This is a very old JAR ATPL question which has been around for about 15 years.
For max climb angle we need max excess thrust. For max roc we need max excess power. These conditions occur at vx and vy respectively.
During any acceleration we are using some of our thrust and power to accelerate, so this thrust and power is not available for climbing. So while we are actually accelerating our climb angle and roc will be reduced.
If you have difficulty with this idea just imagine that we have two identical aircraft, same weight, same altitude, same configuration, same power setting. One is flying at a steady speed at Vx. The other is accelerating through Vx. How will the angles of climb compare? Now repeat the test at Vy, how will the rates of climb compare. In both cases we would find that the climb performance of the accelerating aircraft would be less than that of the aircraft at steady speed. If we now repeat that test at any speed we will find the same result.
The question is not saying that continuous acceleration causes a continuous reduction in climb angle and roc. It is saying that at any moment during an acceleration our climb performance is less than it would have been if we had not been accelerating.
Last edited by keith williams; 20th May 2017 at 10:29.