Originally Posted by
BraceBrace
The rise in temperature is connected to the assumed, and therefore there is a performance margin. If it turned out to be a performance limiting takeoff, the question would not be asked, it would be visible as the temperature would rise above the assumed temperature. With a different QNH this is not that visible. The 10 is an arbitrary number. You cannot simply ignore.
I understand and agree on your reasoning in performance cases with large assumed temperatures. But there are also many takeoffs done close to the limit performance. And basic rules have to cover all situations.
Previously Airbus had limitation on flex temp being higher than OAT by 6° or 9°C to avoid a situation where during taxiing the OAT could rise above flex. But Airbus removed that after it carried out a mod by which if that situation arises then then on setting flex pilot is warned to set TOGA and if not done TOGA is set automatically. As far as QNH is concerned it's corrected in blocks of 10hPs and not interpolated so it's not that critical