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 .