The snag with the SOP of "positive RATE", is that the only cockpit instrument that gives a
rate reading is the VSI/IVSI. Concentrating on the rate parameter might cause some pilots to only look at the rate indication, which might be false.
As the aircraft rotates, the static ports - usually situated around the front of the aircraft - will climb, even when the main gear is still on the runway, so this indication does not necessarily mean that the aircraft is climbing. Ditto the altimeter.
The Rad Alt aerials - on Airbus situated well behind the main gear - will inititally show a descent when the aircraft rotates.
Only when all three: VSI, Altimeter and Rad Alt are showing positive or increasing readings, can one be sure that the whole aircraft is climbing up above the runway. Take a moment to be sure all three indications are solid and not a momentary glitch, before calling positive CLIMB.
Another good reason for checking three instruments instead of one or two, is to check against a faulty instrument. All three should agree the aircraft is actually going up !
