Q Why is it that on a contaminated Runway a take off with Derate is allowed but Assumed temperature is not?
Derate is a limit power setting, which in the context of the question would be maximum thrust.
Assumed temperature calculates reduced thrust settings, which are not authorised
CS 25 Large Aircraft Amendment 5 AMC 25-13 Reduced And Derated Take-Off Thrust (Power) Procedures (page 739),
f. (1) Are not authorised on runways contaminated with standing water, snow, slush, or ice, and are not authorised on wet runways unless suitable performance accountability is made for the increased stopping distance on the wet surface.
Also, see:
Q why isn't autoland allowed on a contaminated runway?
Autoland performance usually has a specific landing performance section - The landing distance required must be established and scheduled in the aeroplane Flight Manual if it exceeds the distance scheduled for manual landing. (JAR-AWO 142).
CS 25 AMC 25.1591 (p 665), provides the basis for landing performance on contaminated runways. There are assumptions about flare distance/time which may not be achievable with an autoland system, thus specific calculations / tests would have to be conducted to establish the performance. It could be argued that this should also apply to HUD landings and non HUD manual landings when using a long body 3 bar VASI.
Contaminated landing performance can claim credit for reverse thrust (differing from wet), if so then this might require demonstration (proof) of a higher reliability for obtaining reverse during an autoland (the Midway landing accident report might refute wisdom of considering this).
Info:
AIC 86/07. 1.1 Operations from contaminated runways, by all classes of aeroplane, should be avoided whenever possible.
2 6 It is difficult to measure, or predict, the actual coefficient of friction or value of displacement and impingement drag associated with a contaminated runway. Therefore, it follows that aeroplane performance relative to a particular contaminated runway cannot be scheduled with a high degree of accuracy and hence any 'contaminated runway' data contained in the Flight Manual should be regarded as the best data available.
8.3.
A landing should only be attempted in these conditions (
wet contaminated) if there is an adequate distance margin over and above the normal Landing Distance Required and when the crosswind component is small. The effect of aquaplaning on the landing roll is comparable with that of landing on an icy surface.
Autoland on Contaminated Runways.