The report:
Investigation: AO-2012-022 - Inadvertent thrust lever asymmetry during the take-off roll involving an Airbus A320, VH-JQX
It is well written and well worth close study. It took a year to publish which is surprising for such a relatively simple incident. Seems during the takeoff roll a small asymmetry between the two thrust levers was noted. The left seat pilot was under training for a command while the right seat occupant was his instructor. Without going into details here (see the ATSB full report), the captain under training heard what he thought was the word "Rotate" from the instructor (PNF) and commenced rotation before realising the airspeed was still 20 knots below the correct VR. The aircraft became airborne but the pilot was able to pick up speed by reducing the climb attitude.
Rotating on the call from the PNF of "Rotate" is something often seen in simulator training even though the airspeed indicator of the PF shows well below the correct VR. It goes to show that the word "Rotate" in a firm voice can very much sound like a command, rather than just a support or advisory call by the PNF. Of course, in real life and no significant airspeed difference between the two main ASI's, "Rotate" is invariably ties in with what the PF expects to see on his ASI.
In the old days, the calls were simply "V1.....VR". Boeing changed "VR" to the new term of "ROTATE" and while that has been in use for decades I wonder how many unreported incidents have ocurred where "Rotate" called in a firm voice has caused momentary confusion if an ASI defect is present and the tendency is to respond to a firm voice (in this case "ROTATE") rather than on a correct bugged airspeed.
But there is a powerful tendency to start rotating at the word "Rotate" depending how forcefully the word is annunciated and despite the fact the airspeed on the side of the PF has not yet reached the correct VR. In other words, because things happen so fast approaching VR, it is a human factor problem where it is all too easy to act on the PNF's support call rather than rotate on the bugged VR on the PF's ASI.
In the simulator, this is most likely to occur when the instructor introduces an airspeed error between the two main ASI's with the PNF calling "rotate" on his own ASI despite the airspeed of the PF being nowhere near VR. Invariably we observe the PF then start pulling back on the control column in momentary confusion even though his ASI may be nowhere near his bugged airspeed. This is where it is good airmanship to be aware of the expected ground speed at VR as a double check.
ATSB missed a good opportunity to emphasise the need for the PF to plan on rotating on his own ASI bugged VR and treat the PNF support call of "Rotate" as purely advisory. if the PF reaches his own bugged VR speed but fails to get a support call from the PNF for whatever reason, he simply rotates (a quick glance at the ground speed indication is wise at that point). There should be no confusion and the problem sorted out at a safe altitude.