Derate v assumed temp
PRO:
With a Derated thrust, the thrust set is the maximum that is available within the EEC/DEEC etc or by the TAT/EPRL limiting system.. etc. That precludes to an extent the firewalling of the thrust levers and introducing a greater asymmetry issue in the event of a thrust loss. Directional stability not he runway may be constrained where there is low friction coefficients.... In an Assumed Temp case the thrust can be increased to the rating limit, and the friction coefficient may exacerbate directional control on the ground. hmmm. maybe..
CON:
Derate removes the opportunity to get more thrust beyond the derate level that is selected. That is, considerable thrust potential is being removed. That shouldn't be a factor in the normal course of operations, but then blowing cowls off, having compound failures etc are a repetitive theme is aviation.
AC25-13 was issued in 1988, and was a fair GM at that time.
§4(c) defines reduced thrust takeoff,
§4(e) describes contaminated runways.
§5(f)(1) gives the prohibition.
§5 is an AMOC, that doesn't prohibit an operator making a case for using an ATM instead of DRT case.
This happens to be a point of interest in my current testing, as I am impacting thrust output of the engine considerably, for the same N1 setting the aircraft was getting ~20% more thrust output, and to get normal thrust output took a 5% reduction in N1. This then makes the case that the thrust itself has not reduced at that point, even though N1,N2, EGT, ff have been reduced, the thrust itself is being set at the same Fn output as the normal engine. That then permits up to a. further 25% reduction in thrust, within the GM of the AC, at which point the N1 etc is rather low, low enough to have to look at some cold weather issues with GPWS mode alerting.
The AC's position was to provide a rational prohibition of using an ATM reduced thrust in substantially adverse conditions. That is not a bad position. It is not rational to preclude a use of a DRT which is otherwise a legal rating that performance is predicated on, so the AC doesn't restrict the use of a DRT, an AC cannot be more restrictive than the regulation which the DRT meets in principal. The argument on the impact of increasing thrust doesn't survive cursory examination, as the contrary case is a greater concern to controllability, that is, higher thrust levels lead to greater potential control difficulty on a contaminated runway, on the sudden loss of an engine...
The balance of risks on thrust setting being higher or lower for a contaminated runway relate to the potential of an engine to fail at any time, of reduced acceleration that may be random from contamination, of reduced deceleration from variations of friction coefficients, impacting braking, and the issues of control with asymmetric T/R. On the day, the guidance needs to consider the runway width, excess runway lengths, risks of an overrun condition etc, which are bespoke to the day and location, weight and weather. And then there is the MD-80 inboard wing snow feeder system to the engine intakes... as found on CRJ's, CL600's, most business jets etc... All adding to the fun of operating under uncertainty, although, hint, anti icing HOTs on aircraft with aft engines, or without leading edge devices takes on a certain piquancy.
That's why the manglers get paid the big bucks to guess what the best answer is for the lawyers years later, and why the pilot gets paid the big bucks to take the fall of such policy and training, and to be the first on the scene of an accident.