I would di-ice with any frost present or likely between stand and getting airborne, making sure the holdover time was enough for the conditions, but I fly a TP and maybe the 767 / 757 has a system to warm the area of near the cells??
If the book says it is ok - it is ok....ours does not.
With high lift slats / LE flaps the boundary layer is extremely energized and separation leading to wing stall is unlikely on that aircraft, with frost on the cells anyway. Other aircraft, however, are not so fortunate and with rear mounted engines and a T tail - a definite no-no.
The fuel is heated from access heat from a heat exchanger from either excess bleed air or from the engine oil. This thaws any ice crystals that would clog the filters, bringing the fuel to approx room temp, which may be enough to thaw the hoar frost on the cells on the 767 / 757.
Last edited by PAPI-74; 31st Jan 2009 at 12:44.