...ps6pnifwju.jpg
David Gearing collection

Alitalia's DC-8s were mostly Series 40 (as is the one on the photo).

So were most of KLMs. but the one in the earlier photo is a Series 63, stretched by nearly 40 feet compared to the Series 40.

The DC-8-40's were powered by the Rolls Royce Conway engines, which links nicely with the aircraft climbing away in the background with the same powerplant - a VC-10 of the RAF, probably operating a trooping flight.

KLM DC-8's were 30, 50 and 63 series. Only Air Canada and Canadian Pacific bought the 40 series apart from Alitalia, with the US carriers remaining faithful to the P&W engines.

You're right, my mistake, KLM had the 30 and 50 whereas Alitalia had the 40 (all dimensionally identical).

If you zoom in on the registration in the picture the original identity N736US can be read indicating its Northwest Orient ancestry.

As it is a diversion day, would the VC-10 be BA?
Note the height of the VC-10. Not many aircraft would be that high so soon, very good short field performance when on a light weight positioning flight to Heathrow.

Up on the Ponderosa there is G-BCBA B720-023B in Invicta colors. This a/c went to Air Nuigini in Feb 16. It returned to the UK in 1977 and then operated for Monarch.

What airline is the one behind the BMA viscount with the tail sticking out?
Looks like a dc-9
Great to see the old Aer Lingus decals or as the would say now Retro!!

I make the line of aircraft:-

DC-8 Alitalia
B737 Aer Lingus
Trident BA
B737 Aer Lingus
Viscount BA
DC-9 Alitalia
Trident BA
BAC 1-11 BA
B720 Invicta
BAC 1-11 unknown

Also, an Aero Commander, something small and a Cessna twin on the left.

I would hazard a guess the 1-11 is Monarch with the yellow and black cheatlines with monarch titling all in lower case on that tailplane. I

I believe the furthest BAC111 may be a Monarch aircraft.

Oops too late. Compton spotted it already!

This is the last of the photos taken on 15 December 1995
http://i1320.photobucket.com/albums/...psrorp3atx.jpg
David Gearing collection

1995? Presume 1975?