Originally Posted by
8che
There seems a lot you haven't found. Did you not notice the part that explicitly states only OAT not TAT (or SAT) is to be used for determining icing conditions on the ground. Its in the FCOM and AFM which forms part of the certification of the aeroplane.
And what is OAT? And what is TAT?
The phrase you are referring to is a phrase you will find in any Boeing commercial jet manual. You don't have to turn around the phrase and create limitations out of it. The reason why the phrase is written down like this in any manual is to point out that INFLIGHT you have to use a much more SPECIFIC tool: TAT, which is an exact indication on the flightdeck. ON THE GROUND, you use OAT. I tend to feel some people think OAT is a term relating ONLY to METAR temperature. If you would read the books even more detailed, you would notice that in performance manuals Boeing sometimes refers to Airport OAT - not OAT - to make a distinction. There is also referred to reported OAT. And actual OAT.
SInce you like to refer to books. I don't find Boeing anywhere specifying the OAT mentioned is METAR temperature. The term OAT is mentioned in their books multiple times on all fleets:
B737 classic/NG: in the FCOMs the difference between aspirated and unaspirated probes is explained, as well as the effect of the use of pitot heat and packs. I think there are also FCOM's that explicitely state indicated TAT on the ground is not to be used as an indication of OAT.
B744: FCOM states "TAT indication on the ground approximates OAT. TAT probe must be aspirated by bleed air".
B777: FCOM explains TAT is a calculated value by ADIRU and SAARU. It uses temperature probes and calculates. Take your FMC and look at page thrust limit page, top line. What does it say? Does it say OAT or does it say TAT? Where do these values come from? And my question remains: show me the line in the books about temperature calculations being unreliable on the ground. I'll admit 777 has been 7 years, I don't mind being told I missed certain points.
BTW: you do the same thing inflight with your crewmembers - telling them to go study the books because you are right above all?