Altitude constraint during an approach

Altitude constraint during an approach


Since panel v1.50, FD-FMC use a « predictive » gradient calculation during approach. This new feature has some consequence with approach data in CSV file. Indeed, now, altitude value of each waypoint of an approach must be consistent, otherwise, aircraft behavior can be unrealistic.

There are cases where you need to be careful when entering altitude information for waypoints in your Excel worksheet :

  • When there isn’t a information altitude for individual waypoint
  • during an DME-ARC or a teardrop turn


Exemple with LFPG – Paris Charles de Gaulle Approach charts :




As you can see, the IAF LORNI, altitude constraint is FL110 minimum for a turbojet aircraft and the altitude at IF ILS 08R waypoint is 5000 feet. But the 3 intermediates waypoints, BUNOR, PG501 and PG554 have no altitude information.

In the FD-FMC system, each waypoint of an approach must have an altitude data; this is mandatory. With the previous version of FD-FMC (before 1.50), it was possible to set BUNOR, PG501 and PG554 with 5000ft altitude constraint, because FMC used a fixed vertical speed during descent with a standard gradient of 5,2%.

From v1.50, FD-FMC calculate for each leg the optimal gradient to join the next waypoint at the right altitude. So, in this case, if you set all these intermediates waypoints to 5000ft, when BUNOR becomes the active waypoint in the FMC, FD-FMC calculates the gradient to join BUNOR at 5000ft and aircraft will dive with a too much descent gradient.

Solution is very simple :

For each part of the appraoch that has no altitude information, calculate the distance and the altitude differential.

In our exemple, from LORNI to IF ILS 08R,

  • distance is 37,3 + 8 +10 + 11,5 = 66,9 Nm
  • Altitude differential is 11000-5000 = 6000 feet
  • gradient is 6000 / 66.9 = 89,68 ft per Nm

So now, altitude that you must enter for each waypoint is :

  • BUNOR : 37,3 x 89,68 = 3345 ft : you can rounded so altitude at BUNOR is 11000-3500 = 7600 ft
  • PG501 :  8 x 89.68 = 717 ft : Altitude at PG501 is 7500 – 700 = 6900 ft
  • PG554 : 10,1 x 89,68 = 905 ft : altitude at PG554 is 6800 – 900 = 6000 ft
  • IF ILS 08R : 11,5 x 89,68 = 1031 ft Altitude at IF ILS 08R = 6000 – 1000 = 5000 ft




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