Question ASI 1 asks what is indicated by the “Barbers’ Pole”. The first three options (MMO, Vne and TAS) are all incorrect. The fourth option “temperature and VMO” is partly correct, in that the Barbers’ Pole does indicate VMO. So this is the best option in this question.
Question AIRSPEEDS 20 also asks what the Barbers’ Pole indicates. In this question the first two options relate to the autothrottle, so they are incorrect. The third and fourth options are “Maximum speed in VMO operation versus altitude” and “Maximum speed in VMO operation versus temperature”.
To select the best option we need to consider how the Barbers’ pole will respond as an aircraft climbs to altitude.
Between ground level and the altitude at which VMO is equal to MMO, the air temperature will decrease, causing the TAS and CAS values of MMO to decrease. But at these low altitudes MMO is considerably greater than VMO, so the reducing temperature will have no immediate effect on the Barbers’ Pole indication. But increasing altitude will also cause the air pressure and air density to decrease. This will increase the value of CAS at any given EAS. The pressure loads acting on the aircraft are determined by EAS, so the CAS value of VMO will increase slightly. This increase will be very small so the movement of the Barbers’ pole may be negligible. So overall, in this part of the climb altitude will increase, temperature will decrease, but there will be no significant movement of the Barbers’ Pole.
As the climb continues above the altitude at which VMO is equal to MMO, the decreasing temperature will cause the TAS value of MMO to decrease. The reducing air pressure and density will also cause the CAS at any given TAS to decrease. The overall effect will be that the Barbers’ Pole indication will decrease. So overall in this part of the climb altitude will increase, temperature will decrease and the Barbers’ pole indication will decrease.
As the aircraft climbs above the tropopause the temperature will remain constant. This will cause the TAS value of MMO to remain constant. But the reducing air pressure and density will cause the CAS at any given TAS to decrease. So the Barbers’ Pole indication will continue to decrease. So overall in this part of the climb the temperature will be constant, while the altitude increases and the Barbers’ Pole indication decreases.
The above discussion shows that movement of the Barbers’ Pole is related to temperature only at altitudes between that at which VMO = MMO and the tropopause. But the relationship between altitude and the position of the Barbers’ Pole is evident over a much wider altitude range.