look at these numbers., page-4

Pigs,

The instruments are only right if the Pitot Tube is clear, you dont have a leak in the Static source, and that they are calibrated for the type of flight you are about to conduct

Then the next problems for a pilot are weather briefing, weight and Balance and conditions varying on route, with finally making an approach VFR or IFR, only after all of that can we say we have had a sucessful flight

I think the POH analogy is similar, but we have only just checked we have the correct instruments, they are still doing their Rating and do not have permission to take off yet, so we can calculate all the wind variations and en route conditions that we like, but its of no benefit without the rest being in place