order to regain the ideal profile . Note that , due to aerodynamic equations , the effectiveness of the speed-brake is a function of speed and therefore its effectiveness becomes less as the speed is reduced . At speeds below 250 knots the effectiveness of using the speed-brake to recover from being above the ideal descent profile is often negligible . The mindset that the speed-brake can always be used to recover from being above the ideal descent profile should not therefore be encouraged , particularly when the situation in question is close to the final approach . During or approaching the final approach , if the aircraft is above the ideal descent profile , often the undercarriage will be lowered earlier than it would otherwise be as this creates a great deal of drag and therefore significantly steepens the descent path . This is often the case when turning onto the final approach and being above the ideal final approach path ( e . g . intercepting the ILS and being “ above the glide ”) or when flying downwind and changing to a ‘ visual circuit ’ from the originally planned ( longer distance ) ‘ instrument pattern ’.
FMS Descent
The FMS will produce a ‘ Top Of Descent ’ ( TOD ) point . This is typically displayed on the Navigation Display ( ND ) as a ‘ TD ’ symbol . The calculated TOD point will be such as to produce a continuous idle power descent from the cruising level until a point on the final approach where the undercarriage and landing flap are deployed and the thrust is increased as required to maintain the final approach profile . The exception to this being where descent restrictions are entered into the FMS , the FMS will therefore alter the descent profile accordingly in order to achieve the descent restrictions . The calculation of the TOD point will account for the distance required to reduce the speed to the final approach speed and will include other factors that affect the distance required for descent such as tail / head wind component and the descent speed ( as given by the cost index ). Note that , due to the laws of physics , an increase in descent speed produces a decrease in the distance required for descent . Although with such an increase in descent speed this will therefore produce an increase in the distance required to reduce speed to the approach speed ( or zero flap speed as used in the descent distance formula ), an increase in descent speed will produce an overall decrease in the descent distance . Therefore , the cost index will have an affect on the TOD point .
The correct descent winds must be entered into the FMS by the flight crew in order for the FMS to calculate a realistic TOD point . If the descent winds are entered incorrectly , and therefore do not replicate the conditions that will be experienced in reality , any such predicted TOD point will therefore be inaccurate . If the inaccuracy is such that the actual tailwind experienced is greater than that entered into the FMS then this will mean that the FMS calculated TOD point occurs later than the required TOD point for the actual conditions . In such a case , if descent is commenced at the FMS calculated TOD point , the aircraft will be above the descent profile required for the actual conditions and may therefore be unable to achieve subsequent descent restrictions or be unable to become stabilised on the final approach ( see later section with regards to the criteria for a ‘ stabilised approach ’). Depending on the severity of the inaccuracy , the use of other measures , such as using the speed-brake , may still render it impossible to achieve descent restrictions or a stabilised approach . It is therefore important that the flight crew check that the FMS descent speed is appropriate and that the FMS descent winds are correct in order for a successful and comfortable descent and final approach