to be accomplished. With regards to achieving descent restrictions, should the flight crew forget or incorrectly input the descent restriction or the descent speed or descent winds as described above, then the aircraft may be unable to achieve the descent restriction. Descent restrictions are entered into the FMS by typing the required numbers in the FMS scratchpad, selecting the appropriate waypoint to which the restriction applies, then pressing the‘ Execute’ button on the FMS keypad. If the restriction is to be‘ above’ or‘ below’ a level then, typically,‘ A’ or‘ B’ is added to the numbers respectively to achieve this e. g.‘ 230B’. Although the process of entering descent restrictions into the FMS is, in general, simple, when such descent restrictions are given by ATC they must still be given in sufficient time for the flight crew to enter( and check) the details in the FMS. It can also be seen that if a descent restriction is not given until very close to the( normal) TOD point but on this occasion a tailwind exists which has not been accounted for in the controller’ s calculation of the TOD point, then upon entering the details in the FMS the aircraft may have passed the ideal TOD point for those actual conditions. As explained, achieving the descent restrictions may therefore prove difficult or impossible. This situation is also exacerbated when the frequency is busy and therefore, after entering the details in the FMS in order to be given the TOD point prediction, there is an added period of time before the flight crew are able to make a transmission to ATC to request descent. There are also occasions when the process of entering descent restrictions into the FMS becomes a little lengthier. This often occurs when the restriction is given relative to an unknown waypoint or a waypoint that is different to that specified on the( flight crew’ s) arrival chart and is therefore different to what the flight crew are expecting. Should this occur on fam flights, controllers will observe that this often results in the flight crew having a lengthy period of‘ heads down’ FMS button pushing and confusion. It may be useful for controllers to note that not all descent restrictions are given on the( flight crew’ s) arrival charts and so in this case the flight crew are unable to pre-empt or appropriately prepare for any such earlier descent until such details are received from the controller’ s transmission. Also, as stated, some arrival charts show restrictions relative to a waypoint that is different to the waypoint that the controller specifies in their instruction( although the restriction may be physically at the same point in space).
As an aside, but a human factors point worthy of note, controllers may observe during fam flights that when controllers issue an instruction that includes a descent restriction, then the same or a different controller issues a subsequent descent instruction that does not include the previously stated restriction, flight crews are often confused as to whether the previously issued restriction still applies or not. Although it is standard procedure that the present instruction cancels all previous instructions, when controllers apply this procedure pilots can often wonder whether the controller has simply forgotten to re-state that the restriction still applies. In such cases flight crews will therefore usually still aim to achieve the previously stated restriction in order to err on the safe side. Finally, should the flight crew, after much effort of entering details into the FMS, forget to‘ execute’ the FMS entries, an appropriate TOD point will not be displayed on the ND. Consequently, should the flight crew not realise that the TOD symbol has not( suddenly) appeared on the ND or that the originally displayed TOD symbol has not moved by the required amount, then it is likely that the descent will not be commenced at the required point.